What is time? My friend and I are having an argument about the nature of time.

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What is time? My friend and I are having an argument about the nature of time. If I understand her position correctly my friend believes that time is simply an artefact of changes in the universe, or that change is itself synonymous with time. This seems to be a commonly accepted position, however I replied that if two spheres were moving in space in parallel to one another but one sphere was moving faster than the other then that would mean that that sphere would traverse a greater distance in less time but with an equal degree of change in the universe. Thus time and change are two separate things. I guess the physics of that claim are debatable but I suspect that it demonstrates something true in a way that is a priori irrespective of the physics because even if there were a physical difference in the amount of forces which change it would be hard to see how those imperceptible forces would contribute to a difference in how we perceive the time effect of those different objects. What do you philosophers think? Which of us is closer to the truth or is the nature of time better answered by physicists than by a priori speculation?

Some good questions.

One view is that if there is change, there is time. However, it doesn't follow from that that time has a "metric" -- that there is an answer to questions of the sort "how much time?" If all that existed were two solid spheres in relative motion, then someone might say that there's no answer to the question "What's the relative velocity?" (hence how much time has passed between varying degrees of separation) even though there is change going on, and hence there's time.

More generally we can at least imagine a universe where time _order_ among events -- what happens before, after or simultaneously with what -- is definite, but without there being any correct answers to questions such as "How much time passed between event x and event y?" Insofar as that's right, it provides a way around your objection.

Whether time requires change is yet another question. Not everyone agrees that it does. Sidney Shoemaker, in a paper from some years ago, argues that there could be definite periods of time with no change. The reference is: “Time Without Change,” Journal of Philosophy 66 (1969), pp. 363-381. You might also look at the article on time in the Stanford Encyclopedia of Philosophy:

http://plato.stanford.edu/entries/time/

Good luck with your explorations!

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