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Assume that an Alcubierre drive is feasible. In fact, assume that one has been built. A superluminal starship can propel itself through space by distorting space in front of it and behind it. However, other than an Alcubierre drive, there are no other known technologies that allow anything to travel faster than light (no wormholes or anything).

Utilizing this same technology, is there any way to make a communications system that does not involve a vessel? An electromagnetic wave carrying information (like modern radio or cell phone signals) won't have any feasible way to bend space around it; you would need a drive and power source, themselves inside a vessel.

If Alcubierre drives exist, does this imply that communications can travel no faster than the Alcubierre driven vessels, just as news could not travel faster than sailing ships in ~1700?

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    $\begingroup$ "probably not", but that kinda depends on how you control your warp field... $\endgroup$ – Starfish Prime Aug 23 at 17:17
  • $\begingroup$ Do you want an answer based entirely on mainstream physics theory? If so, note the comment in the "Physics" section of the Alcubierre drive wiki article pointing out that getting the bubble to move faster than light requires either "tachyonic" exotic matter (i.e. matter with negative mass that can move locally faster than light), or it requires devices to be placed all along the travel path in advance. In the 1st case the exotic matter itself could be used for communication, in the 2nd maybe the devices could boost an "empty" bubble. $\endgroup$ – Hypnosifl Aug 23 at 17:21
  • $\begingroup$ @Hypnosifl In that case, the drive would have to be able to bend space light years away, around an EM "beam" carrying information. Explain how that would work, and that is an answer. $\endgroup$ – kingledion Aug 23 at 17:23
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    $\begingroup$ The idea of the external devices laid along the path is that they'd be the ones that bend spacetime in the necessary way for the bubble to move FTL, not any sort of drive inside the bubble. The "placement of matter" section of the wiki article mentions "The pilot inside the bubble is causally disconnected with its walls" presumably meaning any signal they send out at the speed of light or slower will fail to reach the walls (they are not in his future light cone), or at least certain regions of the walls. $\endgroup$ – Hypnosifl Aug 23 at 17:30
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    $\begingroup$ @stix - Read the wiki section I linked (which has a reference to a scientific paper), it was shown that in addition to negative mass density, the exotic matter would also have to be tachyonic if it was controlled from inside a bubble moving at FTL speeds. Also, dark energy is not thought to have negative mass/energy density in most models, just negative pressure, see this explanation of dark energy from a physicist which says "The data say the dark energy density is positive". $\endgroup$ – Hypnosifl Aug 23 at 18:02
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Not necessarily. The Alcubierre metric merely describes a space-time that is compressed in front of your spacecraft and expanded behind it. The assumption has always been that the warp field comes from the craft inside the bubble, but this isn't required to be the case. An interesting thing about the Alcubierre metric is that it looks almost identical to the shockwave produced by aircraft traveling at supersonic speeds. An aircraft has air compressed in front of it because the air can't get out of the way fast enough, and rarefied behind it because it hasn't had time to diffuse back into the wake created by the aircraft. The difference with an Alcubierre drive though is that this compression/expansion is actively created by the drive itself and used to carry the ship along, whereas a sonic boom is just a side effect of exceeding the speed of sound.

If you imagine taking a string or a rug, and whipping it to create a traveling wave in it, this would be equivalent to your FTL communicator. It might be possible to derive a solution to the Einstein equation where this "wave" travels faster than the speed of light, as there is no limit on how fast space-time itself can expand (in fact the current research suggests at the edge of the Universe's visible horizon galaxies are moving away from us at 3x the speed of light). The caveat here is that what I've described is also essentially a gravitational wave, and we know gravitational waves move at the speed of light, so it's questionable whether or not you could do this without a vessel.

However, even if it's possible, what this does mean is that your comms system is point to point. It's essentially like a laser beam, and wouldn't be able to do broadcast like we do with radio waves. In order to communicate with a space colony or a ship, you'd have to know where it is to send your FTL beam. For colonies, the motion of planets are known well in advance, so it's not as big a deal, but attempting comms with a ship if you don't know where it is would be an issue. I would imagine in this case FTL comms would be similar to how submarines work, in that the spaceship would be out of communications except for certain windows where the ship is in a known location or it drops out of FTL to call home. Since the ship knows where the home base planet is, they'd have no problem setting up their point to point comms, and could radio back their position to allow the planet to communicate back.

Now, assuming what I've described is possible, there are some things we can say about the physics involved:

  • The amount of negative mass density you need to form an Alcubierre bubble is related to the size of the warp bubble you need to create. If we're just creating warp bubbles around photons in our beam, the amount of mass needed would be drastically smaller than that needed for a ship, it just has to be bigger than a few wavelengths of your photons.

  • More than likely you would have to send this negative mass density along with your photons, so we might think of a kind of "exotic energy" used to carry information instead of photons. In this case, your Alcubierre bubble itself becomes the information carrier.

There are only two things we know of that exhibit this exotic matter/energy property: The Casimir effect and Dark Energy. The Casimir effect can only occur between small plates, and is more akin to how holes/electrons function in a semiconductor than what we might call "real" exotic matter. Dark energy however is prevalent in the Universe itself and is our best guess at what is driving the accelerating expansion of the Universe. If you can find a way to harness dark energy, you can not only build your warp drive, but you can use the "dark quanta" of the energy to send information faster than light. In your story you might say scientists have discovered an exotic matter particle that is responsible for Dark Energy, and manipulating these particles is how they form a warp bubble. You could then go on to say that sending beams of these particles is how your civilization communicates at FTL speeds, perhaps by "bundling" dark energy into a long-lived packet that travels at FTL. The "range" of this packet would depend on how long it can stay together.

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If Albucierre drives exist, does this imply that communications can travel no faster than the Albucierre driven vessels?

Yes. But that doesn't actually mean anything.

Because Alcubierre drives can travel through time.

If the only technology in your setting not backed by modern hard science is the Alcubierre drive, then there is no other technology in your setting for FTL communication, by definition. You could easily add one, of course, but that's getting off-topic.

Any FTL travel technology can send matter and information backward in time if and only if it is capable of making FTL jumps in different directions in different frames of reference moving relative to each other. Alcubierre drives are actually one of the hypothetical FTL drives most amenable to exploitation in this manner, since the simplest way for them to work is for each FTL jump to take place in the ship's proper frame (that is, the reference frame moving with the same velocity as the ship, in which the ship is stationary) before the jump.

Thus, a ship could fire up its Alcubierre drive, zip off into interstellar space, drop out of warp (moving at the same speed it was before the jump), fire up its conventional thrusters, accelerate away from Earth to nearly the speed of light, hit the Alcubierre drive again, and return to Earth several years before it departed. Because special relativity.

That explanation made no sense at all, so here's a video from PBS Space Time that explains the FTL time travel process much more clearly.

I will note that, with this system, it is much easier to send small packages back in time, since that crucial conventional-thruster step in the middle will require burning a huge amount of propellant. Your engineers may find that sending people back in time by this method is economically infeasible, but sending a couple of SSDs in a tiny Alcubierre-enabled ship could be profitable.

If you want to forbid FTL time travel, you need to ensure that all FTL travel occurs in the same universally agreed-upon frame of reference. This is probably easier to justify with a technology that teleports ships instantaneously from one place to another, a lá the Teraport in Schlock Mercenary. Just declare that all teleportations are instantaneous in the same specific frame of reference (e.g. the frame of the cosmic microwave background), and you're good to go.

If you're OK with FTL time travel, well... go wild.

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  • $\begingroup$ Nooooooo alcubierre drives are one of the only FTL deives that cirvumvent this time travel problem! PBS spacetime actually has a video about it! Because its the space that moves faster and the ship inside can be stationary relative to its space it does not experience time dilation! $\endgroup$ – Demigan Aug 23 at 19:18
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    $\begingroup$ @Demigan You're talking about the twins paradox and other, related consequences of special relativity that crop up with conventional sub-lightspeed travel. These tend to result in the ship and its occupants effectively being flung forward in time. The Alcubierre drive does indeed circumvent that, but it also creates issues of its own. FTL time travel (which can send a ship backward in time) is one of them, and I linked to the PBS Space Time video on it in my answer. $\endgroup$ – Someone Else 37 Aug 23 at 21:25
  • $\begingroup$ I find the video not that enlightening. They first show how you could overtake the light send out during construction and then travel back to the earth and tadaa! rather than finding the light send out after it they went back in time. They even mention it's a trick based on multiple perspectives, and with how many time's I've heard that time is relative to the observer and not multiple obervers I can't say if this is a 'real' way of time travel or one of those fake tricks like "if you divide a number in infinitely small pieces and add them up it becomes infinite!" which is plain bullcrap. $\endgroup$ – Demigan Aug 24 at 6:34
  • $\begingroup$ It’s dangerous to go alone! Here: Take this! $\endgroup$ – Joe Bloggs Aug 24 at 14:09
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    $\begingroup$ Maybe this is a more interesting question... If anything traveling faster than light compared to you is actually traveling in time, does that mean that all the universes beyond the visible universe boundry are traveling in time compared to us RIGHT NOW? $\endgroup$ – Demigan Aug 24 at 14:52
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One detail about Alcubierre drives, which we handwave away as quickly as the setting allows for convenience, is the trouble with controlling the warp from within the warp. The most "practical" way to achieve this (for definitions of "practical" that involve megastructures) is to build what amounts to a tunnel or track, or analogous system, that controls the warping of space along its path. This would function like an FTL railroad, and would therefore be point-to-point, but could be used for both cargo and communication.

Now, building such a thing that could fit cargo would doubtless be absurdly expensive compared to building one for communications. So you could have both communications networks and free-roaming vehicles, since the costs and benefits involved would vary enough to conceiveably justify both.

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