The time does not change in Galilean transformations precisely because there is a global time that can be defined.

We now summarize this discussion. The key points are: time has to be realizable physically, i.e. we have to construct some clock that tells us what time means. Relating times at different points requires a well defined, physically realizable method of synchronization. In order to construct a time that is well defined everywhere, we need an infinite number of observers with synchronized clocks. Newtonian mechanics requires the assumption that light travels infinitely fast, which is a physical assumption about how the world works.

My hope is that you see the key roles the requirement of physical clocks and the assumption about the speed of light play. If you think about time as a family of physical clocks ticking, then the fact that time changes between Newtonian mechanics and relativity is not so mysterious or surprsing. Eventually, you want to get to the point that *of course* if light doesn't travel infinitely fast then the Galilean transformations can't hold. *Of course* if we measure the speed of light to be the same for all observers Newtonian concept of time can't quite be right. *Of course* if we take the limit that the speed of light be large we translate from relativity back to Newtonian mechanics.

Next set: space, coordinate systems, and distances.

Sets after: metrics, Lorentz transformations

Then: black holes, entropy, black hole entropy

This set has been all conceptual. As we develop the conceptual underpinnings we will move on to more sophisticated technical aspects.

Everyone: go find extra free reading on this topic. If you find something you like, put it in the discussion of this note for your fellow Brillianteers.

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## Comments

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TopNewestYay! Great questions.

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@David Mattingly, Great job...it was a well defined series of questions and thank you to provide it... Cheers!!!!

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Thank you , this was very helpful But does this mean that there is a universal time but we can't measure it ?

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How can absolute time be defined? Because if light travels certain distance in a sec, that distance is also not absolute I.e relative

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what happens to this argument if you take the speed of light to approach infinity?

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Relativity

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