Time travel

Theoretically, the only way that I have come up with time travel would be to travel faster than the speed of light. The speed of light is relevant only because it measures “time” as we know it. Nevertheless, would travel at such high speeds actually allow for time travel? What would actually happen, if one would travel faster than the speed of light (besides the obvious speed thing)? Is there an alternative way(s) to time travel?

I don’t know if this helps, but you could do some research on the first neutrino telescope that has recently become operational. As I understand it, this is relevant because it could prove the existance of multiple other undiscovered dimensions, opposed to our known and defined 4 dimensions, by proving the existance of microscopic black holes that “rain” all over the universe, including the earth. This is all theory, but it’s intriguing nonetheless. Apparently the scientists working on it are attempting to find why gravity is not as strong, relatively, as the bond between the nucleus of an atom and its electrons. Good luck with the time travel though buddy.

What makes you think that ‘faster than light’ is a possibility, or even a meaningful concept at all, even if it were possible, what makes you think that it would cause time travel and what do you mean by time travel anyway?

I guess it all depends on how you define “travel faster than light”. The most “plausible” (I use the term loosely) possibility for time travel involves wormholes. Now, wormholes, as described by curent theories, don’t actually allow FTL travel… they just act as a “shortcut”, so you can get somewhere before light does without actually having to travel faster than it.

Of course, creating a wormhole has its own problems… sure, get a crapload of negative mass/energy and throw it together, then accelerate one end of the wormhole away from the other, and voila! Time travel! And then you need all sorts of other Funkytown technologies, like the ability to “stretch” the opening of the wormhole… but I’m sure if you just modulate the quantum subspace transponder frequency dilation compensator, it’d work like a snap. And, oh yeah, you can only travel back in time to any point subsequent to the creation of the time machine…

We just hashed this out in the “Great Debates” forum, under the thread ‘time travel’. I’ll say what I said there: google for ‘Tipler cylinder’, a theoretical time machine proposed by mathematician?physicist? Frank Tipler.

I am not a physicist, and there are lots of threads on SMDB on time travel, but here’s my two bits.

I’d like to challenge the idea that time travel only occurs faster than light.

You’re always time travelling.

You’re travelling forward at the temporal rate of one second every second. In fact, relative to you, no matter where you are in the universe or how fast you’re moving, to you, you’re always travelling at one second per second.

So obviously we’re talking about time travel that has your personal time moving differently than someone else.

Well, head away in a rocket ship at nearly the speed of light for a year, and then turn around and come back. While you still will have personally passed through time at a rate of one second per second, relative to you the Earth and its occupants will have passed the time away at a much faster rate (say, a few seconds or even minutes for every one of your seconds). Of course, the occupants of Earth will disagree, thinking they’ve moved through time at one second per second, and you’ve been moving through it more slowly.

This is highly complicated by the fact that if you watch Earth continuously during your trip, you’ll both think each other is going slowly, so you’d think you’d arrive back at Earth thinking the same amount of time had passed (the Twin paradox) so my entire above paragraph assumes that you only pay attention to the average rate of time passage over the whole trip (don’t want to turn this into a dissertation on Special Relativity and the Twin Paradox).

In the above scenario, you’ve time travelled forward into the future relative to where you would subjectively be.

If you were to turn this around and send the Earth away at an amazing speed, and then turn the Earth around and bring it back, the opposite is true (again, the Twin paradox, but we’ll gloss over this point). You end up on the Earth in the subjective past from where you would be if you’d stayed on Earth.

But this really isn’t ‘science fiction’ time travel where you go back into the past really. For one thing, you can’t go back further than when you start the experiment (because you’re really only advancing yourself, or the Earth, at a different rate, but it’s still all forward).

The problem with true ‘backward’ time travel is all the annoying little niggling details of paradox. However, even backward time travel may be allowable by way of orbiting wormholes. In a rather deft bit of logical cleverness by physicists, one end of a wormhole orbiting another can provide a gateway to the past without requiring any speed of light problems to arise. The trick? Well, it couldn’t be used as a gateway to the past to before the wormhole existed, but this isn’t a big deal if the wormhole has existed for 10,000 years – it would still give you a 10,000 year potential window for travelling. The other catch though is more extreme. It must be impossible for any information from the wormhole-end-in-the-past to get to the wormhole-end-in-the-future the long roundabout way. This would enable you to avoid such ridiculous results as travelling through it once only to go ‘the long way around’ and stop yourself from travelling through it the first time. This means the past end has to be at least 10,000 light years away if it is a gateway 10,000 years into the past.

This, of course, presumes the existence of wormholes, but we’re talking about time travel, so I hope I’ll be forgiven.

It is a fascinating, if somewhat speculative topic.

On a more light-speed-related note, the problem is the energy requirements to achieve near-light-speed. In fact, to achieve light speed for any actual non-zero mass (like you) would take an infinite amount of energy. This is highly important. In layman’s terms (all that I can handle), your apparent mass relative to wherever you started from appears to increase as you approach the speed of light. Time issues aside, this means it takes more and more energy as you get faster just to increase your speed. You don’t see this at low speeds, where you can add x energy to add y meters-per-second to your speed. But as you get faster, you need 2x energy to add y meters-per-second, and then 4x energy to add another y meters-per-second, and so on. (Very simplistic view). Sooner or later, the energy requirements just ramp off to infinity.

What this means is that while travelling faster-than-light might imply backward time travel mathematically, there’s no way for us to get a mass past light-speed to test it. I suspect you’d look like you were simply going forward-in-time, but backwards-in-direction. Everything else’d look that way to you, if your eyes and brain and relevant signal processing all operated correctly in this warped state.

So the speed of light isn’t just relevant because of time. Time and space combine in such a way to make it impossible to actually achieve light’s speed. We may someday get close, or find a highly localized way around it, but for now, it’s just an impenetrable constant of the universe.

Cool, I’ll have to go check out the other thread.

The master speaks.

prove that time exists - then prove that the speed of light measures time

Proving time exists is fairly trivial – you posted your messages after everyone else, so there’s a sequence of events, constituting time.

If you don’t buy that argument, well, there’s not much point in having a discussion about time travel. The OP presumably assumes time exists, so bringing up its tentative existence doesn’t give us anything.

As for the speed of light measuring time, it doesn’t. It only places a limit on propogation of information across a given distance relative to a non-inertial frame – and you can measure the passage of time in all sorts of interesting ways if you’re moving relative to that frame.

A book that may be relevant to the discussion:

How to Build a Time Machine, by P. C. W. Davies.

If we were to shoot a capsule into a cometary orbit around a black-hole, we could slingshot the capsule around the hole close to the event horizon and along curved space to re-emerge out of the gravity-well of the black hole into real space at a different point in time than what it should be based upon when we launched the thing, what trajectory, its speed, etc. It has to be an orbit which will eventually escape the gravity of the black-hole for obvious reasons. This would allow us to travel in time, but it is so unpredictable at this stage in the game that we would not be able to tell in what direction we travelled in time, or how far we went.

I’m sure that faster than light is not possible. But if we ignore that little inconvience, an object traveling faster than light in one reference frame will be traveling backwards in time in some other reference frame. Traveling faster than light is time travel.

Well sure, but if we’re going to ignore the basic properties of the universe (as we understand them), then we might just as well say that time travel can be achieved by heading for the second star to the right and straight on 'till morning…

Actually, you can have something travel faster than light. The catch is that it cannot be a material object. The contact point of the edge of a long pair of scissors can move faster than light. If the edges are parallel, the edge moves at infinite speed. If the contact point is moving faster than light in one frame, there are frames where the contact point moves in the other direction – backwards in time.

Isn’t that just the same as saying that 123456789 is 987654321 backwards?

Here’s the catch… you can’t use your scissor-contact-point to communicate information faster than light. You’d have to have all portions of the long pair of scissors already closing (synchronizing them across their length beforehand for a future period in time, which would be a signal sent at the slow and pedantic speed of light).

So this doesn’t really help you either, time or space travel wise. :slight_smile:

…thinking a little more…what I think I meant by my last post in this thread is:

If it fire a gun away from myself, from the point of view of the bullet approaching me, time is going backwards, but that’s not terribly helpful because the reference frame of [the bullet approaching me] is entirely imaginary.

There is one other ‘minor’ problem with travelling faster than the speed of light (for the moment assuming you could actually do this impossible thing).

Anything going faster than the speed of light cannot EVER go slower than the speed of light. It’s as if the speed of light is a wall…once you are on one side or the other of that wall there is no switching to the other side.

do you have any cite for that Whack-a-Mole? I am curious…

Also, to the many that I often see saying “You can’t go faster than the speed of light” and “The speed of light is the speed limit of the universe”, etc. I propose this:

(not directly quoted from another poster but adding to their proposal) Several hundred years ago, we “knew” that the earth was the center of the universe; a few hundred years ago, we “knew” that the sun was the center of the unvierse; what if right now we “know” that we cannot go faster than the speed of light?