IRREVERSIBILITY AND FLUCTUATIONS.

Film Loop: Reversibility of Time

Length: 3:40 Min., Black and White, No Sound

It may sound strange to speak of "reversing time". In the world of common experience we have no control over time direction, in contrast to the many aspects of the world that we can modify. Yet physicists have been very much concerned with the reversibility of time; perhaps no other issue so clearly illustrates the imaginative and speculative nature of modern physics.

The camera gives us a way to manipulate time. By projecting the film backwards, the events pictured "happen" in reverse time order. This film has sequences in both directions, some shown in their "natural" time order and some in reverse order.

The film concentrates on the motion of objects. Consider each scene from the standpoint of time direction: Is the scene being shown as it was taken, or is it being reversed and shown backwards? Many sequences are paired, the same film being used in both senses. Is it always clear which one is forward in time and which one is backward? With what types of events is this clear, and in what events is it difficult to tell the "natural" direction?

The Newtonian laws of motion do not depend on time direction. Any filmed motion of particles following strict Newtonian laws should look completely "natural" whether seen forward or backwards. Since Newtonian laws are "invariant" under time reversal, by changing the direction of time, it would not be possible to determine whether the sequence is forward or backward. Any motion which could occur forward in time can also occur, under suitable conditions, with the events in the opposite order.

With more complicated physical systems with extremely large number of particles, the situation changes. If ink were dropped into water, it would not be difficult to determine which sequence was photographed forward in time and which backward. Thus, certain physical phenomena at least appear to be irreversible, taking place in only one time-direction. Are these processes fundamentally irreversible, or is this only some limitation on human powers? This is not an easy question to answer. It could still be considered, in spite of a fifty year history, a frontier problem.

Reversibility of time has been used in many ways in twentieth century physics. For example, an interesting way of viewing the two kinds of charge in the universe, positive and negative, is to think of some particles as "moving" backwards in time. Thus, if the electron is viewed as moving forward in time, the positron can be considered as exactly the same particle moving backwards in time. This "backwards" motion is equivalent to the forward moving particle having the opposite charge! This was one of the keys to the space-time view of quantum electrodynamics developed by R. P. Feynman, described in his Nobel Prize lecture (in Project Physics Reader 6).

For a general introduction to time reversibility see the Martin Gardner article in the Scientific American of January, 1967. (in Project Physics Reader 6).