sparhawk on 11/5/2007 at 11:44

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I was thinking about time traveling and I think I should share my thoughts on them to see what others think of it. :) I know that physical laws currently have no prefered time direction (except in some obscure cases of particle physics, but this is IMO not completely confimed). Our current understanding of the laws does not neccessarily mean that it is technically possible. I know there are several ideas out there, which might or might not work (some of them involving negative energy, wormholes, etc.). But even if these things might be found, that does not neccessarily mean that time travel, as it is usually portrayed in popular movies or stories, will be what we can expect from it.

I think there are two fundemental approaches to time traveling. One would be the reversal of time (TR) and the other would be "normal" traveling only taking the fourth dimension into account as well.

The first approach would be pretty tough. Considering the second law of thermodynamics, this would mean that all these processes would be reversed. This alone would be an unsurmountable task, but lets consider what would happen if we could achieve this with technology. You have to keep in mind, that, for entropy to be reversered, not only would we have to find a process which DECREASES entropy (which has never been observed), we would have to do this maybe for every macroscopic particle. I don't consider quantum states as relevant for the moment, because on this scale particles can be replaced without any consequences, as they are all to be considered to be the same. A photon is exactly the same as any other photon, and the same is true for quarks.

So what would happen if we would manage to reverse entropy and in turn reverse time along with it. A timetraveller (TT) would enter the machine that is supposed to make this happen. Up to this point, the flow of time would be in the normal direction. So now the starts the engine, and the flow of time would still be in the same direction as before. As the engine starts to gain momentm, in whatever it needs to be doing in order to achieve TR, time is probably slowed down, just like proposed with relativistic physics, so we are still working. Now we have two effects taking place similarily. The first one is that, because of the slowdown of time for the TT, the time outside the machine woudl start to pick up. As we approach the zero point, where time starts to tilt over, all the time in the universe outside would go by. This is the same effect that would happen to a photon as well. So instead of traviling backwards in time, the TT would be travilling far into the future.
The second effect that would take place is near the zero point (where time flips over). So what would happen there is, that time reaches a zero point, and then start to reverse. Up to this point the machine always works correct according to physical and mechanical laws. And here is the catch. In the first nanonsecond (or even less), that time starts to spin in the reverse direction, the process, that the machine generats to let time flow backwards, the machine itself would also be reversed. This means that, the first action that the machine would achive in reversing the time, would also reverse the process in the machine itself, because this is part of the past of that machine, which also has to be reversed. This means that the machine gets now into the state where it was a nanonsecond before it started to reverse time, and this means that time can now not go backward any longer, because the machine needs to operate, whatever it does to achieve that, but it can not operate past the point where time starts to reverse. From the point of view of an outside observer it would probably look as if the machine didn't work, because it just stops working at this point. Of course any observer would be frozen in time by then anyway. :)

So my logical conclusion from this thought experiment is that timetravel by simply reverting the physical laws and trying to achieve timereversal by that, is not only technically impossible, it is also logically impossible. Any action that would cause time to reverse, would also cause it's own undoing and thus stops the process of time reversal.


Now we look at the second method of time traveling. We consider spacetime to be a fourdimensional vector. Mathematically it follows that this would need a static universe, because if we would be able to reach ANY arbitrary point in this fourdimensional spacetime, it follows that not only has the past to be fully determined, also the future has to be fully determined. If the future were not fully determined, it would follow that any future observer might find that one specific point in that spacetime would have two different events at the same time associated with it, which can not happen, because we assumed that we not only go this particular point in space but also in time, thus everything must be the same in this spacetime at each point and stay there.
From this it also follows, that, suppose we COULD go to such a point, then if we try to modify the events, we can not do this. If we could do this, then we would have the effect that we would see two different events at the same point, which can not happen, or we would be at a different that just looks like the same.

I don't really think that the universe is static like this, but from my conclusions, I have to think that timetravel is logically impossible, even though our current understanding of physical laws have no such preference.

rachel on 11/5/2007 at 12:47

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If we start from the idea that such a machine is possible, one can imagine that creating an embedded device that effectively "folds" space and time around the machine, isolating it from the outside world, is also possible, at which point the machine would work and remain unaffected by its actions.

The second approach is tricky. I'm fond of the idea that if you modify the past, you create a new, parallel universe in which you see you action's consequences (since you are present in that universe), while your original universe follows its course, but out of your reach. Returning to this original universe would then depend on the sophistication of your time travelling machine.
Most time travel devices in fiction appear to "move" following a single timeline, making for all those "I have to repair the past" stories, but hypthetically attaching (again) a device ala Sliders to move from one universe to the other could be done, and since we're talking about time travel, it's not that far fetched. Time is a dimension like any other, so moving from one dimension to the other, once you achieve that, comes down to just one problem: find and target the correct dimension.

Vivian on 11/5/2007 at 13:18

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Wouldn't traveling back in time mean literally traveling back to a previous state of the universe? From the universe-as-an-object's point of view the future and the past are just this complicated shape that we happen to be stuck travelling along in the same direction as entropy, yes? And as our memory (amongst other things) is entropy-driven, wouldn't reversing time (i.e. reversing the increasing entropy of the universe) also reverse our memory of time?
It could be possible and we'd not know a thing about it. Frinstance, I could have travelled back from next tuesday to today, but because I travelled back to the state of myself today I have no memory of the intervening days and so there is no way of proving whether I did or did not come from next week.

i.e. no biff's pleasure palace for me.

Rogue Keeper on 11/5/2007 at 14:46

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Because future people are careful enough not to reveal that fact to inferior previous generations! Proof of inferiority of you folks? You live in naive consumerist society which teaches you from the crib that everything can be eventually bought for entertainment purposes of common masses. Well forget about that! Can you comprehend the importance and sense of time travelling in the future? No. The first law of time travelling is :

Do not interfere!

Now that I have revealed this biggest secret in human history to you, they won't let me back to the future!

I am doomed to live with these savages until the rest of my days. Oh drat... :nono:

sparhawk on 11/5/2007 at 15:14

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Yes, but seem my other post. What is this "stuff" that is moving along this axis, to give us the sensation of NOW?



Yes. That was the line of thought that made me thinking what would happen with such a machine, and it lead to my conclusion that such a machine couldn't work, because it would immediately cancel itself out. Even if we could assume a machine that is protected against this effect, then it shouldn't work farther then the machine started to get to the point where the actual time reversing process would start, because I would have to take it with me, to get any further backward in time, but since I'm not protected against this, and the machine is, then it wouldn't work. Now if the machine would also protect me, where does it stop? What about the environment the machine is located in? How much of that would need to be preserved. That goes soon into the "Bigger then the universe" problem again. :)