- #71
Hurkyl
Staff Emeritus
Science Advisor
Gold Member
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There is one thing that bugs me - and maybe you could clear it up for me -, and that is when people talk about the particle and the anti-particle "meeting" at a certain point in time. It seems like this would have to, literally, be instantaneous.
Really, it's just as astonishing that the particles (whether they're going the same way through time or not) manage to meet at the same point in space.
But I think you meant that in the sense that "only the present exists". There are a couple of ways that could be explanationed.
The first and most obvious is simply that all particles really do exist in the past, present, and future simultaneously; reality is trajectories, not particles. Causality needs to be slightly modified from the linear sense; the state of a particle can only influence events in its future or past lightcones and nothing outside (i.e. it can only influence events with a timelike separation not spacelike).
Another possibility is more in the spirit of the reverse parametrization idea I mentioned; a lot of physical equations have the lorentz factor &gamma which is dt/d&tau (&tau is proper time)... &gamma involves a square root, though, so if one desired they could take the negative square root to get &gamma, which would correspond to a negative dt/d&tau (i.e. proper time is running backwards with respect to coordinate time). This allows one to continue with the "only the now exists" philosophy, and the modification from ordinary SR is that clocks are permitted to run backwards, not just slow down. In this sense, the equations really do permit you to smoothly go from "forward in time" travel to "backwards in time" travel (but it still doesn't let you get to your "past", nor does it provide a way around the infinite energy requirements).
Hurkyl