# Special & General Relativity Questions and Answers

## Why can't matter travel back in time?

We don't know why nature seems to have a problem with this, but our reality seems to be predicated on the basis that no information can travel back in time. There is an exception.

A basic feature of modern physics a revealed by the equations used in quantum electrodynamics and announced by Baron von Stuckelberg and Richard Feynman back in the mid 1940's is that matter and anti-matter enjoy a peculiar time-reversal symmetry. A matter particle traveling forward in time between times T1 and T2, is MATHEMATICALLY equivalent to the same anti-matter particle traveling BACKWARD in time between T2 and T1.

The following world line diagram:

```     -                    .
-                  .
-               .
-             .
T2       .*.         .
.    .      .
.       .   .
T1    .          .*
.             -
-
-

```
Shows a dotted electron entering from the lower left, and at T2 colliding with its anti-matter twin and annihilating to become a pair of dashed photons. The anti-electron was created at T1 when a second dashed photon was transformed into an electron and anti-electron pair. This diagram, using time reversal symmetry, can be mathematically re-interpreted as an electron entering from the lower left, traveling to T2 where it is 'scattered' into an anti-electron state that travels backward in time to T1 where it is again scattered into an ordinary electron which then continues its journey to the upper right. This later situation is much easier to handle mathematically, because there is only one particle involved rather than two!

We cannot, however, use the antics of the electron in the second scenario to relay a message from T2 to T1....alas!

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.