In my story, I have two huge generation ships racing each other to another star system. Assume they are on an exactly parallel path, neck-and-neck, about one astronomical unit apart. They are traveling at exactly the same speed - point eight cee. They know the other ship is there, somewhere. They launched at the same time, although from separate in-system facilities.

The form of drive is important. They are traveling in a modified Alcubierre drive - that is, using the vernacular, they are in a space-time bubble that preserves relativistic effects within the ships. The drive creates a low-pressure space-time area in front, and a high pressure space-time area behind. The drives also interplay with the Higgs field to reduce the effects of inertial mass. Thus, the bubble is effectively 'surfing' through space, being pushed by the high pressure from behind. Sort of like an airplane stays in the air. There is no intention to use the system to go faster than the speed of light, just close to it.

Because of this, the ships are huge. They have huge fusion reactors, and a complete physics laboratory. Power is not an issue. They each carry tens of thousands of people, and have academic institutions that train new physicists born on the ships. Think in terms of a complete city. Knowledge and the ability to build new equipment is not a problem.

The question is, traveling at such extreme speeds, wrapped up in a bubble, can they communicate with each other? If so, then how?

Because they are adversaries, there are no entanglement possibilities between them.

Some additional background to consider - light takes about 500 seconds to go one AU. The trick is not in the relative speed between the ships, but that they are both traveling at point 8 cee relative to where the light was first emitted. That is, there would be a Doppler effect as the light arrived at the other ship.

The point of protecting the inhabitants from relativistic effects is that I sincerely doubt biological processes could function in the relativistic effects of point 8 cee.

For consideration: Would there be a null point in the bubble between the front negative pressure and the rear positive pressure that could be used?

A point of physics. When you project a beam of light from a spaceship, the point of origin effectively drops dead. That is, the beam origin does not gain or loose momentum from the spaceship. At the point in space-time where you 'drop' it, it stays there. Perfectly dead in the - well - space-time. The ship continues on. It does not 'follow' or 'trail' the ship. If you beam it behind you, then you travel away from the originating point of the beam, the point of origin just hangs there, and the speed of the light is determined relative to that 'dead drop'. Not the speed of the ship. Not the speed of anything around it. If it is a completely uni-directional laser beam, and it comes out the extreme rear of the ship, the ship occupants will never be able to detect it unless they come around. Baring gravity, it goes in a perfectly straight line from the original point of origin in space-time.

Please Note I use point 8 cee because c is constant in all inertial frames, but the speed of a spaceship certainly isn't. I use cee as a unit of speed, as in miles per hour.

  • $\begingroup$ Their relative speed is ≈0±0.1c or sth like that, right? $\endgroup$
    – Mołot
    Dec 27, 2017 at 12:48
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    $\begingroup$ Just a little nipticking : why wouldn't you preserve relativistic effects within the ship ? The only effect of relativity in there is that the trip will seem faster to them so why not keep it that way ? Also, what are "the effects of inertial mass" to you ? $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 13:20
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    $\begingroup$ @JBiggs It serves two purposes. One is to create the delta v necessary to achieve point 8 cee (surfing, as it were, using the pressure difference 'squeeze' to push it forward without reaction mass, like an airplane wing stays up without any reaction mass push downwards as required by a helicopter) and the second is to ensure that mass does not approach infinity as the speed increases. Sort of (in the vernacular) to counter the Lorentz factor. Einstein was perhaps close, but Planck was even closer. We are even still closer today with more up-to-date models. $\endgroup$ Dec 27, 2017 at 20:31
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    $\begingroup$ @Keelhaul 'traveling at point 8 cee' is traveling at point 8 cee period. Not relative to any other point but the speed of light. If you started from a ship that was traveling, say, point 2 cee, you are not traveling at point 8 cee PLUS point 2 cee, you are going point 8 cee. That is, point 6 cee faster than the ship you started from. At speeds approaching cee, the velocities are not purely additive. $\endgroup$ Dec 27, 2017 at 20:38
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    $\begingroup$ There is no "not relative to any other point", and no "relative to the speed of light". Speed IS relative. An object is moving at .8c RELATIVE to another object (you go .8c away from your departure and .8c towards your arrival). In your example, the first ship is traveling at .2c relative to, say, the Earth. The second ship is traveling in the same direction at .8c relative to the first ship. Does that mean it is traveling à 1c relative to Earth ? No, because of relativity and the Lorentz tranformations (look it up), the second ship is moving at around .86c from Earth. It is ALWAYS relative. $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 20:50

4 Answers 4


They totally can

They may travel at extreme speed relative to their departure/arrival/medium in-between (think space dust), but relative to each other, they are mostly immobile. So any communication device able to cross 1AU (like a simple radio or laser beam) would do it, their "speed" doesn't matter.

However, the Alcubierre bubble is another story. Since it's a fictionnal device it's hard to predict, but since it's your story, do what is convenient for you. My intuition would be that the bubble effectively distorts the signal, kind of like how light is distorted by the refractive index of water, especially if the water is not still. But I believe that, using this bubble as a drive, your colonists have it fine tuned and can precisely compute the distortion effect, and reverse it to get a clear signal.

So, speed is not an issue and the bubble problem can be circumvented. The fact that you state "They know the other ship is there, somewhere" would tend to assume they don't know exactly where the other ship is but let's face it : at 1AU apart, with really big ships, it's not that hard to find each other. And once they do, they won't lose track of the other ship. Therefore aiming a radio signal is not an issue either.

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    $\begingroup$ I know this can be hard to grasp at first, but your ships are immobile relative to each other. There is indeed a 8 minute delay to cross the 1AU of distance, but no doppler effect, and no aiming forward. That would be the case if one sheep was moving relative to the other, but they are not. $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 17:58
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    $\begingroup$ @JustinThyme The fact that both ships are moving at 0.8c relative to Earth has no bearing on the question. The ships aren't moving relative to each other. The 1 AU distance results in an 8 minute one-way delay (16 minute round trip, assuming instantaneous reply). That's it. If you want to debate the point further, then I'll let you in on a little known secret: there is no experiment you can perform within your reference frame to determine if your reference frame is moving. $\endgroup$ Dec 27, 2017 at 19:40
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    $\begingroup$ @JustinThyme Nope, not how physics works. All velocities are relative to to some reference frame (e.g. Earth). There is no universal cosmic background reference frame from which light is moving at c. Light moves at c in all inertial reference frames. Also, if there is such an experiment, link it. Because if its true, you've proven known science wrong. $\endgroup$ Dec 27, 2017 at 21:30
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    $\begingroup$ @Justin Thyme I suggest you do some research, "relative to the speed of light" is a nonsense. You have a velocity relative to a reference point (for example : the Earth), not relative to a speed. That just doesn't make sense. And yes, nothing can go past the speed of light : but this is not contradicted by having multiple frames of reference, thanks to relativity and more particuarly the Lorentz transformations. $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 21:35
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    $\begingroup$ @JustinThyme Rotational motion is also known as "acceleration" within the earth-surface reference frame. Acceleration is definitely visible via experimentation. It is, however, not the same as inertial motion. the difference corresponds to the net rotation of the loop with respect to some external preferred frame, that frame might alternatively be referred to as 1) the non-rotating gravitational frame of the earth, or 2) what is commonly referred to as the frame of the "fixed stars". The "fixed stars" are not an absolute reference frame. Our local group is moving with respect to other grps $\endgroup$ Dec 27, 2017 at 22:09


Everything inside an Alcubierre bubble is causally disconnected from everything outside the bubble, but only if you're going faster than light. Since you mentioned that you're using subluminal velocities, you're not causally disconnected from the rest of the universe. You are surrounded by a bubble of compressed space-time, however, and that's going to give you some problems. It's easily observed that compressed space-time refracts light passing through it. This is going to pose some issues when you try to project something out of the bubble. Since this bubble is going to be concave around the transmitter, it's going to tend to diffuse any transmission out of the bubble, so the signal is going to have to be both extremely strong and very concentrated. It's also putting off some serious radiation from the front end, so you'll need to compete with that to get a signal out.

But this is science fiction!

Well then, let's say that you can manage to get a signal out of the bubble. Making the problem worse, the universe has stuff in it [citation needed]. This is probably going to be a fairly lossy connection, so you're going to have to figure out a way to deal with interference and lost bits. These are surmountable problems.

So actually, let's not use a transmitter inside our bubble at all!

I posted this answer too soon, and I've just thought of a fairly novel way to approach this. As I mentioned earlier in the post, the bubble is creating a great deal of radiation. Maybe we can harness that! Let's turn our entire ship into an AM transmitter! I'll be the first to admit I haven't slogged through the math on this one, but I'd put some money on the fact that increasing the speed of your bubble will increase the amount of radiation being emitted from the bubble. So if you can effectively modulate the speed of the ship, you can modulate the energy of the radiation!

Now once your message reaches the other ship, the problem posed by the bubble actually becomes a benefit! I can't speak to the actual shape of the bubble, but I'm betting it's roughly spherical (in the front that is). The back is under negative pressure, so that's going to mess with things, but if you can use the front of the bubble to refract the signal back in towards the ship, you may actually get some gain.

Beware that if you're looking for hard science, this last paragraph is rampant speculation, and I'd recommend you move over to the physics stack exchange to get some answers there.

  • $\begingroup$ But I am not going FTL. There is no disconnect with 'normal' relativity. Would there be a null point between the front and the back that would connect to 'normal' space? I agree there would be a Doppler effect. $\endgroup$ Dec 27, 2017 at 17:28
  • $\begingroup$ See the edit. After some further thinking, I was definitely too quick to judge this one. $\endgroup$
    – bendl
    Dec 27, 2017 at 18:06
  • $\begingroup$ "but the speed of light is constant, so you're actually going to have to point it way out ahead.". Nope, not at all. The speed of light is indeed constant relative to any observer, including your ships, so you'll just have to aim right to the other one. Remember that one of the core principle in relativity is that speed is always relative. The ships are immobile relative to each other, so we don't care about their speed relative to the orther things (like their departure and arrival). $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 18:09
  • $\begingroup$ Otherwise, juste because the earth is roaming at ludicrous speed relative to some distant galaxy, we would have to throw anything ahead taking in account this "speed". But wait ! We should also take in account another totally different speed from another galaxy ! What a nigthmare... That's why it doesn't work that way. Your ships go in the same direction with the same speed ? Then to all extents, we don't care what happens outside. Your ships can communicate fine. $\endgroup$
    – Keelhaul
    Dec 27, 2017 at 18:12
  • $\begingroup$ @Keelhaul Then how do you explain red shift in the light hitting us from distant galaxies? $\endgroup$ Dec 27, 2017 at 19:56

They can if you say they can.

The Alcubierre bubble is a mathematical solution consistent with the Einstein field equations, but that solution may not be physically meaningful. Even if it is, it requires negative energy to create, and we have never seen negative energy. Negative energy may not be possible within the laws of physics. There is so much uncertainty about how a real Alcubierre drive would work, that you can fill in a lot of the blanks however you like.

I can imagine that radio waves pass through the bubble without any issues. The two ships can have a high bandwidth connection, albeit with an 8 minute delay due to the 1 AU of distance.

I can imagine that the Alcubierre bubble scrambles all radio communications in or out. If we assume that the radio waves are transmitting a digital signal, the 1s and 0s might have to be very long and sent with a very powerful antenna to be received. Otherwise the radio signal gets lost in the radio noise. You could also say that the Alcubierre bubble gives off a lot of EM radiation (radio waves, micro-waves, light etc). This could limit the two ships to text messages as opposed to Skype calls.

I can imagine that the Alcubierre bubble completely destroys all conventional EM signals. If this is the case, you could use your warp drive to vibrate your bubble in something akin to morse code. The other ship could detect the vibrations and decode them into a text message. This sounds expensive, slow and risky, only to be used in a dire emergency.

  • $\begingroup$ I am not talking about the Alcubierre drive going FTL. It just has to go point 8 cee. The purpose in part is to reduce relativistic effects inside the bubble. Can life even EXIST going at that speed, otherwise? I can envision that, somehow, the bubble could be intentionally vibrated in such a way as to be able to send a signal, but how does it get detected? Someone outside the bubble might detect it, but someone inside another bubble? $\endgroup$ Dec 27, 2017 at 17:05
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    $\begingroup$ You don't need anything to protect from relativistic effects. There is no difference to life or anything else whether it is standing still or moving at 0.8c. Indeed, stating that raises the question of "standing still or moving at .8c relative to what?" There is no universal reference frame, so all speeds are relative to something else, which alone implies there can't really be a difference. $\endgroup$
    – Elukka
    Dec 27, 2017 at 18:10
  • $\begingroup$ @Elukka Traveling at point 8 cee relative to the speed of light. The speed of light is the universal reference frame. Relativistic effects means that you can not have a beam of light traveling faster than the speed of light, so if the bubble is already going point 8 cee, the light beam can not travel faster than point 2 cee in the direction of travel. But if you take a bubble of space-time along with you, the light can travel at cee WITHIN that bubble in any direction. It just can't ESCAPE from that bubble going at the additive velocity of cee. $\endgroup$ Dec 27, 2017 at 20:53
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    $\begingroup$ @Justin Thyme You just contradicted yourself when saying "the light beam can not travel faster than point 2 cee in the direction of travel". I though the speed of light was always c (and it is) ? Again, even if the ship is going .8c relative to the Earth, your light beam will go at c relative to both the Earth and your ship. Time dilation is a weird stuff, but it works that way. $\endgroup$
    – Keelhaul
    Dec 28, 2017 at 10:18

You're using one mathematical concept that doesn't completely fit with our empirical universe, so let's use another.

Tachyons are theoretical particles that travel faster than the speed of light, which (I think) overcomes all the intrinsic problems associated with the Alcubierre drive.

But what about causality?

Disclaimer: I am not a theoretical physicist. If a PhD in physics suggests that what I'm about to say is hooey, I defer to their judgment.

In the examples of communciation using Tachyons (such as the tachyonic antitelephone), the example is always communication between one relativistic position and one non-relativistic position... allowing (theoretically) the message to be received before it was sent.

However, I've not found (and didn't look hard for) an example of communiction between two relativistic positions. In the case of your ship, both ships are moving within the same intertial frame despite being physically disconnected. Due to this, causality should be preserved (IhopeIhopeIhope).

  • $\begingroup$ I am NOT traveling faster than the speed of light, No tachyons necessary. No worries about causality. Normal rules apply, except that I am enclosing the ship in a bubble that carries its own space-time inertial frame. That part you are correct in. Two issues that I can see remain: one is the Doppler effect, and the second is getting a signal in and out of the bubble. A beam of light sent from one spaceship does not travel with the spaceship. It continues on from its original starting point, A light beam has no inertia. It does not 'gain' momentum from the moving spaceship. $\endgroup$ Dec 27, 2017 at 22:19
  • $\begingroup$ By the time it gets to the other spaceship, the other spaceship is about point 8 AU away from the starting point. That is some Doppler shift. $\endgroup$ Dec 27, 2017 at 22:19
  • $\begingroup$ On a side note, one interesting theory has time going forward, but an an increasingly faster rate. Eventually, the limit is reached where you have instantaneous time throughout all eternity. Everything happens at once. $\endgroup$ Dec 27, 2017 at 22:26
  • $\begingroup$ Mind you, I AM assuming that, at 1AU, the two ships are still within the light cone of each other. Otherwise, yes causality may be a factor. That, really, is related to the Doppler effect. If light can not breach the gap, given the speed of the ship away from the point of origin of the light, then yes causality could be a factor. That is some interesting math. $\endgroup$ Dec 27, 2017 at 22:36
  • $\begingroup$ Ah! The tachyons are necessary because you're losing bandwidth. As you mention elsewhere, at .8c there's still .2c to work with, but that means you can only modulate the signal within that remaining 20%. From a wave propogation point of view, that's a ton of loss. But, introduce FTL tachyons and this entire issue vanishes. (I wasn't suggesting that your ships were FTL.) $\endgroup$
    – JBH
    Dec 27, 2017 at 22:55

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