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Einstein proved that Time is not absolute and said something about how we experience time in different ways depending on how we are moving. But he hardly explained what Time is. And nobody else ever has.
British physicist Julian Barbour has a theory that Time does not exist, and that most of Physics' troubles arise from assuming that it does exist. We have no evidence of the past other than our memory of it. We have no evidence of the future other than our belief in it.
Barbour believes that it is all an illusion: there is no motion and no change. Instants and periods do not exist. What exists is only "time capsules", which are static containers of "records". Those records fool us into believing that things change and events happen.
There exists a "configuration space" that contains all possible instants, all possible "nows". This is "Platonia." These instants -- We experience a set of these instants, i.e. a subset of Platonia.
Barbour is inspired by Leibniz' theory that the universe is not a container of objects, but a collection of entities that are both space and matter. The universe does not contain things, it "is" things.
Barbour does not answer the best part of the puzzle: who is deciding which "path" we follow in Platonia? Who is ordering the instants of Platonia? Barbour simply points to quantum mechanics, that prescribes we should always be in the "instant" that is most likely. We experience an ordered flow of events because that is what we were designed for: to interpret the sequence of most likely instants as an ordered flow of events.
Barbour also offers a solution to integrating relativity and quantum mechanics: remove time from a quantum description of gravity. Remove time from the equations. In his opinion, time is precisely the reason why it has proved so difficult to integrate relativity and quantum theories.