Time travel is a common topic in science fiction works. It in included in the Hitchhikers's Trilogy, as well as many other popular science fiction books. But could time travel be real? The classic answer to this questions is no, time travel can't happen. After all, if time travel really did exist, our decedents would surely have come back to visit us. This may be true. However, this is hardly a definitive proof. Proving or disproving time travel requires more information. The rest of this entry is devoted to various potential time travel methods, and their consequences.

The Wormhole Theory.

One classic method of achieving time travel is the wormhole theory. The concept of wormholes may need some explaining. Here's an analogy to help. Imagine an ant living on a piece of paper. As far as the ant is concerned, there are only two dimensions - length and width. 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: he first bends the paper almost in half. Then, he takes a pencil, and drills two holes in points A and B. 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 (or she) is able to do this by traveling through the third spacial dimension. The straw is like a wormhole. (It is also similar to the concept of a "tesserect", from the book series "A Wrinkle in Time".) Now, extrapolate this into our universe, which has three spatial dimensions. The wormhole exists in the forth spatial dimension. While this could happen, there are reasons to doubt it. This assumes that there are more than three spacial dimensions. This 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 (spacial) dimensions can affect time (see Time Dilation). However, 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 for an example of this). Perhaps time is not directly related to the other dimensions at all, but is unique. Anyway, another possible explanation for wormholes' impossibility is that only four-spacial-dimensional beings could create them. The ant could not have made the 'wormhole' in the demonstration above. It may turn out that wormholes are fully possible - as long as you work one dimension below your own.

Anyway, for the sake of argument, imagine that wormholes do exist, and that we can make them. According to the wormhole theory, time travel is achieved by doing the following: first, create a stable wormhole. This already causes problems. 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. That is, one would need some sort of 'exotic matter' which, instead of pulling other matter towards it via gravity, actually pushes matter away. Current research into 'dark matter', 'dark energy', 'quintessence', and other unusual entities is looking into this. But for the moment, 'exotic matter' with negative gravity is not known to exist. However, again for the sake of argument, make believe that such matter exists. So, now there are two points in space, X and Y, which are connected by a stable wormhole. Next, move the end of the wormhole that is at point Y around very fast. How? Some on the 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. Now, 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. 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.

The Faster Than Light Theory.

The wormhole theory could work. It is highly unlikely, but it is possible. In contrast, the faster-than light theory could not work. First of, traveling 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 lightspeed 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. However, this is irrelevant to this theory. 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. Even near-lightspeed travel isn't the solution. If you travel at significant fractions of lightspeed (like 999999/1000000), you go into the future (again, see Time Dilation). It's been tested. That's what happens. This is an efficient way to travel into the future, but it does not help to travel into the past. However, this revelation brings up another idea, namely...

The Slower than Anything Else Theory.

Many people, when presented with the above paragraph, begin to wonder, "If going really fast sends me into the future, then wouldn't going really slowly send me to the past?" There are two responses to this: 1. What, exactly, is 'really slowly'? and 2. The definition of 'really slowly' doesn't matter, because going 'really slowly' wouldn't work anyway. Here goes: What is slow? To answer this, you need to define a thing which does not move. As one might expect, this causes its own problems (Why can't I just define myself as stationary, and then fly backwards in time?). However, as was already mentioned, the second response makes the first irrelevant. So, here is the explanation of the second response. 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 not move into the past.

Spinning Dense Columns and Rotating Black Holes.

These are some additional theories about time travel. They all rely on objects even more unlikely than wormholes. The 'spinning dense column' theory requires a column of very dense matter, maybe a mile tall and an inch in diameter. This column must be spinning at very high rates of speed. Theoretically, the column somehow drags timespace around it, and if you fly a spaceship around it in a specific pattern, you will travel into the past. Perhaps this could work. But the technical challenge of creating the column will not be overcome in any of our lifetimes. 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. But it is equally likely that you would collapse the ring and kill yourself. And even finding a rotating black hole would possibly be impossible. Both of these theories could be correct, but neither are very likely.

As you can see, it is unlikely that time travel would work. But this does not preclude one from pondering what would happen if time travel did exist. There are many theories trying to explain...

What Would Happen If Time Travel Was Possible?

Many people wonder about the apparent "grandfather paradox": What happens if you go back in time and kill your grandfather before you were born? The most common theory says that you would never be born, but then you'd never kill your grandfather, but then you'd be born, but then you'd kill your grandfather... This would be so confusing that it would have to cause the universe to end. However, this is not the only possible result of going to the past and killing your grandfather. There are many theories on the ramifications of time travel:

The 'any voyage into the past would cause the universe to end' theory.

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 OK, 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.

The 'you are in the past therefore you were in the past' theory

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.

Oh. You're still wondering about the title of this theory. It's called that because if you find yourself in the past, then you must always have been in the past, and therefore whatever you did led to the present that you come from.

The 'parallel universe' Theory

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 you traveled into the past of 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, though in the universe you went to the past of, you don't exist, 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. 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. 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. You kill your mother. 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 mother. There is no conflict here. However, when one expands this to cover all universes (theoretically, an infinite number), conflicts do arise. Since in all universes you started out going back in time, and in all universes, you kill your mother, how should the multiverse decide which you's will successfully go back in time and kill your mother, and which you's will never exist? If all universes were the same (as stated above), there will be no way to decide. This will have two results: each universe will split into two universes, one where you exist, one where you don't; or the universe will end. Personally, I don't want to take that risk.

What Happens if the Universe Ends?

A common theme in the above paragraphs is that time travel will result in the universe ending. By 'end', I mean 'cease to exist'. Also, it is likely that, if the universe ceases to exist, it will never have existed in the first place. But this may throw us into a 'infinite repeat' loop, where the universe does exist, then I cause it to not exist, but then I never existed, so I couldn't have made the universe end, so the universe does exist... So, this may invalidate all of my theories. What is the truth, and what is mindless drivel? At this point, I'll leave it up to you.

Any other ideas?

If you have any other theories about traveling back in time, post them in the Forum below this entry. If I don't respond to an inquiry after a while, email me; my address is accessible from my personal space.