# How many photons does it take to determine the existence of a distant object?

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This applies to any object, but I see the recent discovery of the oldest, most distant galaxy and it started me wondering what the limits are. Presumably you can do better with a bigger telescope and longer exposure, but I'm curious about literally how many photons you need to collect to decide whether or not something is actually there. And then presumably you need a bunch more to determine its red-shift and angular size and rotation etc.
I have absolutely no feel for these numbers. Is it tens of photons or a millions of photons?

What matters is how many photons you collect versus how many you would expect to see if the object wasn't there.

Photons would be present, without a source, for a variety of astrophysical (e.g. diffuse background) or non-astrophysical (night sky brightness, dark current) reasons, so you need to be able to rule out the null hypothesis that what you have seen is consistent with no object at all. In principle, if you expect no background counts, then the detection of one photon is significant.

The lowest backgrounds tend to occur in space-based X-ray and gamma ray observations, where the detection of a few photons is often taken to be evidence of the presence of an X-ray source.

The backgrounds tend to be higher in optical observations. More photons from the source are needed to get a significant detection, because the variance in the background is proportional to the expected background.

Determining anything beyond detection typically needs an order of magnitude more photons because you are slicing your data into several positional or wavelength bins and you need significant detections in those individually. For rotation you need both position and wavelength, so I would say you need yet another order of magnitude increase to be able to say anything.

It is hard to write anything but a vague, general answer, because the details depend exactly on the telescope, the detector and the type of object being observed.

For example, one way of looking for very distant objects is to look for mere detections of high redshift Lyman alpha emitters. To minimise the background you use a set of narrow-band filters which would select the Lyman alpha line over a narrow range of redshifts. Detection of a source through one filter combined with non-detection in adjacent filters tells you it is an emission line object, and assuming it is Lyman alpha you then also know the redshift.

## Why are massless Mass photons affected by gravity & gravitational lensing effect Explained

If photons are massless, why does light get attracted to black holes? This is one of the most common question asked by students.

We know that black holes exert a gravitational force on the objects. How can a massless photon go into black hole and not escape from it? The gravitational force is related to mass isn't it? Is there another force in the black hole or does light has mass?

You are right that according to Newton's gravity, the force of gravity on particle that has 0 mass would be zero, and so gravity should not affect light. In fact, according to Newton's gravity Black holes should not exist: no matter how strong gravity is, light would always be able to escape!

However, we know that Newton's gravity is only correct under certain circumstances, when particles travel much slower than the speed of light, and when gravity is weak. This is certainly not the case near a black hole! When we try to understand how black holes work we need to consider the more general law of gravity which is Einstein's General Relativity.

According to General Relativity, gravity is not a force! On the contrary, gravity just affects how distances are measured, and says what shape has the "shortest" path from one place to another. All particles then follow these "shortest path" routes in their motion. Notice that nowhere so far have I mentioned mass, this rule applies for all matter and energy, whether they have mass or not!

It turns out that very close to the black hole, these shortest paths never cross the event horizon. As a result neither light nor anything less can escape from the gravity field of a black hole!

## Early viewing of space

In Grade 7 learners were introduced to indigenous knowledge about the stars and planets under the historical development of astronomy. This section focused primarily on the practical uses of star observations, such as timekeeping and navigation, along with an introduction to starlore associated with the Moon, Milky Way and other celestial bodies. In this section the focus will lie in the observations of constellations (and the planets) and starlore associated with one example constellation.

A good way to introduce the topic of the early viewing of space, is to ask learners if they know of any stories about famous constellations or the planets. This facilitates discussions about constellations visible in the sky and how the stars are actually related in space.

These workbooks were created by Siyavula with the help of contributors and volunteers. Read more about Siyavula here.[link]www.siyavula.com

In dark conditions away from city lights, thousands of stars are visible in the night sky. Early cultures around the world gazed at the stars in wonder. They noted the movement of the stars and planets across the sky and used this to mark the passage of time. People often grouped the stars they saw into patterns called constellations. Early cultures tended to associate the stars and planets they saw in the night sky with animals or gods and told stories, which were passed on from generation to generation, about the patterns in the sky which were passed down from generation to generation.

Early telescopes were were used by merchants to spot approaching trade ships or pirates. Telescopes also gave rise to the first high-speed telecommunications networks, as spyglasses were used to observe signals from kilometers away.

Today professional astronomers formally recognise 88 constellations, 23 of which are in the southern hemisphere.

The stars that are visible depend upon your location on Earth and also the time of year. The southern sky, which we see from South Africa, is full of beautiful stars and several prominent constellations are visible in the sky including the Southern Cross or Crux, Orion and Pavo the Peacock.

In the following activities you will have the opportunity to observe the night sky and familiarise yourself with some of the most famous southern constellations.

Learn how to view the night sky with Google Earth.

## Pi in the Sky 7

This composite image of the Kuiper Belt object Arrokoth was compiled from data obtained by NASA's New Horizons spacecraft as it flew by the object on Jan. 1, 2019. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko | › Full image and caption

Flying onboard a Gulfstream IV aircraft, CORAL records the spectra of light reflected from the ocean below to study the composition and health of Earth's coral reefs. Image credit: NASA | + Expand image

The star Beta Pictoris and its surrounding debris disk are shown in near-infrared light in this composite image. The outer part of the image shows the reflected light on the dust disc. Image credit: ESO/A.-M. Lagrange et al. | › Full image and caption

#### Mars Maneuver

Long before a Mars rover touches down on the Red Planet, scientists and engineers must determine where to land. Rather than choosing a specific landing spot, NASA selects an area known as a landing ellipse. A Mars rover could land anywhere within this ellipse. Choosing where the landing ellipse is located requires compromising between getting as close as possible to interesting science targets and avoiding hazards like steep slopes and large boulders, which could quickly bring a mission to its end. In the Mars Maneuver problem, students use pi to see how new technologies have reduced the size of landing ellipses from one Mars rover mission to the next.

#### Cold Case

In January 2019, NASA's New Horizons spacecraft sped past Arrokoth, a frigid, primitive object that orbits within the Kuiper Belt, a doughnut-shaped ring of icy bodies beyond the orbit of Neptune. Arrokoth is the most distant Kuiper Belt object to be visited by a spacecraft and only the second object in the region to have been explored up close. To get New Horizons to Arrokoth, mission navigators needed to know the orbital properties of the object, such as its speed, distance from the Sun, and the tilt and shape of its orbit. This information is also important for scientists studying the object. In the Cold Case problem, students can use pi to determine how long it takes the distant object to make one trip around the Sun.

#### Coral Calculus

Coral reefs provide food and shelter to many ocean species and protect coastal communities against extreme weather events. Ocean warming, invasive species, pollutants, and acidification caused by climate change can harm the tiny living coral organisms responsible for building coral reefs. To better understand the health of Earth's coral reefs, NASA's COral Reef Airborne Laboratory, or CORAL, mission maps them from the air using spectroscopy, studying how light interacts with the reefs. To make accurate maps, CORAL must be able to differentiate among coral, algae and sand on the ocean floor from an airplane. And to do that, it needs to calculate the depth of the ocean at every point it maps by measuring how much sunlight passes through the ocean and is reflected upward from the ocean floor. In Coral Calculus, students use pi to measure the water depth of an area mapped by the CORAL mission and help scientists better understand the status of Earth's coral reefs.

#### Planet Pinpointer

Our galaxy contains billions of stars, many of which are likely home to exoplanets – planets outside our solar system. So how do scientists decide where to look for these worlds? Using data gathered by NASA's Spitzer Space Telescope, researchers found that they're more likely to find giant exoplanets around young stars surrounded by debris disks, which are made up of material similar to what's found in the asteroid belt and Kuiper Belt in our solar system. Sure enough, after discovering a debris disk around the star Beta Pictoris, researchers later confirmed that it is home to at least two giant exoplanets. Learning more about Beta Pictoris' debris disk could give scientists insight into the formation of these giant worlds. In Planet Pinpointer, put yourself in the role of a NASA scientist to learn more about Beta Pictoris' debris disk, using pi to calculate the distance across it.

### Procedures

#### Mars Maneuver

When we plan where to land a spacecraft on Mars, we don’t choose a specific spot, but a larger area called a landing ellipse. It's like choosing a parking lot rather than a parking spot. To choose a landing ellipse, we have to compromise between getting as close as possible to interesting science targets and avoiding hazards. As we've created new technology to help direct spacecraft, landing ellipses have gotten smaller and smaller. That means that we're able to land in places we couldn't before and get closer to the stuff we want to study.

In 2012, the Curiosity rover used its sky crane landing system to touch down in a 20 km by 7 km ellipse. When the Perseverance rover lands on Feb. 18, 2021, it will use the same system along with a new technique called Range Trigger that will allow the spacecraft to land in the smallest ellipse yet, measuring just 13 km by 7 km. What percentage of Curiosity's landing ellipse is Perseverance's landing ellipse?

Image credit: NASA/JPL-Caltech | + Expand image

#### Cold Case

In January 2019, NASA's New Horizons spacecraft flew within 3,538 km of the most distant and primitive object explored up-close by a spacecraft. The object was originally known as 2014 MU69, but it was later renamed Arrokoth. It looks like a partially flattened, reddish snowman and is made up of two objects that merged into one. Found 6.6 billion km from Earth, Arrokoth is a small “Cold Classical” Kuiper Belt object, meaning it orbits the Sun in a nearly circular path and has a low orbital inclination. Cold Classical objects make up about one-third of the Kuiper Belt.

One reason scientists are interested in studying Arrokoth and other Kuiper Belt objects is that they are thought to be well preserved, frozen samples of what the outer solar system was like at its birth, more than 4.5 billion years ago. Learn a bit more about Arrokoth by calculating how long it takes the object to make one trip around the Sun.

Image credit: NASA/JPL-Caltech | + Expand image

#### Coral Calculus

Flying aboard an aircraft, NASA’s CORAL mission uses spectroscopy to study the health of coral reefs and the threats they face. To differentiate among coral, algae and sand on the ocean floor, CORAL computes the depth of every point it maps. The water’s depth can be determined using the “absorption coefficient,” indicating how much light is absorbed through a given depth of water.

Imagine CORAL collects a light measurement reflected by white sand covered by an unknown depth of water that is 76% in the blue and 4.5% in the red. Using the formulas below, calculate the water’s depth. Note that sunlight passes through the water twice: when traveling from the Sun to the ocean floor and when reflecting up to the aircraft.

absorption coefficient, α = (4πk)/λ
k = coefficient of the imaginary number portion of the refractive index
λ = wavelength (meters) of light observed

Beer-Lambert law, T = e (-α•d)
T = observed reflectance, or transmittance
(T), of light through a distance (d) of water

Refractive Indices:

• Water in the blue wavelength (450 nm) = 1.3369 + 1.01E-09i
• Water in the red wavelength (650 nm) = 1.3314 + 1.60E-08i

Image credit: NASA/JPL-Caltech | + Expand image

#### Planet Pinpointer

Our galaxy contains billions of stars, most of which are likely home to exoplanets – planets outside our solar system. How do scientists decide where to look for these worlds? Researchers looking at data from NASA's Spitzer Space Telescope found that giant exoplanets tend to exist around young stars surrounded by a disk of debris. A prominent debris disk around the star Beta Pictoris, which is 6 x 10 14 km away from Earth, lead scientists to discover two exoplanets several times bigger than Jupiter orbiting the star! Learning more about the debris disk could give scientists insight into the formation of these giant worlds.

Given the angle of the disk's apparent size is 169 arcseconds, determine the actual distance across it using the formula for small angle approximation, below. (An arcsecond is 1/3,600 of a degree.)

D = dθ
D = distance across the debris disk (km)
d = distance to Beta Pictoris (km)
θ = angle of apparent size (radians)

## Far out new dwarf planet is the most distant known object in the solar system

For a long time, Pluto was the most distant object we knew about in the solar system, but more recent observations suggest there are probably thousands of worlds lurking in the shadows on the fringes. Now astronomers are welcoming the newest and most distant member of the family – a dwarf planet orbiting more than 100 times further from the Sun than Earth. And because its discoverers apparently aren't the most creative bunch, it's been dubbed "Farout."

Officially known as 2018 VG18, the object was spotted at a distance of 120 Astronomical Units (AU) from the Sun. For reference, one AU is defined as the distance between the Earth and the Sun. As the nickname suggests, Farout is the most distant solar system object ever observed, more than three and a half times further than Pluto is right now, and comfortably beating the previous record-holder Eris, orbiting 96 AU out.

This chart illustrates the relative distances of solar system objects, with Farout clearly the most distant

The object was first discovered in images taken on November 10, by the 8-m (26-ft) Subaru telescope in Hawaii. To map its path across the sky and take stock of its physical characteristics, follow-up observations were conducted at the Magellan telescope at Las Campanas Observatory in Chile in early December.

Due to its brightness, 2018 VG18 was estimated to be about 500 km (310 mi) wide, which is less than a quarter the size of Pluto. The color of its observed light is pinkish, suggesting it's an icy world, which makes a lot of sense given how … well, far out it is. While its orbit hasn't been pinned down completely yet, the astronomers estimate that it probably takes more than 1,000 years to complete one trip around the Sun.

Interestingly enough, while it's the most distant known solar system object right now, that might not be the case most of the time. Another dwarf planet nicknamed the "Goblin" was discovered by the same team a few months ago, and while it's currently "only" 80 AU from the Sun, its wildly eccentric, 40,000-year orbit may swing it as far away as 2,300 AU.

That extreme orbit, as well as those of other dwarf planets on the fringes of the solar system, suggest the existence of a huge planet out there somewhere. Getting a clearer idea of Farout's orbit may bring to light new clues about this enigmatic "Planet Nine."

The images in which 2018 VG18 was first discovered. Taken one hour apart, the dwarf planet can be seen to have moved slightly, while the background stars have not

"2018 VG18 is much more distant and slower moving than any other observed solar system object, so it will take a few years to fully determine its orbit," says Scott S. Sheppard, one of the discoverers of the new dwarf planet. "But it was found in a similar location on the sky to the other known extreme solar system objects, suggesting it might have the same type of orbit that most of them do. The orbital similarities shown by many of the known small, distant solar system bodies was the catalyst for our original assertion that there is a distant, massive planet at several hundred AU shepherding these smaller objects."

Some scientists argue that this hypothetical Planet Nine doesn't actually exist, instead suggesting that those smaller distant objects are simply jostling each other like "gravitational bumper cars." Other teams go in the opposite direction, putting forward the idea that not only does the larger planet exist, but it's joined by a Mars-sized tenth planet.

The new object was officially announced this week by the Minor Planet Center. Along with Sheppard, astronomers David Tholen and Chad Trujillo are also credited with the discovery.

## Can there be time without mass?

However, I do think it is important to define quantities operationally
and to geometrically model the physical measurement processes.
what does it mean "to see (visually)"?
"to determine the distance to a distant event from a worldline"? etc.
And I think these are best understood
by drawing spacetime diagrams [instead of boxcars moving in space]
and trying to reason with the Minkowskian spacetime geometry
visually (light cones, intersections, tangents, etc. ) and
algebraically (4-vector addition and their dot products, etc. )
and interpret physically.

Multiple-representations can help support the various interpretations of what is going on.
(Appealing to certain aspects of one's Euclidean trigonometric intuition
[and demoting other aspects] is a powerful, but overlooked tool if used correctly.)

Well, but how do you understandably argue for the construction of the Minkowski diagram? You can of course deal without Lorentz transformations by just using Einstein's "two postulates" (validity of the special principle of relativity, independence of the speed of light on the uniform motion of the light source) to get the Minkowski pseudometric. Then you can construct the Minkowski diagram (of course restricting yourself to 1+1 spacetime dimensions) and use the geometry of hyperbolae and light cones to define the spatiotemporal measures. Then you can construct everything geometrically, including clock synchronization of clocks at different places being at rest relative to each other, demonstrating the non-synchronicity of clocks in relative uniform motion to each other, and all the kinematics. Also the Lorentz transformation follows from such constructions.

On the other hand, I always struggle with Minkowski diagrams, because it's hard to forget the Euclidean plane, we are all used to from early schooldays on. I always find an algebraic approach much more clear. I usually calculate everything algebraically and then put the reesults in a Minkowski diagram to visualize it.

I think this question needs to be examined in a different light (sorry) than what has been posted so far.
A universe with only photons will still have mass. And because of photon-photon interactions it will, in due course, also have fermions with non-zero rest mass.

But there are a couple of things that are unclear to me:

1) Can a universe of photons become more disordered? If not, then time cannot move forward. If so, would the rate at which entropy increased be so low as to challenge the definition of time?

2) A common (but critically oversimplified) way of modeling a photon is to imagine a particle traveling from a source to a target not much differently than how a baseball travels. But many experiments, including double slit and interferometer experiments, refute this. A "traveling photon", is not spatially-specific - it doesn't have a baseball-like trajectory. In a universe consisting only of photons, each photon might persist quadrillions of years without colliding with another photon - but during this life, its cumulative gravitational affect would be an important contributor to the geometry of the universe. And in a broad statistical sense, that contribution would betray its location.

In such a thought experiment, you don't need to consider every practical issue, such as can we make clocks?
Any two particle interactions can serve as a clock.

Let's phrase it another way: in this universe where all particles are massless, is there anything that prevents every interaction that ever did happen to will happen - from happening all at once? Would a universe where everything happened all at once even last a non-zero length of time?

Of course. You yourself described one mechanism for this: photon-photon interactions will produce fermion-antifermion pairs.

Sure it can: the classical GR model of a radiation dominated universe still has a perfectly good definition of "time" that moves forward. (Note that in such a model, which idealizes "photons" as null radiation, there are no photon-photon interactions and no pair production.)

Any rate that isn't zero would still give you a second law "arrow" of time. But note that this is just a definition of which direction in time is the "forward" direction it's not the same as a definition of time.

There is no "matter" in such a universe, only radiation, which doesn't clump gravitationally the way matter (more precisely, non-relativistic matter) does, but that doesn't mean it doesn't clump at all if it has density variations, there will be some clumping, yes.

Actually, the "land dweller" would have to exit the spaceship first, then board it again some time later.

This is actually an instructive way to construct a "twin paradox" scenario. Suppose we pick a frame which we'll call the "land frame" in this frame, a very, very long spaceship is moving to the right at some speed ##v## which is close to the speed of light. The clocks on the spaceship are all synchronized with each other as viewed in the spaceship's rest frame.

At time ##t' = 0## by the spaceship clocks, the "land dweller" jumps out of the spaceship at ##x = 0## in the "land frame" and immediately decelerates to a stop, so he is at rest in the "land frame". (We will say that the land frame clocks read ##t = 0## at this instant, and that the point on the spaceship where the land dweller jumped out is at ##x' = 0##.) At this instant the land dweller's clock also reads zero, since it was sychronized with the spaceship clocks while he was on board the spaceship. Then the land dweller floats there and waits for a while, while the spaceship flies past him then, after some time ##T## has passed on his clock, he immediately accelerates to speed ##v## and boards the spaceship. The time that he sees on the spaceship clock when he boards (this will of course be a different clock than the one he saw when he exited the spaceship, but all of the spaceship clocks are synchronized in the spaceship frame) will be greater than ##T##.

## How many photons from a distant star hit the Earth at any given moment or per second, and does a unique photon strike every (very small) unit area of the side of the planet that faces the source?

For example, a Sun-like star 1000 ly away. I was wondering if two people standing next to one another see distinct photons, and if so, how does something so far away "cover" the entire side of the Earth (facing the source) with photons? I realize the light is red-shifted and the wave is stretched out as it travels away from the source, but how does this work with respect to photons? Thanks.

I was wondering if two people standing next to one another see distinct photons

Any time you and another person see the same thing, you are actually seeing distinct photons. When you see something, it means photons from that object have been absorbed by your retina the same photon can't be absorbed by the retinas of two different people.

how does something so far away "cover" the entire side of the Earth (facing the source) with photons?

It does by emitting a huge number of photons. In some sense, that's the definition of something which is visible.

I realize the light is red-shifted and the wave is stretched out as it travels away from the source

The light from stars we see is not necessarily red shifted. While on a large scale, the universe is expanding, that's not reflected in the motion of nearby objects. For things in the galaxy, the light can be red-shifted or blue-shifted depending on the local velocities of the stars relative to us.

What photo-resolution do we have the capacity to distinguish with technology? 1? 10? a million? Can we determine the difference between x and y photons with any detection instruments? Can we produce the difference between x and y photons with a laser or LED?

As /u/fishify said, "seeing" something means your eye absorbed the photon, so you and your friend must be seeing distinct photons. How many of them will each of you see given your Sun-like star 1000 ly away? Here's a back of the envelope calculation.

The luminosity of the sun is 3.85x10 33 erg/s. The flux through an area is just given by an inverse square distance law (i.e., the area of a shell into which the flux passes gets larger as d 2).

So F = L/(4 pi d 2). But what is the photon flux? We have to know the energy of each photon. It should be handled with an integral and the star's true spectral energy distribution, but let's just say the average photon has a wavelength of 500 nm

6e14 Hz. The photon's energy is h nu = 4e-12 erg. F/(h nu) gives the number of photons per area per second. Plugging in the relevant numbers, we find the flux from a Sun-like star 1000 ly away is

100 photons/cm 2 every second.

The atmosphere will reflect and absorb

half of those before they ever reach the surface, but the human eye is a couple square centimeters, so let's call it even. Your eye will receive

100 photons every second from this distant star.

Note that the biggest assumption I made was that all photons have wavelength

500 nm. Luckily, the Sun puts out most of its energy in the visible, so it's probably not a terrible assumption. And this is astronomy, where pi is 3 and 3 can be 5 or 2 depending on the situation, so good enough.

## Physicists keep striking out in the search for dark matter

S pace may be the final frontier, but we've barely begun to explore its underlying mechanics. For as much as humanity has discovered since we first looked to the heavens, we've only seen about five percent of the total matter in the universe. The other 95 percent -- the so-called "dark matter" -- well, we can't even figure out how to see yet. But that doesn't mean researchers from around the world aren't devising ways to do so.

The search for dark matter began in earnest back in the 17th century, shortly after Isaac Newton released his theory of universal gravity, when astronomers posited that some celestial objects might not emit light but could still be observed based on their gravitational effects (i.e. black holes). Over the past few decades, thanks to advances in optical and radio astronomy technologies, evidence for the existence of dark matter has continued to mount. At this point, astronomers believe dark matter constitutes about 27 percent of the universe's total mass (and nearly 95 percent if you include dark energy as well). While the scientific community is now certain that dark matter exists, there's no consensus as to what the stuff is actually made of.

There are two leading theories right now. One argues that dark matter is made of Weakly Interacting Massive Particles (WIMPs) -- theorized to have a mass 100,000 times greater than an electron (and therefore behave as conventional particles). The other speculates it's made of axions, elementary particles with a mass a hundred-billionths that of an electron (and that behave as waves). Axions are thought to exert the same Wave-Particle Duality that photons do, just without our ability to observe them directly.

"If dark matter was some new particle, there's really only a couple of ways that it can interact with us," Dr. Philipp Schuster, associate professor at the SLAC National Accelerator Laboratory, told Engadget. "One possibility is that it could be a particle that's actually charged under familiar forces [i.e. WIMPs]. And the other the possibility is that it could just be a particle that's not charged under standard model forces but nonetheless have it its own force.

"In that case, it could interact and through a new vector particle, basically for something akin to electromagnetism," he continued, "or it could interact with us through something that actually doesn't have an analog in nature."

In order to determine what these dark matter particles are made of, researchers devised a number of experiments. These studies can be divided into three general categories: particle detectors should dark matter be made of WIMPs, wave effect detectors if dark matter is actually axions (aka dark photons), and astronomical surveys that study the effects of dark matter on the observable universe, specifically, gravitational lensing.

If dark matter is made of WIMPs, we'll likely discover it with enormous tubs of liquid xenon stored deep beneath the Earth's surface. Because WIMPs, as their name implies, don't readily interact with known matter, detecting them is a tricky process. Anything radioactive -- from cosmic background radiation to the trace amounts of uranium in soil -- can return a false positive reading.

The XENON1T study, for example, is buried deep within a mountain underneath the Gran Sasso National Laboratory in Italy. Every piece of equipment it uses has been hand-crafted from super radio-pure stainless steel, Rafael Lang, associate professor of physics and astronomy at Purdue University, told Engadget. Its one-ton vat of liquid xenon is among the purest on Earth, with just one part per trillion (PPT) of krypton contamination -- orders of magnitude lower than what's found in nature. In fact, the XENON1T is the most sensitive (read: least radioactive) WIMP detector built to date.

"What we do is we take a bucket, we fill it up with liquid xenon and we sit and wait until a particle hits the liquid xenon," Lang explained. The device's primary detector, the Liquid Xenon Time Projection Chamber (LXeTPC), sits in the cryogenically-cooled xenon, itself surrounded by a larger tub of purified water to further shield it from radiation.

The idea is that, with all this shielding, the only stuff that will make it through will be WIMPs. And, should a WIMP manage to strike one of the xenon nuclei, the impact will cause the liquid to scintillate -- that is, create a flash of visible light -- that the LXeTPC will detect.

The LUX-ZEPLIN experiment being conducted in South Dakota by the Lawrence Berkeley National Lab is also looking for WIMPS. "The basic idea is we're building a super fancy Geiger counter to try to detect this one particular type of event," Dr. Daniel Akerib, professor of particle physics and astrophysics at Stanford University, told Engadget.

The XENON1T and LUX-ZEPLIN experiments aren't the only subterranean devices looking for WIMPs. As part of its Cryogenic Dark Matter Search (CDMS), the US Department of Energy is working with SLAC to build out the SNOLAB, a dark matter detector located two kilometers below ground at Vale's Creighton nickel mine in Ontario. Once SNOLAB comes online in 2018, it's expected to be ten times more sensitive than the current CDMS experiment being conducted 2,340 feet below ground at University of Minnesota's Soudan Underground Laboratory.

"We're not only going to be able see lower-mass particles, but we're also going to be much more sensitive than ever before," SLAC senior staff scientist Richard Partridge said in a statement. "This is a huge challenge, one that requires much R&D, very careful fabrication, and high-precision testing. SLAC has a big role in all this, but we're also working closely with many other institutions." Lang, for one, welcomes the competition. "It helps, it helps greatly," he exclaimed. "There's a big need to try out all kinds of crazy different ideas that you can come up with."

These experiments' current failure to positively identify an interaction between WIMPs and xenon nuclei may be due to the theoretical particle's weakly interacting nature, or it may be because researchers are simply looking for the wrong thing. If dark matter isn't made of massive particles, but light ones such as axions, detectors like the LUX-ZEPLIN or XENON1T won't see them. But the Dark Matter Radio, Fermilab's ADMX, or the APEX experiments just might.

"If the dark matter is built out of a spinless particle that is sufficiently light, then it actually behaves much more like something like an electromagnetic wave than a particle," Schuster explained. And by sufficiently light, he means "a billionth the mass of an electron."

"What that means is that there's a lot more of it. There's a lot more particles in order to make up the dark matter of the galaxy," Dr. Peter Graham, associate professor of physics at Stanford, told Engadget. "And what that also means is that you don't, for example, look to see an individual ping from an individual axion on it in your experiments. It's just it's just way too little energy."

Just as a single drop of water can't cut through bedrock while a river can, researchers have to look for axions behaving en mass. To do so, we just have to find their resonant frequency. The Dark Matter Radio experiment out of Stanford University, for example, operates much like a terrestrial radio, just on a cosmic scale. The radio is akin to a basic LC circuit (read: an electronic oscillator) "looking at the hundreds of megahertz or megahertz even down to maybe kilohertz, we're looking at a broad range," Graham said.

This setup provides unique challenges compared to particle detectors. For one, the radio does not have to be buried deep underground to avoid interference from cosmic rays. It does, however, have to be encased in a conducting box to effectively screen out background radio noise. What's more, while particle detectors are turned on and left to run for a year at a time, these radios can cycle through its various frequencies every 10 to 15 minutes. Once researchers do find the resonant frequency, they'll be able to immediately calculate the individual axion mass. "Because we know it's basically nonrelativistic, we know the frequency is equal to the mass of the axion," Graham explained.

Whether dark matter is comprised of WIMPs or axions makes a big difference in our understanding of the universe's mechanics. "Dark matter could just be built out of hidden photons (aka axions)," Schuster said. "That would basically mean that not only is there a new force out there, but the remnants of that force -- the particle carriers -- have a high enough density in the universe that it actually is producing dark matter. I think that would actually surprise the field quite considerably."

Should this turn out to be the case, researchers will have discovered a new fundamental force -- the 21st-century equivalent to electromagnetism.

"The other possibility, of course, is that dark matter could be built out of an existing particle but it could be charged under a new force, that its photons are the mediator particle," Schuster continued. "That possibility is very similar to the WIMP idea, but it's just it's different in that the particle is not charged under [a known] force, they're charged under a new force, the hidden photon being the mediator."

Some astronomers are taking a more direct approach and searching for evidence of dark matter using the cosmos. Astronomers with the Large Synoptic Survey Telescope, which will be coming online in 2021, and the Dark Energy Survey, which is has been collecting data at the Cerro Tololo Inter-American Observatory in Chile since 2013, hope that gravitational lensing might hold the key to observing dark matter directly. Well, as directly as a weakly interacting fundamental particle can be observed.

These surveys hope to observe dark matter much in the same way that we search for black holes: by looking for the light that they deform. "If you put a mass concentration in front of some distant object you're looking at then the light coming from that distant object will come around that mass, the rays will be diverted and you'll get a distorted image," Dr. Steve Kahn, professor of physics at SLAC and the head of the LSST project, told Engadget.

Both the DES and the LSST can and will exploit this effect to potentially find clumps of dark matter in space. "Correlations in the distortion of galaxies which are near each other in the sky will appear distorted in similar ways. " Kahn said. "So this lensing effect is a way of literally seeing dark matter. The dark matter is invisible but we can infer its existence and its distribution of lensing in the background images of the galaxy."

This technique will also help us calculate how far away these mass concentrations are due to their Doppler redshift. Just as the sirens of an ambulance rise in frequency as the vehicle approaches you but then drop as it passes, the photonic frequency appears more blue if the light source is approaching you and shifts to red if the source is moving away.

At this point, however, humanity's journey to discover the secrets of the universe has barely begun. "It would just be very, very surprising if the if the bulk of what's leftover is, you know, some simple single particle with no interesting interactions," Schuster concluded. "I think a much more likely possibility is that there are many more forces, many more new interactions, that are related to dark matter. We just need to figure out what it is."

## Q: How can photons have energy and momentum, but no mass?

Physicist: Classically (according to Newton) kinetic energy is given by and momentum is given by , where m is mass and v is velocity. But if you plug in the mass and velocity for light you get . But that’s no good. If light didn’t carry energy, it wouldn’t be able to heat stuff up.

The difficulty comes from the fact that Newton’s laws paint an incomplete (and ultimately incorrect) picture. is very accurate for slow moving (compared to light) objects with mass, but it’s not true in general. When relativity came along it was revealed that there’s a fundamental difference in the physics of the massive and the massless. Relativity makes the (experimentally backed) assumptions that: #1) it doesn’t matter whether, or how fast, you’re moving (all physical laws stay the same) and #2) the speed of light is invariant (always the same to everyone).

Any object with mass travels slower than light and so may as well be stationary (#1).

Anything with zero mass always travels at the speed of light. But since the speed-of-light is always the speed-of-light to everyone (#2) there’s no way for these objects to ever be stationary (unlike massive stuff). Vive la différence des lois! It’s not important here, but things (like light) that travel at the speed of light never experience the passage of time. Isn’t that awesome?

The point is: light and ordinary matter are very different, and the laws that govern them are just as different.

Light and Matter: different

That being said, in 1905 Einstein managed to write a law that works whenever: . The same year (the same freaking year) he figured out that light is both a particle and a wave and that the energy of a photon isn’t governed by it’s mass or it’s velocity (like matter), but instead is governed entirely by f, it’s frequency: E=hf, where h is Planck’s constant.

For light m=0, so E=Pc (energy and momentum are proportional). Notice that you can never have zero momentum, since something with zero mass and zero energy isn’t something, it’s nothing. This is just another way of saying that light can never be stationary.

Also! Say you have an object with mass m, that isn’t moving (P=0). Then you get: E=mc 2 (awesome)!

Unrelated tangent: It took a little while, but the laws governing the massive and the massless are even more inter-related than the ‘Stein originally thought. He figured out that the energy of a photon is related to it’s frequency (E=hf), but why are photons so special? Why do they get to have frequencies? They’re not special. Years later (1924) de Broglie drew the most natural line from Einstein’s various equations from light to matter. So for a given amount of matter you can find it’s frequency. Holy crap! Everything has a frequency!

On the off chance that anyone out there got unduly excited about that last statement: the frequencies never go out of wack, you can’t tune them, more importantly they are utterly unimportant on the Human scale, or even the single-cell scale, and don’t ever buy a bracelet or anything else with “quantum” in the name.

No, no, no, no, no, no, no, no, no.

### 223 Responses to Q: How can photons have energy and momentum, but no mass?

I apologize if this is a bit unrelated to the article. Based on what I understand, the electromagnetic force is mediated by photons – light. If two electrons were approaching each other with the same speed, they should end up moving in opposite directions, again with the same speeds, and in the intervening time period had to have exchanged a photon. If one of the electrons (while they are still approaching each other) emits a photon that the other eventually absorbs, then clearly that photon must carry momentum to be able to turn both electrons around, but since the KE of the electron does not change, it cannot carry energy, or so it would seem. That would disagree with Einstein’s equation which says that the energy and the momentum of a photon are directly related. Where is my thinking flawed?

If mc2=E=hf isn’t the reverse true (hf=E=mc2)? If not, why not? And if so, then light’s energy can be converted into mass (i.e., has mass).

Okay, next. Do electrons have mass? They certainly have energy. But Maxwell showed that electricity moves at the speed of light, correct? Again, we have an instance of energy but no mass even though mass and energy are supposedly interchangeable.

Please clarify in simple terms if possible. Thank you.

Pls can you be more explanatory on why photon does not have mass and electric charge?

What they are saying is that e=mc^2 only applies to rest mass and the equation when you include “relativistic” or “inertial” mass is e=mc^2+p(hc)^2 or something like that but the upshot is that if m=0 as in the case with light, you can still have energy expressed, in a kinetic way. But it’s just a tool for explanation to save the simpler equation in cases of light. They don’t understand light, and the wave-particle duality, double slit Feynman sum of all routes etc drives them nuts so this is their agreed upon cop out.

Gravity controls it because it is not having mass that affects a photons path through a gravitational field, it is the fact that the space itself that the photon travels through in that gravitational field has been warped (distorted).

“If mc2=E=hf isn’t the reverse true (hf=E=mc2)? If not, why not? And if so, then light’s energy can be converted into mass (i.e., has mass).”

it is true. the problem is the energy associated with mass is far far in excess of what your typical photon will carry. pions, being some of the lightest particles, are likely candidates here – the neutral pion decays to two gamma rays so in theory it should work in reverse under specific conditions.

“Gravity controls it because it is not having mass that affects a photons path through a gravitational field, it is the fact that the space itself that the photon travels through in that gravitational field has been warped (distorted).”

be that as it may, because of the energy-mass equivalence, i could argue that the miniscule amounts of energy each photon has would be equivalent to a mass of the same energy, which would experience and exert gravitational forces, however small. probably too small to factor into gravitational light deflection for every-day energies.

Assigning ANY mass, however negligible, to anything traveling at constant “c” (light speed here) would violate Einstein’s special relativity theory though. My understanding is that since mass increases at it approaches c, and would become infinite at c, requiring infinite energy to propel it.

If the light is made up of photons, and photons are particles illuminated According to Law Article (everything from running into the universe has mass) of that conclude that the mass of photons, what is the mass of light and how it calculated?Is the photon sphere?
If Be a sphere, what size and diameter?
And how to penetrate the human body?
Thank you

I gotta be honest. That sounds like complete crap to a layman like myself. All this effort to explain how light can have energy but no mass or no rest mass or no mass when it is convenient smacks of convenience to me. I thought I could come here and understand something. What I am coming away with is the realization that the scientific community does not understand something. You guys sound like people in 1904 trying to describe the aether. We need a new Einstein.

i want to know the precise explanation about the momentum of QUANTUM PARTICLES like photon,neutrino, anti neutrino.just explain how they posses momentum with zero mass,no charge.moreover please add one thing more to my knowledge what is the basic difference between the classical idea of momentum and quantum idea of momentum.

The way I understand it is photons travel at the speed of light only in a vacuum and at slower speeds through different things. Electricity or electrons to be exact have mass and travel slower (in wires they drift around a few centimeters per second). The electromagnetic field that electricity generates travels near the speed of light moving all of the electrons along the wire. For example the electrons in an antenna only vibrate back and forth but the EM field generated travels near the speed of light in our atmosphere.

Photons always travel at light speed.
No matter what the medium is.

Photons travel at light speed in a vacuum. Physics 101. Gravity slows photons otherwise how would a black hole be black? The electrons in a prism deflect photons (ok absorb and re-emit) but this process does not occur at constant C.

Photons don’t have rest mass but they do have relativistic mass. If you put alot of photons in a box with reflective walls, the box would be measurably heavier.

The way I see it, Light is , in simple terms, supreme and probably the only thing which is absolute. Everything else changes and is manipulated in order to maintain this ‘absolute’ quality of light. All rules, be it Quantum or Classical , accommodate to this exclusive property of light. Like Einstein said, c will always remain constant No Matter What! SO … your length can contract , your time can dilate and loads of other stuff can happen, but the speed of light will remain the same. There are a lot of unexplained phenomenon out there . All left for scientists is to discover yet newer ways in which the space time and the properties of matter are compromised to adjust to Light and it’s manipulating ways, because Light won’t ever deter. No sir, because Light is Supreme…

No light is not supreme. The speed at which things travel is supreme. It’s not just space that is important it’s spaceTIME. We too travel at the constant c but through the time dimension. This is the interesting thing. Everything travels at c through spacetime and it’s simply v+t=c. Light being massless has no t. Neutrinos are no different.

I understand why a photon has to move to be something, but why does it have to move at the speed of life

why (if protons already travel at light speed) cant we cause protons to travel at a speed they can o on their own?

The Physicist says:

I’ll handle this one. There is one speed in the universe, call it C (the constant). Since space and time ante one thing, “spacetime” EVERYTHING moved at C through spacetime. A body with intrinsic mass is slowed down through space so it travels forward at the speed of C, mostly through time. That’s why we age. Light doesn’t. From the perspective of light, it’s still the Big Bang since it has always used it’s “C” in a maximum space speed, zero time. Hope this blows your mind.

I am a P.hD scholar of Entomology, dept of
Zoology, University of Kashmir. I want to
know that if photons doesn’t have mass, how
could they have energy because Einstein’s
equation E-mc2 tells us that mass and
Thank you.

Photons have no rest mass but do have kinetic energy in the form of angular momentum. The full equation is E=mc2(hf)

Does a photon simultaneously exhibit amplitude and frequency in multiple planes about a common axis? If so, could it be described as a vibration? Also, for this discussion are we defining a vacuum as an area of space devoid of the influence of mass/gravity?

Can electricity be generated on planets without an iron core? Do protons and neutrons exchange or share an electron? If a neutron looses an electron, does the mass/energy relationship in the nucleus of the atom chance? Is electrical current the result of displaced electrons or transferred electrons?

The above explanation is very, very weak and makes some “historical” mistakes, too. Although I have studied physics at the university, I am lately getting the feeling that modern physics is now more like a religion, than science. (But it might be my fault, as I am a Greek, who just can give up on logic and being rational.)

You make one statement that to me is seriously mistaken. On the one hand you say that the laws for light and matter are very different. I agree absolutely. You then try to apply special relativity to light! This is ridiculous – you say that “things (like light) that travel at the speed of light never experience the passage of time. Isn’t that awesome?”. No, it’s not, because it’s not true. Light is capable of internal change as it travels, we can in principle observe changes to wavelength, polarisation etc. If matter could travel at c, it would be observed without any internal change, and would look frozen to an observer. Light doesn’t do that, so you can’t apply the rules about time in SR to light.

I have difficulty understanding that Light (photons) do not have rest mass and yet does not go out of Black Holes. Does that mean BH does absorbs the photons to something else? Or the photons are still there hanging around with so much gravity? How does gravity work on photons? Thank you.

I’ve been thinking about the picture that’s been displayed with the beams of light versus a plain red brick. Same idea, I’m told photons are massless and the beams of light picture doesn’t at all resemble a brick. And one can’t feel X-ray photons going through them when they get an X-ray taken. But what about gamma ray photons?
Doesn’t that pack a punch? I’m also told that “resting mass” is just a mathematical
value assigned to make the math work. . I’m glad I am amongst the legions of
folks who agree that in some way, whether black hole dwelling (trapped) photons or
radioactive gamma ray photons there is a measure of mass. I have also been assured that photons emitted from the Sun are massless. Also recall that what we see in a bolt of lightning is just pure energy. But if we can see the lightening…..then why isn’t that a photon light display as well? Sorry to seem as if I have flight of ideas.

If light is not “pulled in” but distorted I’m just guessing not trying to get too deep but we know black holes are so powerful light can not escape if that’s the case then could that mean A) light is stuck in a infinite loop around a black hole because it is so strong even tho technically light is not effected or could that imply B) a black hole is so dense that it creates a hole in space time and light “falls” in to a black hole which could mean there may be a opening on the other side “maybe” but we can not survive the journey to investigate

For Elaine:
A bolt of lightning does not move at the speed of light, it moves at the speed of electricity (much slower). Think of a light bulb. When you flip the power switch on a lamp, the light is produced by a current of electricity flowing through a filament (old-style light bulbs). The filament doesn’t travel at the speed of light nor does the electricity traveling through it. The “bolt” in lightning bolt is simply an electrical current like that in a filament. The light itself travels from the current to your retina at the speed of light.

For Frank:
The laws of physics break down inside a black hole so we don’t know what is going on inside. But it is safe to say that light is not “pulled in” from anywhere since if the warping of spacetime at the event horizon is sufficient to prevent light from “escaping” then the warping of spacetime (or “gravity”) at all points inside the black hole is just as limiting. So, nothing could emit photons, in theory, nor could they travel in any direction beyond the Planck length. But again, laws of physics don’t apply so it could be Alice and the mad Hatter in there.

If anything with mass was to be accelerated to C , then it would approach infinite mass , and would need infinite energy to be propelled to C .Yet a photon travels at C and has no mass . So a photon traveling at C , having no mass , would not experience [metaphorically ] Time . So , if a photon is not affected by time , has no mass , then it can exist forever , and since we do not know what lies beyond the leading edge of the Universe ,which we determine is expanding , and know not what it is expanding into , that photon is [ or should ] be traveling at atleast the same speed as the Expansion . This would seem to be a Constant whether in Classical or Quantum physics , so as we look and observe the distant galaxies , we measure, that the farther they are away , the farther back in time we are seeing , the faster they seem to be travelling , we determine this by the shift in the spectrums of the sources of light from these distant objects .
I am not a physicist , nor a scientist , but simply one who has been studying this stuff since childhood , 55 years have passed since my interest began , and what I have stated is by no means scientific beyond that, it is what my understanding of it all at present encompasses . There is no explanation , which is definitive , of why or how light behaves as both a particle and wave , we have observed it , and have proven that in experiments , but still don’t know why or how . To date , it remains a mystery , as does the Universe and it’s origin . I remain in awe , as I was in first understanding the immensity of what the Universe was . Maybe one day we will have an answer , however IMHO I think those two things , as well as Time itself will remain inscrutable .

Photons want to travel in straight lines but are constantly colliding with bosons (tiny particles that exist everywhere, even in the so-called vacuum of space). If there were no bosons, a photon would travel not only in a straight line but instantaneously. With bosons around, photons zig-zag randomly – light-speed (186k m/s) being their average rate of progress. All moving sub-atomic particles zig-zag randomly and scientists can keep track of them only by assigning probabilities of their being in given places at given points in time (i.e. by applying quantum mechanics). Photons have energy because they spin round.

If E=p*c for photons (m=0)
and P=m*v =0
E=0*c =0
Seems legit ! ^^

Gravitational force can bent light ! can nuclear force ?

My doubt is electron is a specific charge which has specific mass but photon is a particle but why it has no mass. but actually particle should have mass rather than charge but in reality why photon has no mass

If light were composed of waves it would be as though every star in the galaxy was a radio station playing at the same frequency. You would see nothing but static.

Time is simply a measure of distance traveled.

The reason that time appears to slow down near a massive object is NOT because time itself has slowed down as time is not a thing but a measure of distance traveled. So if “time” seems slower for an observer at a distance what he is really seeing is a change in distance travelled.

An observer outside of the gravitational field will find her space uncompressed by a massive object displacing space. It is the compression of space on the plank scale that is causing the confusion.
X
a b c d
()()()()()()()()()()()()()()
( e )( f )( )( )( )( )
( g )( )( )( )

This is a very rude picture of a gravitational field. X being the massive object, ()= plank unit of space, a being closer to the massive object compressing space and g representing an observer outside of the field. Obviously on the plank scale there would be many trillions of discreet bits of space per cubic centimeter.

Now we assume that light has to take a plank time to cross one unit of space no matter how compressed or decompressed it is. So for an observer looking at the ground he sees units a,b,c,d. Even though a,b,c,d are 4 units of space compressed as they are by the massive object the observer will measure them in uncompressed space to be the distance of 1 unit of space. He will then observe that it takes a beam of light on the ground 4 times longer to travel the same distance as a beam of light in his region of uncompressed space takes.

He will incorrectly conclude that time has slowed down near the massive object when in reality the distance that he measures is incorrect.

In the case of a black hole it will emit light but the space around a black hole is so compressed that by the time the light reaches the event horizon it is shifted far to the red because the individual particles when translating from more compressed to less compressed space spread out. The amount of shifts indicates the strength of the gravitational field at the point of emission.

Sorry my diagram had the spaces taken out. The brackets should be spaced further apart as they get farther from X

## The Great and the Small: Is Quantum Foam Losing its Fizz?

We are very pleased to welcome Eric Perlman as a guest blogger today. He led the study setting limits on the foaminess of space-time that is the subject of our latest press release. Eric is a professor at the Florida Institute of Technology. After completing his PhD in 1994 at the University of Colorado, he held postdoctoral fellowships at the Goddard Space Flight Center and Space Telescope Science Institute. He also held research positions at Johns Hopkins University and the University of Maryland, Baltimore County. He has lived in Florida for 8 years and enjoys his family, singing, and playing chess and other board games.

Astronomy has been a tool of discovery since the dawn of civilization. For thousands of years, humans used the stars to navigate and find their place in the universe. Astronomy made possible the travels of the ancient Polynesians across the Pacific Ocean as well as measurements of the Earth’s size and shape by the ancient Greeks. Today, astronomers search for hints about what the universe was like when the universe was much younger. So imagine, for a second, what life would be like – and how much less we would know about ourselves and the universe – if the microscopic nature of space-time made some of these measurements impossible.

This is the puzzle posed by our observations of very distant objects in X-rays and gamma-rays, and for me as an astronomer, it is a mind-blowing concept I never thought I would have to deal with. I have always been interested in quasars, both because of the extreme physics that can occur near their central supermassive black holes or in their energetic outflows called jets, as well as because of the unique insights that can be gained because of the fact that they are the brightest and most luminous things that we can see from the earliest stages of the universe’s evolution.

I got interested in the structure of space-time almost by serendipity. Fifteen years ago, I wrote a paper on Hubble Space Telescope observations of a quasar, PKS 1413+135, where the galaxy’s nucleus was so bright that one could easily see Airy rings – artifacts that are always observed when one takes high-quality images of an unresolved source. Two years later, Richard Lieu and Lloyd Hillman of the University of Alabama in Huntsville, wrote a paper claiming that the mere fact that those images existed ruled out models of quantum gravity. This was then disputed by two of my co-authors, Y. Jack Ng and Wayne Christiansen of the University of North Carolina. In 2005, I was asked to review one of their papers by the editor of Physical Review Letters. I became so fascinated with the topic that after their paper was published, I contacted them with an idea of my own. Jack, Wayne and I, along with other collaborators, have worked together on this subject since that time.

What is it about space and time that might determine whether we could even take images of a distant object? The answer lies in the nature of space-time itself and how light travels. Light travels along a path called a null geodetic. This is the shortest distance between two points. In the absence of gravity, a null geodetic is represented by a straight line. But if a mass is present, the null geodetic represents the shortest-distance path through the curved space-time. Albert Einstein’s theory of general relativity (GR) describes how mass curves space-time, an effect we can see in such varied ways as gravitational lensing by distant galaxies, the precession of Mercury’s orbit, and the relativistic frame-dragging that is observed in the variability of X-ray binaries.

Illustration of Space-time foam

In GR, there are four dimensions, three of space and one of time. One of the greatest puzzles of modern physics is why gravity (and GR) cannot be unified with quantum mechanics (which has been immensely effective in describing the structure of atoms), as the electromagnetic and other forces have been. Quantum gravity models predict that space-time has a foamy structure (see artist’s conception, above), with tiny bubbles that are constantly fluctuating. These bubbles – which are quadrillions of times
smaller than atomic nuclei and last for infinitesimal fractions of a second (far shorter than the femtosecond bursts from the fastest lasers) – must be described by additional dimensions. Because these bubbles are so small and last for such a short time, they can never be observed directly. But they would affect light in an interesting way: because of the fluctuating nature of space-time, each photon would take a slightly different path to us, and hence the distance that it travels would be different. We could still never observe a single deflection imposed by quantum foam. But if you observe a very distant object, each photon will go through many of these fluctuations, and the fluctuations could add up – although exactly how depends on the model of quantum gravity. These fluctuations would then cause the wavefronts we observe to be distorted ever so slightly. If the accumulated distortions become comparable to the wavelength of light we’re observing, it would be impossible to form an image, no matter how good your telescope is – just like trying to distinguish sounds emitted by many out-of-phase loudspeakers, they would become pure noise.

This was the effect that we wanted to search for, which previous papers had not described properly. We modeled these effects, and then used observations of distant quasars by the Chandra X-ray Observatory, Fermi Gamma-ray Space Telescope, and the Very Energetic Radiation Imaging Telescope Array System, known as VERITAS, to look at archival images of the most distant quasars. We wanted to look in the X-rays and gamma-rays because of their short wavelength – allowing us to possibly observe the smallest possible distortions in the wavefronts by quantum gravity. But even the most distant quasars (see examples, below) appear to form sharp X-ray and gamma-ray images. Two models of quantum foam had predicted that these images would disappear at short wavelengths. Chandra’s X-ray images rules out one model, according to which photons diffuse randomly through space-time foam in a manner similar to light diffusing through fog. And gamma-ray images with Fermi and VERITAS, demonstrate that a so-called holographic model with less diffusion does not work.

So where does this leave us? Space-time appears to be smooth, at least on scales of 10-16 cm, a thousand times smaller than an atomic nucleus, and space-time must be much less foamy than most models predict. And, right now the only model that seems to hold up, predicts that space-time fluctuations should be anti-correlated with one another and so would not add up over long distances. While philosophically, it may be reassuring to not have to think of space-time as having a foamy nature that can affect one’s ability to see distant objects, on another level it makes the small scale structure of the universe even more puzzling.

## An experiment seeks to make quantum physics visible to the naked eye

Photon pairs are produced with a source (green point). A photon from each pair is emitted upwards the other is directed into a semi-transparent mirror (black circle). Following the mirror, the photon exists in two entangled states (symbolized by the yellow figure of eight). The photon is then detected by a detector (top right) or by the eye of the human observer (bottom right). In order for the photons to be detectable by the human eye, they are amplified by laser beams (boxes with yellow triangle symbol). The amplitude and phase of the laser beams can be changed during each run of the experiment, with the result that either the detector or the eye can detect the light pulse, and sometimes both simultaneously or neither at all. Through statistical analysis of the perception of light, quantum physicists can then infer the existence of quantum entanglement. Credit: Valentina Caprara Vivoli

Predictions from quantum physics have been confirmed by countless experiments, but no one has yet detected the quantum physical effect of entanglement directly with the naked eye. This should now be possible thanks to an experiment proposed by a team around a theoretical physicist at the University of Basel. The experiment might pave the way for new applications in quantum physics.

Quantum physics is more than 100 years old, but even today is still sometimes met with wonderment. This applies, for example, to entanglement, a quantum physical phenomenon that can be observed between atoms or photons (light particles): when two of these particles are entangled, the physical state of the two particles can no longer be described independently, only the total system that both particles form together.

Despite this peculiarity, entangled photons are part of the real world, as has been proven in many experiments. And yet no one has observed entangled photons directly. This is because only single or a handful of entangled photons can be produced with the available technology, and this number is too low for the human eye to perceive these photons as light.

Entangled photons amplified 100-fold

Nicolas Sangouard, a theoretical physicist at the University of Basel, together with two quantum physicists from Delft, Netherlands, and Innsbruck, Austria, has now shown in the scientific journal Optica how it may be possible to detect entangled photons directly. The basic idea of the experiment is that an entangled photon is generated and then amplified using a special technique, without destroying the quantum physical entanglement.

In the process, about 100 entangled photons are present, which, according to current knowledge, is the precise number needed to create the impression of light in humans. But although hundreds of photons reach the retina, there are also significant losses: only about seven actually reach one of the 120 million light-detecting rods of the retina. These photons then generate the nerve impulse that triggers the perception of light in the brain.

In the experiment proposed by the three quantum physicists, entanglement is created by a single photon directed at a semi-transparent mirror. Sangouard explains what happens next: "The single photon is not transmitted or reflected by the mirror instead – quantum physics is strange – the photon is simultaneously transmitted and reflected. Behind the mirror, the photon exists in a 'transmitted' and 'reflected' state, whereby these two states are entangled with another."

A photon detector and a human observer are placed behind the mirror. In order for the observer's eye to detect the entangled photons, they are amplified 100-fold with a type of magnifying glass before they reach the eye. Technically speaking, this is achieved by a displacement in phase space using a laser. Whether the human observer and/or the detector actually detect the entangled photons is not revealed directly but rather through determination of the probabilities. For this, the experiment is repeated many times and the data obtained statistically analyzed.

Very long observation period

It is not yet certain if Sangouard's group will conduct the experiment or if other quantum physicists will implement it. The required technologies – special photon sources and special lasers – are generally available today however, the biggest obstacle is the practical implementation of the experiment. The human eye is about a billion times slower at counting weak light pulses than modern photon detectors. "According to an initial estimate, several hundreds of thousands of runs would be necessary until we have enough data to determine if we've actually detected entangled photons. This means that the test person in our experiment would have to note at one-second intervals over the course of several hundreds of hours if they have just detected a light pulse or not."

If these obstacles were overcome, the experiment would demonstrate that the human eye is able to detect quantum entanglement directly and achieve what until now has required complicated and expensive detectors. Scientists are currently working to use the principle of entanglement to build secure digital communication links and for quantum computers. According to Sangouard, these applications could benefit from the new experiment.