Theories of Time

Past, present, and future. Which is real and which is not? Philosophers have had various theories, and a century ago, philosopher JME McTaggart classified two common theories as the A series and the B series. He hoped to prove that time is unreal by arguing that time satisfies both the A series and the B series, and that those two theories contradict each other. Thus, time has no reality.

Here are all the possible theories. I have added the growing block universe, a halfway theory between presentism and eternalism, and also four other theories which I have named, and which seem to have no advocates.

TheoryPast?Present? Future?
A series, presentismX
Growing block universeXX
B series, eternalism, block universeXXX

What do we find from modern science? At first, it offered no challenge to common-sensical views of time. Time is universal, and there was no evidence otherwise. Or so it seemed.

Newtonian mechanics was a great success in the eighteenth and nineteenth centuries, but when James Clerk Maxwell worked out his famous equations of electromagnetism, problems became evident. According to Newtonian mechanics, it is possible to catch up to anything, including a ray of light. But according to Maxwellian electrodynamics, that ray of light has a fixed speed in a vacuum, so it is not possible to catch up to that ray.

This discrepancy led to a variety of hypotheses, with the most prominent one being the ether theory, that there is some reference frame that Maxwell’s equations hold, with motion relative to it giving correction terms. Physicists like Albert Michelson and Edward Morley tried looking for such effects, but they got upper limits below expected values.

This made the difficulty even worse, and theorists experimented with ways to avoid this difficulty. Henrik Antoon Lorentz was the most successful with his proposal that moving relative to this “ether frame” causes distortions of space and time. Albert Einstein worked from it and showed that there was no way to work out what this “ether frame” was. He also worked out the consequences for momentum and energy. This was “special relativity”, modifying Newtonian mechanics to be consistent with Maxwellian electrodynamics. Einstein later worked out “general relativity”, generalizing to curved space-time.

This means that time is relative, just as space is, and that there is no “universal time”. That causes a lot of difficulty for presentism, because it requires some slicing of space-time to create each “present time”, a slicing that is totally unmotivated. But eternalism does not have that problem.

Thus being another counterintuitive aspect of relativity.

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