Special & General Relativity Questions and Answers
What is time?
We don't really know. Physically, time is an essential parameter that we seem to need to sort out configurations of matter and energy in our universe. We also need exactly 3 dimensions of space. For thousands of years, these four dimensions have circumscribed how we classify external events in nature. Some physicists in search of the 'Theory of Everything' have suggested that at the sub-atomic scale, many more 'dimensions' are needed to mathematically sort out how particles interact, and account for the various patterns they see at these scales.
Quantum mechanics sometimes plays havoc with our understanding of how events are ordered in time. At the atomic scale, past and future seem not to have much meaning because the mathematics suggest that much of the activity would appear the same way if matter moved backwards and forwards in time ( provided that the so-called 'CPT' Theorem is satisfied). It is said that the direction of time has no real physical meaning for sub-atomic systems. It is only for large ensembles of particles that a direction for time emerges. This is very much like the concept of temperature which is meaningless for systems consisting of only a few particles. Like time, temperature is an emergent quality of nature defined only for large enough systems. In some sense, time may also be an emergent quality having no real existence in the quantum world of individual particles and systems.
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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.