What Came Before the Big Bang? Exploring Pre-Universe Matter and Energy

[Chalnoth]

Care to elaborate? I don't know why you wouldn't use something like potential energy in this situation.

Well, since you'd usually use Newtonian gravity in this situation, you probably would. But my point is that when you work with General Relativity, there simply isn't any potential energy term there, and furthermore there is not even any good way to define one in general.

 

[Drakkith]

Well, since you'd usually use Newtonian gravity in this situation, you probably would. But my point is that when you work with General Relativity, there simply isn't any potential energy term there, and furthermore there is not even any good way to define one in general.

Alright, then is this even an issue in GR?

And is GR the only thing to look at here in regards to conservation of energy?

 

[Chalnoth]

Alright, then is this even an issue in GR?

And is GR the only thing to look at here in regards to conservation of energy?

Well, it's a conceptual issue, to be sure, as many people have a hard time understanding that conservation of energy is only an approximate, local law, one that simply does not hold globally.

The main issue here is that conservation of energy stems from invariance in time. That is, if you can take a system and examine it at two different times and the system remains the same in some specific mathematical sense for those two times, then energy is necessarily conserved between those two times.

Most of the physical laws we know and love, whether they be Newtonian physics, or electricity and magnetism, or quantum mechanics all are unchanged in time, and so they all follow energy conservation. General Relativity throws a wrench into this whole thing because it makes the time coordinate arbitrary: if you can change your very definition of what you mean by "time", then the very idea of "time invariance" ceases to have meaning, and so while you might be able to write down a specific system where energy conservation holds, in general it just won't.