Imagine a spaceship capable of achieving faster than light travel uses several transporters to allow crew members to move around its interior at the constant speed of light. What happens to the crew using the transporter when the spaceship goes to warp 2? BTW warp 2 means twice the speed of light in this context. Is there a clever way to work around this issue or I need to update the operation manual correcting warp 2 to move only at 20% speed of light?

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    $\begingroup$ There is no conundrum. Surely you know that? Using an internal transporter only means travelling at lightspeed relative to the transporter starting and end points. This is irrespective of the motion of the spaceship -- either less than or faster than light. The clever way to get around the issue is to realise there is no issue. Nice question though. $\endgroup$
    – a4android
    Nov 30, 2019 at 4:59
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    $\begingroup$ Relativity is the key here - relative to the ship, the crew doesn't move faster than lightspeed. $\endgroup$
    – IronEagle
    Nov 30, 2019 at 5:30
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    $\begingroup$ It's not about light catching up. It's the same as man walking at 5 km/h on a otrain moving at 90 km/h, & he is m0ving forwards in the same direction as the train. he doesn't go backwards at 85 km/h. Because he is moving within the same frame of reference as the train, which is at relative rest with respect to him. He still walks forward at 5 km/h inside the train. $\endgroup$
    – a4android
    Nov 30, 2019 at 6:35
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    $\begingroup$ "Would Headlights Work At Light Speed" is a good watch. $\endgroup$
    – Schwern
    Nov 30, 2019 at 21:28
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    $\begingroup$ @GwenKillerby I know the difference between Gallilean and Einsteinian relativity. I know the trainwalker is moving at 95 km/h to an external observer. I merely didn't mention it. There may be better analogies, but this was at attempt to point out the faulted reasoning in the question. $\endgroup$
    – a4android
    Dec 1, 2019 at 23:44

5 Answers 5


The real answer is "whatever you want." We have no known physics for what happens past the speed of light, so you are writing your own rules.

However, if we are to use our own intuitive laws to try to write what happens faster than light, most of our laws of physics state that if every point that you care about is moving at the same pace, the laws of physics work as though they were all standing still. Thus you should expect the most natural behavior of light possible. No issues at all here.

If you wanted to work relativity in, light travels the same speed for everyone, so everyone on the ship should see light moving at the speed of light in their reference frame. (This unintuitive behavior is where effects like time dilation and space compression form).

The only thing which is "different" is that the whole contraption is traveling at fictional speeds. You write the rules here. However, if you want to draw from real physics, you can use supersonic aircraft as an excellent example.

In regions where everything is traveling at subsonic speeds, air behaves like a fluid obeying the rules of wave mechanics. Information of an event ripples outwards on these waves in a very intuitive way. Go supersonic, and the story gets more interesting. The air near the aircraft travels at subsonic speeds with respect to the aircraft. All the normal wave mechanics behaviors work close to the craft. The air far from the aircraft travels at subsonic speeds with respect to the ground (as you would expect). In between, there's a region a few nanometers wide where the fundamental assumptions of wave mechanics simply fall flat. The air behaves differently in this region. The result is a slightly more complicated version of a high pressure* front which handles all of the odd discontinuities between these two regions. This thin layer is what forms a sonic boom. This thin layer is also responsible for consuming an astonishingly large number of CPU cycles in simulation, so if predictability is something your characters need in your FTL scenarios, that could play a major part.

You could do similar. Have light travel "at the speed of light" everywhere except in a thin shock cone around the craft, where light does something different. It does whatever is required to stitch together the discontinuity that is caused by traveling faster than light.

After all, light isn't a particle. It isn't a wave. It's just light, and if you blink, you'll miss it!

* In this boundary region, "pressure" gets to be tricky. We typically assume that pressure is the same in all directions. This is critical for deriving the wave behavior of sound waves. In the bow shock of a supersonic aircraft, that breaks down. "Pressure," if you call it that, becomes a directional thing. Needless to say, that complicates things rather quickly.

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    $\begingroup$ 👍having done that, I should also mentioned that the only phenomenon to upset locality is entanglement so you are right I have to invent rules if my world accepts ftl. $\endgroup$
    – user6760
    Nov 30, 2019 at 7:45
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    $\begingroup$ If EM didn't behave the same inside an FTL ship, the "transporter conundrum" would be irrelevant because your brain wouldn't work, your computers wouldn't work, and as a matter of fact, molecules would fall apart and the ship would turn into a plasma. So you're better off just leaving it alone :) $\endgroup$
    – hobbs
    Nov 30, 2019 at 20:25
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    $\begingroup$ @user6760 - As long as you're making up your own rules, there's a very simple solution to any real or imaginary problems - if the transporter tech would be unworkable while the warp drive is on, simply have the main computer disable the transporters while that is the case. A plot point then could be someone disabling the safeguard and ending up as a splat on the side of the intergalactic highway. $\endgroup$
    – Vilx-
    Dec 1, 2019 at 16:30
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    $\begingroup$ +1 for "if you blink you'll miss it!" $\endgroup$ Dec 2, 2019 at 8:27
  • $\begingroup$ @user6760 Some physicists consider entanglement to upset locality. It probably doesn't, and there really isn't much of a reason to believe it does, beyond our intuitive understanding of the universe. But regardless, things like the Alcubierre drive or traversable wormholes don't require any special physics and do not violate locality - they work as solutions in general relativity (mind, that doesn't mean they're right solutions - corresponding to the universe we live in - but they don't violate GR, the lightspeed limit or locality). $\endgroup$
    – Luaan
    Dec 2, 2019 at 12:13

Warp 2, eh? Well, you're clearly using a warp drive, like some actually functional and practical descendant of Alcubierre's ideas. Such systems don't involve the ship travelling faster than light at all, but only the warp in space around it. That's why at sublight speeds the occupants of a ship using a reactionless space-warping drive don't experience any relativistic effects like time dilation.

It also means in this context that beaming at lightspeed from bow to stern of your ship is exactly the same with the warp drive on as it is off.


If whatever FTL trick you're using affected the inside of the ship, it wouldn't be very healthy for the crew. It's not just teleportation. Imagine you are walking forward or backward in the ship. What would happen to you if you could experience the warp speed you're travelling at?

Obviously, any transport method used necessarily mustn't be felt inside the ship. Your transporters, however they are working, will do so exactly as on Earth (which isn't standing still either, btw.).


Try not to think about it too hard. It's not possible in this universe to go faster than the speed of light, it is irrational, so justifying how transporters will work in a rational way is also not possible.

The answers you have here that say that it would make no difference are all completely wrong in RL btw. They are based on notions that only work at slow speeds and with objects that have mass. In a universe governed by relativity, one cannot add one's speed to the speed of light -- that is, if you are travelling at 1000kmh, then the light bouncing off you isn't doing c+1000kmh, it is still only moving at c. This is partially because of the nature of photos (they don't actually 'bounce' off things but are rather absorbed and reemitted) and a misunderstanding of what the speed of light represents (clue: it's named badly, it isn't a speed limit, it's actually about the fundamental nature of reality).

If I were writing a sci-fi book (and I am) I wouldn't have anything travelling faster than light, just like Alistair Renyolds' Revelation Space. Relativistic speeds are sufficient for most purposes. You can circumnavigate the whole universe in a little over 50 subjective years if you want, without breaking the 'speed limit'.

I don't have a problem with teleportation though. That's just an engineering problem, though a moral/philosophical one too, obvs. -- you die every time you step into a transporter, like in Dan Simmons' Hyperion Cantos.

The best way to write sci-fi is to stick to what's scientific. Otherwise it is just another boring fantasy novel. (not that you said that you're writing a sci-fi book, it's just that whenever I read about teleporters and faster-than-light travel I assume the author is aiming for sci-fi, despite being so far outside the realms of science that you may as well put elves and witches in it).


  • $\begingroup$ We can more or less handwave away the engineering challenges associated to both supplying enough energy to reach velocities with high $\gamma$ (just as an example "subjective 10c" means "objective 0.995c", to the extent that objective speed even makes sense) and having your ship not be destroyed while doing so. But even with that handwaving done, you still have a bit of a problem in a scifi setting that obeys relativity in the sense that you can reach a destination tens of lightyears away, but it will likely be completely different than it was when you left. $\endgroup$
    – Ian
    Dec 1, 2019 at 22:18
  • $\begingroup$ This question is not marked hard-science so real world science can be bent for the purpose of the story. The question is what happens within the constraints of the story, which are looser than the constraints of the real world. $\endgroup$
    – CJ Dennis
    Dec 2, 2019 at 2:09
  • $\begingroup$ I actually don't dislike this answer, even if the OP hasn't tagged hard-science, the question itself and OP's comments shows a consideration for some scientific accuracy. It's easy to fall in totally unrealistic schemes using FTL. I really loved "The Forever War" for this, even without FTL, there are already many problems that arise with relativity to cope with.. $\endgroup$
    – Kaddath
    Dec 2, 2019 at 9:53

Another approach would be to look at the current theories of "realistic" teleportation. They basically amount to the belief that it isn't viable to actually transmit matter like a Star Trek teleport, instead teleportation would work by mapping a body, killing you and then effectively printing a new copy wherever you want to be. At which point the speed you're moving doesn't matter.


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