Antimatter: matter going backwards in time?

thequantumdot:

  In 1928, Paul Dirac published a paper that promised to unify quantum mechanics, special relativity, and the new concept of spin. He predicted that electrons could have both a positive and negative energy. Unknowingly, Mr. Dirac predicted (although he didn’t explicitly state it) a new class of matter: antimatter. A few years later, Carl Anderson discovered the first signs of the positron. The image above is the cloud chamber photograph of the first positron ever identified.

  Antimatter is created when high-energy collisions occur between particles. This happens all the time in the upper atmosphere when cosmic rays collide with atomic nuclei. Antimatter has similar properties of matter: the positron has the same mass as the electron; an antiproton and a positron can form antihydrogen. However, if a particle has a charge, it’s antiparticle has the opposite charge. In addition, when matter collides with antimatter, they annihilate each other and energy is released.

   But what does antimatter have to do with time? Consider this. An electron with charge -e moves along a wire with some velocity v. It creates a current:

I = qv = -ev.

But this would be equivalent to the current created by a particle of charge +e moving with velocity -v:

I=+e(-v) = -ev.

That is to say, the current is the same if we reflect both the charge and the time of the system; it exhibits charge-time symmetry. This is mathematically equivalent to a positron moving backwards in time.

   CPT symmetry is the product of 3 symmetries: charge, parity (flipping an odd number of spacial coordinates) and time.  Violations of CP symmetries have been experimentally observed in certain exotic situations, which lend some explanation as to why there is more matter in the universe than antimatter. T symmetry is violated in macroscopic thermodynamics and the microscopic weak force.