We can walk forwards or backwards, slide left or right, and go up and down stairs, but it seems we can only move forwards in time. This hasn't stopped people from imagining about time travel, though. Time travel is a common topic in science fiction works. It is included in The Hitchhikers's Guide to the Galaxy and the Doctor Who television series as well as many others. But could time travel be real? It has been suggested that time travel could not possibly exist as we are not surrounded by sightseers from the future. While this might be true, physicists have developed a few more scientific theories about the possibilities of time travel, resulting in many theories about how it could happen.

Wormholes

One classic method of achieving time travel is the wormhole theory. A wormhole is a rip in the space-time fabric, sort of like a tunnel, as its name would imply. Theoretically, it joins two points of the universe, and can be used for instant travel, without actually travelling the full distance. Once you enter the mouth of a wormhole, you instantly pop out of the other side, even if it is a million light years away.

It can be explained by imagining an ant living on a piece of paper. As far as the ant is concerned, there are only two dimensions. Joe comes along, and sees that the ant wants to go from point A to point B without passing the space in between. To help the ant, Joe does the following:

1. He first bends the paper almost in half.

2. Then, he takes a pencil, and drills two holes in points A and B, which have been brought close together.

3. Then, he takes part of a straw, and sticks it in the two holes.

Now the ant can just walk the length of the straw to get from A to B, rather than walking on the paper. He is able to do this by traveling through the third spacial dimension. The straw is like a wormhole.

Extrapolate this into our universe, which has three spatial dimensions. The wormhole exists in a theoretical fourth spatial dimension. While this could happen, there are reasons to doubt it. The theory assumes that there are more than three spacial dimensions, which may or may not be true. Also, time may distort things. According to relativity, time is the fourth dimension. Experiments have proven that actions which occur in the first three dimensions can affect time (see Time Dilation).

However, time might not distort things after all. Common logic says that, if anything, time is the zeroth dimension (after all, you could have a two-spacial-dimension world with time - see the book Flatland by Edwin A Abbott for an example of this). Perhaps time is not directly related to the other dimensions at all, but is unique.

Another possible problem with wormholes might be that only four-spacial-dimensional beings could create them. The ant could not have made the 'wormhole' in the demonstration above because it could not bend the paper itself. It may turn out that wormholes are fully possible only as long as you work one dimension below your own.

Time Travel Using Wormholes

According to the wormhole theory, time travel can be achieved by first creating a stable wormhole. This already causes problems as natural wormholes (if they existed) would be very unstable; they would want to collapse the instant they were created. So, in order to prop them up, antigravity would be needed. Some sort of 'exotic matter' would be needed which, instead of pulling other matter towards it via gravity, actually pushes matter away.

Current research projects into 'dark matter', 'dark energy', 'quintessence', and other unusual entities are looking into this, but for the moment, 'exotic matter' with negative gravity is not known to exist. However, for the sake of argument, let's believe that such matter exists. Now there are two points in space, X and Y, which are connected by a stable wormhole.

1. Move the end of the wormhole that is at point Y around very fast. But how? Some people on Internet message boards have suggested dumping a large supply of magnetic material into the wormhole at point Y, and using electromagnets to move it. This is not guaranteed to work: quite possibly the magnetic material would simply go through the wormhole and pop out the other side, but this is not important. Imagine that point Y has been moved around very rapidly.

2. According to Einstein's theory of relativity, the moving end should go forward in time faster than the stationary end. As a result, Y is farther in the future than X.

3. So, if one travels from Y to X, one travels back in time.

Sadly, so many parts of this theory are merely speculation that it would probably never work.

Einstein's Relativity

Einstein's Theory of Relativity clearly allows for time travel, or at least discrepancies in perceived time. This is because time is relative to speed.

Travelling Faster than the Speed of Light

Would you be able to move about in time if you could travel faster than the speed of light? Well no, because travelling at or above the speed of light is impossible. Because of E=mc², mass equals energy. So the faster you go, the heavier you get. The heavier you get, the more energy you need to accelerate. But then you get heavier... The closer to light speed you get, the closer to infinity your mass gets. To travel at the speed of light, you would need an infinite amount of energy. Since infinite amounts of energy don't exist, speed-of-light travel is impossible.

Even if you could travel at the speed of light, you wouldn't go back in time. The opposite would happen: if you looked out the window of your lightspeed vessel, you would see every event in the entire universe happening at once, and probably in the same place.

Travelling at Near Light Speed

Whilst you can't time travel by moving at light speed or faster, you can make time pass at different rates. This is demonstrated in the famous example of the 'twin paradox'. There are a pair of identical twins. One twin stays on Earth, while the other is sent away on a spaceship travelling close to the speed of light. The travelling twin returns after one year as measured by her on-board clock, only to find that everything on Earth, including her twin, has aged half a century or so. The effect is a form of time travel. This has been tested by scientists with underground particle accelerators, and might be an efficient way to travel into the future, but it does not help to travel into the past.

Travelling Slower than Anything Else

Many people, when presented with the twin paradox, begin to wonder, 'If going really fast sends me into the future, then wouldn't going really slowly send me to the past?' The main response to this is 'what, exactly, is 'really slowly'?'. To answer this, you need to define a thing which does not move, which is quite diffucult. What would the opposite of 'going infinitely quickly into the future' be? It's not 'going infinitely quickly into the past'. It is 'not moving at all'. To understand this, think of regular motion. The opposite of 'moving really quickly' is 'moving really slowly'. It is not 'move really quickly, backwards'. So it is with time: Were you able to stop moving completely, to cease all motion, you would cease to move into the future, but would not move into the past.

The Tippler Method

The Tippler time travel method, which depends on neutron stars, was invented and designed by Frank Tippler from the University of Texas. A neutron star is a star that contains about as much mass as our sun, but is only ten or so kilometres across, instead of 1.5 million. It is extremely dense and has a very strong gravitational field, which is so intense that it warps the very time-space continuum.

Time Travel using the Tippler Method

Get a bunch of neutron stars and fashion them into a cylinder about 100 kilometres long and with a 10 kilometre radius. Then spin the cylinder about 2,000 times per second. The surface should be moving at about half the speed of light by now. According to Tippler's theory, it should be dragging the space-time fabric with it, with enough force to mix up space and time. Now fly your 'spaceship' in a specific orbit around the giant cylinder. This should, in theory, take you back in time, but only untill when the machine first started spinning (since that was when the space-time continuum was first warped). As you can imagine, this requires some practically impossible engineering, but there are no theoretical reasons why it wouldn't work.

Rotating Black Holes

The rotating black hole theory is equally tenuous. The center of a black hole is known as the singularity. It is a single point which contains all of the black hole's mass. Close to the singularity, all known laws of physics break down. But once you are within that region, you will never escape. Some theories state that if the black hole is rotating, the singularity is not a point, but a ring. It is possible that, if you jump through the ring, you would travel into the past. Some theories even state that the ring would form the entrance to a wormhole of sorts; you would find yourself deposited at a 'white hole,' the opposite of a black hole. However, no evidense has been found supporting these 'white holes.' It is quite likely that if you were to jump into a spinning black hole, you would collapse the ring and kill yourself. And even finding a rotating black hole will most likely be impossible. Both of these theories could be correct, but neither are very likely.

What Would Happen If Time Travel Were Possible?

Many people wonder about the apparent 'grandfather paradox'. What if, when we had invented time machines, you went back in time, and accidentally landed on your grandfather, who had not yet had children. You would simply pop out of existence because you could not have been born. Then, you could not have killed Grandpa, and so he survives and you are eventually born and go back in time and land on him again, so you instantly pop out of existence... you would be stuck in an endless loop. There are a few theories around how this could be resolved.

End of the Universe

This theory involves two types of temporal loops. One type is the loop mentioned in the last paragraph, the 'grandfather paradox'. For the rest of this paragraph, let's call it the 'infinite repeat' loop, because it results in two different possibilities, infinitely repeating after one another. Another type of loop exists. It is the 'infinite possibilities' loop. In this loop, the loop changes every single time that the loop repeats. Think of this: Imagine that you ask your best friend to go back in time to before you were born and kill your granddad. Also, you had enough forethought to tell him to, while he's back there, write a note to his future self to go back in time and kill the man who would be your granddad. Everything's Okay, right? Maybe not. When your friend is given the instruction to go an kill your granddad from you, he might do one thing. When he receives a note from his future self, he might do another. And if he does another thing during the second repeat, he must do a different thing the third. And the forth. And the fifth. A change in one iteration of the loop would result in a change in the note, which would result in a change in the next iteration. Eventually, he'll do something that ends up breaking down the loop (ie, forgetting to write himself a note). This will result in a infinite repeat loop starting. And as was already mentioned, infinite repeat loops may cause the universe to end.

Fate

This theory allows for time travel to occur without any fear of paradox. But the price is not cheap. Under this theory, free will is an illusion. Nothing that you do is truly your own choice; everything was preordained at the beginning of the universe. If you accept this, then the 'grandfather paradox' vanishes. No matter how hard you tried, you could not go back in time and kill your grandfather. You would slip, or the gun would jam, or something else would pop up, but you could not kill him.

This theory allows for some interesting events to happen, however. Let's say that, while reading a history textbook, you noticed that a person who looked exactly like you saved George Washington during the Battle of Trenton. To meet this person, you go back in time. However, much to your dismay, you can't find them anywhere. But, since you're already in the past, you decide to see George. As you approach, you see that he is fighting the Battle of Trenton, and he is losing. You save him. When you return to the future, you ponder what has happened. Apparently, the history book was talking about you. Assuming that free will is an illusion, this could very easily happen.

Here's another example. One evening, you find a floppy disk on your desk which you're sure you didn't put there. You look at it, and sure enough, it's the blueprints to a time machine. So you build the time machine, and send the blueprints to your past self. Now, where did the blueprints come from? There is no answer to this question. The blueprints simply 'are'.

This theory is often used in science fiction programmes because it avoids the paradoxical problems and allows the plot to tie itself up neatly in the end.

Parallel Universes

The above two theories assume that when you travel in the past, you travel in the past of the same universe. This causes problems, as mentioned above. But what if the universe whose past you travelled into wasn't your own? If this were true, you could go back and kill your granddad, without fear of causing the universe to end. The 'grandfather paradox' would be solved. This is because although you don't exist in the universe you went to the past of, in your own universe, you do.

For the sake of argument, assume that there are parallel universes. One thing to note about these universes is that without inter-universe interaction, all parallel universes would be the same (barring quantum effects; see next section for more). After all, if they started out the same, and since no outside force changed them (a force outside the multiverse?), they would remain the same.

This changes with inter-universe interaction. Let's say you live in universe A and you've just developed a time machine. Because of the above stated theory, the 'you' in all of the multiverses must also have developed a time machine. You go back in time (actually, back in time and universe) and end up in the past of universe B, where you kill your grandfather. Therefore, in universe B, you never existed. However, in universe A, you did exist, and could therefore have gone back to kill the universe B you's grandfather. There is no conflict here.

However, when one expands this to cover all universes (possibly an infinite number), conflicts do arise. Since in all universes you started out going back in time, and in all universes, you kill your grandfather, how should the multiverse decide which versions of you will successfully go back in time and kill your grandfather, and which versions of you will never exist? If all universes were the same (as stated above), there will be no way to decide. This will have two results: either each universe would split into two universes, one where you exist, one where you don't; or the universe would end.

Quantum Parallel Universes

In the above discussion of parallel universes, it is assumed that all parallel universes are the same. In a multiverse governed by Einstein's relativity, this would be true; no force would be acting to differentiate the universes. However, in a multiverse governed by quantum mechanics, this would not be true. However, before going into this, a discussion of quantum physics is needed.

The key idea of quantum mechanics is that certain particles, such as electrons, can exist in more than one state at once. For example, the electrons of an atom do not have specific locations; rather, they are located within the 'electron cloud' region. They are, in fact, at every point in the electron cloud at once.

One of the most commonly cited hypothetical experiments is the case of Schrodinger's Cat. A cat, a Geiger counter, a vial of acid, and a radioactive atom are sealed in a box. If the radioactive atom decays, the Geiger counter will break the vial of poison and kill the cat. After about an hour, there is an equal chance that the atom has or has not decayed. So, (because the decay of the radioactive particle is a quantum event), the cat exists in two states, both alive and dead. Of course, if you open up the box, you will either find the cat to be alive or dead, not both. Somehow, the duality is broken down. Some say that the act of observation breaks down the duality, but this is probably taking Heisenberg's Uncertainty Principle too literally (saying that the act of observation affects the thing being observed). A more logical explanation is that there are actually two universes; one in which the cat lives, and one in which the cat dies. Until you open up the box, you don't know which of these universes you are in; therefore, the cat is both dead and alive. Such quantum dualities are being broken down all the time, by physicists if by no one else. This would create a large supply of non-identical parallel universes -- and allow for time travel and free will to coexist peacefully.

Just because you discover time travel wouldn't mean that every you in every universe would discover time travel. Of those who did, not all of you would be bent on killing your grandfather; some might want to kill their mothers, or maybe just sight-see. You would all end up in different universes. some of you might succed in killing your grandfather; some might not. And if it so happened that one particular universe had both a succesful patricide and a time-traveler of its own, than that particular individual would cease to exist, without causing any major fuss. While it is possible that the non-existance of some you's might disrupt whatever other universe that they would have visited, leading to a chain reaction ending with (you guessed it!) the end of the multiverse, it is more likely that only a few universes would cease to exist, causing no major problem overall.

However, a quantum multiverse does create problems with time-travel. If you go into the past, do your best not to change history, and then return to the present, will you be in your present? For that matter, were you in your past, or the past of another universe? You couldn't merely plot a four-dimentional destination (space and time); you would need to plot a five-dimentional point (including the universe number). Then, of course, you could purposely go back into the past of your own universe, and kill your own grandfather, and end your own universe. What effect this would have on other universes is diffucult to say. Suffice it to say, the multiverse would be much more interesting were it quantum-based.

Because there would be no firm limits on what you could and could not do, free will would not be forbidden in a quantum multiverse. Of course, this does not guarentee that it does exist. Perhaps some other factor would come into play. Nonetheless, one should still obey the cardinal rule of time-travel: don't go back in time and kill your grandfather!

Web Resources

Here are some helpful sites on the Internet on the topic of time travel.

• Travelling through time... - This site has a similar layout to this Guide entry; it goes into more detail about how General and Special Relativity can be used to manipulate time.

Your Theories?

This is merely a sampling of the many theories about time travel. If you have theories of your own, feel free to discuss them in the Forum accompanying this entry.