I've read a good number of FTL equals time travel questions and answers both here and on other sites, but for the life of me I just do not get the connection.

Let's take a really simple case: say at 12:00 I teleport to some place 30 light minutes away from Earth. If I then pull out a telescope and take a look at Earth I will see the past from 30-ish minutes ago, but if I then teleport back to Earth I'll be back at 12:01 or whatever. I'm not going to end up in the past just because I was looking at it a minute ago. And if I go to a place where time flows differently due to gravity or some other time shenanigans there might be a mismatch, such as spending only 10 minutes there but returning to Earth to find years have passed (or vice versa) but either way it'll still always be in some sort of future.

I read several explanations involving "light cones", still don't quite get what they are (so if you bring them up, imagine you're talking to small child) or why should I even care about them, if I have a teleporter then the light comping from an object is not the most up-to date information I can get about it.

Several articles also brought up a "light on a train" thought experiment where a light is flashed in the middle of a moving train and is picked up by light detectors, and a stationary observer and an observer on a train moving in opposite direction will see the detectors going off in different order which is supposed to prove something about different frames of reference. But the same thought experiment can be done with sound instead of light and it would have the same results, but as far as I know it's quite possible to break the sound barrier without time travel.

Anyway so the question is: assuming a magical teleporter exists is there any logical reason that it has to lead to time travel? And if so, what is the exact mechanism?

  • $\begingroup$ You and some may be over thinking this by a huger amount. FTL/Teleport from point a to point b is not time travel. Teleport implies mater to energy conversion then energy to mater conversion. Worm Hole would energy be more likely to survive the journey or would mater be more likely. Current physics does not treat time as having a direction. Hence Worm Holes are often used as the method for Time Travel. While FTL is hard to sell, I suspect a FTL Drive being a PHYSICAL Time Travel Device would be even harder to sell. Just my 2cps in case it helps you tune your question. $\endgroup$ Mar 24, 2017 at 17:12
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    $\begingroup$ Written specifically to address this issue, for this site $\endgroup$
    – Joe Bloggs
    Mar 24, 2017 at 18:18
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    $\begingroup$ Also: Duplicate question, but on a different stack exchange $\endgroup$
    – Joe Bloggs
    Mar 24, 2017 at 18:19
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    $\begingroup$ I think the problem here is the assumption that teleportation is instantaneous. For a "real" teleporter, teleporting to a location 30 light-minutes away takes 30 minutes (as seen by an outside observer, though to the person being teleported it appears to take no time). That is, if Alice teleports to a place 30 light minutes away, and Bob back home is watching the spot through a good telescope, he sees Alice appear 1 hour after she left. And if she immediately punches the return button on the teleporter, she's standing next to Bob a few seconds after he sees her appear at the destination. $\endgroup$
    – jamesqf
    Mar 24, 2017 at 23:25
  • $\begingroup$ It all boils down to definition of simultaneity. What is simultaneous and what is not, depends on your velocity. Most simply speaking, assume that with FTL you can jump to any simultaneous place. So you jump from A to B and at B modify your velocity so that past of A is now simultaneous to you, and jump to that point. If your FTL is less instantaneous, all that changes is distances you need to jump, and velocities you need to reach, but this effect will be manifestable in ANY form of FTL, no matter how infinitesimally FTL your FTL is. $\endgroup$
    – M i ech
    Mar 25, 2017 at 2:52

7 Answers 7


This is one of the best explanations that I've ever read, courtesy of Wikipedia. It deals with FTL communication rather than strictly teleportation but the principle is the same.

Numerical example with two-way [faster-than-light] communication

As an example, imagine that Alice and Bob are aboard spaceships moving inertially with a relative speed of 0.8c. At some point they pass right next to each other, and Alice defines the position and time of their passing to be at position x = 0, time t = 0 in her frame, while Bob defines it to be at position x′ = 0 and time t′ = 0 in his frame (note that this is different from the convention used in the previous section, where the origin of the coordinates was the event of Bob receiving a tachyon signal from Alice). In Alice's frame she remains at rest at position x = 0, while Bob is moving in the positive x direction at 0.8c; in Bob's frame he remains at rest at position x′ = 0, and Alice is moving in the negative x′ direction at 0.8c. Each one also has a tachyon transmitter aboard their ship, which sends out signals that move at 2.4c in the ship's own frame.

When Alice's clock shows that 300 days have elapsed since she passed next to Bob (t = 300 days in her frame), she uses the tachyon transmitter to send a message to Bob, saying "Ugh, I just ate some bad shrimp". At t = 450 days in Alice's frame, she calculates that since the tachyon signal has been traveling away from her at 2.4c for 150 days, it should now be at position x = 2.4×150 = 360 light-days in her frame, and since Bob has been traveling away from her at 0.8c for 450 days, he should now be at position x = 0.8×450 = 360 light-days in her frame as well, meaning that this is the moment the signal catches up with Bob. So, in her frame Bob receives Alice's message at x = 360, t = 450. Due to the effects of time dilation, in her frame Bob is aging more slowly than she is by a factor of [formula], in this case 0.6, so Bob's clock only shows that 0.6×450 = 270 days have elapsed when he receives the message, meaning that in his frame he receives it at x′ = 0, t′ = 270.

When Bob receives Alice's message, he immediately uses his own tachyon transmitter to send a message back to Alice saying "Don't eat the shrimp!" 135 days later in his frame, at t′ = 270 + 135 = 405, he calculates that since the tachyon signal has been traveling away from him at 2.4c in the −x′ direction for 135 days, it should now be at position x′ = −2.4×135 = −324 light-days in his frame, and since Alice has been traveling at 0.8c in the −x direction for 405 days, she should now be at position x′ = −0.8×405 = −324 light-days as well. So, in his frame Alice receives his reply at x′ = −324, t′ = 405. Time dilation for inertial observers is symmetrical, so in Bob's frame Alice is aging more slowly than he is, by the same factor of 0.6, so Alice's clock should only show that 0.6×405 = 243 days have elapsed when she receives his reply. This means that she receives a message from Bob saying "Don't eat the shrimp!" only 243 days after she passed Bob, while she wasn't supposed to send the message saying "Ugh, I just ate some bad shrimp" until 300 days elapsed since she passed Bob, so Bob's reply constitutes a warning about her own future.

My understanding of what is happening here is that, for both Alice and Bob, the other person is "aging more slowly" due to the effects of time dilation. This effect is symmetrical, so somewhat counter-intuitively, Bob appears younger to Alice at the same time that Alice appears younger to Bob.

Since each person exists in the "past" of the other - appearing younger in age - if you send a message from one to the other then it will get there before you sent it from your frame of reference. Replace "FTL communication" with "teleportation" or even just "FTL travel" and the paradox is the same.

I believe in the above example, Alice's and Bob's given velocities are so fast (0.8 C) in order to exaggerate the effects of relativity and make it easier to grasp. But the paradox still holds for trivial velocities.

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    $\begingroup$ Welcome to the site, jered. Great first answer for a newbie! Deserves a +rep indeed. We would appreciate a lot if you could invest more energy to your answers and include some detail by yourself, instead of just copying information (you have copied useful and precise information btw) from other channels. Have a very nice and productive stay at World Building :) $\endgroup$ Mar 24, 2017 at 20:29
  • $\begingroup$ @YoustayIgo Thank you for the feedback. $\endgroup$
    – jered
    Mar 24, 2017 at 20:49
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    $\begingroup$ Is this kinda stuff happening basically the reason we can't have nice things (i.e. FTL)? $\endgroup$ Mar 25, 2017 at 0:30
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    $\begingroup$ @htmlcoderexe precisely why. When Einstein said that going faster than light implied time travel he wasn't just saying why we couldn't detect or see FTL travel (as it would immediately convert into a temporal speed combined with some STL speed). He was saying that the mathematics of anything FTL implied contradictions with the earlier assumptions. In essence, he proved there was nothing faster than light by assuming there was such a particle and then going on to show that by his model of physics there was a mathematical contradiction. I.E. "time travel". $\endgroup$
    – user64742
    Mar 25, 2017 at 1:30
  • $\begingroup$ But is that relative aging sound? Time dilation is a result of absolute speed difference with the speed of light. $\endgroup$ Mar 25, 2017 at 8:46

The issue is that, thanks to relativity and everything it brings, things that occur simultaneously in one reference frame do NOT occur simultaneously in all reference frames (or even most of them). The "light on a train" thought experiment is just an effort to prove this, but we'll hold off on that and accept it to be true.

Let's say I have two synchronized clocks on Earth and Mars. If we have a spaceship travelling fast enough in the direction from Mars to Earth, the clock on Mars will appear to be running behind the Earth clock. The faster the spaceship is traveling, the farther behind the Mars clock will appear (hours, days, even YEARS if you're going fast enough).

So if you decide to teleport (instantly) from Earth to the spaceship, then to Mars, you'll get to see how slow that clock was running. But weren't the clocks synchronized from the perspective of Earth and Mars? So you just teleport back to Earth (instantly), and you've arrived back where you started, but in the past.

I emphasized that the Mars clock could appear years (millenia, any amount of time) behind the Earth clock so you know that the time it takes for information to go between Earth and Mars (like with a telescope) doesn't really matter.

Now, I'll resummarize the "light on a train" thought experiment, just using Earth/Mars again. Suppose we have a broadcast tower halfway between Earth/Mars, airing some TV show at exactly 5PM. Again, the clocks on each planet are synchronized. The boadcast tower is placed exactly halfway so everyone gets to enjoy the show at the same time.

But suppose we're on our super-fast 0.99c spaceship. We're traveling near lightspeed from Mars to Earth. From our perspective we feel quite stationary though, and it's Earth and Mars and the broadcast station that are zooming past us at near lightspeed. Because light travels the same speed in every reference frame (which is not true of sound), from our perspective the broadcast will struggle to reach Mars. Because Mars appears to be running away from the broadcast at 0.99c and the broadcast can only travel at c, it would take 100x longer than usual for the signal to reach Mars. Earth appears to receive its signal almost twice as fast from our perspective. But since we know that the show starts at 5pm, the signal must reach Mars at 5pm, and so Mars' clocks must be running slow. But only from our perspective.

Final note, I believe you can have instant teleportation without allowing time travel if you agree on a single reference frame (say, our solar system) to be able to teleport instantly within. The only consequence of this is that teleportation won't appear instant from all reference frames. From the perspective of our same spaceship, people teleporting "instantly" from Earth to Mars will take quite awhile, and people teleporting from Mars to Earth will show up before they left.

EDIT: I think I've messed up some numbers. The time difference between Mars and Earth wouldn't be anywhere near 100x thanks to length contraction and time dilation and such. But aside from the actual numbers, I think everything else remains correct.

  • $\begingroup$ Lord Frith 5 down in this thread has another version of the causality violation produced by teleportation here. arstechnica.com/civis/viewtopic.php?t=1138124 It is all about hopping from one reference frame with one time speed to another one with a different time speed. Heady stuff. Welcome Geoffrey! $\endgroup$
    – Willk
    Mar 24, 2017 at 18:47
  • $\begingroup$ @Will I don't believe that example is correct. Both Alice and Bob should agree that 4 years have passed on Earth. It's just that Alice will only have aged 4 days while Bob has aged 4 years. This is because time on Earth seems to rapidly accelerate when Alice slows down on approach to Alpha Centauri. $\endgroup$
    – Geoffrey
    Mar 24, 2017 at 18:52

FTL does not lead to time travel if there is a preferred absolute frame of refererence -- an "Ether" equivalent -- against which FTL (or instant transportation) is measured.

Lacking that, FTL ends up being relative to your current frame of reference. And when you move "normally", time and space being to bleed into each other a bit via a rotation like mechanism.

You say "the same time" at two far apart locations, but how do we measure it? In relativity, there is the future (things light can reach from now), the past (things light could have reached you from), and elsewhere.

If we define a "at this moment" plane through space-time, when you move the angle this plane cuts through space time changes. If you move towards Alpha Centauri, the "same time" there moves backward in time relative to being "stationary" relative to Alpha Centauri. So you send a message to Alpha Centauri "instantly" in your current frame, and then they send a message back, and it arrives in your past.

We can avoid this by inventing an absolute frame of reference against which "the same time" is measured. Doing so would require rebuilding most of physics and somehow deriving our current physics as a special case.

  • $\begingroup$ Hopefully said special case is not due to a false vacuum... $\endgroup$
    – wizzwizz4
    Mar 25, 2017 at 7:11

The problem is that simply knowing what time it is on Earth doesn't let you know what time it is 30 light-seconds away, because the answer to that depends on your inertial frame of reference. It could be anywhere between 11:59:30 and 12:00:30 (from the Earth's point of view, which isn't necessarily the same as yours, because you may be moving with respect to the Earth). So you can enter a frame of reference where 12:00:00 on Earth is 11:59:45 at your destination, and teleport there. Then you spend 10 seconds changing your frame of reference, after which it's 11:59:55 where you are, but now it's 11:59:40 on Earth because of your new frame of reference. Then you teleport back, arriving 20 seconds before you left.


This is a fun question to consider. I look at it this way: both time and distance travel must be perceived in order to be measured, and the measured result is going to depend on from whose perspective you choose to view.

If you are the one perceiving the change in time, you will always be measuring from your own now. Even if you go back into your own past, you are also in your own present, because you remember your past the first time you perceived it. If you are traveling alone, no one except you can measure time from your own perspective.

Someone observing your change in time to your own past where you do not return, will perceive you disappearing from their own time. If you return to your time starting point, they will not be able to perceive any change in time existed for you at all and it well be as if nothing happened, even though for you, you experienced some change in time.

Distance. Same conditions apply for distance as time, with one important difference: you cannot physically be at a previous place and subsequent place at the same time like you can in time travel. Therefore, you would think both you and an external observer can perceive your change in distance at the same time with the same results. We know this to be untrue though because of the limitations of the speed of light and that with two different points of perspective, you each will make different measurements of distance you traveled. The distance traveled from your perspective will be different than that of the other observer. Einstein called this the relativity of simultaneity.

Explanations of your question proposed that say you reappear in the past are only seeing the answer from the perspective of someone perceiving from the past point in time. You, measuring from your own timeline, will know that you are in your present, which is sometime in your future as compared to your starting measurement point.


FTL travel IS time travel, as seen from some observers. Consider this experiment.

Joe is a peaceful fellow, but he sees a TV program that tells him that people on Mars have cooties. Joe is now radicalized, so he puts a bomb in his FTL teleporter, sends it to Mars, and blows it up.

Sally, an observer in a spaceship floating above Mars sees this sequence of events: first a bomb appears, destroying Mars, and then, later, light from Earth reaches her, showing Joe watching the TV program. Joe's machine appears to send the bomb back in time.

This is why FTL and time travel are deeply irrational concepts--if they exist, they destroy causality, and a large portion of rational thought and science stop making sense.

This is the meaning of relativity: there is no special, privileged reference frame. We are on a planet that is moving, around a star that is moving, and the galaxy is moving. Nothing is standing still. Causality means that every cause comes before its effect, no matter who is observing it. This must remain true for all observers, no matter how they are moving. This is only possible if nothing moves faster than light.

Joe's FTL teleporter vanishes in a puff of logic. Now he has to send his bomb on a rocket traveling slower than light, and Sally will see the rocket launch before Mars explodes. Everyone will see that the cause happened before the effect.

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    $\begingroup$ Note that just because something destroys causality or rational thought does not mean it isn't true. A lot of quantum physics stuff doesn't seem rational. $\endgroup$
    – Zan Lynx
    Mar 25, 2017 at 21:46

The simple truth is in order to invent a time machine you also have to invent a teleportation device.

Here is why.

Supposing you invent a time travel machine, and set it up in your living room, and decide you need to go back six months because you forgot your anniversary and the wife has been giving you heck ever since.

So you get in, dial back the date, sit back and press GO.

POOF you jump back six months. Then you immediately die. Why?

Because six months ago the earth was on the other side of the sun. You went back in time but not in space. You re-emerged in a vacuum.

So, if you invent a time machine you need to not only move in time but also reappear at a different location... aka.. teleport.

OK so what about the other way around.

If I want to teleport from say the moon to the earth and I look up from the moon and see the spot I want to land. I can't just dematerialize and re-appear at that spot. Why? Because the spot I see is not in the same time frame as I am. Light takes time to reach me. So the earth has moved and turned a little bit more than what I can see or measure. If I want to land on that spot I have to move through space and also go back In time a little to "back up the earth".

Of course all this is even more complicated by the fact that you have to match arrival velocities and need incredible accuracies to not materialize 1mm too low.

  • $\begingroup$ I think it's all wrong. Earth movement isn't an absolute thing; Earth only moves from a certain point of view (that's why people didn't believe in it for a long time). The room, the time machine in it and you are resting in a frame of reference bound to the Earth. The time machine has same space coordinates now and 6 months ago relative to that frame of reference. Why would it send you to some different coordinates? $\endgroup$
    – avek
    Mar 25, 2017 at 9:21
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    $\begingroup$ @avek The reference frame of the Earth is non-inertial (it's orbiting the sun!), so over a long enough time it does move with respect to any arbitrary inertial reference frame, so you're not really 'at rest', except in your instantaneous inertial rest frame (which is always true, by definition) $\endgroup$ Mar 25, 2017 at 9:37
  • $\begingroup$ @avek. That's the point, You can not just move in time, despite what Hollywood shows us. Six months ago, even six seconds ago, we were not here. We have moved in space since then. So your time-coordinates must ALSO include a space-coordinate. Perhaps you want to think of it another way $\endgroup$
    – Trevor_G
    Mar 25, 2017 at 16:20
  • $\begingroup$ @avek . You discover the earth is about hits an alien space vessel that happened to be floating in its orbit path. It's too late to discover what it contains. SO you decide. lets go back a day so the earth is not "HERE" yet and visit the ship. Then you would WANT to go back in time without moving in space. $\endgroup$
    – Trevor_G
    Mar 25, 2017 at 16:21
  • $\begingroup$ This is a comment tangental to the topic, not an answer! $\endgroup$
    – JDługosz
    Mar 26, 2017 at 9:38

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