# No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light.

There is also speculation that it might be possible to open wormholes or travel faster than light with an Alcubierre Drive.

Wouldn't the first point render the second impossible? If both were valid points, you would get a contradiction because you could encode a message in the Alcubierre Drive payload (or send a messenger through a FTL spaceship) and therefore transmit information faster than light? Or is this a misunderstanding of the impossibility of transmitting information faster than light?

With wormholes, this is pretty much sidestepped because nothing is actually traveling faster than light, but rather moving through a shortcut in spacetime. You're essentially cheating, so this isn't particularly interesting and doesn't really break any laws of physics from what I understand.

• There's no reason to believe that wormholes or the Alcubierre Drive allow you to reach your destination faster than light in flat spacetime. Don't forget that you're bending spacetime - the distance appears shorter, but time passes slower. Spacetime is almost perfectly flat, there's no "shortcuts". Wormholes would still have benefits to traveling in flat spacetime (e.g. you don't need to accelerate all that much), but they don't allow FTL travel. It's just good for suspension of belief in sci-fi; don't think it works that way in the real world. – Luaan Dec 11 '18 at 9:27
• @Luaan That would make a nice answer. – rob Dec 11 '18 at 14:06
• "Spacetime is almost perfectly flat" -- until you bend it, which is the whole point. – Peter - Reinstate Monica Dec 12 '18 at 6:50
• @Luaan "don't think it [wormholes] works that way in the real world" is a funny assertion ;-). – Peter - Reinstate Monica Dec 19 '18 at 15:12
• Coming to the actual question the answer is of course yes. Information is encoded in the actual objects that you send FTL (like the piece of paper with “the Red Sox won the World Series” written on it”). – lcv Dec 21 '18 at 8:15

Note that I am far from an expert in this area, so sorry if I do not go as deep as you would like.

The general consensus in the scientific community is that it is impossible to transmit information faster than light.

This idea comes from special relativity. If information traveled faster than the speed of light, then causality becomes all messed up. This doesn't take into account the solutions of general relativity equations of things like wormholes.

There is also speculation that it might be possible to open wormholes or travel faster than light with an Alcubierre Drive.

The idea of wormholes or an Alcubierre Drive comes from solutions to equations in general relativity. We do not currently know whether or not these things are physically possible, but they do not violate the first point because within the local space-time of the object nothing is is actually traveling faster than the speed of light. Space-time itself is manipulated in order for this "faster than light travel" to occur.

As mentioned in the comments, the main "obstacles" to producing wormholes or Alcubierre drives isn't from your first point, but rather it comes from not having yet observing the means necessary to produce them (negative mass, infinite energy, etc.)

• "they do not violate the first point because within the local space-time of the object nothing is is actually traveling faster than the speed of light." But with respect to other frames of reference objects certainly are traveling faster than light, which is the whole point, – Peter - Reinstate Monica Dec 12 '18 at 6:56
• @DanYand Thanks for the clarification in your answer. I made my comment because as I understood the OP he is particularly interested in the implications of the large-scale violation of c (e.g. doesn't traveling faster than light "usually" travels cause causality problems). In other words, that c is not exceeded on the small scale was not the point of the question (but is the point of the physics, as you and the other Q&A you linked to in your answer explain). – Peter - Reinstate Monica Dec 19 '18 at 7:57
• @PeterA.Schneider You're raising good points, and you're making me think twice about this -- which is good. Large-scale causality violations are prevented if the spacetime is globally hyperbolic, but I failed to check that condition in the wormhole / Alcubierre cases, so you're right; there really is an interesting question here, and I need to think about it more carefully. – Chiral Anomaly Dec 19 '18 at 14:35

Alcubierre Drives also cheat. They essentially shorten the distance to your destination, so even though you're not locally moving faster than c, you can get there in less time than it takes light to get there through flat space.

• So the distance is also shortened for everyone else who isn't in your spaceship? Sounds like a good way to mess up the universe. – user253751 Dec 11 '18 at 3:04
• @immibis Where are you getting that from? Even in special relativity, the spatial distance between points is not necessarily the same in two different frames. – probably_someone Dec 11 '18 at 3:07
• @probably_someone "They essentially shorten the distance to your destination" - if it's only shortened for the people in the spaceship, but not for the people outside, then the people outside still see you carrying information faster than light don't they? – user253751 Dec 11 '18 at 3:25
• @immibis Yes, which doesn't contradict anything in this answer. This answer only says that things inside the region of warped space aren't locally moving faster than $c$. – probably_someone Dec 11 '18 at 9:24
• Another way to look at it is to note that if you were to stand beside an Alcubierre-drive craft as it departed and shine a lamp parallel to its direction of travel, the light from that lamp would arrive at the same time the ship did, which would be faster than it would have arrived had the ship not been there (because it would be caught in your "warp bubble", for lack of a better term, and travel the same - shortened - path as the ship). – anaximander Dec 11 '18 at 12:15

A couple of other answers already emphasized that FTL is only supposed to be impossible locally, and that's true whether we're talking about transmitting information or transporting a physical object. This answers the question.

If we drop the "local" caveat, then FTL becomes mundane. In the context of cosmology, this was emphasized in another post:

What does general relativity say about the relative velocities of objects that are far away from one another?

Here I'll mention another example to drive home just how boring FTL becomes if we drop the "local" caveat.

Consider two points $$A$$ and $$B$$, both with radial coordinate $$r=3M$$ in Schwarzschild spacetime (in the usual coordinate system). At this radius, a circular orbit is possible only for something that is moving at the speed of light. Now, suppose that the angular coordinates of these two points are $$\phi_A=0$$ and $$\phi_B=2\pi/1000$$, respectively. We can send a circlarly-orbiting light pulse from $$A$$ to $$B$$ in either of two directions: the short way (increasing $$\phi$$), or the long way around the black hole (decreasing $$\phi$$). If light-pulses $$1$$ and $$2$$ both leave $$A$$ at the same time but pulse $$1$$ goes the short way and pulse $$2$$ goes the long way, then pulse $$1$$ arrives at $$B$$ before pulse $$2$$ does. This is an invariant statement, true for all observers. This is (boring) FTL, because pulse $$1$$ arrived before pulse $$2$$ even though pulse $$2$$ was traveling at the speed of light the whole time.

Here's the point: if we disregard the all-important "local" caveat in the usual statement that information/objects can't be transported FTL, then we don't need anything as exotic as a wormhole to achieve FTL.

Appendix

This answer was written under the assumption that the spacetime does not contain any closed timelike curves. According to Alcubierre's report (https://arxiv.org/abs/gr-qc/0009013), the Alcubierre Drive spacetime does not contain any closed timelike curves. In such a spacetime, causality is safe despite the (mundane) type of FTL described above.

In contrast, the Kerr metric for an eternal rotating black hole does contain closed timelike curves (hidden behind the event horizon), and that does mess with causality — at least it would if we continued to trust general relativity under those conditions, which might not be the right thing to do.

• With "local" you mean "distances small with respect to the spacetime curvature", right? Because one could consider the vicinity of a black hole "local" in cosmological terms. And hypothetical wormholes as well as black holes make FTL "possible" precisely because they (hypothetically) bend spacetime on scales small enough to become interesting for humans. – Peter - Reinstate Monica Dec 19 '18 at 7:00
• Since this answer is perhaps partly inspired by our comment exchange above, let me add that it was news to me that the lightspeed threshold is not valid (or rather, meaningless) on the large scale (I suppose, as above, "large" with respect to curvature). I suppose it was also news to the OP, which is why I made the comment above. The max = c from special relativity is so ingrained that you would think "violating" it on the large scale caused all kinds of havoc to causality and stuff. I think that's what the question was about. Apparently it doesn't; the world proceeds just fine. Why? – Peter - Reinstate Monica Dec 19 '18 at 7:17
• @PeterA.Schneider Good questions. I really should have defined what I meant by "local" since I claimed that it's so important. Maybe the safest way to define "no local FTL" would be "events at $p$ cannot influence events at $q$ if $p$ and $q$ are not connected by any timelike or lightlike worldline." This protects causality. In a small region where curvature is negligible, this condition reduces to the usual intuition. Self-consistency is built into this condition, at least if there are no closed timelike curves... – Chiral Anomaly Dec 19 '18 at 14:25

In 2001, using Gain Assisted Superluminal Propagation, Lijun Wang, Arthur Dogariu, and Alexander Kuzmich at the NEC Research Institute in Princeton, N.J., excited cesium to a gaseous state that resonates with light frequencies and pushed laser pulsed light waveforms through the gas. Light there reforms at the opposite end of the test chamber at the instant the tip of the pulse touches the chamber entry. According to Wang, "... the net effect can be viewed as that the time it takes a light pulse to traverse through the prepared atomic medium is a negative one." The laser waveforms arrive before they fully depart, but not before they start to depart. They appear to exist simultaneously in two places.