SF deals with FTL on a regular basis, and while fictional, many authors actually are Physicists.
Oddly enough, some of the best descriptions that I’ve read of how FTL travel would be accomplished involve the assumption that an object with a mass>0 cannot go faster than c.
In “Primary Inversion” Catherine Asaro describes a phenomenon much like the recent (and well-discussed) experiment. It involves adding an imaginary element to one’s velocity, and more or less going around c without actually hitting it. Bizarre, I know, but who knows? It fits in with idea of “imaginary time” at FTL speeds.
The other author that has presented (IMHO) an acceptable method of FTL travel is David Weber. He kinda takes the easy way out though. In his imagination, since one cannot go faster than light, simply manipulate oneself into another dimension, where c is apparantly faster. Thus one can travel at what appears to be FTL speeds, without ever actually going FTL.
The preceding is all copyright it’s respective authors and is only presented here to furthur intellectual discussion, and is not to be taken as anything remotely resembling truth, fact, or reality.
I don’t see the basis for your objection. All other points are moving, but the maximum velocity is WRT the preferred frame at the point the particle/person is at. It doesn’t matter that the velocity is different WRT the comoving coordinate system at some point a distance away because the particle isn’t there. If you can define a universal time coordinate, and the particle never travels backwards in time WRT that coordinate at the particle’s location, there can’t be a causality problem. If the universe is homogeneous on a large enough scale, as it appears to be, such a coordinate is definable.
I’ll agree that David Weber is pretty good with his physics, compared to a lot of authors, but he’s no Heinlein-- he has things like FTL propagation of gravitational effects, a common misconception. The key in his system, is that although there is no preferred reference frame in n-space or in any single band of hyperspace, there is a preferred frame in any system of two or more bands, in order for the speed of light to remain invariant in each band-- Hence, he’s indirectly taking advantage of that same chink that I mentioned earlier. Incidentally, the notion of a hyperspace where c is faster is by no means unique to Weber-- It’s probably the most commonly used FTL method in science fiction, and is also used in Star Wars, most of Asimov’s works, and a few of Heinlein’s. I’ve never read anything by Catherine Asaro, but the idea sounds interesting; I’ll have to look into that book. I’m pretty sure that her method would still be subject to the time-travel effect, though.
ZenBeam: You can’t use a universal time coordinate because of things like the Twin Paradox… Were there a Universal Time, the twins’ watches would have to agree after the travelling one returned. You might try using time since Big Bang as your coordinate, but when a high-energy cosmic ray slams into the Earth’s atmosphere, for instance, that time is a lot lower for the cosmic ray than it is for the atmosphere.
The twin paradox isn’t relevant. The universl time coordinate is the time coordinate for an observer at rest in the comoving frame. At least one of the twins leaves this frame, so he can’t expect to use the universal time. Likewise, the high-energy cosmic ray is not stationary in the comoving frame. From it’s frame of reference, the universe is anisotropic. The observable mass in the universe is moving predomnantly in one direction. The CMB is blue shifted in one direction, red shifted in the opposite direction.
Here’s a version of the twin paradox which may be relevant (and hopefully interesting enough to make up for its length): Consider a two dimensonal universe with one time dimension, and with a closed space dimension. You can represent this as a cylinder, with time along the axis, and space around the cylinder.
Two twins are in a frame of reference where their timeline runs along the cylinder, parallel to the axis. One gets into a spaceship, and heads off in one direction at a high (constant) velocity. Eventually, he circumnavigates the universe, and returns to the stay-at-home twin. The travelling twin is younger due to relativity, even though both were in an inertial frame during the entire trip*. You can’t use General Relativity to resolve the issue, because all accelerations occurred when the twins were together. The resolution is that the stay-at-home twin is in a preferred frame of reference. He can define a spatial axis such that it forms a closed loop at a constant time T in his frame. For any other frame of reference, the spatial axis is a helix, spiraling around the cylnder. For this situation, there must be a preferred frame of reference. I don’t see any way to avoid it. Yet any experiments the twins performed without circumnavigating the universe would just show normal Special Relativity, with no preferred frame of reference.
Further, in a spatially closed 3+1, spatially homogeneous dimensional universe, I don’t see how you can avoid the same issue. If there can (or must) be a preferred frame of reference in a closed universe, it’s hard to imagine why it isn’t at least possible for a flat or open universe.
(*) You could symmetrize this by having both twins start out with some velocity around the cylnder, and have the two accelerate equally away from each other, such that one of the twins ends up in the preferred frame, and likewise have them decellerate equally at the end.
You have a point there with your cylindrical universe, ZenBeam… As it happens, I have access here to one of the foremost experts on non-simply connected cosmologies… I’ll have to ask him about it. Give me a little time to look into it.