TillEulenspiegel said:
Richard Feynman theorized that in QED backward and forward time movement of particles and anti-particles is possible.
Well, up until around 1958, if memory serves, it was believed that charge was a complete symmetry of the universe, which would mean that everything was reversible. Then weak beta decay was found to violate parity symmetry. Then sometime in the 60s (I think) k-mesons were discovered to violate time symmetry, so now we have to use CPT symmetry. This really bugged me for a long period of time, because I can't see how just one particle could do this. There's a sort of aesthetic to this stuff, and it always seemed to me that there should be an even number of them. Then someone, I forget who (maybe it was you), pointed out on this board that much more recently another meson has been found to violate T symmetry, which makes me feel a lot better.
CPT, or any group of three, forms a nice symmetry system, for some reason, and seems to crop up a lot.
Feynman's view was not precisely a belief, but an interpretation and popular description of CPT symmetry, that an antiparticle, which has CP reversed, in indistinguishable from its corresponding particle travelling backward through time. Of course, where one goes from there may involve beliefs. I find highly interesting his dodge for the backward photons from the Maxwell's equations, that essentually, every "backward" photon is involved in a virtual photon exchange with something in the past. Which got me speculating--it seems to imply that a photon has to hit something back in time, which is consistent with Big Bang ideas, because if you go back far enough in any direction, you hit something. So, does the fact that radio does work forward in time provide evidence that there will be no Big Crunch? Or is it a probabilistic thing, and at this time in the universe, the probabilities are just overwhelmingly forward? Some hard SF has been written assuming that it is probabilistic. Greg Egan, maybe, or Stephen Baxter. But I digress.
Tachyons are proposed particles that exhibit -t behavior . The original idea was evoked as a property of 10-dimensional string theory which has all but been dismissed altho there is a new reconsideration of this hypothetical beast as an adjunct of 11 dimensional string theory as an open string.
Tachyons are
way older than that. Just take v greater than c and calculate gamma. Then all the annoying infinities go away, but you're left with imaginary numbers in all your mechanics equations. This is not as bad as it sounds, because standard Minkowski space used in relativity is just like plain space if you assume that the time coordinate is imaginary.
There are possibilities that these things may exist , but they appear to all be special cases and most physicists agree that any backward movement in time cannot transmit information ( and what is a human or a machine that got him there but information ) and that any construct used for time travel prohibits traveling to a point before the construct existed ( at least in the same universe).
The latter is true. Plus, these things tend to be made of completely uniform materials of infinite length, which we don't think are possible.
As to the former, it's my opinion that transmitting information back in time would violate relativity. But I'm not certain about that, and I've seen people who aren't entirely stupid disagree. It is a handy mathematical trick to assume that something like information is going back in time. But its usefulness is limited to quantum interactions, which seem not to be able to transmit information even superluminally, despite "spooky interactions over a distance." (This point completes Feynman's idea with Maxwell's equations.
Bottom line being for now special class HE physics can demonstrate -t behaviors, but the translation to macroscopic scales is something altogether different.
That's one of those quantum things. It just so happens that, at those temperatures, quantum behavior gets obviously big. As with, to take an earlier and simpler example, Einstein-Bose condensates.
Trouble is, I'm not sure where the microscopic ends and the macroscopic begins. People have been using time-reversal (call it a mathematical trick or reality, whatever) when drawing Feynman diagrams for decades. But there's always the caveat that, well, this is small, so I don't have to think about it when ordering a beer. There are plenty of macroscopic things that could not work in a classical universe, such as this computer and the eyeballs I use to see the screen. Light itself wouldn't work over distances of parsecs the way it does if it weren't for QED. But these are all indirect. The HE experiments just make a quantum operation directly large enough that it cannot be ignored.
Plus, there are about a half dozen mainstream interpretations of QM and about a bazillion others, none of which, so far, anyone has been able to convince me is in any way empirically distinguishable from the other. (Someone tried once; the best attempt I've seen, but it had a fatal flaw.) When I see this, I get an intuitive feeling that the fact that there are so many means that none of them is very good, and there might be some other way of looking at it.
I like to play a sort of ultra-skeptical game sometimes, wherein I make a distinction between what I really think I know, what I just kinda think I know, and what I just sorta assume I know. I really think I know some aspects of information theory, the whole Shannon schtick with irreversible computing engines, and that that it can be tied directly to thermodynamics. I really think I know that the best evidence for a macroscopic arrow of time is also thermodynamic and information theory in nature. I really think I know that I have a brain, and that this brain operates as a theremodynamically irreversible computing engine. I really think I know it because my head gets hot.
Put that together, and what I kinda think I know is that my brain operates according to the same set of statistical laws that provide the best evidence on a macroscopic scale for the arrow of time. So, I kinda think I know that my brain, being an entropy engine, is going to perceive other entropy engines, like the macroscopic universe, as going the same direction in time. My process of forming memories involves entropy-increasing operations, so the time when there was more entropy is in my memory, and the time when there will be less entropy is not in my memory.
However, I only sorta assume I know that this perception is an accurate perception of what's going on. If some demiurge or Maxwell's demon started playing tricks with the entropy of the universe, jogging it back and forth like the time slider on a PC DVD player, I wouldn't notice at all, because at all times my memories would always be of lower-entropy times. Again I digress.
But anyway, if I were writing an SF story, I'd either use a many-worlds interpretation, where the arrival of a time traveler in the universe he and/or she arrives in is just a big fat hairy quantum fluctuation with respect to that universe, or one wherein paradoxes are averted because of feedback both ways between the past and the future which will settle down to some stable configuration in which the paradox does not occur.
