If you have studied (or are studying) Feynman diagrams in school you may have heard that particles with a downward trajectory (backward in time) are interpreted as an antiparticle moving forward in time. Well, if you read Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures it is clear that Feynman took this very literally (also look here).

If you are into physics you should contemplate this for a couple of days. It is a fascinating topic. Some great discussion on this topic took place on the sci.physics.research news groups last year... it is a great read.

It is clear that because of simultaneity in Special Relativity (or lack of) observers on different inertial reference frames can see different sequences of events. What Feynman shows is that what appears as a particle to one observer can appear as an anti-particle to another. Think about this... when a particle-antiparticle pair are created in a particle accelerator are they really the same particle viewed going forward and backwards in time . If I happened to be passing by the lab at relativistic speeds could I see the event as a particle scattering?

One word of warning though. Don't fall into the trap that I (and others) did. A particle traveling backwards in time appears to "have opposite electromagnetic, weak, and strong charges, as well as spin" (see reference). But an attractive force, viewed backwards in time, is still an attractive force. Think about it for a while. Think of a small body passing a large body in space and being diverted by gravity. Now run the film backwards... do you see it. Yes, it is still an attractive force. Observers in different reference frames can see different events but you can't have something accelerating towards an object in one reference frame and away in another. Remember the The Barn and the Pole Paradox? Think about a film of a falling object. The object is accelerating towards the ground. If we reverse the film the object starts near the ground with an upward velocity. But notice something.. the object is slowing down. In fact the object is accelerating towards the ground. If it shot up into space at accelerating speeds we would all be surprised.

Now the very keen observer may have notices that I've done a big no-no. I've been invoking arguments based on special relativity to discuses gravity which requires general relativity. But as far as I know the arguments hold true.

I'd love to hear comments on this topic.