0
$\begingroup$

We know from the first law of thermodynamics that the amount of energy in the universe is conserved. However, doesn't this law presuppose that the amount of energy in the universe is a finite quantity of Joules?

As David Hume reasoned, all of science depends on inductive generalization. But wouldn't it be a fallacy of hasty generalization to conclude from models and experiments where the quantity of joules are finite and contained by an adiabatic wall that therefore the universe at large is like this?

Improve this question
$\endgroup$
0

1 Answer 1

Reset to default
1
$\begingroup$

Ideally, yes. Energy is conserved, so all interactions just release (e.g. chemical reactions) shift and distribute energy around. But the sum should stay the same. All stored chemical energy sources will eventually be exhausted, heat exchanges between any bodies at different temperatures will keep happening until the whole universe is in thermal equilibrium. Maximum entropy. No more thermal exchanged. Heat death of the Universe.

However, energy is not conserved in the Universe.

This is because of General Relativity and the (accelerated) expansion of the Universe. The universe is expanding, which means that something is driving the expansion - dark energy. But the expansion is accelerating, so this dark energy is somehow increasing. From where? Nobody really knows.

If you have physics background, then you will know that energy conservation is linked to time invariance of the Hamiltonian. From Noether's theorem, the conserved charge in time translations is the energy of the system. Since the accelerated expansion of the universe breaks the time translation invariance of the system (i.e. the whole universe), energy in said system is not conserved.

Improve this answer
$\endgroup$
7
  • $\begingroup$ Thanks SuperCiocia. I was just going to write something on these lines. $\endgroup$
    – Sayan Mandal
    Commented Oct 1, 2017 at 1:07
  • $\begingroup$ This is full of mistakes. The universe is expanding, which means that something is driving the expansion - dark energy. No, this is an elementary misconception. Cosmological models without dark energy also feature expansion. GR has local energy conservation but not (in a general spacetime) global energy conservation. This has nothing to do with Noether's theorem (which basically just can't be used in GR for technical reasons). It simply wouldn't make sense to have global conservation of energy in GR because energy-momentum is a vector, and we can't say how to add [...] $\endgroup$
    – user4552
    Commented Oct 1, 2017 at 2:19
  • $\begingroup$ [...] vectors that are in different tangent spaces, due to ambiguities caused by the path-dependence of parallel transport. $\endgroup$
    – user4552
    Commented Oct 1, 2017 at 2:20
  • $\begingroup$ I was under the impression that Noether's theorem could still be used, with the use of Killing vector fields. I was not aware of what you said about parallel transport. This is how I was told in my introductory GR lecture course, which I believed to be true. I will follow up on your information. $\endgroup$
    – SuperCiocia
    Commented Oct 1, 2017 at 2:27
  • $\begingroup$ @SuperCiocia: The stuff about Killing fields would relate to conserved quantities for the motion of a test particle, which is a much more restricted thing than mass-energy conservation for the entire spacetime. $\endgroup$
    – user4552
    Commented Oct 1, 2017 at 16:59

Not the answer you're looking for? Browse other questions tagged or ask your own question.