Is Time Travel Possible?
20-Feb-2009

Geez, going from "Warm Up" to "More Challenging", in one question. My answer -- time travel to the "future" is possible, while time travel to the "past" is not.

To rephrase the question a bit, I define "time travel" with respect to a given individual; i.e., can a given individual travel into what he sees as the past or future?

Travel to the Future

The Theory of Special Relativity, discovered by Einstein, and as most eloquently described by the so called "Twin Paradox", gives us time travel to the future. This result isn't controversial. Basically, a clock in motion will run slower than a clock at rest (this has been verified by atomic clocks on aircraft), w.r.t. the frame of reference of the resting clock.

So, if I fly to Miami and back, I arrive back with my twin brother (as well as everything else) a little older, and thus have moved into my future. The effect is not noticeable on a commercial aircraft, but would be if I flew at speeds closer to the speed of light (traveling at about 86% of the speed of light, I would age about half as fast as my twin, so if the trip took 5 of my years (10 Earth years), I'd move 5 years into my future, and all things associated with time travel to the future, such as 10 years of interest accruing on my bank accounts, would be true after the trip).

A couple of notes. This is not "instantaneous" time travel. It becomes "more" instantaneous the closer to the speed of light you travel, but the question doesn't ask us to consider instantaneous time travel (FWIW, I find no evidence for the possibility of instantaneous time travel; I'm not sure if the Prof addresses this or not. And as in previous answers, I am not appealing to metaphysics).

Secondly, this explanation posits the possibility of building a machine that can travel near light speeds. In Question 3, I argued true virtual reality was not possible because complexity theory indicates that it impossible for the necessary machine to exist. It is possible that that a similar objection could be raised here. I don't know if there are theoretical objections to the possibility of such a machine traveling at meaningful percentages of the speed of light. But, as time travel has been demonstrated on aircraft, it is not really necessary. We are not asked to discuss degrees here. The last question, however, by its nature, required complete virtual reality, various degrees of it were not acceptable.

Travel to the Past

As for the past, I find time travel to the past impossible because the past does not exist, nor will it ever exist. There is nowhere to travel to.

Imagine being on a speeding train that passes under a highway. After it passes, you desire to go back to when the train passed under the highway. It simply never happens. You can't go back there (OTOH, you could take a very fast plane ahead to where the train passes under a highway in its future).

In physics, this is called the Arrow of Time. Conventional physics postulates that time flows from "the beginning" towards "the future", unidirectionally. It cannot be reversed. The irreversibility of time is based in part on The Second Law of Thermodynamics which basically says a closed system (such as the universe, as postulated in Question 1), always degenerates forward. In other words, you can't unscramble eggs (as Bertrand Russell put it). There is no evidence to suggest that the Second Law and the Arrow of Time are wrong. We will not be appealing to metaphysics.

We could speculate on undiscovered laws and physical principles. Perhaps time is actually a circle, and it will cycle back on itself, meaning a leap really far to the future will actually be a leap to the past (I believed this when I was a kid). We could postulate some sort of "quantum of time"; the smallest indivisible atomic increment, that time clicks along at, and that at each click, the state of the universe really exists "outside of time" and independent of our perceived movement of it. If so, it would seem theoretically possible to go to the past, as it would actually exist. We could speculate on any number of things like this, including a god that can teleport us to any point in time (and, as in the case of the past, make it exist). In this sense, any sort of time travel is possible, but there is no evidence to support any of these speculations, thus there is no reason to believe they exist.

Logic, not Physics

I have argued this from the point of view of my understanding of physics. The Prof implies he will be considering the logical arguments in this space. I'm not sure I'm up to that. I'm not sure of what they are, and part of the ground rules are to not look it up or do direct research. (And one other note on the physics -- the Theory of Special Relativity is tricky, and I have simplified things by assuming a stationary frame of reference on the Earth. Of course, the Earth is also moving, and perhaps we need to think about time travel relative to other points in the universe. I really don't think it matters, but my grasp of the physics isn't strong enough to be sure).

One of the arguments you hear against travel to the past is that if it were possible, why haven't people from the future come to visit us? This is a pretty stupid argument. First of all, how do we know they have not? How do we know we won't see evidence of these travelers next month? Why are we so sure society will evolve to this level of technology, even if the technology is theoretically possible? We could destroy ourselves next month, the day before the critical breakthru is made.

What about "time travel paradoxes"? You know, the one about going back in time and killing my grandfather, thus meaning I would never be born to do the deed. Well, the easiest way to dispense with this is my already argued belief that the past does not exist, thus such deed is impossible, resolving all such paradoxes. In fact, the existence of such thought paradoxes reinforces the idea that the past does not actually exist.

I don't have a good answer to time paradoxes like this. I'm more comfortable with the idea of positive paradoxes, such as a member of a race going back in time to found that race, as opposed to negative ones, such as the grandfather paradox.

It is easy for me to answer these questions, tho, as I believe free will is an illusion. In a system without free will, the will to act out this impossible grandfathericde would never exist, as it is against the laws of possibility. This is a pretty lame answer, but it is the best I can come up with.

Well, the questions are clearly getting harder. Time to see what the Prof thinks. I love this topic, yet it is one I am very poorly read in. As for the last question on brain in the vat, I liked it, but I don't think I wrote it up all that well. B+ for the philosophy, C for the writing on Question 3. I would have liked if the Prof spoke to the issue of reality for the vat masters and the vat, and the complexity theory surrounding the VR computer.

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