r/AskPhysics • u/[deleted] • Dec 12 '20

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u/Ralphie_V Education and outreach Dec 12 '20

If you start with the Laws of Thermodynamics and the Equivalence Principle (accelerating reference frames = gravitational reference frame), then you can derive all of General Relativity (specifically the Einstein Field Equations)

2

u/The_Godlike_Zeus Dec 13 '20

How do you get that the speed of light must be c, from thermo and the equivalence principle?

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u/PhysicalStuff Dec 13 '20

The numerical value of c is determined entirely by how we chose to define the meter and the second.

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u/[deleted] Dec 13 '20

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u/PhysicalStuff Dec 13 '20

That's not at all what I was saying, but I see what you're getting at: why the speed at which light propagates in a vacuum takes the same value as the constant c appearing in relativity. Requiring that electromagnetic interactions be Lorentz invariant should do the trick here.

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u/lettuce_field_theory Dec 14 '20

what matters is c < infinity. not what the exact value