--- On Sat, 9/24/11, Stephen A. Lawrence <sa...@pobox.com> wrote: > From: Stephen A. Lawrence <sa...@pobox.com>
> I believe I alluded to something like this earlier. In reading back over previous emails, yes, you're right. > > In a universe which adheres in general to the SR model, you > can, none the less, allow instantaneous information transfer > in a single, distinguished "universal rest frame" without > leading to any causality violations. Well, as far as I can tell (and remember from the countless space-time diagrams I sketched out) it is of equal, isotropic velocity in that rest frame. From other frames' perspectives, the speed (of some superluminal motion) is different in differing directions. The difference between conventional special relativity and theories including an absolute rest frame, seems to me, to be that effectively, in the absolutist framework, "time" is universal, or put another way, "propagated" instantaneously. In SR, "time" is apparent propagated at c. Relativity of simultaneity and all that. I can see how an /apparent/ causality violation could happen; if a body exceeds c, it outruns its own light signal, and a suitably positioned observer could detect photons emitted from the body at the destination before photons from its departure position reached it. It would LOOK like the thing moved acausally, but it is just a trick of the light in this case. But whereas in one case it is just an illusion, in the other case, it is assumed to be something real. > It's when you allow the "instantaneous" transmitter to move > at an arbitrary velocity, and send information to an > arbitrary receiver in the same inertial frame as the > transmitter, with arrival time being "instantaneous" in the > (arbitrarily selected) rest frame of the transmitter, that > you run into trouble. Yes. There should be, for superluminal velocities, an anisotropy in different directions of propagation velocity. It would seem, if I am thinking this correctly, that if we have thing that can travel at v > c, that we can build an 'ether compass', to borrow an outdated term, to determine our velocity with respect to an absolute rest frame, and determine the direction in which we are moving against it. Unless something weird happens.... see my upcoming response to Jouni's post. > Note well: Time travel is just fine (entails no > contradictions) as long as the destination is outside the > backward light cone of the starting point. It's > getting the destination into the backward cone of the > starting point which requires the frame hopping. This > becomes clear if you try to draw the "contradiction" on a > space time diagram. You can move from certain > positions which are outside the backward light cone of an > event to inside it, if we allow "single-frame" FTL travel, > but to move from the event to a position outside either of > its cones from which you can still get to a point inside its > backward cone, you need to frame-hop. Right. Which is why I said, if you do some frame switching, you can cause real problems within the scope of conventional special relativity if FTL is allowed. > (I hope this made at least a little sense...) It did. Many thanks! > > If an assumed absolute frame is present, > > Which, BTW, is the case according to at least some modern > theories of cosmology. Which theories in particular? Robertson-Walker is one I've heard about in the past. If I even remembered the name right. Don't remember much to be honest. > Any model in which you can see yourself if you look far > enough out into space has an implicit absolute frame in > it. As I recall, there was a major search, using > Hubble, for just such a situation a while back (no luck, > tho, the universe may still be open for all that experiment > showed). That gives me something to think about. --Kyle
