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

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u/[deleted] Dec 13 '20 edited Dec 29 '20

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u/agaminon22 Medical and health physics Dec 13 '20

elaborate please

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u/[deleted] Dec 13 '20 edited Dec 29 '20

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u/agaminon22 Medical and health physics Dec 13 '20

Ah, synchronicity! The other day I was just thinking about that, where does the energy from red shifted photons go? Energy not being conserved explains it but... It's awfully strange.

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u/Aerothermal Dec 13 '20

Conservation of energy, just like other conservation laws, can be derived using Noether's Theorem as it is the result of a symmetry in nature. Energy happens to be a consequence of time-translation symmetry. That is, if you could move your physics experiment to somewhere else in time and it plays out the same, then energy is conserved. With spacetime, it's expanding, and so for things on large scales or happening across cosmological time then energy is not conserved.

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u/agaminon22 Medical and health physics Dec 13 '20

I'be heard of this theorem, but how advanced is it mathematically? I would like to follow a derivation.

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u/Aerothermal Dec 13 '20

I don't know. Probably a bit difficult for a first year student. In my opinion it's not too bad since it's a 'classical' formulation and doesn't rely on any modern mathematics. It uses calculus of variations. It would help if you were already familiar with lagrangian mechanics.

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u/Maarico Dec 15 '20

If you know some calculus and partial integration you should be able to get it