The fastest thing that travels (as we know it) is light. But there are distances in our universes that even time takes billions of years to reach. So traveling at the speed of light is simply not enough. So, assuming that time traveling has been invented, what if we set out towards our destination (assuming that we know of a place where we can reach and can run some personal errands) and time travel a billions light-years ahead so that we can skip the journey time and be there. What are the possible arguments that prove that this is not possible?
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1$\begingroup$ Correction the fastest thing isn't light it is space if you can compress the space between the destination and yourself so that only by landing a single foot you are literally in 2 places at once this is approved by GR. $\endgroup$– user6760Commented May 2, 2015 at 13:01
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1$\begingroup$ Edit: The fastest thing that travels (as I knew it) is light. lol.. $\endgroup$– You_Shall_Not_PassCommented May 2, 2015 at 17:39
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$\begingroup$ @user6760 True, but I think the compression effect propagation would be limited to light speed. And compressing the space in between is probably indistinguishable from stretching the conveyance, which would probably limit interactions with your surroundings. So it isn't really different from travelling at light speed? Although travelling at light speed would actually be very useful and enough to make interstellar travel practical. $\endgroup$– Ville NiemiCommented May 2, 2015 at 22:40
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$\begingroup$ Check this out! $\endgroup$– user6760Commented May 3, 2015 at 7:21
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$\begingroup$ @user6760 Cool! I misunderstood what you meant, sorry. In my defense that would almost certainly destroy both your origin and destination, so it didn't occur to me to think of that as a mode of travel. Certainly would make for a convincing rationale to only jump well outside inhabited systems,which is usually a bonus. $\endgroup$– Ville NiemiCommented May 3, 2015 at 8:03
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6 Answers
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Assuming time travel and teleportation are possible, the scenario you mention is also possible.
First, let me note: a light-year is not a measure of time but of distance: it is the distance light can travel in a year, approximately $ 9.4608 \times 10^{12} \text{ km} $. If you're travelling at the speed of light, the relevant unit of time that it takes to cover this distance is just a year.
Travelling at the speed of light to reach somewhere billions of light-years away, as you say, takes billions of years. With the aforementioned technologies, it would be possible to simply jump forward in time those billions of years to get to the end of your journey straight away. However, you also need teleportation because your ship to get you there will not remain in the same place: it will have travelled billions of light-years, which you also need to travel to catch up with it.
However, if you have teleportation, I will argue that time travel is unnecessary - instead, you can just teleport yourself to your destination.
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$\begingroup$ I asked this question assuming that teleportation is traveling to distance as light or in the speed of light. I was wrong to have assumed that. If teleportation is something different, then you don't need time traveling for achieving this scenario. +1. cheers. $\endgroup$ Commented May 2, 2015 at 11:43
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1$\begingroup$ @You_Shall_Not_Pass "teleportation" tends to be defined in sci-fi as moving matter instantaneously from place to place with no displacement in time. That's not to say you can't use a different definition, but I answered using that because that seems fairly well accepted. $\endgroup$ Commented May 2, 2015 at 11:44
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$\begingroup$ Right.. "Without displacement in time".. $\endgroup$ Commented May 2, 2015 at 11:51
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$\begingroup$ If teleportation take splace without displacement of time (relative to the fram of reference of the origin transporter pplatform?) and can cover long distances (several light-years, say), then it is automatically usable as a method of backwards time travel. $\endgroup$ Commented May 2, 2015 at 17:05
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$\begingroup$ @HagenvonEitzen only if you're using actual science and not the sci-fi definition of teleportation. I'm using the latter. $\endgroup$ Commented May 2, 2015 at 17:25
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Time travel is not necessary.
The answer is because of time dilation. As the speed of the craft approaches the speed of light, to the outsider at the source or the destination the ship can travel no faster than the speed of light. However, to a passenger on the ship, the subjective experience of time is shortened. Thus the time elapsed for them during the journey can be made arbitrarily short, by traveling closer and closer to the speed of light. Of course the universe may look very different when they arrive.
Also, the space travel in the Ursula LeGuin's sci-fi books works like this.
If, however, you mean you want to get there FTL in the frame of reference of the outside observer, then, yes, time traveling at the end of the journey would work. Your example actually illustrates the close connection between FTL and time travel according to our current understanding of relativity.
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In addition to the other answers, you could consider the following.
If you time-travel but stay in one place, even going a day forward in time, you will find the Earth has traveled FAR and you're in outer space. The Earth, solar system, indeed galaxy - are all traveling remarkably fast from each other and with each other.
If your characters could get the extremely detailed precision of predicting when a destination will be in your place in space, you may jump forward, undertake what you need, and then jump backwards to home. But you don't get to pick any destination, just one that will be flying by your point of space at some point in time.
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$\begingroup$ @You_Shall_Not_Pass - thanks, it's something I always think about when people tell time traveling stories. If I go forward 1 hour, I will no longer be in Portland, but closer to New York :) Who's to say that's a bad thing. $\endgroup$– MikeyCommented May 6, 2015 at 21:43
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It's true that by traveling at nearly lightspeed, time dilation will result in the trip taking very little subjective time for the traveler. (At lightspeed, in fact, time ceases to flow, an interesting phenomenon enjoyed only by photons.)
The downside of traveling at, say, 0.999c is the horrific amount of energy needed to accelerate you to such a velocity (and an equally horrific amount of energy to decelerate at the destination). And even if you had the energy of a small star at your disposal, the amount of time needed to get to cruising speed, at an acceleration that won't reduce you to a puddle of salt water and sediment, is measured in millennia. (This, too, has to be doubled if you want to decelarate at the end of the trip.)
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$\begingroup$ I think the millennia-to-accelerate statement needs to be backed up. Constant acceleration of 1g felt by the passengers within the ship's reference frame, gives what graph of speed by t as seen from an outside observer? $\endgroup$– JDługoszCommented May 2, 2015 at 20:09
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$\begingroup$ upload.wikimedia.org/math/8/9/7/… $\endgroup$– JDługoszCommented May 2, 2015 at 20:14
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$\begingroup$ Back of the envelope, I get 0.7c after 1 year, .89c after 2 years, and .995c after 10 years. $\endgroup$– JDługoszCommented May 2, 2015 at 20:26
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$\begingroup$ For easy reference, that Wikipedia Math link posted by @JDługosz is $ x(t) = \frac{c^2}{g} \left( \sqrt{1 + \frac{\left( gt + v_0 \gamma_0 \right)^2}{c^2}} - \gamma_0 \right) $ $\endgroup$– userCommented May 4, 2015 at 9:13
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$\begingroup$ Starting from rest, and plugging in values, what I used is $ x(t) = \frac{ \left (3\times 10^8 \right) ^2} {9.8} \left( \sqrt{1 + \frac{\left( 9.8 t \right)^2}{\left (3\times 10^8 \right)^2 }} \right) $ so it's clear that you take the normal Newtonian formula and then back off by scaling by a factor which also grows. By coincedence, $t$ of one year gives about 1 $ c $ by Newtonian accelaration. $\endgroup$– JDługoszCommented May 4, 2015 at 17:09
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I just had a thought: if you can choose a reference frame that is extreme in making "your space" into "my time", then time travel will translate into spqce travel, directly. That sort of thing happens near black holes.
Blue is a different reference frame than black, on a s-t diagram. Then I added red to be even more extreme: the time axis approaches a 45° angle. On this relativistic moving ship, he jumps back in time.
From the rest frame, he jumped back quite a bit less in time and also jumped backward (along his direction of travel) in space.
That makes sense: he returned to an earler point on his world-line, which occupied a different place (from our point of view). He was there the when he existed at that time, and will be there again when he revisits it.
But, it works with the projected (current) motion even if he returns to a time before he adoped that velocity. You would have to, otherwise the position is already occupied by your previous self.
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Other's have covered this nicely except for one point I wanted to make clear.
According to our current understanding the physics of our Universe, if you have FTL travel, you necessarily also have time travel.
It seems to me that it also makes the reverse true too.
