"The reason why we perceive a preferred direction has to do with the fact that it is much easier to go from a simple initial state then from a very complicated state," said Van Kolck. So time can be reversed in physics equations without affecting the result, but the effects of time reversal remain unperceivable in our everyday lives. "Until the 1960s, physicists thought that the laws of physics were exactly invariant in the transformation of time going to minus time," said Van Kolck. "Then it was discovered that there are some phenomena involving subatomic particles where there seems to be a very tiny violation of this symmetry." In other words, if you made a movie of the billiard balls and played it in reverse, the backward version actually would be a little bit different from the forward version - like a "glitch in the matrix." "It's like the process of some things happening in one direction versus the opposite one doesn't happen at the same rate," said Van Kolck. This phenomenon is known as time reversal violation. When time reversal is violated, the equation doesn't balance out; your car doesn't go as fast on the way back. It is this imbalance that physicists believe may explain the unequal amounts of matter and antimatter in the universe. Since physicists first started looking for sources of time reversal violation in subatomic particles, they have been measuring properties of particles known as electric dipole moments, or EDMs. An EDM is generated by a property of subatomic particles known as spin. Spin can be visualized as a particle spinning around its center rather like the Earth rotates around its axis. With time reversal, the spin would appear to reverse, like a movie of the particle played backward. But for the equations to balance, the EDM would have to equal zero. Any non-zero value would generate a different outcome of the equations - the backward version of the movie actually would be different from the forward version.
Time Reversal: A Simple Particle Could Reveal New Physics
I'll forgive the Matrix remark, and count it as two Invisibles jokes for the price of one.