Explaining the Drake Equation on a smaller scale

Explaining the Drake Equation on a smaller scale

So me and my friend were planning a video to explain the Drake Equation (within a time limit of 5 minutes), and we needed some help. This video is aimed at explaining the concept to an age group between 13-18 years, and having gone through loads of articles, we felt that a lot of the common audience would not be able to comprehend the concept.

So our question was, can we take the Drake Equation and try to explain it with maybe a real life example and on a smaller scale? It is an incredibly interesting equation and we felt like doing a good job of explaining it to a common teenager.


No need to make it complicated: what about this…

Just scribble a rectangle on a piece of paper, and say "there are 100 billion stars in our galaxy"…

Then, color off (let's say) 1/3 of the rectangle, and say "only one third of those are the sort of star that could have life, so that's blah billion"

Then, color off (say) 9/10ths of that box, and say "we believe about 90% of those have planets - so that's blah billion"

Then, color off (say) 1/20th of that box, and say "of those with planets, it seems that about 1 in 20 have Earth-like planets. Now we're down to blah billion… "

and so on.

(Note: the Drake equation has a number of fairly silly terms relating to "nuclear war!", which were added as political sops in that era; suggest ignore these unless you want to sound 90 years old!)

So just scribble a box or draw a line on a piece of paper… or maybe use "a bag of marbles" as the other answer suggests.

Just BTW there is in fact an entire documentary (I noticed it on "Netflix") called "The Drake Equation" which does exactly what you say…

… it is not really very good as I remember. I think the guy simply draws a line in the ground, to do the "fractions" demo, you know? (ie, they just erase more and more of the line). It doesn't need to be more complicated than that.

It's worth noting that the Drake equation simply points out:

(i) if you multiply those three or four fractions together, you get the number of civilizations in the galaxy. Which is self-evident.

but, the whole point is

(ii) we have utterly no clue - not even vaguely - what most of the fractions are,

You could say it's a written formula, which, helps clarify our thinking on, something we are utterly clueless about. So rather than just vaguely saying "we're utterly clueless," we can speak more clearly about the nature of our cluelessness!

although interestingly,

(iii) very admirably, the issue of "How many stars have planets?"… one could say that issue has been somewhat settled these very years, as we speak - that's great.

As the Drake identity (it's not an equation) is just a trivial exercise in combinatorics, I'd suggest the simplest, most commonly used model in combinatorics: The urn.

You have a number N of balls in an urn. Those represent the stars in the galaxy. Of those only a fraction is green, the rest is red. Green signifies "has planet", red "doesn't have a planet". You take the fraction of green ones that then host a planet in it's habitable zone and so on, for every characteristic the Drake identity describes.

In the end you just count how many balls with all desired characteristics on them you've taken out of the urn, relative to the total number of balls in the urn.

If you write the corresponding fractions in your video side-by-side with the undesired balls disappearing, it should increase the understandability further.

What is the Drake Equation: the math that predicts how many alien civilizations are out there

Are we alone in the universe? This is one of the biggest questions science is trying to answer. Many are inclined to think that there is indeed life beyond Earth. After all, there are billions of galaxies each with billions of stars. Surely, among them, there must be other Earth-like planets and sun-like stars capable of seeding life.

But however groundbreaking finding microbes on another planet would be, that would pale in significance to making contact with another alien civilization.

There’s actually a way to estimate how many alien civilizations may reside in the Milky Way thanks to a statistical model developed in the 1960s known as the Drake equation, named after astronomer Dr. Frank Drake.

What is the Drake Equation

The Drake equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester.

The Drake equation is a probabilistic method for estimating the number of advanced extraterrestrial civilizations (N) that harbor technology capable of communicating their existence.

The Drake equation itself is:

  • R * represents the average rate of star formation in our galaxy
  • fp is the fraction of stars that have planets
  • ne is the fraction of planets that orbit their parent stars in the habitable zone, also known as the Goldielocks zone, i.e. can potentially support liquid water at the surface and life
  • fl is the fraction of planets that could support life and actually do develop life at some point
  • fi is the fraction of planets that harbor life that evolves into an intelligent species capable of founding civilizations
  • fc is the fraction of civilizations that develop technology that emits detectable signals of their existence into space (i.e. artificial radio signals)
  • L is the length of time for which extraterrestrial civilizations release detectable signals into space (before they may go extinct, for instance)

The story of how the Drake equation can be traced back to the early 1950s when radio astronomy — the study of celestial objects’ radio frequencies — became more widespread. If they could detect radio signals from pulsars and far-away galaxies, they should also be able to detect artificial extraterrestrial signals, scientists thought at the time.

Frank Drake posing beside his famous equation. Credit:

First, scientists listened to artificial radio signals from Mars. Then, in the late 1950s, physicists Giuseppe Cocconi and Philip Morrison argued in a milestone paper that radio telescopes had become sensitive enough that they could detect radio transmissions from other star systems. The pair of scientists further argued that some of these messages would likely be transmitted at a frequency of 1420.4 Mhz, which corresponds to the wavelength of neutral hydrogen. Since hydrogen is the most abundant element in the universe, it would only be logical for an advanced civilization to broadcast its existence at this frequency to other star systems.

Dr. Frank Drake, a young astronomer at the time, had independently reached the same conclusion as Cocconi and Morrison. In spring 1960, Drake embarked on the first microwave radio search for signals from another solar system, aiming the 85-foot antenna of the National Radio Astronomy Observatory in Green Bank, West Virginia, tuned to 1,420 Mhz, in the direction of two nearby Sun-like stars.

Shortly after, at a meeting at the Green Bank facility in 1961, Drake had a speech in which he revealed for the first time his famous equation as a way to stimulate scientific discussion and interest around the search for intelligent alien life.

“As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms,” Drake said.

These pioneering efforts sparked the Search for Extra-Terrestrial Intelligence (SETI) movement spearheaded by Soviet and American scientists. By the late 1980s, large-scale SETI projects were established that examined thousands of sun-like stars at a time, culminating with NASA’s Project Phoenix — the world’s most sensitive and comprehensive search for extraterrestrial intelligence. Project Phoenix eventually moved to the now-defunct Arecibo observatory, which collapsed last year in Puerto Rico, and scanned nearly 800 stars, all within 200 light-years distance, at frequencies between 1,200 and 3,000 MHz.

Although SETI scientists came back empty-handed and interest (along with public funding) waned, the search for intelligent life continues. The privately-funded SETI Institute is currently building a dedicated array of telescopes that will equal a 100-meter radio telescope, known as the Allen Telescope Array (ATA). This will be the first radio telescope designed from the ground up for the sole purpose of performing SETI searches. The first 42 elements have already been installed at the Hat Creek Observatory, situated in the Cascade Mountains about 300 miles north of San Francisco.

So how many advanced alien civilizations are out there?

The Drake equation isn’t exactly rooted in hard science and is more of a speculative framework. Over the years, the equation took a lot of flak from the scientific community due to the many assumptions it makes. For instance, the first exoplanet was only discovered in 1992, more than 30 years after Drake proposed his equation.

Although we can estimate some factors of the equation with relatively high confidence — we know for instance that there are about two trillion galaxies in the known universe and that the Milky Way is home to 200 to 400 billion stars — other variables are far more uncertain. In particular, the odds of an exoplanet in the habitable zone actually hosting life is perhaps the most difficult to gauge since we know of only one planet so far capable of doing so, Earth.

Drake’s equation was made famous by the late Carl Sagan, who featured it during an episode of his timeless series Cosmos. But since Sagan first talked about Drake’s equation, much has changed. Thanks to observations by the Kepler telescope, we now have a much firmer grasp of how many Earth-like worlds may be out there.

Crunching the Drake equation with various values, the number N of advanced civilizations in the Milky Way ranges from as low as 0.000000000091 (we are probably very much alone in the Milky Way) to as high as 15,600,000 (the Milky Way is home to millions of distinct intelligent civilizations).

In a 2016 study published in the journal Astrobiology, Adam Frank, professor of physics and astronomy at the University of Rochester, and Woodruff Sullivan, an astrobiologist at the University of Washington, looked at this question from another angle.

Rather than asking how many civilizations may exist in the Milky Way — the main premise of the Drake equation — the two scientists calculated the odds that humans represent the only technological species that has ever arisen. Flipping the question means that rather than guessing at the odds of advanced life developing, the two calculated the odds against it occurring in order for humanity to be the only advanced civilization in the entire history of the observable universe.

“This shifted focus eliminates the uncertainty of the civilization lifetime question and allows us to address what we call the ‘cosmic archaeological question’—how often in the history of the universe has life evolved to an advanced state?” said Sullivan.

By applying the most recent data on exoplanets at the time, Sullivan and Frank found that the odds that human civilization is unique in the cosmos (2吆 22 stars) is about one in 10 billion trillion, or one part in 10 22 .

“One in 10 billion trillion is incredibly small,” says Frank. “To me, this implies that other intelligent, technology producing species very likely have evolved before us. Think of it this way. Before our result you’d be considered a pessimist if you imagined the probability of evolving a civilization on a habitable planet were, say, one in a trillion. But even that guess, one chance in a trillion, implies that what has happened here on Earth with humanity has in fact happened about a 10 billion other times over cosmic history!”

The Drake equation and Fermi’s Paradox

But if that were true, where are all the aliens? This is the question that physicist Enrico Fermi, the inventor of the world’s first nuclear reactor, asked as well when he posited his famous Fermi Paradox — the notion of how there are a virtually limitless number of stars, but you don’t see much life floating around.

The question is a valid one when considering:

  • There’s nothing special about our sun – it’s young, medium-sized and similar to billions of other stars in our galaxy.
  • It’s believed there are between 100 and 400 billion planets in the Milky Way. Considering intelligent life appeared on one, it’s reasonable to consider there should be at least some other kind of intelligent life elsewhere in the galaxy.
  • Millions of years of technological progress mean that an intelligent species should have the capability to travel to distant stars and even other galaxies. Just look at how our world has changed in the past 100 years alone.
  • According to mathematicians Duncan Forgan and Arwen Nicholson from Edinburgh University, self-replicating spacecraft traveling at one-tenth of the speed of light — admittedly a quick speed — could traverse the entire Milky Way in a mere 10 million years. This means that civilization could potentially colonize the whole galaxy in a mere couple of millions of years. Except it didn’t happen.

The most straightforward explanation for this paradox is that the vast majority of these advanced alien civilizations, if not all, went extinct. Perhaps we too will go extinct as fast as we came into this world.

“The universe is more than 13 billion years old,” said Sullivan. “That means that even if there have been a thousand civilizations in our own galaxy, if they live only as long as we have been around—roughly ten thousand years—then all of them are likely already extinct. And others won’t evolve until we are long gone. For us to have much chance of success in finding another “contemporary” active technological civilization, on average they must last much longer than our present lifetime.”

“Given the vast distances between stars and the fixed speed of light we might never really be able to have a conversation with another civilization anyway,” said Frank. “If they were 20,000 light years away then every exchange would take 40,000 years to go back and forth.”

The inevitability of self-annihilation of intelligent life is an opinion shared by scientists at NASA’s Jet Propulsion Laboratory and Caltech who also made their own spin on Drake’s equation in a 2020 study.

The researchers found that life was most likely to emerge around 13,000 light-years from the galactic centers, where there is the greatest density of sun-like stars. The optimal time frame for the development of alien civilizations was estimated at 8 billion years after the formation of the galaxy. For comparison, Earth is about 25,000 light-years from the galactic core and complex intelligent life evolved around 13.5 billion years after the Milky Way formed.

According to the researchers, most civilizations that have appeared before us have likely self-annihilated. Other civilizations that are still active in the galaxy are likely young, due to the propensity of intelligent life to eradicate itself. Over a long enough timeframe, the probability of self-annihilation borders on certainty.

“As we cannot assume a low probability of annihilation, it is possible that intelligent life elsewhere in the Galaxy is still too young to be observed by us. Therefore, our findings can imply that intelligent life may be common in the Galaxy but is still young, supporting the optimistic aspect for the practice of SETI (search for extraterrestrial intelligence),” the authors wrote in their study.

“Our results also suggest that our location on Earth is not within the region where most intelligent life is settled, and SETI practices need to be closer to the inner Galaxy, preferably at the annulus 4 kpc (kiloparsec) from the Galactic Center.”

Other possible explanations for the apparent silence in the universe include the possibility that life is exceedingly rare (let alone the intelligent variety) or that humanity is simply too early to the party. We may be alone but only for the time being.

Explaining the Drake Equation on a smaller scale - Astronomy



Thoughts regarding the potential for advanced extraterrestrial civilization range from deeply considered speculation from well regarded scientists to inane comments on internet forums that turn every intelligent back-and-forth into an unfunny joke about how stupid humans are in general or, more specifically, how stupid supporters of the opposition political party are. I'm going to take a closer look at three of the most famous scientific ideas that have developed from the ongoing conversation regarding ET and sprinkle in some of my own lowbrow thoughts on the matter.

The Drake Equation is, far and away, the most famous of the three concepts I'll cover. It is a fairly straightforward formula for estimating the number of advanced civilizations in the Milky Way galaxy capable of interstellar communication and was developed by Frank Drake, a SETI pioneer and professor of Astronomy and Astrophysics at the University of California, Santa Cruz, as he was preparing for an early conference on the subject of communication with ET in 1961.

R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
fp = 0.5 (half of all stars formed will have planets)
ne = 2 (stars with planets will have 2 planets capable of developing life)
fl = 1 (100% of these planets will develop life)
fi = 0.01 (1% of which will be intelligent life)
fc = 0.01 (1% of which will be able to communicate)
L = 10,000 years (which will last 10,000 years)
N = the number of civilizations in our galaxy with which communication might be possible

Drake's initial values, listed above, resulted in N = 10.

Somewhere between 100 and 120 thousand light years in diameter and 1000 light years top to bottom, the Milky Way is home to an estimated, a very broad estimate I might add, 200 to 600 billion stars. I like keeping my math simple, so I'll go with the smaller diameter estimate of 100 thousand light years as I move along. Breaking the galaxy into 100 sections, we get zones 10 thousand light years side to side with the thickness, of course, staying the same. Obviously, there's nothing in those corner sections, but if scientists can give themselves 400 billion stars worth of wiggle room, I'll give myself a few empty sections of space. Regardless of the broad range of estimates, the galaxy is mind-numbingly huge.

One strength of Drake's formula is that it recognizes the obvious: not every star will

So let's assume all the planets in the habitable zone have life of one form or another. How do we get to the essential N number? Well, a recent survey of Earth put the number of species of animals here at 7.7 million. We know only one of them is a very smart monkey species so let's say that for every 7.7 million planets with life, there's one with a smart monkey, or smart something else, capable of building a civilization that can communicate with our's. My reason for that number? It's the Drake Equation, I can plug whatever number I want into that variable. So can you or anyone else. Looking at the problem that way give us the answer N = 65. A little more generous than Drake's. Of course, Drake's result might be generous. He may have been incredibly conservative too.

Playing with the Drake Equation is a fun little exercise whether you come up with only one other advanced ET planet or 1000, but is the Drake equation pointless? Of course not, but it was only developed as a useful device to spur conversation and while it still serves that purpose to this day that's as far as it goes. It's basically just a tool for bullshit sessions on the subject of intelligent life in the galaxy.

A confirmed answer of one would change our world forever and that brings us to the Fermi Paradox and . . . UFOs. The Fermi Paradox, as generally understood, raises the question, "If our galaxy is teeming with civilized life why haven't we discovered it?" It's certainly a valid question.

Enrico Fermi was a Nobel Prize winning physicist whose work led to the development of the first nuclear reactor, quantum theory, nuclear and particle physics, and atomic weapons. He was joined on this day by fellow scientists Emil Konopinski, Herbert York and Edward Teller.

UFO fever had gripped the United States and the subject of flying saucers, as they were then called, came up. The scientists expressed doubt that the UFOs of the day were of ET origin and the subject quickly turned to our own ability to travel beyond light speed within the next ten years. According to Teller, Fermi thought there was a ten percent chance of that happening . . . back in 1950! Obviously, he was a little optimistic on that projection.

While the conversation shifted off to other matters, Fermi suddenly blurted out, "Where is everybody?" causing a laugh from his party. In recounting the conversation in correspondence, everyone knew Fermi was talking about advanced extraterrestrials. He went on to, according to York's account, perform some quick calculations, his own on-the-fly version of the Drake Equation a decade before the fact, and "concluded on the basis of such calculations that we ought to have been visited long ago and many times over."

Some might think, me included, that Fermi's apparent skepticism of ET visitation in his day conflicts with his own calculations of the odds, that maybe we have been visited long ago and many times over. Just because they have maintained a fairly low and ambiguous profile in their survey doesn't mean they aren't visiting. Other answers to Fermi's question over the years have included thoughts like they just aren't there, faster than light travel isn't possible, they haven't found us and so on.

The problem with finding an ET signal via listening to the sky, the method pioneered by Drake, is obviously the size of the galaxy. Let's say there are 1000 advanced civilizations in the galaxy, a very optimistic number, and they are fairly evenly distributed, which won't be the case but it makes the calculations easier, with 10 in every 1 percent of galaxy space.

You can see for yourself just how much space exists between them. That one percent block of galaxy space is 10000 light years across and 1000 light years top to bottom. Each civilization is a good two or three or four thousand light years from its' nearest neighbor. Some sort of waveform communication is going to have to be in existence a few millennia for any of them to hear each other. Also in this block are two to six billion other stars and 500 million other planets with maybe 5 million of them capable of harboring life. It's both very big and very busy.

But as time has worn on, the conversation of that crew of scientists has risen to the level of legend . . . a legend born from a lunchtime bullshit session among scientists.

Lastly, I'll discuss the Kardashev Scale, developed by Russian astronomer Nikolai Kardashev, in which he categorizes advanced ET civilizations but before I get to the details of that I must mention this paper written by Kardashev in which he suggests an alien supercivilization could build a space telescope powerful enough to see life on Earth from the center of the galaxy. The diameter of the reflector of such a device? It would need to be 0.1 light years across . . . about 586 billion miles! That's roughly 6,300 times the diameter of the Earth's orbit around the sun. The guy does not speculate conservatively!

To explain the scale in greater detail, I'll leave it to Michio Kaku, the theoretical physicist who co-founded string field theory and author from his speech at the Global Competitiveness Forum in Saudi Arabia early in 2011.

I have to wonder what value the scale really has? Why would any civilization need or even want to harness the power of the galaxy? What's the point? Kardashev didn't say. I have to think the most advanced ETs would be incredibly efficient and be able to generate whatever energy they need with surprising simplicity.

I applaud Kardashev's wild imagination but don't find his scale especially compelling in a practical sense but it is also a fine launching pad for bullshit sessions.

Kaku's comment about the European Union being the beginnings of a Type I economy is laughable just a few months after the fact considering the current debt crisis they're going through. I think a Type I, II or III civilization might be developed enough and managed by smart enough individuals to not have any economy at all.

Kaku's cliched trotting out of the martyrdom of the 16th century astronomer and friar Giordano Bruno at the hands of the Roman Inquisition more than 400 years ago seems ironic considering he gave his speech in a country that still has public beheading and dismemberment as part of its' 21st century criminal justice system.

Another cliche often repeated by scientists in general is recounting the sad tale of Galileo's mistreatment by the Catholic church as a result of his book in support of Copernican heliocentric theory. What often isn't mentioned is that when Copernicus' theory was first presented to then Pope Clement VII, about 100 years before Bruno and Galileo, the pope was incredibly excited by this new concept and Cardinal Nicholas Schonberg praised him and offered financial support, writing directly to Copernicus:

What this clearly demonstrates is a historical lesson none of us should ever forget. No human advance is safe. We can always fall back rather than move forward. But I'm sure the check for Kaku's appearance fee cleared . . . the beginnings of a Type I economy.

Of course, the very premise of such a conference, competitiveness, might very well be an anachronism to a truly advanced civilization if it has ever been embraced at all and the forum's hosts, members of the Saudi royal family, are leaders of what has long been considered by forward thinking individuals an anachronistic system here on the Earth. It's cooperation that would be needed to take a society from a level analogous to where we're all at today to the point of supercivilization. We may not be able to provide undeniable proof of ET visitation, despite compelling evidence, but we should be thinking AS IF it has been occurring. The reasons are obvious . . . look at the current condition of our world. We need to deduce how a truly advanced supercivilization manages itself and become that supercivilization. Admittedly, we've got a long way to go, but if they can do it, we can do it.

The truth is, if an alien civilization has been visiting us, quite possibly for thousands of years, it has happened and the how of it all is a fact even if we can't duplicate the feat and the why of it all must be considered. We clearly know the distances are incredible, the technology of actually accomplishing interstellar travel extraordinary are well beyond our current capacity. There can be no other conclusion . . . there must be some level of affection and admiration for us from them driving their intellectual curiosity, a characteristic shared by all higher order species here on Earth. No thinking, rational species would go through so much trouble to cover so much distance to study something they had utter contempt for. It just doesn't make any sense. Whether it is because of our past, our present or our potential, we must, we must embrace this idea if we want to move forward and not back.

They also wouldn't have pursued their own technological progress as aggressively if they had no passion for it. Quite simply, they wouldn't do it if they didn't like it. I have some sense that there is this great passion, that a really advanced society would also have fun with and even make a game of, their technological advances.

And what do they see in us? What does any Western anthropologist see when investigating primitive cultures or archeologist see when digging into the past? Being the most advanced doesn't necessarily make you the most interesting. Our science might not impress them all that much, maybe it's our art.

We can't really be sure a surveying ET would even be aware of the significance of a masterpiece hanging on a wall in a museum. Have they read Moby Dick or The Odyssey too? It seems most likely to me that they've seen some marvelous examples of human expression in many great structures spread across our planet and I really can't imagine, no matter how advanced they are, their not being admirers of them.

The Great Pyramid, Chicken Itza, the Taj Mahal, the Eiffel Tower, the Gateway Arch, the Sydney Opera House. These are the structural expressions that are most likely to attract ET attention, I think, for obvious reasons. It's hard to miss them. An advanced culture is going to be a metalworking culture. Spaceships, whatever their level of function, aren't going to be made of animal skins, stone or wood. I think they'd really like that arch.

Some less ambitious statues become more spectacular because of their placement. The Statue of Liberty and Christ The Redeemer in Rio come to mind. Perhaps the geography surrounding those statues reminds them of home. Or maybe they're really impressed by our commercial billboards. You never know.

And what of their art? The zero sum thinker wouldn't see art in the design of possible ET vehicles on a visit. Of course (if happening) they are both artistic and scientific wonders. Efficient and elegant. As far as any art in an ET world, again perhaps the binary thinking of men excludes great possibilities. Could their art fulfill both an aesthetic and practical role? Could their kinetic art, like mobiles, also generate much of the energy they need and make the idea of harnessing the power of the galaxy laughable? Could their music do the same? Or is it the other way around? Of course our own architecture and vehicle design is both a sight to behold and performs its' function to whatever extent is needed, but in an ET world have they managed to blend these concepts to a greatly advanced level? If they advanced enough to get here, you have to consider that they have.

Of course the Drake Equation, Fermi Paradox and Kardashev Scale have some value in that they drive speculative conversations about ET but there may very well be a cadre of Earthophiles on some alien planet possessing a knowledge base thousands of years old and it's not inconceivable they know us better than we know ourselves.

The Drake Equation

In 1961, at the Green Bank observatory in West Virginia, a small conference was held for astrophysicists. The meeting was organized by Cornell University professor and astronomer Frank Drake.

The subject of the conference was the search for extraterrestrial life.

In preparation for the conference, he jotted down his thoughts in the form of an equation. An equation that has changed how we think about life on other worlds.

Learn more about the Drake Equation and variables which make it up on this episode of Everything Everywhere Daily.

This episode is sponsored by Everything Everywhere Tours.

Thanks right, I’m going to be offering tours for my listeners starting next year.

You will be able to travel with me to get an in-depth experience in some of the world’s most historic cities. The plan is to see things that most tourists never get to see, and most don’t even know about.

Not only are we going to see some amazing things which few people experience, but we are going to have some top experts with us to give everything context.

Obviously, there are issues with the pandemic right now, so it is impossible to set dates or prices, but if you are interested, you can sign up to get more information when it is available.

Go to and leave your email to get future updates.

Once again, that’s and I’ll have more information at the end of the show.

In the late 1950s and early 1960s, the science of astronomy was making huge strides. In particular, the field of radio astronomy had been taking off and many new discoveries were being made.

With these new discoveries and new observation tools, the questions of extraterrestrial intelligence began to come to the forefront and were taken more seriously by scientists.

It was noted that the radio telescopes of the late 50s were sensitive enough to pick up radio waves that were broadcast from other civilizations.

It was in this environment that Frank Drake created the Drake Equation.

The Drake Equation is nothing more than an attempt to try to identify the factors which determine the number of civilizations we could potentially communicate within our galaxy.

The Drake Equation is really nothing more than an educated guess, and there has been criticism of the equation and suggestions that it needs to be updated in light of discoveries in the last 60 years.

With that, let’s get into the equation itself.

The entire equation when written out would read:

N = R* x fp x ne x fl x fi x fc x L

If that sounds complicated, it really isn’t, and each of the variables can be explained in a very easy-to-understand way.

Let’s start with N, which is the entire point of the equation. N is just the number of civilizations in our galaxy we could communicate with. That is what we are trying to figure out.

The other seven variables are all things that would determine the number of civilizations. Many of the early attempts to assign values to these variables were really nothing more than educated guesses. Since 1961 we have gotten a far better idea as to the numbers for a few of the values.

R* is the first variable. It represents the rate of star formation in the Milky Way, or how many new stars are created every year.

Drake’s initial guess in 1961, and it was just a guess, was there were was about 1 new star that formed every year in our galaxy. He assumed that this was a conservative estimate, and he was right.

The Milky Way has somewhere between 100 to 400 billion stars, and the current estimate is that approximately three solar masses of stars are created each year. That could be three stars like the sun, or 1 big star three times the size of the sun, or 5 stars smaller than the sun.

Here I should note that the Drake Equation is only designed for our galaxy. If you wanted to calculate this for the entire universe, you’d have to multiply everything by the estimated number of galaxies in the universe which is somewhere between 200 billion and 2 trillion.

The second variable is fp which represents the fraction of stars that have planets.

Of all the variables in the equation, this is the one where the most progress has been made in the last 60 years. In 1961, no one even knew if other stars had planets, or if they did how common of an occurrence it was.

Since then we have discovered thousands of planets around other stars. In fact, we’ve found so many that it is assumed now that pretty much all stars have planets and that it is a natural part of the formation of stars.

That would make the value of this variable 1 or very close to 1, and really renders it irreverent. In many of the updated versions of the equation, this variable is now eliminated entirely.

In Drakes’s original 1961 estimate, he put this value at .2 to .5, which as it turns out was a very conservative estimate.

The third variable is ne which represents the number of planets per solar system that can support life.

Here we have made very little progress because the tools and techniques we have to detect planets can only detect very large planets which have a measurable effect on the star which it orbits.

Our estimates of this number should improve over the next several decades as new telescopes will be built, and new techniques developed, which will allow us to find smaller planets.

The key thing which astronomers will be looking for are planets inside the habitable zone of a star. Also known as the Goldilocks zone, it is the zone that isn’t too close to the star, like Venus, nor too far away, like Mars.

The 2013 Kepler mission concluded that there could be 40 billion Earth-sized planets within the habitable zone of stars, which would give this a value of 0.4, assuming there are 100 billion stars.

Drake initially thought this value would be between 1 and 5.

The fourth variable is fl which represents the fraction of habitable planets that actually develop life.

Here too, we have no real clue what this number might be because we have yet to actually find life anywhere other than Earth.

However, there is a lot of activity on this front. Researchers on the origins of life on Earth have concluded that basic single-cell life on Earth appeared almost as soon as the planet was formed and cooled.

Evidence from Mars has shown more water and other factors necessary for life than we originally thought.

Future missions to Mars, as well as some of the moons of Jupiter, might determine if life, or the building blocks of life, were able to form outside of the Earth.

Drake initially thought that all planets in the habitable zone would develop life, but as of right now we have no clue if that is true or not.

It probably isn’t 100%, but it also probably isn’t zero.

The fifth variable is fi which is the fraction of planets with life that develop intelligent life, and here we really don’t have a clue. If we haven’t even found single-cell life outside of Earth, we can’t really even make a good estimate on how likely intelligence is to arise.

There are a host of other problems with this. What is intelligence? Would dolphins on other planets be considered intelligent? How about a dog?

The estimates for this variable vary widely. Some people think this might be close to 1, but others think it might be very close to zero.

One hypothesis is called the Rare Earth Hypothesis. This contends that simple cellular life might in fact be very common throughout the universe, but intelligent life like humans might be extremely rare.

Life doesn’t inexorably evolve to create intelligent life. The odds of an intelligent species evolving could be billions to one.

Drake assumed this was near 100%.

The sixth variable is fc which is the percentage of intelligent civilizations which develop the ability to communicate through space. Basically, have they developed radio.

Again, we have no clue what this might be. Could an intelligent species just plateau at a stone-age level of technology? Or maybe even a level like ancient Rome?

Or maybe the level of technology required is something that we haven’t figured out yet and it isn’t electromagnetic radiation. It could be that we are the ones in the equivalent of the stone age by galactic standards.

The final variable is L, which is the lifetime of the civilization. Even if a species were to evolve to become intelligent, and if they were to develop sufficient technology, there might only be a finite period of time where they are able to broadcast.

They could be destroyed by war, environmental collapse, get hit by a meteor, or maybe their star explodes.

Again, this is a huge unknown and estimates are all over the place. Estimates range from a few hundred years to infinite.

So, if you take all seven variables, and make some best-guess estimates for each, what do you get?

Well, you get pretty much anything you want. Estimates have ranged from Earth being the only intelligent, technical civilization, to there being millions of technical species.

You might be thinking that this is all pretty useless if we can’t even guesstimate an answer. However, it was really designed to be a way of thinking about the problem and starting a discussion, not necessarily an attempt to find a solid answer.

One of the major criticism of those who use the equation to argue that there are many advanced civilizations, is “where is everyone?”

This is known as the Fermi Paradox, and it will be the subject of a future episode.

The Drake Equation was the first step towards trying to get beyond science fiction and really trying to understand if we are in fact alone in our galaxy. Over time, as we learn more, we’ll have a better idea of the values of the variables in the equation, and how likely it is that there are other intelligent civilizations.

The associate producer of Everything Everywhere Daily is Thor Thomsen.

I’ve traveled a lot and I’ve been on quite a few tours with tour groups. The way they normally work is that you spend a few days in one city, and then go to the next city, and so on.

There is nothing wrong with this, but I’ve been thinking of what I could offer that would be different.

In the course of researching many of the episodes I’ve done, I kept coming across interesting things, many times in cities I’ve already visited. I made a mental list of all the things to see the next time I go back, and eventually, it dawned on me, that many other people would like to see these things too.

For example, in Rome, most people see the Colosseum and the forum, the Trevi Fountain, and the Vatican. There might be some other things, but they hit the highlights and move on. However, few people bother to visit Ostia Antica, the old port city of Rome, or Hadrian’s Villa, just outside of town in the hills. Or the necropolis under the main altar of the Vatican, or Nero’s Palace, or the catacombs around Rome, or the 2,000-year-old sewers under Rome.

Basically, do an incredibly deep dive into the history of a single city. Unpack your bags once, and get tours of these places with genuine experts with PhDs in the fields of archeology, history, and art.

A tour for the true geeks who really want to see and hear the details.

So far, I’ve identified three cities that would be good candidates for this type of tour: Rome, Istanbul, and Jerusalem.

The tours will be quite small. Only 8 to 10 people max. If there is more demand, I’ll just do a second tour and do them back to back.

Obviously, given conditions around the world right now it would be impossible to set dates for a tour, and without dates, you can’t set a price, and no one can commit to anything without that.

But, if this is something you might be interested in, go to just leave your email address, and I’ll notify you when I know more information and conditions are such that it is possible to actually commit to a date, and this might not happen until 2022.

You can also click on the link in the show notes.

Everything Everywhere is also a podcast!

Solution to the Drake Equation

The Drake Equation, also known as the Green Bank Equation after the 1961 conference where it was first presented, is a formula created by American physicist Frank Drake to estimate the number of intelligent alien civilizations which exist in the Milky Way and which, given sufficient technology and time, humanity might one day be able to communicate with. Although it takes the form of a mathematical equation, it is important to note that Drake never intended it to be solved precisely. Instead, he offered it as a sort of thought-experiment, taking into account all of the factors which would go into the likelihood of intelligent civilizations existing elsewhere in the universe:

N = R* x f(P) x n(E) x f(L) x f(I) x f(C) x L

In his work, Drake assumed that all life would emerge independently, and naturally, through evolution. As a result, one can imagine the universe as a collection of billions of sets of dice, all of them being rolled over and over again. Everywhere the right set of numbers turns up, intelligent life will emerge. The Drake Equation is an attempt to think about those numbers.

– Physical Variables for Life –

The first set of three functions in the Drake Equation relate to the fundamental physical conditions necessary for life to develop. R* is the number of stars which form in the galaxy per year (some variants of the equation say the total number of stars in the galaxy). In a young galaxy, new star births can number in the thousands per year. In a middle-aged galaxy like our own, however, the Max Planck Institute for Astronomy says that only about one Sun-like star (not too large or small to support life) is born per year. The Milky Way Galaxy currently holds about 100 billion stars. This is the only function in the Drake Equation that can be known with any degree of mathematical certainty.

The second function, f(P), refers to the proportion of stars that have planets around them. Drake reasoned that life could not evolve without a planet for it to be based on, orbiting a star capable of supporting life with light and heat. How many stars have planets is unknown, mostly because we are only now developing the technology to find Earth-sized planets around other stars. According to the Scientific American, recent estimates are that 7-30% of stars have planets large enough for us to detect them (roughly Saturn-sized or larger). However, Rich Townsend at the University of Wisconsin-Madison says that if we factor in smaller planets, virtually every star is likely to have some – if we only knew how to look for them.

The third function, n(E), refers to the proportion of planets that are actually capable of supporting life. If we did not know the number of total planets, we certainly don’t know the number of life-sustaining planets. It’s tempting to set this number at 10% – after all, one in eight of the planets in our solar system (Earth) is known to have life on it. On the other hand, so far as we can tell, Mars, Jupiter’s moon Europa, and Saturn’s moon Titan all have the capability to support microbiotic life, either in the distant past (Mars), the present (Europa), or the distant future (Titan). Townsend says we should figure that one-quarter of planets supports life.

So far, with three variables accounted for, the Drake Equation suggests that one potentially life-supporting star comes into existence every four years, and that there are 25 billion candidate stars in the total history of the Milky Way Galaxy, which might plausibly have life on a nearby planet.

The final four variables of the Drake Equation assume that life is actually capable of first coming into existence from non-living organic compounds, and then of evolving into more advanced forms. How likely any of these things are to happen is truly unknown, since all we have to work with are the bits and pieces of the Earth fossil record which paleontologists and evolutionary biologists have been able to piece together.

The first of these, f(L), refers to the likelihood that life will develop from non-living matter, a process called abiogenesis. There is no scientific consensus on the exact process by which this happened in the first place, so there really is no way of knowing how likely it is to occur somewhere else. However, this is another variable which Rich Townsend says is effectively 100%. After all, he reasons, life has billions of years to work with during the lifetime of a Sun-sized star. Sooner or later, if it’s possible for life to originate at all, it will do so.

The second variable, f(I), is far more problematic. This variable refers to the likelihood that intelligence will evolve. Paleoarchaeologists do have some idea how this might have happened among apes and humans, in terms of gradual increases in mental and social complexity. (So far they have less to offer in the instance of other somewhat intelligent species, like dolphins, Humboldt squid, and social insects.) Townsend, ever the optimist, says that this number is 100% as well.

But that may not be the case: the Earth has benefited from several unusual conditions, like our large Moon, our active geology, the lack of unstable stars nearby, and the helpful gravitational influence, which pulls most potentially dangerous comets and asteroids out of danger. All of these factors go into making the climate and the surface of the Earth relatively stable. If that weren’t the case, life simply wouldn’t have time to evolve far before one or another stellar or natural disaster simply wiped it all out. So, rather than taking Townsend’s figure, it seems appropriate to assume that only 10% of life-supporting planets will actually develop intelligent life. Even this figure may be a high one.

The third life variable, f(C), estimates what proportion of otherwise intelligent life will eventually go on to develop advanced technology. The best measure of this is the ability to communicate by radio waves. Here, again, there is much speculation. What is the likelihood that such technologies will be invented? What percentage of intelligent civilizations will never invent them, or, having invented them (at least theoretically), decide never to use any with sufficient power to broadcast radio waves out into space. Our current Search for Extra-Terrestrial Intelligence (SETI) involves listening for alien radio signals. If they aren’t sending any, though, we will never find them. Once again, optimists like Townsend say that 100% of intelligent alien species will eventually invent and use radios. This is probably optimistic, but let us accept it for the moment.

The final variable in the equation, L, is the most speculative of all. Advanced, radio-using civilizations probably have limited lifespans, predicted Drake: at least a large percentage of them will eventually succumb to climate change and go extinct, die out in a vast pandemic of contagious disease, or blow themselves up with nuclear weapons. (Drake was writing at the height of the Cold War, which explains his pessimism in this respect.) Townsend, unusually pessimistic himself, says that the average advanced civilization probably only survives for 200 years. That seems very pessimistic: after all, a civilization which survives its equivalent of events like the Cold War could plausibly survive for many thousands or even millions of years.

There is another factor to consider, however. Even if a civilization survives for many thousands of years, there is no guarantee we would be able to hear their radio signals for that entire time. The strongest signals being sent on the Earth are television carrier waves these would also reach the farthest out into space, where aliens might hear them. But very shortly we will no longer be using television carrier wave signals: they’re just not necessary in the new age of digital and satellite communications. Civilizations much more advanced than ours might communicate in ways we haven’t even imagined yet.

Because these figures are so speculative, it’s hard to solve the Drake Equation with any certainty – which is just as he intended it to be. Using the numbers here, though, we can say that some sort of intelligent civilization could be born in the Milky Way Galaxy as often as once every forty years. If each one survives for about 200 years, there are currently five civilizations in the galaxy with intelligence equal to or greater than our own. Optimists say there are actually far more: Townsend, for example, figures there are 25. The pessimists say there are less – in fact, we may be the only ones.

Accepting Townsend’s figure, though leads to another thought experiment problem, called the Fermi Paradox. If there are intelligent civilizations elsewhere in the galaxy, the odds are that at least some of them are older than ours. Given the rate at which our technology is progressing, even a century or two’s difference in ages could put them far ahead of us in terms of their technology. And if that’s true, we should be able to detect them. So where are they?

There are several possible solutions to the Fermi Paradox. Only one of them is an optimistic one: all of the advanced civilizations are so advanced that they’ve progressed beyond our ability to see what they’re doing. The other is pessimistic: civilizations tend to destroy themselves very quickly. Pessimists say that the reason we can’t see any alien civilizations is because, even if they did once exist, they’re already dead.

The “Great Filter” Hypothesis:

But perhaps the best known explanation for why no signs of intelligence life have been found yet is the “Great Filter” hypothesis. This states that since that no extraterrestrial civilizations have been so far, despite the vast number of stars, then some step in the process – between life emerging and becomes technologically advanced – must be acting as a filter to reduce the final value.

According to this view, either it is very hard for intelligent life to arise, the lifetime of such civilizations is short, or the time they have to reveal their existence is short. Here too, various explanations have been offered to explain what the form the filter could take, which include Extinction Level Events (ELEs), the inability of life to create a stable environment in time, environmental destruction. and/or technology running amok (some of which we fear might happen to us!)

Alas, the Drake Equation has endured for decades for the very same reason that if often comes under fire. Until such time that humanity can find evidence of intelligent life in the Universe, or has ruled out the possibility based on countless surveys that actually inspect other star systems up close, we won’t be able to answer the question, “Where is everybody?”

As with many other cosmological mysteries, we’ll be forced to guess about what we don’t know based on what we do (or think we do). As astronomers study stars and planets with newer instruments, they might eventually be able to work out just how accurate the Drake Equation really is. And if our recent cosmological and exoplanet-hunting efforts have shown us anything, it is that we are just beginning to scratch the surface of the Universe at large!

In the coming years and decades, our efforts to learn more about extra-solar planets will expand to include research of their atmospheres – which will rely on next-generation instruments like the James Webb Space Telescope and the European Extremely-Large Telescope array. These will go a long way towards refining our estimates on how common potentially habitable worlds are.

In the meantime, all we can do is look, listen, wait and see…

There are some great resources out there on the Internet. Check out this Drake Equation calculator.

We have recorded an entire episode of Astronomy Cast about the Drake Equation. Check it out here, Episode 23 – Counting Aliens with the Drake Equation.

A Drake Equation for Alien Artifacts

Jim Benford’s study of ‘lurkers’ — possibly ancient probes that may have been placed here by extraterrestrial civilizations to monitor our planet’s development — breaks into two parts. The first, published Friday, considered stars passing near our Sun in the lifetime of the Solar System. Today Dr. Benford looks at the Drake Equation and sets about modifying it to include the lurker possibility. Along the way, he develops a quantitative way to compare conventional SETI with the strategy called SETA — the search for extraterrestrial artifacts. Both articles draw on recently published work, the first in JBIS, the second in Astrobiology. The potential of SETA and the areas it offers advantages over traditional SETI argue for close observation of a number of targets close to home.

by James Benford


“To think in a disciplined way about what we may now be able to observe astronomically is a serious form of science.” –Freeman Dyson

I propose a version of the Drake Equation for Lurkers on near-Earth objects. By using it, one can compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring for artifacts to the conventional listening-to-stars SETI strategy, which has thus far found no artificial signals of technological origin. In contrast, SETA offers a new perspective, a new opportunity: discovering past and present visits to the near-Earth vicinity by ET space probes.

SETA is a proposition about our local region in the solar system. SETA is falsifiable in its specific domain: ET probes to investigate Earth would locate on the nearest objects down to a specified resolution. SETI, on the other hand, is about messages sent from distant stars. For example, one can falsify a proposition such as “Are signals being sent to Earth at this moment within 100 ly?” But there is the region beyond 100 ly and beyond 1000 ly, etc. So SETI is falsifiable only within larger and larger domains. Of course other factors can also weaken falsification: our sensitivity might be inadequate, duty cycle might be small, and of course frequency coverage will always be incomplete.

Rose and Wright pointed out the energy efficiency of an inscribed physical artifact vs. an EM signal, because the artifact has persistence and the EM signal has to be transmitted indefinitely (Rose & Wright, 2004). Here I point out that artifacts are not only energy efficient, but increase the chance of contact. Rose and Wright did not explore where to locate the artifact so it would be identified here I suggest there are attractive locations near Earth where they might be readily observable.

In a recent paper, I introduced the term ‘Lurker’: an unknown and unnoticed observing probe from an extraterrestrial civilization, which may well be dead, but if not, could respond to an intentional signal. And/or it may not, depending on unknown alien motivations (Benford, 2019). Lurkers include self-replicating probes, based on von Neumann’s theory of self-replicating machines, which is why they are often called von Neumann probes (Von Neumann, 1966). Recently concepts have appeared for self-replicating probes that could be built in the near future (Borgue & Hein, 2020).

Another pioneering work on this concept was of course famously developed in � A Space Odyssey” (Clarke, 1968). A ‘solarcentric’ Search for Extraterrestrial Artifacts advocated by Robert A. Freitas, who coined the term SETA for it in the 1980s (Freitas & Valdes, 1985). There are also the papers from the mid-1990s by Arkhipov (Arkhipov , 1995, 1998a, 1998b). Scot Stride has shown that autonomous instrument platforms (i.e. robotic observatories) to search for anomalous energy signatures can be designed and assembled using commercial off-the-shelf hardware and software. That provides an economical, flexible and robust path toward collecting reliable data (Stride 2001a and 2001b). Further analysis has appeared recently (Haqq-Misra & Kopparapu, 2012, Lingam & Loeb, 2018, Cirkovic et al. 2019, Shostak, 2020).

Near-Earth objects could provide an ideal way to watch our world from a secure natural object (Benford, 2019). They are attractive locations for extraterrestrial intelligence to locate a platform to observe Earth while not being easily seen.

2. Drake Equations

2.1 The Standard Drake Equation estimates the number of radiating civilizations that are detectable, NC , as the product of the rate of creation of such radiating civilizations (Drake, 1965),

This modified Drake Equation is:

I replace the usual Drake Equation symbol for time over which they radiate L, with TR. And I also multiply by:

fR = fraction that actually do radiate signals that might be observable at Earth. That is, they radiate with the intention of trying to communicate. Leakage radiation is unintentional, but comes in two types: radar, which has no message, and broadcasts, which come from many incoherent sources which cancel out, such as TV.

These parameters are listed in Table 1:

Table 1: Drake Equation Parameters. Subscripts are italicized letters in definitions

2.2 A Drake Equation for Alien Artifacts An equivalent to the Drake equation for the number of Lurkers in our solar system, NL, can similarly be expressed as the rate of creation of radiating civilizations, times the fraction that also develop interstellar probe technology fip, times the sojourn that Lurkers would be in the solar system, TL:

fip = fraction that also develop interstellar probe technology and launch them

TL = time that Lurkers could reside in the solar system

(Note that for such civilizations, fC =1 a civilization with the capability to build such probes surely can build interstellar transmitters.)

Then a Drake equation for alien artifacts is

The new parameters are listed in Table 2:

Table 2 Drake Alien Artifact Equation Parameters

In the ratio of equations 1 and 2, of the number of Lurkers in our solar system to the number of radiating civilizations, most terms, in the first bracket, cancel so:

This initial result is that the ratio of civilizations sending probes that are now resident in our solar system to the number sending messages is the product of two ratios: A ratio of motives:

the fraction that also develop interstellar probe technology and launch them, divided by fraction that only radiate, so fip/fR < 1,

the time Lurkers are present in the solar system/ the time ET civilizations release electromagnetic signals. Surely a civilization with the capability to build such probes can build interstellar transmitters, so I will argue that TL/TR > 1.

Our own civilization has been capable of radiating for about 50 years, including message-free Cold War radar transmissions and inadvertent leakage radiation has been emitted for a long time (Quast, 2018). Intentional messages have also been sent but are difficult to detect with Earth–scale receiver systems (Billingham and Benford, 2014). We cannot yet build interstellar probes capable of traveling to and decelerating into a star system and conducting operations there. But that may be possible in the next century. If so, relatively soon we will be capable of both radiating to the stars and sending probes to explore nearby star systems.

However, equation 4 does not take account of the space volumes that the two groups operate in.

2.3 Space Volume Factor

Another factor must be included: Equation 4 must be modified for VL, the volume over which Lurkers can travel, and its corresponding range RL vs. VB, the volume over which Beacons can transmit and be plausibly detected, and its corresponding range RB. Lurker probes traveling at a small fraction of the speed of light should be compared to the transmissions from an interstellar Beacon propagating at the speed of light. That means that the volumes from which signals can be detected from Beacons is much larger than the volume over which Lurker could travel.

For example, assume that interstellar probes could operate at

10% c, the speed of light, as contemporary concepts of fusion rockets are designed for. An example: for the Icarus Firefly magnetically confined Z-pinch concept at 4.7% c, traveling 10 ly would take about two centuries (Freeland & Lamontagne, 2015). Starshot, which is a flyby probe concept, at 0.2 c takes more than 20 years to arrive at the Centauri system. Assuming the attention span of the civilization is measured in centuries, a rough estimate of the distance over which probes will be launched is tens of lightyears. (The signal from the probe reporting back to its origin would travel at the speed of light, of course.) If it is possible for probes to move close to c, then the beacon volume to probe volume would be close to unity.

In contrast, the electromagnetic waves of an interstellar Beacon, be it light, millimeter-wave or microwave, propagate

20 times faster, at the speed of light. For example, we can estimate the range over which a Beacon would be used to be hundreds of light years. By that I again mean that the attention span of a civilization might be measured in centuries.

I define the volumes and ranges in Table 3:

Table 3 Space Volume Factor Parameters

Therefore equation 3 must be multiplied by the ratio of these 2 volumes, VL/VB:

As volume scales as the cube of the distance to them, RL/RB:

This is a ‘Success Ratio’ of searching for artifacts compared to listening to stars. It allows us to quantitatively evaluate their relative merits. Although the volume ratio would argue that long-range Beacons will be much more likely to be detected than probes that come to observe Earth, the time ratio tends to mitigate that advantage.

2.4 Decision Tree Parameters

The ratio of the number of lurkers to the number of radiating civilizations can be estimated using the three factors in equation 5, which have the following’s sizes:

So the ‘Success Ratio’, Eq. 5, will depend on choices for these parameters.

The key parameters making up these factors can be divided into objective and subjective components, where ‘objective’ means it can be quantified or at least estimated and ‘subjective’ means it’s a matter of opinion. Here is a Table of the parameters:

Table 4: Objective and subjective SETA Parameters and determining factors

The issues determining the objective parameters are listed subjective parameters are a matter of taste and underlying assumptions.

By making choices among the objective and subjective parameters, one constructs a decision tree: A set of parameter choices leads to a conclusion about the success ratio for SETA and SETI strategies, as embodied in equation 6. Because ET civilizations will vary enormously in motivations, we can expect a variety of outcomes for the Success Ratio.

2.41 Estimates of TR, time that ETI Beacons radiate

In the literature, estimates of TR fall between a hundred and 100 million years, a very wide range. Michael Shermer estimated TR by averaging the lifespans of 60 Earth civilizations, getting 420 years, (Shermer, 2002). Using 28 civilizations since the Roman Empire, he gives

300 years for “modern” civilizations. But Shermer’s number for the lifetime of societies is not relevant if new societies arise to replace old ones. In that case, one should take the summation of existence times for all the technological cultures on a planet. Note that the longest operating institution still existing on Earth is the Catholic Church,

2,000 years. We’ll take the times to be 300-10,000 years, an order of magnitude range.

2.42 Estimates of TL, time Lurkers could reside in the solar system

A key point is that Lurkers will still be discoverable even though dead for a long time. That’s not true of an EM transmission, which is simply passing through at the speed of light. That fact weighs to the advantage of the Lurker search strategy.

The time that Lurkers would be in the solar system, TL, will be limited by the lifetime of the orbits they are in, which provides an upper bound. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

Our Moon is thought to have formed about 4.5 billion years ago. For TL we use the time that life became evident in our atmosphere, 0.65 10 9 < t1 < 2.5 10 9 years.

Earth Trojans

There may be many objects in the Earth Trojan region (Malhotra, 2019). Their lifetime in Trojan orbits is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable orbits about the Lagrange Points (Ćuk et al., 2012, Dvorak et al., 2012, Marzari & Scholl, 2013, Zhou et al. 2019). Orbital calculations show that the most stable orbits reside at inclinations < 0° to the ecliptic there they may survive the age of the solar system,

Earth Co-orbitals

Morais and Morbidelli, estimate lifetimes to run between 1 thousand and 1 million years (Morais and Morbidelli, 2002). With a mean lifetime of 0.33 million years. Morbidelli says that no further studies have been done on their approach (A. Morbidelli, personal communication).

3. Scenarios for Success Ratio Estimates

Here we show several scenarios, some of which show that the two strategies, SETA and SETI, are competitive.

Scenario 1: Choosing via relative costs at equal ranges:

1) The ratio of fractions of ET civilizations would be proportional to the cost of interstellar probes vs. Beacons. The cost of interstellar probes will be substantially more than the cost of interstellar Beacons. Stated differently, Beacons will have substantially longer range for a fixed cost.

If we take as an example a Beacon at 100 ly and a Lurker probe launched from 100 ly, then RL and RC in Eq. 5 cancel out. For Beacons that have a range of 100 ly the cost is of order $1 billion. This is from extrapolations, based on current cost scaling and costs (Benford, 2010, Billingham and Benford, 2014). The Firefly interstellar fusion rocket has an estimated cost of $60 billion. Two thirds of that cost is fuel to accelerate and decelerate (A. Lamontagne, personal communication). Therefore the cost ratio is

100 in favor of Beacons. If cost is the deciding factor, then fP/fR = 1/100 and Eq. 5 reduces to

Next, one chooses an orbital location for the Lurker: Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. So we use the time life became evident in our atmosphere, 0.65 10 9 < TL < 2.5 10 9 years.

Next, one guesses the transmit time of the Beacon: estimates of civilization radiating times TC vary from

300 -10 5 years. Here the ‘dash’ means the range of credible values:

So for these parameter choices, a Lurker search is much more likely to be successful. Note, however, that if we assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly, a factor of 1/1,000 reduces the ratio to 0.1 to 100.

Scenario 2: What if cost doesn’t matter? That would be at variance with all we know of economics on Earth, but is a hypothetical we could consider. If cost doesn’t matter, then a civilization wanting to investigate the life of Earth or whether civilization was here could build probes to investigate the ecosystem, visible in spectra of our atmosphere, and also build Beacons to broadcast to us. In such a case, fP/fR = 1, and, as we’re talking about a single civilization, RL/RC = 1. Consequently the Success Ratio NL/NC = TL/TC, which would surely be >>1. Again, lurker strategy is likely to be more successful. In this scenario, the time ratio is the important factor.

Scenario 3: Early spacefaring civilizations: A civilization such as ours, which is presently capable of only interplanetary speeds, cannot build interstellar probes as envisioned by some of our starship concepts. Starships are centuries into our future and will always be more expensive than Beacons. They could be only a radiating society and might build Beacons. In this case the success ratio NL/NC = 0, and a listen-only strategy is appropriate.

Scenario 4: Supercivilizations capable of fast interstellar flight: The opposite extreme from scenario 3 is a civilization where starships can travel at a large fraction of the speed of light. In this case, Beacons, although still cheaper, would serve to reveal our civilization only if we respond by sending a message back to them. At about the same time their probes would be arriving and could be reporting the existence of our civilization. This could’ve occurred over geological time frames, so in this case NL/NC >>1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.1, and we would expect to find dead Lurkers on the nearby objects described in 2.42.

Scenario 5: Lurkers in Co-orbitals and short radiating time: Instead of a Trojan or the Moon, we choose one of the co-orbitals, which have a mean lifetime TL

0.33 million years. 1) For TR , choose the 300-year lifetime estimate of Shermer for the Beacon to radiate. Then TL/TR = 1,000. 2) Let’s assume that starship probes are launched from a civilization 10 ly away. (A probe such as Firefly, traveling at 0.2c and decelerating into our solar system, would take 50 years to come 10 ly.) 2) Assume the Beacon civilization is at 100 ly, and the probe-building civilization is at 10 ly. So RL/RB = 0.1. 3) Further, again assume that the willingness of civilization to undertake the expense would be determined by economics. A continuous Beacon at hundred light-years would cost about $1 billion and a Firefly probe is estimated to cost $60 billion (M. Lamontagne, personal communication), so fP/fR = 0.01. Therefore the Success Ratio, eq. 5, is:

For this case listening-to-stars has a higher success ratio. But if one assumes that the radiating civilization also develops interstellar probes, fR

fp, the two strategies have a roughly equal success ratio:

So one’s assumptions of the parameters in the Table determine the answer.

Scenario 6: Lurkers in Co-orbitals and long radiating time: If we use the band of estimates in the literature for co-orbital lifetime,

10 5 years, and estimates of civilization radiating times TC vary from 10 2 – 10 5 , then TL/TR varies from 1 to 1,000. For the previous 100 ly/10 ly distance ratio, Eq. 5 then gives a Drake Equation ratio of

And the listening strategy will be preferred.

It is clear from these scenarios that 1) the two strategies, SETA and SETI, are competitive, 2) the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.

5. Research for Finding Alien Artifacts

I advocate a sequence of tasks:

  • We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has taken about 2 million images at high sub-meter resolution (M. Revine, personal communication). We can see where Neil Armstrong walked! The vast majority of the photos have not been inspected by the human eye. Searching these millions of photographs for alien artifacts would require an automatic processing system. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans or the co-orbitals (Davies & Wagner, 2011, Lesinkowski et al., 2020). Note the recent AI analysis of 2 million images from LRO which revealed rockfalls over many regions of the Moon (Bickel et al., 2020).
  • Conduct passive SETI observations of these nearer-Earth objects in the microwave, infrared and optical.
  • Use active planetary radar to investigate the properties of these objects
  • Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.
  • Launch robotic probes to conduct inspections, take samples of Earth Trojans and the co-orbitals. The low delta-V, 3-5 km/sec, make this an attractive early option, is well within present capability (Stacey & Conners, 2009, Venigalla et al., 2019). China plans a mission to co-orbital 2016 HR 3 in the middle of this decade (Zxiaojing, et al., 2019).

6. Conclusion

Clearly looking for alien artifacts in the region of the solar system near Earth is a credible alternative approach, a strategy of ETI archeology. The formulation given here is a way of discussing the SETA strategy and comparing it to SETI.

The listening-to-stars strategy that SETI researchers have been following for over 50 years, is now being pursued very vigorously by Breakthrough Listen. What has SETI learned so far about life in the universe? Only that there is no intelligent life broadcasting signals toward Earth at the time we’ve listened, within the sensitivity levels, duty cycles and frequencies we have observed. If the ongoing SETI listening program continues to not hear a signal, the case for looking for Lurkers will grow ever stronger.

The SETA strategy was not pursued after it was suggested in the 1980’s, because listening to stars is easier and observing technologies and spacecraft were not sufficiently developed to pursue it. But now SETA is more attractive:

  • Close inspection of bodies in these regions can now be done with 21st Century observatories and spacecraft.
  • The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
  • The Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
  • There are differences in detection in the two strategies: in the artifact case we should listen to those objects and image them in the optical or radar from Earth or send probes to visit them. In SETI, we can only listen.
  • SETA is a concept that can be falsified, a fundamental requirement for a science. SETA can be falsified or verified in practice by precisely specifying what one is looking for. For example, the statement “No artificial objects larger than 1 m exist on the surface of the Earth Trojan” can be verified by observing that object at that resolution. Smaller objects wouldn’t be resolved. If we conduct the efforts described in Section 5, and don’t find artifacts, the SETA concept is disproven for the near-Earth region, where it is most credible. If we find them, it’s verified.

Arkhipov A V, Lunar SETI. Spaceflight 1995 37: 214.

Arkhipov A V, New Approaches to Problem of Search of Extraterrestrial Intelligence. Radio Physics and Radio Astronomy 1998 3: 5-12.

Arkhipov A V, Earth-Moon System as a Collector of Alien Artifacts. JBIS 1998 51: 181-184.

Bailer-Jones C. A. L. et al. New Stellar Encounters Discovered in the Second Gaia Data Release, Astronomy & Astrophysics 2018 616: A37.

Benford, J., Benford, G., & Benford, D. 2010, Messaging with Cost Optimized
Interstellar Beacons. Astrobiology 2010 10: 475-490.

Benford, J. 2019 Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables, AsJ 2019 158:150.

Benford, J. 2021 How Many Alien Probes Could Have Come From Stars Passing By Earth?”, JBIS 74 76-80, 2021.

Bickel V. et al., 2020 Impacts drive lunar rockfalls over billions of years, Nat. Commun. 11:2862 |

Billingham, J. & Benford, J., 2014, Costs and Difficulties of large-scale METI, and the Need for International Debate on Potential Risks, JBIS 67:17-23.

Borgue, O., & Hein, A., 2021, Near-Term Self-replicating Probes – A Concept Design, Acta Astronautica, in press.

Christou A.A. et al. 2021 Composition and origin of L5 Trojan asteroids of Mars: Insights from spectroscopy. Icarus 354: 113994-11415.

Cirkovic, M. Vukotić B. and Stojanović M. 2019 “Persistence of Technosignatures: A Comment on Lingam and Loeb”, Astrobiology 19:1300-1302.

Clarke, A. C. and Kubrick, S., 2001: A Space Odyssey, 1968

Ćuk, M., Hamilton, D. & Holman, M. 2012 Long-term stability of horseshoe orbits. M. No. R. Astro. Soc. 426:3051.

Davies, P.C.W. & Wagner, R.V., 2013 Searching for Alien Artifacts on the Moon”, Acta Astronautica, doi:10.1016/j.actaastro.2011.10.022.

Drake, F. M., 1965, Current Aspects Of Exobiology, eds. Mamikunian, G. and Briggs, M H., Oxford University Press, p. 323-345.

R. Dvorak, R., Lhotka, C. and Zhou, L. 2012 The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids. Astronomy & Astrophysics 541.

Freeland, R.M. & Lamontagne, M. 2015, Firefly Icarus: An Unmanned Interstellar Probe Using Z-Pinch Fusion Propulsion, JBIS 68, 68.

Freitas, R. & Valdes, F., 1985. The Search for Extraterrestrial Artifacts (SETA), Acta Astronautica 12, 1027, IAA-84-243.

Haqq-Misra, J. & Kopparapu, R. 2012, On the likelihood of non-terrestrial artifacts in the Solar System. Acta Astronautica 12, 1027, IAA-84-243.

Kaltenegger, X. L., Lin, Z. and Madden, J. 2020 High-resolution Transmission Spectra of Earth Through Geological Time, Astroph. Lett., 2041.

Lesnikowski, A., Bickel, V. and Angerhausen, D., 2020,Unsupervised Distribution Learning for Lunar Surface Anomaly Detection, arXiv:2001.04634.

Lingam, M. & Loeb, A. 2018 Relative Likelihood of Success in the Searches for Primitive versus Intelligent Extraterrestrial Life, Astrobiology 19, 28-39 (LL18).

Mamajek E. et al. The Closest Known Flyby Of A Star To The Solar System 2015 ApJ Lett., 8003 L17.

Marzari, F. and Scholl, H., 2013 Long term stability of Earth Trojans, Celestial Mechanics and Dynamical Astronomy, 117, 91.

Meadows, Y. et al., 2018, Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment, Astrobiology 18, 620.

Malhotra, R., 2019 Case for a Deep Search for Earth’s Trojan Asteroids, Nature Astronomy 3, 193.

Morais, M. and Morbidelli, A., 2002, The Population-of Near-Earth Asteroids in Co-orbital Motion with the Earth, Icarus 160.

Parkin, K., 2018, The Breakthrough Starshot System Model, Acta Astronautica 152, 370.

Quast, P. E., A profile of humanity: the cultural signature of Earth’s inhabitants beyond the atmosphere, 2018, Intl. J Astrobio.,

Rose, C. and Wright, G. 2004 Inscribed Matter as an Energy-Efficient Means of Communication with an Extraterrestrial Civilization” Nature 431, 47.

Shermer, M.,2002 Why Hasn’t ET Called? Scientific American 287.

Shostak S, 2020, SETI: the argument for artefact searches, Intl. J. Astrobiology 1–6. 10.1017/S1473550420000233

Stacey, R.G. & Connors, M. 2009 Delta-v requirements for earth co-orbital rendezvous missions Planetary and Space Science 57, 822.

Stride, S. L., 2001a, An Instrument-Based Method to Search for Extraterrestrial Interstellar Robotic Probes, JBIS 54, 2-13.

Stride, S. L., 2001b, Instrument technologies for the detection of extraterrestrial interstellar robotic probes, Proc. SPIE 4273, Search for Extraterrestrial Intelligence in the Optical Spectrum III.

Venigalla, C. et al., 2019. Near-Earth Asteroid Characterization And Observation (NEACO) Mission J. Spacecraft & Rockets 56 1121.

Von Neumann, A. 1966, Theory of Self-reproducing Automata. ed. Burks, A., University of Illinois Press.

Wiegert, P. A., Innanen, K.A. and Mikkola, S. 1997, An Asteroidal Companion to the Earth, Nature: 387, 685.

Zhou, L., Xu, Y.-B., Zhou, L.-Y., Dvorak, R., & Li, J. 20119 Orbital Stability of Earth Trojans. Astronomy & Astrophysics, 622, 14.

Zxiaojing, Z. et al., 2019 Zhenghe-A Mission to a Near-Earth Asteroid and a Main Belt Comet, 50th Lunar and Planetary Science Conference, 2132.

The logic is clear, and in general, I agree with it. However, it appears that you are confusing the presence of a lurker to be found and actually finding it.

Let me give you some terrestrial examples:

1. Archaelogy. Ruined cities clearly are analogous to lurkers. They exist long after the civilization that built them disappeared. They may have inscriptions to communicate with us [c.f. Shelley’s “Ozymandias”], or just be piles of stones. However, many such cities became buried. Despite billions of humans tramping all over the globe, buried cities continue to be found, thousand of years later. There are likely to be more to be found and therefore the found/total cities ratio is >> p(lurkers), at least with current technologies, although this, in turn, depends on assumptions.

Cost is a strong predictor of choice when the utility offered by different items is the same. The utility provided by beacons and probes is very different. I would caution against using the cost difference as a strong predictor. For example, even though the utility of bicycles and boats overlaps, the fact most bicycles are cheaper than most boats is irrelevant if there is a demand for floating.

Probes offer valuable utility to a people demanding communication with other peoples. A probe within your neighbor’s system provides the security of verification. The cost of a probe would need to be compared to the cost of a beacon, giant telescopes, and the risk posed by revealing a home world. The difference in utility may make the difference in cost irrelevant.

Advances in robotics, Ai, and biotech could fundamentally reshape economics, especially the economics of super projects. The transformation of a planet into billions of probes or a giant telescope could be bootstrapped from a tiny “seed capital”. These same advances may also increase civilization lifetime.

The advantages of probes increases with time. An ancient people will be more likely to invest in probes and over time, will focus investment where it has a high return. Assuming an ancient player 1, the likelihood player 2 makes contact with a bespoke probe increases with time. Imo, the surveillance advantage provided by probes is too big for player 1 to ignore. Arguably, surveillance, verification is the foundation of diplomacy. MAD is impossible without it. However, as player 2 matures, the probability of player 1 being discovered increases. The location of living probes will be important. Perhaps proximity to Earth would be proportional to the general aggressiveness of galactic diplomacy.

Thank you James Benford for the last two articles. Your discipline provides a place for me to be less disciplined.

I don’t find very much meaning in the Drake equation. It makes reasonable assumptions which, taken collectively, are immensely confining.

To begin with, we assume the lifespan of a civilization is finite, and indeed, more evanescent than the Milky Way. Yet as we consider the subjective time of organisms in high energy environments, such as Forward’s cheela, we realize that they should be expected to live very fast. The universe is not exactly an ideal gas, but at a brutish approximation, the smaller it was, the hotter it was, and the faster things happened. Give or take a few orders of orders of magnitude, a voyage across the universe was as long and interesting an affair when that universe was the size of a grapefruit as it is today. There may only be a finite amount of “t”, with our chemistry of atoms, but there may have been an infinite amount of “tau” through which the minds of the cosmos have developed.

Parallel with this is the possibility of alternate regimes of physics, where different laws are of foremost importance, whether at earlier times or in strange environments. Some earlier version of the Drake equation might have discussed the possibility of life evolving in a ripple of quark-gluon plasma, and science fiction writers may glumly have pondered ways to survive the bitter cold end when triplets of quarks would be ripped away from one another by immeasurable distance.

If our conditions were not the first conditions of life, we can’t be sure that our observations are free of “supernatural” interference (at least by our standards). What if vast and complex intelligence lurks in the unnoticed entanglements of nucleons or the magnetic field lines of the sun? What if the positive terms of the Drake equation are offset by negative terms as advanced cultures destructively interfere with radio noise to protect the peace of their evolutionary reserves? What if Lurkers can be reverted to the original source rock of a planet, complete with sediments and fossils, with flawless accuracy? Few who have seen old mining country on Earth would not wish for such. What if the messages between aliens, or the aliens themselves, fly with warp drives like the one profiled in your recent article, hidden from our sight and detectable only as the overwhelming mass of dark matter and energy that lies somewhere outside of our nature preserve? Where does the Drake equation calculate the odds that we are living in the Matrix? We really can’t predict where the breakthrough will be made.

Brilliant analysis. Astrobiology is an subdiscipline of biology without a specimen. SETA and SETI are sadly similar — to date. But the next picture beamed back from the Perseverance rover could shake up many if not most contemporary world-views. Keepin’ muh fingers crost!

Very interesting articles, thank you. In comparing searching for lurkers versus the more traditional radio searches, I have always wondered whether one could get a better idea about the likelihood of success for the latter by constructing various distributions for the radio luminosity versus the number of civilizations emitting signals with that luminosity, that is the luminosity function as it is called.

This would inform what kind of searches to conduct.
For example, if easy to detect radio signals are rare, than an all sky search search would probably be likely to succeed than a targeted search.
This may be why an additional factor why searchiarlng for lurkers may well prove to be more successful than traditional SETI as we have reasons for knowing where they will be so the time spent searching the obvious ones would be the same if you searched for all lurkers. Hope this makes sense.

I can see one using various Monti-Carlo simulations for trying to get a handle on this.

My comment is more on the lifetime of civilizations. If we are to consider the Roman Catholic Church as the oldest institution, then we must also consider the Greek Orthodox Church and the other 4 churches claiming to be Apostolic as the oldest institutions too. Of course there can only be one Apostolic church, but truth is that all 6 can claim a succession of bishops all the way back to the Apostles. That being said, I would not quite consider them as the oldest institutions per se. There are several horse races taking place on St George’s day in Greece that claim to be the uninterrupted Christianized descendants of ancient sporting events. The Tuesday bazaar of the city of Serres has been taking place every Tuesday for centuries, earliest known occurrence is in the 13th century and is likely older and I doubt that it is unique in Greece or the rest of the world. For a more informal event I would note the Carnival celebrations that have been taking place since time immemorial and we incorporated into the Church calendar and Christianized. There are quite a large number of similar folk events dating to time immemorial such as the event of Arapis in Nikisiani on Mt Paggaion taking place on either Epiphany or St John’s that shows its obvious ancient origin of a ritual to bring the end of winter:

Greece is a place where civilizations and states have risen and fallen and the theme is both of continuity and disruption. After prehistory and protohistory which in Greece are important, think Mycenean civilization and the Trojan War, Greek history is divided into Ancient (ca 800 BC-330 AD), Byzantine (330 AD – 1453 AD) and Modern (from 1453 on). We do have a continuity of culture, the whole bickering and backstabbing council that led to the Battle of Salamis, whose 2500 years we commemorated last year, is quite familiar and ordinary, yet also breaks, and not just of the technological kind. Stuff done millennia ago is known to come back and bite us, and I have no doubt that something launched a very long time ago, if it returned, would still be quite recognizable.

Alex: I’m not seeing the connection you’re making please amplify upon it.

Let me state up front that I agree that we should look for lurkers. I would go further and suggest that they may even be on Earth. They could have very small sensors. With miniaturization, “smart dust” sensors could be like grains of sand and maybe infiltrated over deep time into the geology.

I have no argument about the Drake Equation, nor your modified versions for SETA. Both provide a possible number of civilizations or artifacts that could exist. The Drake Equation makes the most sense as a civilization isn’t divisible, and the SETI folks assumed no star travel or colonization. Artifacts however are more problematic. In your modification for SETA, you substitute the time to radiate with the time lurkers are in the system. But your value for numbers assumes 1 lurker per system per civilization. Each civilization might launch N probes and we have no way to make estimates on N. This is important for finding them, as your “Success Ratio” is dependent on this.

Clearly, if there are more than 1 lurker per civilization, then your equation underestimates the “success ratio” – assuming that if any are present they will be found.

With beacons, an all-sky radio survey is possible, and with a wide enough bandwidth, any beacon should ultimately be detectable. [That we haven’t detected a signal so far is blamed on insufficient coverage of the sky and bandwidth limitations.]

However, with lurkers, the problem is much more difficult. A dead lurker does not announce its presence. By analogy, a live bird can be heard singing from some distance, so you know it is present, even if unseen, whilst a dead bird is just a small object somewhere on the ground, perhaps covered in leaf litter. Then there is the issue of stealth. Almost all complex organisms utilize stealth whether they are prey or predator. This is because animals have agency and can respond to other animals. Assuming lurkers would not be stealthed to evade detection just assumes that they will not expect to do any close observation of animals, just make standoff remote observations, preferably of phenomena that have no agency. Whatever the reason, lurkers could be very hard to detect. They may be very small, they may be camouflaged, they may be buried in regolith or sediments.

My point is that any use of numbers to compute a “success ratio” is not suitable to determine the likely success of a search. Even if we assume there is a lurker somewhere nearby and has been so for millions of years, ie N=1, there is no reason to assume that it could ever be found. My analogy to archaeologists finding cities lost for centuries and millennia despite being underfoot is indicative of the problem. They are being found now as a result of droughts on the landscape, and new detection techniques such as aerial lidar.

Now I don’t want to give the impression that the search is so hard we shouldn’t try. I am all for trying for the “low hanging fruit”. A search of the lunar images makes absolute sense. While I doubt we would find anything other than manmade probes and ships, the cost is so low that we should try, just in case there is a shiny, crashed probe on the surface, or a transparent pyramid as depicted in Clarke’s “The Sentinel”. Looking beyond the Moon gets much more expensive. If a lurker is alive and responds to pings, that would be nice, but that may indicate that we were lucky to find one that was relatively recently arrived and not millions of years old.

In summary. There could be a lot more lurkers in the system, most of which are dead, launched successively by a civilization to monitor our system. This would be analogous to our launches of probes that last years and then die, like the rovers on Mars. On its face, this increases the numbers for the success ratio, although it is a guess what the multipliers are. OTOH, despite their numbers, they may be very hard to detect for a variety of reasons. By analogy with the bird, a single person can detect a singing bird, but a village might need to carefully scour an area to find the dead bird. The ease of detecting beacons vs lurkers is not a numbers game as per your equations, but more about the probability of detecting either. That is subject to available technology and a lot of guessing. My sense is that the difficulty of detection is more important than the computed number guesstimates of the modified Drake equations.

This opinion may be colored by my limited fieldwork looking for organisms that should be present, but seem to evade a careful search.

Question: Is it still a matter of debate as to whether or not self-replicating space probes are possible?

To what extent is our current era of technological stagnation related to our ability to access energy? Technological progress increased dramatically as a result of the increased access to energy that came about as a result of the Industrial Revolution. And, as we know, most of the energy that we have used to propel ourselves into the current era has been derived from fossil fuels. Fossil fuels being so energy dense and easily accessible represented a sea-change in our ability to carry out projects and do work as well as synthesize a variety of products such as pharmaceuticals. That said, in addition to being problematic from an environmental standpoint, fossil fuels are finite and represent a very small fraction of the energy available from other potential sources.

Technological stagnation has clearly set in as Peter Thiel and others have identified such that besides the internet and information technology, many areas of technology have stalled. Let’s step back and imagine the possibilities for what might happen when and if the next energy sea-change occurs:

A Solar System Energy Infrastructure & What It Could Accomplish

The greatest source of energy available within the solar system is the Sun. Once space travel finally takes off, we can imagine the construction of huge arrays of space-based solar power that could be used to fuel antimatter production by the ton or power particle accelerators that would make the LHC look miniscule. Advances in basic physics, historically, have come about as a result of experiments carried out at progressively higher energies. So, imagine the technological progress that could result from a massive particle accelerator constructed by robots in the inner solar system. With energy no longer being a limiting factor in technological progress and our ability to conduct experiments and make inroads into fighting entropy, then the Great Stagnation could lift. Right now, we are stuck in the valley of a bimodal distribution of technological progress brought about by our inability to access space cheaply.

Technological stagnation has clearly set in as Peter Thiel and others have identified such that besides the internet and information technology, many areas of technology have stalled.

This may be illusory. It is certainly the case that technologies tend to have a logistic development course which makes highly visible technologies like aircraft appear to have reached their upper limits. However, less visible technologies, like biotechnology, are still undergoing exponential development.

The economist, Brian Arthur, argues that technologies are multiplicative, that each additional technology multiplies the space of development. It is certainly true that information technologies, the software probably more than hardware, have been a very important enabling technology.

The coupling of energy and technology development is more likely an artifact, IMO. Early technologies, like steam power, were about creating work, which requires energy. However, the technology development and use of fossil fuels were mainly correlated with growing GDP. With the oil shock of the 1970s, while this energy/GDP correlation still held, the slope dramatically decreased. Miniaturization of hardware, and a focus on information-rich technologies, rather than work generating ones, have decoupled this relationship even further.

So while our aircraft have stopped traveling faster, they have become more efficient. Our computers are less energy-consuming, and software more capable. Biology manipulation development is still racing ahead (recall how fast the new mRNA vaccines were applied?) and there is a open road ahead in that arena.

As for space technology, the old ELV technology has matured, although it is getting a rebirth with reusability. Electric engines and solar sails are becoming almost “off the shelf”, and fusion engines are looking like possibilities, Human spaceflight seems to have stalled, but we will see whether reducing the cost of access to space and private enterprise leads us to a renaissance in space or not. Clearly, robotic spacecraft and machines are advancing nicely, and I for one am amazed at how fast telescopic techniques and capabilities are advancing.

Overall, I don’t see technology development slowing down, just the obvious visible ones that we experienced in the first half of the 20th century. We just need to attune ourselves to new directions in technology development.

The decoupling of technological development and energy to which you refer is well-captured and critiqued in Peter Thiel’s quote:

“We wanted flying cars, instead we got 140 characters.”

So, yes, software and hardware have steadily/reliably advanced in recent decades with the internet being the single biggest product of this trend, but IT is only one (albeit important) realm of technology. Think about it this way: we were sending people to the Moon when supercomputers were less capable than the processors we have on smartphones today. “Visible” technologies that do work as opposed to merely process information clearly matter greatly in terms of expanding our presence in the solar system let alone beyond, would you agree? And further, it is precisely in the realm of technologies that do work where we have witnessed the greatest stagnation. In order to terraform Mars or build a solar system infrastructure, all of the IT and Apps in the world aren’t going to substitute for fundamental advances in material science, energy storage and generation, and rocket propulsion. My point is that, right now, we are stuck in the valley of a bimodal distribution of technological progress with respect to technologies that do work brought about in part by our inability to access space cheaply. Accessing space cheaply is one of the biggest ways in which I see us breaking out of this period of stagnation in the realm of technologies that perform work. The fact that advances in IT have not resulted, so far, in great strides in technologies that do work is part of the reason why we are still in the valley, so to speak. Our ability to model certain physio-chemical phenomena using classical computers may be part of the reason, in addition to socio-political reasons, why it has been so hard to break out of this holding pattern. To me, an interesting question is that why, despite the continued advances in IT has this not yet translated into a new logistic curve in the realm of technologies that do work? I also wonder to what extent Richard Feynman may be on to something when he talks about the limitations in our ability to model nature:

“Nature isn’t classical, dammit, and if you want to make a simulation of nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem, because it doesn’t look so easy.”

Could it be that part of the reason why advances in “visible” technologies that do work has slowed is because the advances in IT despite their evolutionary improvements do not represent a revolutionary leap in our ability to model nature quantum mechanically? In other words, perhaps quantum computers, due to their ability to model nature at the quantum level, are what will reignite a period of advancement with respect to technologies that do work– advances in energy generation and storage, material science, propulsion, chemistry, and even fundamental physics? Yes, there is a fair amount of hype when it comes to quantum computers, but the advances in this field are undeniable.

This is not a counterargument, more of an exposition of a counter view based on what I see happening.

“We wanted flying cars, instead we got 140 characters.”

Theil’s cute meme is clearly an exaggeration. But consider for a moment. “Do we even want flying cars”?
Personal air transport has been an idea going back to the late 19th century. The “cities of the future” were full of brutalist skyscrapers with the air filled with improbably winged transport. When helicopters became a reality, many cities (like London) banned their use over populated areas keeping their use over the R. Thames. In the more liberal US, police, and traffic helicopters flew over cities with the inevitable misuse issues, not to mention the odd crash. But now we are getting those flying cars – passenger-carrying drones. Let’s just see what happens when the sky is full of these things. How many crashes onto the ground will happen? How much misuse will be evident? We have that pleasure/displeasure to come. Note that cheap IT will make these drones possible, perhaps by constraining their flight paths and maintaining autopilot at all times. What other tech were we promised? Nuclear power “too cheap to bill for”. Not only did this not happen, but nuclear power is also expensive and we have experienced reactor failures, most notably Chernobyl. Luckily, wind and solar power have proven far more benign, and are now cheaper than most fossil fuels and vastly cheaper than nuclear. Just as well we don’t have vast nuclear plant assets that need to be decommissioned and the waster disposed of somehow.

IT OTOH has hugely increased our capabilities. Job categories like typists and telephone operators have disappeared. Many jobs involved in driving will likely disappear. Skilled, but repetitive cognitive work jobs will be reduced. All these changes benefit the end user in providing capabilities that were once expensive, but can now be done cheaply. In the previous post, we noted that machine learning will replace human eyeballs to find potential non-terrestrial artifacts on celestial bodies.
While we haven’t reached Star Trek levels of medicine, it is IT that has increased our understanding and manipulation of biology. The time to develop the vaccine for Covid-19 was largely due to clinical trials. Computer models of human physiology reduce that time and reduce costs by eliminating failures before expensive testing. It may be possible that new drugs and vaccines can be developed in weeks. That seems like a very worthwhile goal. The development of therapies that involve the immune system is already considered breakthroughs in the treatment of common diseases like cancer. Seems like a better outcome than dying earlier but having a shiny flying car to drive.

If you read the popular science press and the journal literature, you will note the rapid advances in many areas due to IT. Much of it is for dealing with the small scale. These improvements and even breakthroughs are not obvious like flying cars, but they have an impact that is cumulative.

You mention terraforming planets and building space infrastructure. I think the model in your head is that of lots of spaceships and astronaut workers. It is the sort of future depicted in Allen Steele’s short story collection: “Sex and Violence in Zero-G”, itself a future that assumes the building of O’Neill habitats and solar power sats. But the future won’t be like that at all. Instead, IT will allow robots to do the work, supervised by humans, mostly on Earth. Interstellar flight will be robotic for the foreseeable future. Terraforming planets (if we must be so crass reinventing colonization and “Manifest Destiny”) will be much more subtle, using robotics to plan out the needed ecosystem successions using engineered organisms designed by IT systems, rather than “bulldozers and tractors driven by Heinleinian super-competent men plowing up the regolith and planting trees and crops”.

Want an anti-matter drive? That will probably need a supercomputer to make sure that the engine doesn’t destroy any nearby infrastructure in a “glitch”. Want a space manufacturing city the size of an O’Neill, churning out hi-tech product and even Enterprise-class spaceships? Again, mostly built by robots, supervised by humans on Earth, perhaps not even commuting to work, a precursor social system to Forster’s “When the Machine Stops”. And those Mars colonists? Less like Heinlein’s colonist ideal, and more like Asimov’s Solarians, I think.

The key to economic growth on earth, for most people, is growing a low energy economy. That means building and making things that use little energy over their life cycle. One of the best ways to do this is to make them “lighter”, i.e. smaller and more information-rich. Yes, a solar system-wide economy would be great (and probably needed for real starships), but most of this work will be done by robots, including the manufacture of more robots.

I really don’t see technological stagnation. Just the more visible things of the 20th century reaching their limits while less visible technologies continue to develop, in some cases, explosively.

The 1960’s vision of space development was exemplified by the movie: �: A Space Odyssey”. I would have loved that future to have happened by 2001. In reality, we don’t really want nuclear-powered Orion spaceplanes. A Moonbase would be nice, but we don’t really need Space Station V or even the Discovery. Discovery’s mission could have been completed by HAL, saving a huge amount of hardware and resources for life support. The Aries 1B Earth-Moon ship would be nice, but do we need a Clavius base of the size and complexity shown? Not a single robot to be seen doing surveys and resource extraction. Just people (men only) pottering about in spacesuits. Should we mine the lunar regolith for He3, it will be more like the scenario in “Moon”, but without the cloned Sam Rockwell, because in reality, he would be on Earth, supported by a team to manage repair robots for the excavators.

As Mies van der Rohe said: “Less is more”. Today I view that as using elegance rather than brute force to achieve goals. Those goals will not be those we once had, and Peter Thiel still seems to want.

I am not necessarily an advocate for flying cars and you certainly make a number of valid points in your response. That said, how can we access space without more advances in these “visible” technologies that do work? And yet, without access to space, it may be harder to advance these visible technologies because we will not have access the the “free energy” of the solar system. So, this represents a paradox of sorts and the irresolution of this paradox partly, IMHO, explains why we have had a hard time expanding our presence in the solar system. The resolution of this paradox, if achieved, will allow us to expand our presence in the solar system and it will increase our ability to make even further advancements in the “visible” that do work. Interstellar travel and even fast interplanetary travel will require substantial advancements in technologies that do work, would you agree? What do you think about the idea of a fully-autonomous solar powered antimatter factory in the inner solar system? Without advances in materials science, energy storage and generation, and rocket propulsion, how can we– without advances in these “visible” technologies that do work– expand our presence into the solar system?

What do you think about the idea of a fully-autonomous solar powered antimatter factory in the inner solar system?

That might be like having an explosives factory nearby. No thank you, they occasionally go boom. Did you see the recent fertilizer explosion in Beirut?

Without advances in materials science, energy storage and generation, and rocket propulsion, how can we– without advances in these “visible” technologies that do work– expand our presence into the solar system?

If flying pigs are not possible (except as air cargo), don’t waste effort trying to engineer them?

I think we will have some of the visible technologies, but they may largely suitable only for machines. I’m reminded of Asimov stories where robots are used to test new technologies. We once tolerated a high human loss rate in test pilots. Today, test flights are far safer. If new aircraft was tested by a semi-autonomous machine (or telechiric?) there would be no need to put a meat person in the pilot’s seat. If the human pilot isn’t needed, why not design the vehicle without the intention of having a human pilot at all? Hence drones aircraft.

If a machine can tolerate 10,000s of g, maybe the best way to build launchers and interplanetary spaceships is to assume machine control instead. Instead of humans running factories in space, PK Dick autofacs.

IOW, perhaps the paradox is solved by taking humans out of the equation. Accept that our “mind children” will inherit the future in space, not we meat humans.

I have used this analogy before: Are we like intelligent Devonian fish trying to build mobile aquaria to colonize the land? Evolution found a better way. I wouldn’t rule out technologies to transfer [copy really] human minds to synthetic brains and bodies, although I suspect AGI embodied in machines will be first.

The latest SETI talk on “Vanishing Stars” had this viewer poll:

“What’s the best way to search for ET?”

1. Search the observable universe for billions of possible ET candidates – 43%
2. Search nearby star systems for a radio signal or flashing lasers – 48%
3. Search for technologies in our solar system – 9%

Not that the survey was indicative of expertise, but it might suggest that the lurker search has some selling to do. I think Seth Shostak was surprised that option 3 ranked so low, so he may be more on board with the idea than I assumed.

[I also think the poll was poorly designed. It seems to me that option 2 is just a subset of option 1, which implies that its score should be lower than option 1.]

I think the problem for searching for lurkers in our system is analogous to the problem of finding a cat, any cat. Is it best to walk the neighborhood and look out for cats, or to do a more intense search of your garage or basement? The article basically argues for the latter as there is the advantage that a long-dead cat’s skeleton may be found in the basement at anytime you search, whilst the neighborhood cats may be out of sight sleeping when you walk the neighborhood in the daytime, rather than for a few hours at night when they are active. )

If any alien probes had explored our solar system in the past it seems to me that they would have had good enough AI to select Earth as an obvious destination. It would be relatively easy for an advanced robotic spacecraft to enter and maintain an orbit around Earth. The probe could photograph our planet and send high quality images and videos back to whoever launched it. From the dawn of life on Earth up until the early 20th century the probe would be undetectable so there would be no danger of discovery or destruction. For the past 100 years or so we’ve had the ability to find such a probe and it’s not there. Since Earth is the obvious candidate for exploration in our solar system, the idea that there might be probes or artifacts elsewhere seems far fetched.

Alex: Thank you for pointing out that my estimate for the number of Lurkers we could find is in fact an underestimate! Yes, I expect an ET civilization would send several probes to look at aspects of that life once it is discovered. If it saw evidence for civilization, which could happen only in the last few centuries, it would then send other probes with sufficiently large apertures to investigate many aspects of Earth, not just biology.

Saying that aliens will disguise themselves or adopt camouflage is one possibility, but we can’t assume that all aliens will!

I do expect there would the problems with swarms of nano probes acting together. Robert Freitas points out that the processing requirements for them to work together would take energy, and with their small mass they’d be bright in the IR.

I don’t think probes would be found on Earth because the active weather and geology would disassemble it or bury it. However, on the moon NASA estimates that our own artifacts will survive for at least millions of years. But on co-orbitals or Earth Trojans the lifetime will likely be longer, unlike the Moon, because they will not be suffering from a rain of micrometeorites falling into their weak gravitational well.

I would like to emphasize that the Chinese are likely to explore the new earth region before we do. That’s because they already are there. They now have a probe at L1 and are sending it on to the Earth Trojan region to explore for Trojans. The cost of such missions is relatively small because the delta-V required is small. Several mission studies have been done to estimate the probe and operations cost for such expeditions and they turn out to be quite straightforward. For example, Mason Peck of Cornell estimated that, with a free piggyback launch by SpaceX, the cost of a probe and operations on Earth would be of order $10 million.

The articles and the comments led me to further thoughts. First, the more I think about search for probes by their own emission in the lunar night, the better it looks.

A region beyond the lunar farside is well shielded against Earth’s emissions. The cone includes L2 point of Earth-Moon system, near which there already is a Chinese relay satellite. During the lunar eclipses (not only total because of

3.1 deg lunar angular diameter from L2), it is shielded also from the Sun, and it is safe both to take very long exposure images of the far side, achieving extremely low SNR over UV-Vis-IR ranges, and to listen in the longwave (radio to terahertz) to sensitivities not achievable anywhere else in the near-Earth space. Maybe SNR in the ionizing range (X-Ray and gamma) is also reduced to considerable extent, which may reveal a nuclear-powered probe even if it is no longer active. The surveyed area is very big – looking at the surface is much simpler than at the interior, and the lunar far side has more surface area than a million of one-kilometer asteroids. Yet, from near lunar L2 a sub-kilometer-scale resolution can be achieved with a compact camera, with resolution increase available by low-orbit satellites. Starlight background from a 100 m-wide area is on the order of 1 milliwatt per visible range. So the lunar far side night reconnaissance indeed looks a very attractive and low-cost program, with a range of possible activities and scientific returns beyond SETA.

The long-exposure photos of farside in the visible range during the coming eclipses could be taken by relay satellites at virtually no cost. At the high end, specialized instruments could be put on some future sats, surveying full electromagnetic domain and yielding additional scientific results, which in this extremely low-noise environment may well be valuable by themselves even in the abscense of ETA detection.

The sooner – the better, the noise floor is rising due to increasing human presense in cislunar space.

Here comes the other side. The Lunar Night Reconnaissance is strongly biased towards the living probes, and there is the very important distinction. If we find a dead artifact, we can relatively safely study it, but finding a living probe would raise all the issues of ethics and safety to their fullest. I don’t believe much in the Dark Forest State, but just not recognizing a warning sign could be enough, be it megayear-old-tech analog of “Danger! High voltage” picto or “Watch the traffic lights!” warning.

But if we find a living probe, it absolutely does not mean that we need to come down on it, because, within this framework, it would almost guarantee that there are many more dead ones.

Products of a technology live orders of magnitude less than the technology itself. A “no-FTL” probe into another stellar system needs to live for centuries or millenia, but it’s hard to imagine multi-million-year-long lifespan – for most purposes it is just too much overhead. But within the initial assumptions it is quite reasonable to assume that first ETIs capable of interstellar exploration appeared billions of years ago, and Earth biological “interestingness metric” was above threshold since at least Cambrian Explosion, so there were many visits in the pasts by probes that are no longer active.

So if we do find a living probe one, we could study it from a safe distance while intensifying our searches elsewhere, assured by the knowledge that there is much, much more to find and we can do it. The way actualy looks straightforward: after finding a living probe, search for dead artifacts, find and study them, gain knowledge that will help to understang the living one, then try to study the probe and/or interact with it if it is considered safe or needed, based on what we learned.

PS I think it’s quite wrong to call all xenoartifacts Lurkers. Aside of ominous connotations, a lurker is the most hard case of xenoarchaeology – something that is deliberately camouflaged and thus hard to find, and demanding most care during studies. Likely also the least common. Two other classes could be identified, which can be described as “active” and “passive”, or “probes” and “artifacts”. First is something that is likely not stealthy, but is living or actively decaying (whatever that means). These are the most easy to find, but demand almost as much care as Lurkers. The last is something that is surely dead – quite hard to find (but not as hard as a true Lurker), but the most common and the most safe to study.

My only issue is why would a probe be on the lunar farside, rather than nearside if the purpose is earth observation? If it was, that would seem to imply that it is/was “hiding” and collecting data from other sensors that were able to view the earth directly.

The Haqq-Misra paper suggests a number of options for detecting both surface and buried technology on the Moon, including anomalous IR signatures in the lunar night [reminiscent of how the buried moon bus ​Selene was detected in Clarke’s “A Fall of Moondust”].

The obvious place to start is the inspection of the LRO images of the Moon’s surface. If nothing, then consider collecting different data that might expose anomalies. This should probably be piggybacked on surveys for other reasons, like detailed mineral prospecting. Unlike the search for life on Mars, we cannot hope to limit the search area by inspecting the “most promising” places as Perseverance is doing.

Far side is just easier to search because of lower backgrounds. While it is of little use for direct Earth observations, it could be a location for something that does not require Earth in sight, like data processing module of transmitter. Of course there is no way yet to tell which bias is stronger, but instruments designed to touch noise floor on the far side could both expand “search space” and yield more scientific results in areas other than SETA.

Lunar poles are another good location – they could attract ETI explorers for reasons similar to our own. They would go there if they need something to build, produce or repair on the Moon and need CNO elements for that. For the locations on the Moon, the lunar polar ice deposits are by far the closest and most convenient place to obtain volatiles. Maybe to the lesser extent even for Earth coorbitals where they are likely very scarce. In addition, the search area is much smaller than the whole lunar surface, and a search for artifacts themselves and for evidence of past activity could be piggybacked on the already-envisioned in-situ exploration. does not go into much details, only proposes search of Diviner data. And I haven’t found yet any papers on comprehensive search of anomalous sources in it. But I can imagine it could be tricky. Nightside thermal emission is not influenced by Earth interference on the near side (like radio and visible, latter because of ashen light), but the background is high and varying with mineralogy and subsurface structure, on all scales. 100 K equals to several watts per meter, enough to swamp even multi-kilowatt source at the Diviner resolution. But it may be much better for non-thermal emission or sources like radiators at 300 K, heated enough to give output in the shorter-wave bands where thermal background is low.

I don’t think probes would be found on Earth because the active weather and geology would disassemble it or bury it.

I would interpret this as saying that the probe would send landers to the Earth’s surface to do the necessary observation, sampling, and analysis of life, but that unless they can take off again, they will eventually be destroyed or buried and this will make them next to impossible to find except serendipitously.

This may well be a case of looking where the light is available rather than where you dropped your keys (a long time ago).

As a commentator mentioned elsewhere, acquiring an alien probe would be immensely valuable, far more so than a short radio transmission sending prime numbers, or a low-resolution picture of some sort, even if it was in a poor condition like the Antikythera device that has recently been reconstructed. However, such a device on an airless world[let] would be in far better condition to be investigated.

I do think looking at the hi-res LRO images of the Moon is a good first step. Both machine learning methods and citizen science would be a good way to start, especially as this could be combined with other searches. I imagine that locating all the manmade objects on the Moon would be a nice project, especially if we found an unexpected or unreported terrestrial probe on the surface. With humans on the Moon again in this decade, what a great opportunity to plan an exploratory trip to investigate a possible artifact up close and bring it back to base for examination.

Alex: Recall that my piece referred to ‘ET lurking in our Backyard’. Using that analogy, the standard SETI staring-at-stars method is to look not in the backyard but in some other country. Are there cats there? Better to start by looking locally.
Seth Shostak actually favors SETA, looking locally. As my reference shows, he wrote a paper about it a year or two ago. However he did not propose any strategy, tasks for doing so or locations to search.
The psychology of SETI practitioners, who I have observed for decades now, is to listen for a message. That’s a passive activity. What I advocate is active exploration, look at photos of nearby sites, then send probes exploring our near space. This is not in the comfort zone of SETI practitioners that I know.Their passive psychology is uncomfortable about missions beyond the Earth. So I’ve found that they do accept that SETA makes sense, but they are not going to engage it themselves. That’s why I think that the SETA concept will be pursued outside of the SETI community, perhaps by the Chinese, who are already exploring Near-Earth Space.

SETA takes a higher budget, and it’s mostly just plain space exploration. Which should be a great thing. Put to effective use in construction, rock in an Earth Trojan orbit or even one of the near asteroids could be worth more than gold on the ground. Prospecting for alien artifacts helps to keep things interesting, even though I doubt anyone can really calculate the odds.

Now when something looks at first photograph like a spaceport in the asteroid belt (the Cererian faculae), it may turn out to be a natural formation – but if that formation happens to be a massive salt deposit and perhaps an entryway to an underground water reserve on Ceres, it still bears further examination.

Even when alien ruins are scarce, these prospecting expeditions test us. Can rival nations and groups possibly reach a less wasteful philosophy than competitive claims of ownership, and work together to bring knowledge and resources that no human made to assure the wealth of all humanity? We’ll need such thinking to avoid woe if they do turn up.

For what little it may be worth, I had an article proposing
a revised Drake Equation for alien artefacts in the Journal of the British Interplanetary Society in 2000, vol 53, No 1/2:

Sadly, I don’t have a subscription for JBIS so I cannot reach around the paywall. Can you flesh out the abstract a little to provide a flavor for the relevant parts that impact the Benford paper?

As your paper was cited by the Haqq-Misra & Kopparapu paper in the references, I gave that paper a read as well. It does an interesting Bayesian estimate of probabilities of probe discovery, assuming a probe of 1-10m in size that is not camouflaged and is present.

I tend to think the authors have vastly overestimated the detection probability as they assume the probes are not buried. This is reasonable for the Moon and asteroids, but not the Earth. That we are now discovering buried caches of coins and other metal objects by magnetic detection should indicate that a probe is unlikely to be on the Earth’s surface in plain sight. Even on the Moon, dust might cover a crashed artifact over time.

In the discussion they state:
[NTA = Non-terrestrial Artifact]

However, it has been suggested that NTAs might be identified by their emission of anomalous microwave or infrared radiation [8], so the discovery of an unexpected temperature anomaly on the lunar surface could be a signature worthy of further investigation. Likewise, the chemical composition of an NTA would presumably be different from the surrounding lunar regolith, so an anomalous spectral signature on the lunar surface could also be a potential indicator of an NTA. This suggests a method by which surface temperatures and mineralogies, such as those available from the LRO Diviner Lunar Radiometer Experiment [41, 42, 43], could be examined for anomalies that are consistent with the presence of an NTA. Such an analysis is worthy of future investigation but is beyond the scope of the present study.

Going beyond a visual search makes sense to me. Just as terrestrial searches for buried metal objects with magnetic detectors is employed by “detectorists”, and other techniques have exposed hidden artifacts, buried cities with LIDAR, satellite spectral photography, and surface vegetation changes with drought. Buried probes on Earth must be far rarer than fossil animals, and yet we have barely scratched the surface for fossils. Locations on celestial bodies or free in space might be easier to detect – if we can effectively search those locations.

I do wonder how easy it would be to detect any one of a swarm of 1 gm Breakthrough Starshot probes that might have crashed into a world in the target system. Would they be detectable at all? Burned up as meteors on planets with atmospheres, or vaporized on impact with an airless moon. What technology would be needed to detect any residual anomalous metals around an impact?

Richard: I’ve gotten your paper from my JBIS library (1996 till now). You do propose strategies for finding artifacts and tasks for doing so. You don’t cover much about where to look, but at that time Earth co-orbitals and Earth Trojans had not been discovered. I’m going to reread it carefully!
It strikes me that Freitas proposed SETA in the 1980’s, you did so in 2000 and I’m doing so now. My proposals are more specific and quantitative, as befits our greater knowledge base. And there is a constant in this: the SETI community continues to ignore SETA!

Hi Jim. Thanks for giving it a read. Your paper is more sophisticated and massively more up to date. I like that these things come round in circles – each time they are more refined and move closer to potential implementation. For me, the most important thing is that the possibility of new ‘local’ forms of SETI gain more credence. The chances of success may be remote, but they are non-zero. Your paper is a huge step in the right direction. Best wishes, Richard

I’m very interested in Alex Tolley’s comments about the use of aerial lidar to find archeological sites on Earth.Is it possible we could do the same on the Moon?Apollo 15 mapped the lunar surface with lasers.Could any possible artifacts be hidden this data?

‘Library of the Great Silence’ invites E.T. to share long-term survival strategies

Intelligent aliens will soon have a space here on Earth where they can share how they made it through their technological adolescence.

We haven’t yet heard from any such beings, of course. Some researchers find this “Great Silence” puzzling, given how old the universe is and how many potentially habitable worlds dot its vast expanse.

One possible explanation is that civilizations tend to destroy themselves once they become “advanced” enough to explore the cosmos in a meaningful way. Such power is inherently hard to control and can burn you to the ground more easily than it can fuel an outward push, the idea goes.

“Although interstellar exchange could take time, a material archive of transformations will have immediate global value that may be sufficient to extend the lifespan of human civilization in the interim,” reads a description of the project, which is a collaboration with the SETI Institute in Mountain View, California. (Keats is currently an artist in residence there, and Hat Creek is the site of the Allen Telescope Array, which SETI Institute researchers use to scan the skies for possible signals from ET.)

“Manipulating existentially significant objects without the use of words — and without the underlying assumptions of language or limitations on who participates in the conversation — may facilitate comprehension of human behaviors that has previously eluded us, or even directly encourage beneficial practices such as cooperation,” the description adds.

Hat Creek may not be the only repository of such artifacts, either.

“We’re starting to reach out to libraries that already exist about whether they would host, potentially, branches,” Keats told “And, simultaneously, we’re actually looking off the planet and starting to look at what it would take to have a branch on the moon, for instance.”

You can learn more about the Library of the Great Silence here:

And the project is getting a formal kick-off of sorts this afternoon (April 29), via a conversation between Keats and his chief advisor on the project, SETI Institute senior astronomer Seth Shostak. The 30-minute discussion starts at 5 p.m. EDT (2100 GMT), and you can watch it live online.

SETI: microbes may already be communicating with alien species – new research

Drake Equation Tutorial

In November 2006, I was a participant in a panel discussion Defining the Drake Equation at the Windycon Science Fiction Convention. My co-panelists were Seth Shostak of the SETI (Search for Extraterrestrial Intelligence) Institute Bill Higgins, a physicist at Fermi National Accelerator Laboratory (Fermilab) and Bill Thomasson. You can see a picture of our panel at MidAmerican Fan Photo Archive Windycon 33 Saturday Panels. I have decided to turn the preparation that I did for that panel, and notes taken during the panel discussion, into a tutorial on the Drake Equation.

Drake Equation History

The year is 1960 and Frank Drake of the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia undertakes the first attempt to find extraterrestrial civilizations. Dubbed Project Ozma, for a period of 6 hours a day for four months the NRAO radio telescope listens for radio signals of intelligent origin. None are found.

Within a year a meeting is hosted in Green Bank to explore the issue of extraterrestrial intelligence. Frank Drake needed to come up with an agenda for the meeting in order to provide some structure to the discussion. To serve as an agenda, he devises the Drake Equation. Sometimes known as the Sagan-Drake Equation in the past, the meeting was attended by approximately a dozen interested parties.

Drake Equation Overview

The Drake Equation is an attempt to encapsulate all the variables that would be relevant to establishing the number of intelligent civilizations that existed in the Milky Way galaxy and which were broadcasting radio signals at this particular point in time. The Drake Equation is composed of seven terms. The first six are used to compute the rate at which intelligent civilizations are being created and the final term identifies how long each lasts on average as a broadcasting civilization. It is worth stressing that the Drake Equation applies only to intelligent civilizations in the Milky Way galaxy. It does not apply to civilizations in other galaxies because they are too distant to be able to detect their radio signals.

The Drake Equation is:
N = R * fp * ne * fl * fi * fc * L

N = The number of broadcasting civilizations.
R = Average rate of formation of suitable stars (stars/year) in the Milky Way galaxy
fp = Fraction of stars that form planets
ne = Average number of habitable planets per star
fl = Fraction of habitable planets (ne) where life emerges
fi = Fraction of habitable planets with life where intelligent evolves
fc = Fraction of planets with intelligent life capable of interstellar communication
L = Years a civilization remains detectable

According to the Wikipedia entry for the Drake Equation, the following values were those used in the original formulation of the Drake Equation:
R = 10
fp = 0.5
ne = 2.0
fl = 1.0
fi = 0.01
fc = 0.01
L = 10000

Plugging Drake's original numbers into the Drake Equation produces a value of 10 for the number of broadcasting civilizations in our galaxy. Now lets go through each of the terms in detail.

R - The rate of formation of Suitable Stars in the Milky Way Galaxy

Estimates for the number of stars in the Milky Way vary from a low of 100 billion to a high of 400 billion. Estimates for the age of the Milky Way also vary from a low of 800 million years to a high of 13 billion years. If we go with the lowest star count and the oldest age for the galaxy, the average rate of star formation works out to 7.7 new stars per year. If we go with the highest star count and the youngest age for the galaxy, the average rate of star formation becomes 500 new stars per year.

An important caveat to the above values is that the rate of star formation in the galaxy is not constant over time. In the galaxy's younger days, stars were being formed at a much higher rate. Today, estimates for the overall star formation rate range from 5 to 20.

Another caveat is that not all stars are created equal. For example, very massive stars are not considered suitable. Some versions of the Drake Equation use the R term for the overall rate of star formation and then add a second term to estimate the fraction of these stars that are like our own Sun. A suitable star would be one that has a reasonably long life (approximately 10 billion years for our Sun which is now in midlife) and sized so that the fusion process that powers the star produces the right amount of energy to sufficiently warm the planets but not turn them into toast. Estimates are that the rate of formation of Sun sized stars is on the order of 1 per year.

Fp - The Fraction of Stars with Planets

At the time the Drake Equation was created, the only planets that were known were those of our own solar system. Since that time approximately 200 extrasolar planets have been discovered.

When the Drake Equation was created, it was thought that planets would only be found in single star systems. It was believed that gravitational disruptions in multiple star systems would prevent planets from forming. This hypothesis removed approximately 50 percent of the stars from consideration. It has now been shown theoretically that these multiple star systems can have planets. For example, if a planet is in orbit around a star that is X units of distance away, then the planet's orbit can be stable if the companion star is more than 5X units away. Alternatively, if two stars are X units away from one another, then a planet that orbits these stars from a distance of more than 5X units should have a stable orbit.

So what fraction of stars have planets? Estimates range from a low of 5% to a high of 90%. If you use a value of 0.1 you are saying that you believe that 1 in 10 stars will have planets. Alternatively if you use a value of 1.0 you are saying that every single star will have planets.

Ne - The Average Number of Habitable Planets per Star

In his original equation, Drake optimistically assigned a value of 2 to this parameter meaning that there are on average two Earth-like planets per star for those stars with planets. Factors that must be considered in arriving at a value for this parameter are the chemical composition of the solar nebula from which the planets were created (the presence of sufficient quantities of the necessary elements) and the idea of a star's habitable zone (the range of orbital distances within which liquid water can exist)

Something else to consider is that our idea of habitable may be too restrictive. Does life require an Earth-like planet? This is a question of life as we know it versus life as we don't know it. However, from a biochemical standpoint, it is hard for us to imagine life that does not require liquid water.

Choosing a value of 1.0 for this parameter means that you think that every star with planets will have one habitable planet. A value of 0.5 means that there will be one habitable planet for every two stars with planets.

Fl - The Fraction of Habitable Planets Where Life Emerges

This parameter is something of a wildcard in that we only have one example of life. It is difficult for us to say how easy or hard it is for life to start given suitable environmental conditions. One interesting point to consider is this:

  • the Earth is approximately 4.5 billion years old
  • the period of heavy bombardment during which the planets were pummeled by debris left over from the birth of the solar system ended about 3.8 billion years ago
  • the oldest known sedimentary rocks and deposits, found in northwestern Australia, are estimated to be 3.5 - 3.8 billion years old
  • the oldest known fossil evidence of life is of cyanobacteria found in these deposits dated at 3.5 billion years old.

The implication of this is that life got started rather quickly on Earth. The big unknown is just how common are the conditions which resulted in life. This is one reason why the search for evidence of past life on Mars is so important. Finding or not finding evidence of past and/or present life on Mars will help us to better answer the question of the likelihood of life elsewhere in the galaxy and universe.

Choosing a value of 0.01 for this parameter means that you think that life develops on only 1 of every 100 habitable planets whereas a value of 1.0 means that life develops on every habitable planet.

Fi - The Fraction of Planets With Life Where Intelligence Life Evolves

Given that life evolves on a planet, how likely is it that intelligent life will appear? This is another big unknown. Of all the millions of species that have ever existed on Earth, only one has evolved the level of intelligence necessary to develop technology.

Further, while very simple life appeared very quickly on Earth, complex life took far longer to develop. Given that there is not a parameter to distinguish microscopic life (which lacks the complexity to develop intelligence) from the development of complex macroscopic life, this aspect must be taken into account in the context of this parameter.

Whereas Drake believed that life would develop on every planet that had habitable conditions, he estimated that intelligent life would emerge on only 1 of every 100 of these planets

Choosing a value of 0.001 for this parameter means that you think that intelligent life will appear on only 1 of every 1000 planets with life. A value of 1.0 means that the development of intelligent life is a certainty on those planets where life develops

Fc - The Fraction of Intelligent Civilizations with Interstellar Communication

So what if aliens have no equivalent of a Maxwell or a Morse or a Marconi or an Edison? They may be smart enough to construct towns and transportation but do they ever invent radio? Drake was of the opinion that 1 out of every 100 civilizations would discover radio. What do you think?

A value of 1.0 means that every civilization develops radio and a value of 0.001 means that only one in a thousand civilizations develop radio.

L - The Number of Years an Intelligent Civilization Remains Detectable

The L parameter turns the equation from a rate into a number. It is also a number for which there is no real basis for the assignment of a value. We are the only intelligent civilization we know of and we do not know how long we will remain detectable. A conservative estimate for this value would be 50 years based on our own experience to date. Drake felt that 10,000 years was a good guess.

N - The Answer is the Number of Detectable Civilizations at this Time

And the answer is N - the number of intelligent civilizations that are broadcasting their presence to the Universe.

Experiment with the Drake Equation

To facilitate your own experimentation with the Drake Equation, I have created an OpenOffice Calc spreadsheet and a Microsoft Excel spreadsheet. If you do not have OpenOffice, I strongly encourage you to get it. OpenOffice is the free, open source alternative to Microsoft Office. You can learn more at the OpenOffice web site

In the spreadsheet you will find that I have inserted my own values for the seven parameters. Following is an explanation for the values I used.

R = 2 which is double the estimated rate of formation of Sun-like stars but well below the maximum estimate of 20 new stars per year in the galaxy.

fp = 0.45 which is 1/2 the high estimate of 90% of these stars having planets.

ne = 0.50 because I do not believe that every star that has planets will have habitable planets. Recall that Drake assigned a number of 2 for this parameter. My optimistic estimate is that for every two stars with planets, there will be one habitable planet.

fl = 0.2 with no sound basis, I decided that life will emerge on only 1 in 5 habitable planets.

fi = 0.05 again guessing that intelligent life will develop on only 5 out of every 100 planets with life.

fc = 0.5 because I am optimistic that if there is intelligent life, there is at least a 50-50 chance that they will develop the technology necessary for interstellar communication.

L = 500 because I am not as optimistic as Frank Drake about the number of years for which an intelligent civilization will be broadcasting its presence by way of radio transmissions.

I was very much surprised to see that the combination of values that I used yielded a result of 1.13 currently broadcasting civilizations. That makes us the one. Going back and changing only the L parameter to Drake's value of 10,000 yields 22.5 broadcasting civilizations. If we were to assume that the Milky Way is a cylinder with a radius of 50,000 light years and a thickness of 1,000 light years, then there would be one broadcasting civilization for every 349 billion cubic light years of space.

Now consider this. Let's make the following assumptions:

  • the radius of the Milky Way is 50,000 light years
  • there are currently 22.5 broadcasting civilizations
  • all civilizations lie on the galactic equator in a 2 dimensional distribution

Given these assumptions, this means that on average each of these civilizations are separated by a distance of just over 21,000 light years. That means that any civilization that began broadcasting less than 21,000 years ago, like us for example, would not yet be detectable.


The Drake Equation must be one of the swaggiest (SWAG being the acronym for Scientific Wild-A** Guess) equations ever created because of the uncertainty associated with its parameters. The Drake Equation does do a great job of identifying and categorizing the relevant parameters. It also accomplishes the task of providing structure to the ongoing debate about the search for extraterrestrial intelligence and the likelihood of its existence. The large degree of uncertainty associated with so many of its parameters does tell us one important thing: that we have a lot more to learn.

Need someone to talk about space to your group? Check out the Chicago Society for Space Studies Speakers Bureau

For space art, astronomy, and digital photography stories, visit the Artsnova Blog

Drake Equation

How many alien societies exist, and are detectable? This famous formula gives us an idea. The Drake Equation, which was the agenda for a meeting of experts held in West Virginia in 1961, estimates N, the number of transmitting societies in the Milky Way galaxy. The terms are defined as follows:

N : The number of civilizations in the Milky Way galaxy whose electromagnetic emissions are detectable.

R*: The rate of formation of stars suitable for the development of intelligent life (number per year).

fp: The fraction of those stars with planetary systems.

ne: The number of planets, per solar system, with an environment suitable for life.

fl: The fraction of suitable planets on which life actually appears.

fi: The fraction of life bearing planets on which intelligent life emerges.

fc: The fraction of civilizations that develop a technology that produces detectable signs of their existence.

L : The average length of time such civilizations produce such signs (years).

There are 100 scientists at the SETI Institute, working on nearly 100 research questions. But each of these topics can be related to one of the terms in the Drake Equation.

N: Are technological societies common or not? Our ATA radio telescope is looking for signals coming from other star systems that would help answer that question.

SETI Institute and GNU Radio Join Forces

Qualcomm Founder Franklin Antonio Funds Allen Telescope Array Upgrade Program

Learn more about the Allen Telescope Array (ATA)

fp: Scientists involved in NASA’s TESS mission are hunting for nearby star systems housing planets with signs of life, such as oxygen in their atmospheres.

Breakthrough Listen to Collaborate with Scientists from NASA’s TESS Team to Look for Signs of Advanced Extraterrestrial Life

NASA’s TESS Mission Uncovers Its 1st World with Two Stars

Learn more about our research being done with TESS

An Astronaut Smart Glove to Explore The Moon, Mars and Beyond

Martian Moon’s Orbit Hints at an Ancient Ring of Mars

Mineral Link Identified Near Mars 2020 Landing Site

Evidence of Lakes or Seas on Early Mars

NASA Haughton-Mars Project: New Partnership on Mars Drone Application Research

Watch the video: ΤΟ ΣΥΝΕΔΡΙΟ ΤΗΣ ΡΩΜΗΣ - ΕΞΙΣΩΣΗ DRAKE 1961 (September 2021).