Time

We perceive time as always proceeding forward into the future; there is no way we know of to travel into the past (except through memories). Time reversal refers the attempt to understand whether a process is moving forward or backward in time. For example, if you watch a movie of two isolated billiard balls colliding on a billiard table, can you tell if the movie is being shown forward or backward? If the two balls are exactly the same, and you do not see one of them being hit to start the process, probably not. If we were to film the process of an egg being dropped and hitting the ground, and then show the film, you could definitely determine in which direction the process continued. The ability to distinguish between forward and backward processes, between past and future on all scales, is crucial for scientific research.

To see if there really is an "arrow of time," or preferred direction in which time flows, researchers have been conducting many different types of experiments for more than 40 years. According to theory, we should be able to tell forward time processes from backward time processes. Experimental verifications of this theory delineate how difficult some experiments are to design and perform. The latest are designed to observe atomic particles as they undergo different processes. If the processes are then "run backward;" any differences that show up mean that we can tell a forward process from a backward running process.