No FTL information implies no FTL travel?

Question

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.

Answer 1

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.)

Answer 2

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.

Answer 3

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.

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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.

Related: How does "warp drive" not violate Special Relativity causality constraints?

Answer 4

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.