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As far as I know (and I don't know much), Noether's theorem claims that time translation invariance of the laws of physics leads to the conservation of energy. The way I understand it is that if we imagine a universe where all the laws of physics always require time as one of the inputs, then energy in this universe is not conserved.

But the laws of physics do not necessarily only describe energy, do they? There's at least linear/angular momentum, and maybe other things as well. So why is it that only energy is not conserved in the above universe? What makes time translation invariance specific to energy?

Parzh
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As far as I know (and I don't know much), Noether's theorem claims that time translation invariance of the laws of physics leads to the conservation of energy.

That is incomplete, but not incorrect. As you noticed, there are other conservation laws, and in fact they also come from symmetries. What Noether's theorem states is that for every continuous symmetry of a physical system there is a corresponding conservation law. For example, translation symmetry leads to the conservation of linear momentum, and rotational symmetry leads to the conservation of angular momentum.

The way I understand it is that if we imagine a universe where all the laws of physics always require time as one of the inputs, then energy in this universe is not conserved.

It it worth pointing out that energy is not conserved in our Universe, because the Universe is in expansion (hence, it gets bigger, and time translation symmetry does not apply). At local scales this is negligible, but it is relevant at large scales.

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Does Noether theorem apply only to energy conservation?

No.

The way I understand it is that if we imagine a universe where all the laws of physics always require time as one of the inputs, then energy in this universe is not conserved.

When there is explicit time dependence in the system, the energy is generally not conserved. Generally, the Hamiltonian energy will change with time as: $$ \frac{dH}{dt} = \frac{\partial H}{\partial t}\;,\tag{A} $$ where the RHS is generally not zero when there is explicit time dependence.

But the laws of physics do not necessarily only describe energy, do they?

That's right, there are other "integrals of the motion," as they say.

There's at least linear/angular momentum, and maybe other things as well.

Yes, linear momentum conservation follows from invariance under spatial translation, just like energy conservation follows from invariance under time translation.

What makes time translation invariance specific to energy?

It just is the way it is. For example, consider a classical real scalar field theory. The conserved charges associated with space-time translations are the $0\mu$ components of the stress-energy tensor, which can be written in this case as: $$ T^{\mu}_{\nu} = \frac{\partial L}{\partial \partial_\mu\phi}\partial_\nu\phi - L\delta^{\mu}_\nu\;, $$ from which it follows that $$ H = \int d^3x T^{00} $$ is the conserved charge associated with time-translation invariance, i.e., the energy.

And $$ P^i = \int d^3x T^{0i} $$ is the conserved charge associated with space-translation invariance, i.e., the momentum.


A simple way to see how time-translation invariance leads to energy conservation is to just use Eq. (A) above.

To put a little more color on that equation, recall that the Hamiltonian energy is defined for a system with a single degree of freedom as $$ H = p \dot q - L(q,\dot q)\;,\tag{1} $$ where $$ p = \frac{\partial L}{\partial \dot q}\;, $$ is the momentum, and where $q$ is the generalized coordinate.

Note that I wrote $L(q,\dot q)$ in Eq. (1) to indicate that there is no explicit time dependence in the system (so we suspect the energy will be conserved).

From Eq. (1) we have: $$ \frac{dH}{dt} = \dot p \dot q + p\ddot q - \frac{\partial L}{\partial q}\dot q - \frac{\partial L}{\partial \dot q}\ddot q $$ $$ = 0\;,\tag{2} $$ since the second term cancels with the fourth term, by definition, and the first term cancels with the third term, by the Lagrange equations of motion. Eq. (2) says that the energy is conserved. Thus, we see that time translation invariance leads to conservation of energy.


Similarly, suppose the Lagrangian was translationally invariant. Then we have $$ \frac{\partial L}{\partial q} = 0\;, $$ which, by the Lagrange equations of motion says: $$ \frac{d}{dt}\frac{\partial L}{\partial \dot q} = 0 \equiv \frac{dp}{dt}\;. $$ Thus, we see that space translation invariance leads to conservation of linear momentum.

hft
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I would just like to point out that energy "not being conserved" in our Universe depends on the details of what you mean.

If you take the expanding universe to be god given then phenomena within that background will be described by a system whose "physical laws" depend on time. Hence you can say this system (which does not include the universe itself) does not conserve energy. But really it is better to say that it has no definition of energy.

The above is often what we do in practice but it can lead to confusion for somebody new to Noether's theorem. Indeed it is strange to say "the laws of physics depend on time" when most relativists who think about cosmic expansion every day don't believe this to be the case. Someone who tries to be as fundamental as possible will write down a theory which attempts to describe the expansion of the universe itself (e.g. Einstein's equation). And in this case, the fact that our universe happens to be expanding has no bearing on the symmetries. Symmetries are properties of a system and they lead to quantities that are conserved by any solution to the equations of motion. So the properties of a particular solution can't be used to rule out what symmetries were there in the first place.

The statement that our universe does not conserve energy (while true in one sense) has been thrown around fairly quickly ever since Sean Carroll wrote a blog post about it. We should also draw attention to the more fundamental statement (as Carroll does elsewhere in his writing) that the Hamiltonian vanishes in general relativity.

Connor Behan
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I'm not sure if you know special relativity or not, but time can be described as another dimension. As you stated, time translation invariance gives us conservation of energy. However, spacial translation invariance gives us conservation of momentum. Effectively, time is to space what energy is to momentum. This is why energy is not a vector, just as time is not a vector. It may help to consider the 4-vectors of position and momentum so I will link the wikipedia page here

Nic
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