How far into the future can we go by traveling close to a black hole?

How far into the future can we go by traveling close to a black hole?

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If we sent someone on a path that passed as near as possible to a black hole without getting pulled in, how far into the future would they go due to time dilation? Let's assume the black hole is 5 solar masses (I am assuming the mass will affect the calculation). I would like to know from two considerations:

  • An observer that can survive anything (e.g. ideal case)

  • The forces that a human can tolerate (e.g. a human would not be able to get as close to a black hole as would be ideal)

The way that you have specified the question, the answer is as far as you like. You simply put your spaceship into any orbit around the black hole and wait.

A more sensible question is what is the largest time dilation factor that can be accomplished - i.e. that maximises your travel time into the future for a given amount of proper time experienced on the spaceship.

This in turn is governed by how close to the black hole you can come and still tolerate the tidal forces. If you don't put a limit on this (your first case), then the answer is again infinite; you can hover as close to the event horizon as you like, using an enormous amount of rocket fuel, and the time dilation (see below) can be arbitrarily large.

Your second case is more realistic. Roughly we can say that the tidal acceleration across a body of length $l$ is given by $GMl/4r^3$, where $M$ is the black hole mass and $r$ is the distance from the black hole. If we make this acceleration equal to say $1 g$, and your body length $l sim 1$m, then for a $5M_{odot}$ black hole $r simeq 2500$ km (well outside the Schwarzschild radius of 15 km).

If you could "hover" at this radius, then the time dilation factor would be $$frac{ au}{ au_0} = left( 1 - frac{2GM}{rc^2} ight)^{1/2},$$ where $ au$ is the time interval on a clock on the spaceship and $ au_0$ is the time interval well away from the black hole.

For $M=5M_{odot}$ and $r = 2500$ km, this factor is 0.9970.

If the spaceship is in a circular orbit, the factor is $(1 - 3GM/rc^2)^{1/2} = 0.9955$.

These factors are perhaps not as big as you might have imagined!

As far as we know, once beyond the event horizon, someone might experience all of time in the blink of an eye until the hawking radiation makes the black hole disappear and the person comes out again. More likely though you'd just get spaghettified…

In all seriousness though, there is no real limitation, although they aren't really transported to the future, the outside world just moves faster.

Time travel into the future is totally possible

Believe it or not, time travel is possible.

In fact, you're doing it right now. Every single second of every single day, you are advancing into your own future. You are literally moving through time, the same way you would move through space. It may seem pedantic, but it's a very important point. Movement through time is still movement, and you are reaching your own future (whether you like it or not).

And what's even cooler is that you can skip forward in time if you feel like it.

Well, let me be clear, you need to do a little bit of engineering first.

We know through the physics of Einstein's special theory of relativity that you can trade motion in space for motion in time. If you're standing perfectly still, you're moving through the dimension of time at a particular speed (the speed of light, for those of you who are curious). As soon as you start moving through space, however, you slow down your rate of moving through time.

In other words, the faster you move in space, the slower you move in time.

This means that moving objects, like a clock on a rocket, run a little bit slow. One second for someone in a moving spaceship lasts a little bit longer than a second for someone staying still.

The trick is that in order for this to have any sort of noticeable impact, you have to get close to the speed of light, which is really hard to do—to give you some perspective, astronauts that orbit the Earth at tens of thousands of miles per hour are off by only a microsecond or so from our clocks on the ground.

Our fastest human spacecraft don't even crack a tenth of a percent of the speed of light. But if you could somehow spend a good amount of time hugging close to that ultimate speed limit in the universe, the slower your clock will run. You will travel through time into the future. Nothing will feel different for you, but after a couple of years' journey you would return to the Earth to find our clocks advanced by thousands or even tens of thousands of years, depending on how fast you go.

The speed of light plays a decisive role in time travel

"We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now," wrote Millington. "But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us."

The highest speed at which a signal or — physically speaking — an electromagnetic wave can propagate is what is known as the speed of light. It is exactly 299,792,458 meters per second. Albert Einstein postulated within the framework of his theory of relativity that the speed of light is a universal constant, i.e. that light always moves at the same speed in a vacuum — and independently of the observer.

It is precisely this condition that plays a decisive role in the question of whether time travel is possible. The law of causality follows from the fact that nothing can be faster than the speed of light. The law states that the effect of an action can only occur after the cause, which would make time travel into the past impossible. "For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth)," explained Millington.

Iranian Scientist Claims to Have Built "Time Machine"

Ali Razeqi says his time machine uses "complex algorithms" to see the future.

It's not quite Back to the Future, but a young Iranian inventor claims to have built a time machine that can predict a person's future with startling accuracy.

Ali Razeqi, who is 27 and the "managing director of Iran's Center for Strategic Inventions," claims his device will print out a report detailing an individual's future after using complex algorithms to predict his or her fate.

According to the Daily Telegraph, Razeqi told Iran's state-run Fars news agency that his device "easily fits into the size of a personal computer case and can predict details of the next 5-8 years of the life of its users. It will not take you into the future, it will bring the future to you."

Razeqi says Iran has decided to keep his prophetic time machine under wraps for now out of fear that "the Chinese will steal the idea and produce it in millions overnight."

Iran's Deputy Minister of Science, Research, and Technology dismissed Razeqi's claims on Friday in an interview with Fars—a sign of just how much attention the story has received.

We talked to Thomas Roman, a theoretical physicist at Central Connecticut State University and a co-author of the book Time Travel and Warp Drives, to ask about the possibilities for a Razeqi-like time machine and to debunk popular misconceptions about time travel. Here's an edited version of our interview:

What do you think of Razeqi's claim that he's built a time machine that can predict a person's future?

Does his alleged time machine break any laws of physics?

It's hard to know because it's so wacky.

What are some popular misconceptions about time travel?

One popular misconception is that you could go back to any time in the past. And that's not true. You can only go back as far as the time when the time machine was invented. So if I invent my time machine today and I wait 30 years and go back to the past, the farthest back in the past I can go to is today when I turned my time machine on.

Another major misconception—and you see this a lot in time travel movies—is the idea that you can go back in time and change the timeline. In these stories, the time traveler goes backward in time and does something that mucks up the future and subsequently has to do something to "restore the timeline." However, that can't be the case, since we can't have the same event both happen and not happen in the same universe. You can't change the past.

For example, suppose I go back in time and try to kill my grandfather. If I succeed, then of course I'm never born and I could never have made the trip back using the time machine.

Once again, we can't have the same event—the killing of my grandfather—both happen and not happen in the same universe.

Is there any way of getting around this "grandfather paradox"?

There are two possibilities. One is what's sometimes called the self-consistency scenario, in which all events along the time loop that I make are adjusted to be self-consistent.

So for example, if I go backward in time and try to shoot my grandfather, something will always prevent me from doing so. The recoil on my shoulder makes me miss, or my grandfather ducks, or I change my mind. It's like the universe and the laws of physics are conspiring to make things consistent.

The other possibility is that when I shoot my grandfather the universe splits and there's one universe in which I shoot my grandfather and there's another universe in which I did not shoot my grandfather.

Didn't split timelines play a role in the latest Star Trek reboot by J. J. Abrams?

Yeah, there was something along those lines. In the movie, the Romulan bad guy Nero goes back to the past to get revenge against Spock, who he claims is responsible for the destruction of his home planet Romulus. So he's going to get even by going back into the past to destroy [the planet] Vulcan.

But since Vulcan wasn't destroyed in the original timeline—the one Nero came from—then upon going back into the past, he causes the universe to branch.

So the Vulcan he destroys is not the one in his original timeline, but the one in the new branch. So he's not really getting revenge on the original Vulcan from his timeline. But I suppose revenge is revenge.

That aside, I thought that [using the concept of a split timeline] was a clever way of rebooting the franchise because then you have the same characters but you don't have to slavishly follow the past history of the episodes since you're in a new timeline where everything can be different.

Okay, so you might not be able to travel to the past. But is future time travel possible?

There's no problem with that. In fact, we know how to do it in principle. If you travel very close to the speed of light, time slows down for the space traveler compared to someone on Earth.

Another way of traveling to the future is by orbiting very close to a black hole. For example, if you orbit around the black hole at the center of our galaxy, you could also have your time stretched relative to observers on the Earth.

If future time travel is possible, then could a time machine like the one the Iranian businessman claimed to have built actually work?

Going to the future is no problem. A mechanism for traveling into the future is afforded by [Einstein's] special theory of relativity. It's when you try to go backward that you run into the grandfather paradox. However, that said, what the businessman claims to have built is still nuts.

One thing that's rarely mentioned in time travel stories is that if you travel back only in time but stay in exactly the same point in space, the Earth won't be there anymore. So wouldn't time travel require traveling through space as well?

Yes, it would have to. The Earth is turning on its axis, and it's orbiting the sun. So the Earth isn't always in the same spot in its orbit. So if you're staying in the same place and traveling back to the past, the Earth is gone from underneath you. When you stop your time machine, you'll be in a bit of a pickle.

Why do you think time travel is so popular in books and movies?

You have to admit, it's a pretty tantalizing idea. Part of the appeal is that you can go back and see things for yourself that you only know through history books and the geological record. I think everybody would think it'd be really cool to go back and see dinosaurs or go back and visit ancient Greece.

I think another appeal is we all have things in our past that we wished that we hadn't done, or that we wished hadn't happened. And I think there's the desire to be able to go back and prevent those things from having happened.

How far into the future can we go by traveling close to a black hole? - Astronomy

Carl Sagan, the astronomer, Pulitzer Prize-winning author, and legendary popularizer of science, gave this interview during the making of "Time Travel." True to form, he discusses arcane aspects of the field—from how you define time to what it might look like inside a wormhole—with flair and a refreshing dash of humor. Sagan was David Duncan Professor of Astronomy and Space Sciences and director of the Laboratory for Planetary Studies at Cornell University when he died in 1996.

NOVA: Let's start with the crux of the matter. What for you is time?

Sagan: Ever since St. Augustine, people have wrestled with this, and there are all sorts of things it isn't. It isn't a flow of something, because what does it flow past? We use time to measure flow. How could we use time to measure time? We are stuck in it, each of us time travels into the future, one year, every year. None of us to any significant precision does otherwise. If we could travel close to the speed of light, then we could travel further into the future in a given amount of time. It is one of those concepts that is profoundly resistant to a simple definition.

NOVA: Do you think that backwards time travel will ever be possible?

Hear Sagan via RealAudio
Sagan: Such questions are purely a matter of evidence, and if the evidence is inconsistent or insufficient, then we withhold judgment until there is better evidence. Right now we're in one of those classic, wonderfully evocative moments in science when we don't know, when there are those on both sides of the debate, and when what is at stake is very mystifying and very profound.

If we could travel into the past, it's mind-boggling what would be possible. For one thing, history would become an experimental science, which it certainly isn't today. The possible insights into our own past and nature and origins would be dazzling. For another, we would be facing the deep paradoxes of interfering with the scheme of causality that has led to our own time and ourselves. I have no idea whether it's possible, but it's certainly worth exploring.

NOVA: Would you like it to be possible?

Hear Sagan via RealAudio
Sagan: I have mixed feelings. The explorer and experimentalist in me would very much like it to be possible. But the idea that going into the past could wipe me out so that I would have never lived is somewhat disquieting.

NOVA: On that note, can you describe the "grandfather paradox?"

Hear Sagan via RealAudio
Sagan: The grandfather paradox is a very simple, science-fiction-based apparent inconsistency at the very heart of the idea of time travel into the past. It's very simply that you travel into the past and murder your own grandfather before he sires your mother or your father, and where does that then leave you? Do you instantly pop out of existence because you were never made? Or are you in a new causality scheme in which, since you are there you are there, and the events in the future leading to your adult life are now very different? The heart of the paradox is the apparent existence of you, the murderer of your own grandfather, when the very act of you murdering your own grandfather eliminates the possibility of you ever coming into existence.

Among the claimed solutions are that you can't murder your grandfather. You shoot him, but at the critical moment he bends over to tie his shoelace, or the gun jams, or somehow nature contrives to prevent the act that interrupts the causality scheme leading to your own existence.

NOVA: Do you find it easy to believe the world might work that way—that is, self-consistently—or do you think it's more likely that that there are parallel universes?

Hear Sagan via RealAudio
Sagan: It's still somewhat of a heretical ideal to suggest that every interference with an event in the past leads to a fork, a branch in causality. You have two equally valid universes: one, the one that we all know and love, and the other, which is brought about by the act of time travel. I know the idea of the universe having to work out a self-consistent causality is appealing to a great many physicists, but I don't find the argument for it so compelling. I think inconsistencies might very well be consistent with the universe.

NOVA: As a physicist, what do you make of Stephen Hawking's chronological protection conjecture [which holds that the laws of physics disallow time machines]?

Hear Sagan via RealAudio
Sagan: There have been some toy experiments in which, at just the moment that the time machine is actuated, the universe conspires to blow it up, which has led Hawking and others to conclude that nature will contrive it so that time travel never in fact occurs. But no one actually knows that this is the case, and it cannot be known until we have a full theory of quantum gravity, which we do not seem to be on the verge of yet.

One of Hawking's arguments in the conjecture is that we are not awash in thousands of time travelers from the future, and therefore time travel is impossible. This argument I find very dubious, and it reminds me very much of the argument that there cannot be intelligences elsewhere in space, because otherwise the Earth would be awash in aliens. I can think half a dozen ways in which we could not be awash in time travelers, and still time travel is possible.

Hear Sagan via RealAudio
Sagan: First of all, it might be that you can build a time machine to go into the future, but not into the past, and we don't know about it because we haven't yet invented that time machine. Secondly, it might be that time travel into the past is possible, but they haven't gotten to our time yet, they're very far in the future and the further back in time you go, the more expensive it is. Thirdly, maybe backward time travel is possible, but only up to the moment that time travel is invented. We haven't invented it yet, so they can't come to us. They can come to as far back as whatever it would be, say A.D. 2300, but not further back in time.

Then there's the possibility that they're here alright, but we don't see them. They have perfect invisibility cloaks or something. If they have such highly developed technology, then why not? Then there's the possibility that they're here and we do see them, but we call them something else—UFOs or ghosts or hobgoblins or fairies or something like that. Finally, there's the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization will destroy itself before time travelers invent it.

I'm sure there are other possibilities as well, but if you just think of that range of possibilities, I don't think the fact that we're not obviously being visited by time travelers shows that time travel is impossible.

NOVA: How is the speed of light connected to time travel?

Hear Sagan via RealAudio
Sagan: A profound consequence of Einstein's special theory of relativity is that no material object can travel as fast as light. It is forbidden. There is a commandment: Thou shalt not travel at the speed of light, and there's nothing we can do to travel that fast.

The reason this is connected with time travel is because another consequence of special relativity is that time, as measured by the speeding space traveler, slows down compared to time as measured by a friend left home on Earth. This is sometimes described as the "twin paradox": two identical twins, one of whom goes off on a voyage close to the speed of light, and the other one stays home. When the space-traveling twin returns home, he or she has aged only a little, while the twin who has remained at home has aged at the regular pace. So we have two identical twins who may be decades apart in age. Or maybe the traveling twin returns in the far future, if you go close enough to the speed of light, and everybody he knows, everybody he ever heard of has died, and it's a very different civilization.

It's an intriguing idea, and it underscores the fact that time travel into the indefinite future is consistent with the laws of nature. It's only travel backwards in time that is the source of the debate and the tingling sensations that physicists and science-fiction readers delight in.

NOVA: In your novel Contact, your main character Eleanor Arroway travels through a wormhole. Can you describe a wormhole?

Hear Sagan via RealAudio
Sagan: Let's imagine that we live in a two-dimensional space. We wish to go from spot A to spot B. But A and B are so far apart that at the speed of light it would take much longer than a generational time or two to get there as measured back on world A. Instead, you have a kind of tunnel that goes through an otherwise inaccessible third dimension and connects A and B. You can go much faster through the tunnel, and so you get from A to B without covering the intervening space, which is somewhat mind-boggling but consistent with the laws of nature. And [the theoretical physicist] Kip Thorne found that if we imagine an indefinitely advanced technical civilization, such a wormhole is consistent with the laws of physics.

It's very different from saying that we ourselves could construct such a wormhole. One of the basic ideas of how to do it is that there are fantastically minute wormholes that are forming and decaying all the time at the quantum level, and the idea is to grab one of those and keep it permanently open. Our high-energy particle accelerators don't have enough energy to even detect the phenomenon at that scale, much less do anything like holding a wormhole open. But it did seem in principle possible, so I reconfigured the book so that Eleanor Arroway successfully makes it through the center of the galaxy via a wormhole.

NOVA: What do you think it would be like to travel through a wormhole?

Hear Sagan via RealAudio
Sagan: Nobody really knows, but what Thorne has taught me is that say, for example, you were going through a wormhole from point A to point B. Suppose point B was in orbit around some bright star. The moment you were in the wormhole, near your point of origin A, you would see that star. And it would be very bright it wouldn't be a tiny point in the distance. On the other hand, if you look sideways, you would not see out of the wormhole, you would be in that fourth physical dimension. What the walls of the wormhole would be is deeply mysterious. And the possibility was also raised that if you looked backwards in the wormhole you would see the very place on world A that you had left. And that would be true even as you emerged out of the wormhole near the star B. You would see in space a kind of black sphere, in which would be an image of the place you had left on Earth, just floating in the blackness of space. Very Alice in Wonderland.

NOVA: Your inquiries about space travel for Contact sparked a whole new direction in research on time travel. How does that make you feel?

Hear Sagan via RealAudio
Sagan: I find it marvellous, I mean literally marvellous, full of marvel, that this innocent inquiry in the context of writing a science-fiction novel has sparked a whole field of physics and dozens of scientific papers by some of the best physicists in the world. I'm so pleased to have played this catalytic role not just in fast spaceflight but in the idea of time travel.

NOVA: How do you feel being responsible for bringing time travel perhaps a step closer?

4. Wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

The Closest Black Hole to Earth Has Been Discovered by Scientists

Astronomers have discovered the closest black hole to our solar system found to date, located "just" 1,000 light-years away.

According to a study published in the journal Astronomy & Astrophysics, the black hole forms part of a triple star system known as HR 6819, which is so close, in astronomical terms, that you can see it without binoculars or telescopes from the southern hemisphere on a dark, clear night.

"We were totally surprised when we realised that this is the first stellar system with a black hole that can be seen with the unaided eye," Petr Hadrava, a co-author of the study from the Academy of Sciences of the Czech Republic, said in a statement.

The previous record holder is a black hole candidate known as V616 Mon that could be as near as 1,200 light-years away, although most astronomers consider a figure of 3,000 light-years to be more realistic.

Black holes are astronomical objects that are so massive and dense that nothing, not even light, can escape their gravitational pull.

First predicted by Albert Einstein's theory of general relativity, black holes contain a singularity, a single point of infinite density and gravity where space and time as we understand them break down. Surrounding the singularity is the event horizon, the boundary beyond which nothing can escape.

Stellar-mass black holes, those with masses tens of times that of the sun, form after massive stars die as supernova explosions and collapse into themselves under the influence of gravity. Other black holes meanwhile, can form when incredibly dense star remnants, called neutron stars, collide. Similarly the merger of two black holes, or one neutron star and a black hole, can also generate a new, larger black hole. In addition, there are supermassive black holes, thought to be present at the center of many galaxies, whose origins are more mysterious.

"Stellar-mass black holes must not be confused with the supermassive black holes lurking at the center of most massive galaxies," Dietrich Baade, an author of the study from the European Southern Observatory (ESO), told Newsweek. "For instance, [the one] in the Milky Way has a mass of 4.2 million suns. Supermassive black holes formed early in the universe and probably keep growing through the accretion of stars and mergers with other supermassive black holes."

Astronomers think that there are vast numbers of black holes in the galaxy, but to date, only a couple of dozen have been identified.

"If you take the age of the Milky Way, the number of stars it contains, and the life expectancy of these stars, it is only a back-of-the-envelope effort to realize that there must be very many stellar-mass black holes in the galaxy," Baade said. "More complex models predict between 100,000,000 and 1,000,000,000 of them."

Black holes cannot be observed directly with telescopes that detect X-rays, light, or other forms of electromagnetic radiation. However, scientists can infer their presence by looking for their effects on the matter that surrounds them. Any matter in the immediate vicinity of a black hole will gradually be drawn inwards in a process known as accretion, creating an "accretion disk" of orbiting material. For example, if a star ventures too close, it will be consumed by the black hole, violently ripped apart by its powerful gravitational forces.

Nearly all of the black holes that have been identified so far have revealed themselves due to their strong interactions with their immediate environment. However, the authors of the latest study say that the black hole they identified in HR 6819 is one of the first stellar-mass black holes that does not interact violently with its environment, appearing truly black so to speak. This makes it extremely difficult to detect.

The team only discovered the black hole after observing its two companion stars using a telescope at the ESO's La Silla Observatory in Chile. Initially, they were monitoring the pair as a part of a study on double-star systems, but were shocked to uncover the previously hidden black hole while analyzing the data they had collected.

"We found that there was a third object whirling around one of the two luminous stars that weighs in at five suns or more," Baade said. "That fairly massive star's velocity changes with a period of 40 days. However, in spite of the strong gravitational pull exerted on this star, the third object does not emit any appreciable amount of light. Therefore, it can only be a black hole. The exciting thing is that it is one of the first&mdashperhaps even the very first&mdashabsolutely dull black holes that do not make themselves known through the violence in their immediate neighborhood."

"Other black holes were detected because gas that is transferred to them from a companion star heats up to very high temperatures and radiates strongly in X-rays, which are readily observed," Baade said. "The new black hole is really black because it is not fed by its companion. This makes it so much more difficult to discover: instead of a single X-ray image, it takes many observations suitably distributed over a long time to detect periodic velocity changes."

According to Baade, the latest discovery is surprising for two main reasons: Firstly, the fact that the team found possibly the first instance of a true black, or "non-accreting," black hole. And secondly, that it was discovered so nearby, relatively speaking. This indicates that there are many more similar black holes to be found in the future, the researchers say, with this system likely representing just "the tip of the iceberg."

Roberto Saglia, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany, who was not involved in the latest study, told Newsweek that the important aspect of this research is the detection a "non-active" stellar mass black hole.

"Most stellar mass black holes are first discovered because they have a hot accretion disk around them that shines in the X-ray/ultraviolet range and is detected by X-ray satellites. Here there is no X-ray emission and the inference of the presence of a black hole comes just from dynamical measurements," he said.

"This is important, because we expect from stellar evolution that many more stellar mass black holes should be around compared to the number of detected ones," he said. "This system provides an alternative way to probe this 'unseen' family of black holes, as gravitational wave detections can also provide."

The fastest manned vehicle in history was Apollo 10. It reached 25,000mph. But to travel in time we'll have to go more than 2,000 times faster

Now, I realise that thinking in four dimensions is not easy, and that wormholes are a tricky concept to wrap your head around, but hang in there. I've thought up a simple experiment that could reveal if human time travel through a wormhole is possible now, or even in the future. I like simple experiments, and champagne.

So I've combined two of my favourite things to see if time travel from the future to the past is possible.

Let's imagine I'm throwing a party, a welcome reception for future time travellers. But there's a twist. I'm not letting anyone know about it until after the party has happened. I've drawn up an invitation giving the exact coordinates in time and space. I am hoping copies of it, in one form or another, will be around for many thousands of years. Maybe one day someone living in the future will find the information on the invitation and use a wormhole time machine to come back to my party, proving that time travel will, one day, be possible.

In the meantime, my time traveller guests should be arriving any moment now. Five, four, three, two, one. But as I say this, no one has arrived. What a shame. I was hoping at least a future Miss Universe was going to step through the door. So why didn't the experiment work? One of the reasons might be because of a well-known problem with time travel to the past, the problem of what we call paradoxes.

Paradoxes are fun to think about. The most famous one is usually called the Grandfather paradox. I have a new, simpler version I call the Mad Scientist paradox.

I don't like the way scientists in movies are often described as mad, but in this case, it's true. This chap is determined to create a paradox, even if it costs him his life. Imagine, somehow, he's built a wormhole, a time tunnel that stretches just one minute into the past.

Hawking in a scene from Star Trek with dinner guests from the past, and future: (from left) Albert Einstein, Data and Isaac Newton

Through the wormhole, the scientist can see himself as he was one minute ago. But what if our scientist uses the wormhole to shoot his earlier self? He's now dead. So who fired the shot? It's a paradox. It just doesn't make sense. It's the sort of situation that gives cosmologists nightmares.

This kind of time machine would violate a fundamental rule that governs the entire universe - that causes happen before effects, and never the other way around. I believe things can't make themselves impossible. If they could then there'd be nothing to stop the whole universe from descending into chaos. So I think something will always happen that prevents the paradox. Somehow there must be a reason why our scientist will never find himself in a situation where he could shoot himself. And in this case, I'm sorry to say, the wormhole itself is the problem.

In the end, I think a wormhole like this one can't exist. And the reason for that is feedback. If you've ever been to a rock gig, you'll probably recognise this screeching noise. It's feedback. What causes it is simple. Sound enters the microphone. It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers. But if too much of the sound from the speakers goes back into the mic it goes around and around in a loop getting louder each time. If no one stops it, feedback can destroy the sound system.

The same thing will happen with a wormhole, only with radiation instead of sound. As soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole. So although tiny wormholes do exist, and it may be possible to inflate one some day, it won't last long enough to be of use as a time machine. That's the real reason no one could come back in time to my party.

Any kind of time travel to the past through wormholes or any other method is probably impossible, otherwise paradoxes would occur. So sadly, it looks like time travel to the past is never going to happen. A disappointment for dinosaur hunters and a relief for historians.

But the story's not over yet. This doesn't make all time travel impossible. I do believe in time travel. Time travel to the future. Time flows like a river and it seems as if each of us is carried relentlessly along by time's current. But time is like a river in another way. It flows at different speeds in different places and that is the key to travelling into the future. This idea was first proposed by Albert Einstein over 100 years ago. He realised that there should be places where time slows down, and others where time speeds up. He was absolutely right. And the proof is right above our heads. Up in space.

This is the Global Positioning System, or GPS. A network of satellites is in orbit around Earth. The satellites make satellite navigation possible. But they also reveal that time runs faster in space than it does down on Earth. Inside each spacecraft is a very precise clock. But despite being so accurate, they all gain around a third of a billionth of a second every day. The system has to correct for the drift, otherwise that tiny difference would upset the whole system, causing every GPS device on Earth to go out by about six miles a day. You can just imagine the mayhem that that would cause.

The problem doesn't lie with the clocks. They run fast because time itself runs faster in space than it does down below. And the reason for this extraordinary effect is the mass of the Earth. Einstein realised that matter drags on time and slows it down like the slow part of a river. The heavier the object, the more it drags on time. And this startling reality is what opens the door to the possibility of time travel to the future.

Right in the centre of the Milky Way, 26,000 light years from us, lies the heaviest object in the galaxy. It is a supermassive black hole containing the mass of four million suns crushed down into a single point by its own gravity. The closer you get to the black hole, the stronger the gravity. Get really close and not even light can escape. A black hole like this one has a dramatic effect on time, slowing it down far more than anything else in the galaxy. That makes it a natural time machine.

I like to imagine how a spaceship might be able to take advantage of this phenomenon, by orbiting it. If a space agency were controlling the mission from Earth they'd observe that each full orbit took 16 minutes. But for the brave people on board, close to this massive object, time would be slowed down. And here the effect would be far more extreme than the gravitational pull of Earth. The crew's time would be slowed down by half. For every 16-minute orbit, they'd only experience eight minutes of time.

Inside the Large Hadron Collider

Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be travelling through time. Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had.

So a supermassive black hole is a time machine. But of course, it's not exactly practical. It has advantages over wormholes in that it doesn't provoke paradoxes. Plus it won't destroy itself in a flash of feedback. But it's pretty dangerous. It's a long way away and it doesn't even take us very far into the future. Fortunately there is another way to travel in time. And this represents our last and best hope of building a real time machine.

You just have to travel very, very fast. Much faster even than the speed required to avoid being sucked into a black hole. This is due to another strange fact about the universe. There's a cosmic speed limit, 186,000 miles per second, also known as the speed of light. Nothing can exceed that speed. It's one of the best established principles in science. Believe it or not, travelling at near the speed of light transports you to the future.

To explain why, let's dream up a science-fiction transportation system. Imagine a track that goes right around Earth, a track for a superfast train. We're going to use this imaginary train to get as close as possible to the speed of light and see how it becomes a time machine. On board are passengers with a one-way ticket to the future. The train begins to accelerate, faster and faster. Soon it's circling the Earth over and over again.

To approach the speed of light means circling the Earth pretty fast. Seven times a second. But no matter how much power the train has, it can never quite reach the speed of light, since the laws of physics forbid it. Instead, let's say it gets close, just shy of that ultimate speed. Now something extraordinary happens. Time starts flowing slowly on board relative to the rest of the world, just like near the black hole, only more so. Everything on the train is in slow motion.

This happens to protect the speed limit, and it's not hard to see why. Imagine a child running forwards up the train. Her forward speed is added to the speed of the train, so couldn't she break the speed limit simply by accident? The answer is no. The laws of nature prevent the possibility by slowing down time onboard.

Now she can't run fast enough to break the limit. Time will always slow down just enough to protect the speed limit. And from that fact comes the possibility of travelling many years into the future.

Imagine that the train left the station on January 1, 2050. It circles Earth over and over again for 100 years before finally coming to a halt on New Year's Day, 2150. The passengers will have only lived one week because time is slowed down that much inside the train. When they got out they'd find a very different world from the one they'd left. In one week they'd have travelled 100 years into the future. Of course, building a train that could reach such a speed is quite impossible. But we have built something very like the train at the world's largest particle accelerator at CERN in Geneva, Switzerland.

Deep underground, in a circular tunnel 16 miles long, is a stream of trillions of tiny particles. When the power is turned on they accelerate from zero to 60,000mph in a fraction of a second. Increase the power and the particles go faster and faster, until they're whizzing around the tunnel 11,000 times a second, which is almost the speed of light. But just like the train, they never quite reach that ultimate speed. They can only get to 99.99 per cent of the limit. When that happens, they too start to travel in time. We know this because of some extremely short-lived particles, called pi-mesons. Ordinarily, they disintegrate after just 25 billionths of a second. But when they are accelerated to near-light speed they last 30 times longer.

It really is that simple. If we want to travel into the future, we just need to go fast. Really fast. And I think the only way we're ever likely to do that is by going into space. The fastest manned vehicle in history was Apollo 10. It reached 25,000mph. But to travel in time we'll have to go more than 2,000 times faster. And to do that we'd need a much bigger ship, a truly enormous machine. The ship would have to be big enough to carry a huge amount of fuel, enough to accelerate it to nearly the speed of light. Getting to just beneath the cosmic speed limit would require six whole years at full power.

The initial acceleration would be gentle because the ship would be so big and heavy. But gradually it would pick up speed and soon would be covering massive distances. In one week it would have reached the outer planets. After two years it would reach half-light speed and be far outside our solar system. Two years later it would be travelling at 90 per cent of the speed of light. Around 30 trillion miles away from Earth, and four years after launch, the ship would begin to travel in time. For every hour of time on the ship, two would pass on Earth. A similar situation to the spaceship that orbited the massive black hole.

After another two years of full thrust the ship would reach its top speed, 99 per cent of the speed of light. At this speed, a single day on board is a whole year of Earth time. Our ship would be truly flying into the future.

The slowing of time has another benefit. It means we could, in theory, travel extraordinary distances within one lifetime. A trip to the edge of the galaxy would take just 80 years. But the real wonder of our journey is that it reveals just how strange the universe is. It's a universe where time runs at different rates in different places. Where tiny wormholes exist all around us. And where, ultimately, we might use our understanding of physics to become true voyagers through the fourth dimension.

'Stephen Hawking's Universe' begins on May 9 on Discovery Channel (HD) at 9pm

Here's How Black Holes Cause Time Travel

A researcher explained how time would be distorted inside a black hole. According to the researcher, entering a black hole would be like traveling through time.

When it comes to space and time travel, many believe that this idea can be achieved through dilation. The concept of time dilation refers to the different times measured by two clocks if one of these devices is on Earth and the other one is on a spaceship.

As the spaceship carrying one of the clocks moves at light speed, time here would appear to be much slower compared to the time measured by the clock on Earth. Theoretically, the same phenomenon would occur if the same spaceship enters a black hole.

In a post on Quora, researchers Atharva Palshetkar of the CTES College in India explained that for the spaceship in the black hole, time would appear to be moving at a normal pace. However, looking outside the black hole from within would reveal that time is actually moving at a much faster rate.

“Now if someone was supposed to see you falling down a black hole, he would see you going slower and slower, taking weeks, years and even decades, until you reach a point where light can’t escape the black hole’s event horizon,” Palshetkar explained.

“Meanwhile, while you enter the black hole everything you see outside will begin to speed up outside,” he continued. “Your family, kids, grandkids, hundreds of generations will rise and fall in just matters of minutes or hours.”

According to Palshetkar, those inside the black hole will eventually get to witness billions of years pass by on the outside without getting affected since, in their current state, time is still moving at a regular pace.

Of course, Palshetkar’s explanation of how black holes cause time travel is just a theory. Due to several factors, not much is known about the nature of black holes. For one, it is also impossible to visit a black hole to study it since the nearest one over 6,500 light-years from Earth. In addition, scientists have already stated that humans will most likely not survive if they get sucked into a black hole.

A team of astronomers found a way to get clear images of the black hole. Pitured: Black hole, Results when the core of a massive star collapses the gravitational force is so strong that not even light can escape. Photo: QAI Publishing/Getty Images

Using Gravity as a Means of Time Travel

In much the same way that traveling at speeds close to the speed of light can slow down perceived time, intense gravitational fields can have the same effect.

Gravity only affects the movement of space, but also the flow of time. Time passes more slowly for an observer inside a massive object's gravitational well. The stronger the gravity, the more it affects the flow of time.

Astronauts on the International Space Station experience a combination of these effects, though on a much smaller scale. Since they are traveling quite quickly and orbiting around Earth (a massive body with significant gravity), time slows down for them compared to people on Earth. The difference is much less than a second over the course of their time in space. But, it is measurable.