8 ms·

No, it doesn't, because the concept of "gravitational potential energy" is not meaningful for an expanding universe considered as a whole. It's only meaningful

by pdonis 3y ago

No, it doesn't, because the concept of "gravitational potential energy" is not meaningful for an expanding universe considered as a whole. It's only meaningful for isolated systems within the universe.

thx-2718 3y ago
Wouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? Which that amount should be increasing as space-time expands. Thought experiment, if you could place a mass of an arbitrary amount at any one point in space, how much mass would you need such that all the mass of the universe is now falling towards it. Or could you bend space-time to a point that all mass falls into it.
- pdonis 3y ago
  > Wouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? No. The universe is not an isolated system that we can operate on from the outside. You can't treat it as though it is. So your thought experiments aren't meaningful.
  - thx-2718 3y ago
    Obviously the thought experiment requires energy that doesn't 'exist' or doesn't have meaning in the sense that it could happen literally. It's a what-if and that does have a number and that does have meaning. So there is meaning to the previous persons question which is what the thought experiments were meant to show but obviously that's something you can't imagine.
    - pdonis 3y ago
      > Obviously the thought experiment requires energy that doesn't 'exist' No, it requires energy to be added to the system from outside the system. Which is precisely what you cannot do with the universe as a whole. That's what makes such thought experiments meaningless for the universe as a whole.
      - thx-2718 3y ago
        > it requires energy to be added to the system from outside the system. Yes that is exactly what energy that doesn't 'exist' means.
        pdonis 3y ago
        But you can't do that for the universe as a whole. Asking what would be the case if you could is meaningless; it's like asking what would be the case if 2 + 2 were 5. No consistent model exists of such a situation, so the question is meaningless.
        thx-2718 3y ago
        > you can't do that for the universe as a whole. Yes correct, that's why that energy doesn't 'exist' because it's outside of the universe.
        pdonis 3y ago
        There is no such thing as "outside of the universe". If you disagree, please show me the consistent model on which your scenario is based.
        thx-2718 3y ago
        > There is no such thing as "outside of the universe". Yes that's correct in the general accepted sense for the term 'universe'. Edit: question if the initial thought experiment was posed as 'observable universe' would it make a difference to you?
        pdonis 3y ago
        > if the initial thought experiment was posed as 'observable universe' would it make a difference to you? It would address my "can't operate on it from the outside" objection, yes. But it still wouldn't make the thought experiment meaningful, for the reasons I gave in response to wyager elsewhere in the thread.
  - wyager 3y ago
    How is your objection not a useless and unfalsifiable catch-all that applies to calculating any quantity over a volume?
    - pdonis 3y ago
      Um, what? I can operate on an ordinary volume (say a beaker in my lab or a planet that I am in a distant orbit around) from the outside. I can't operate on the universe as a whole from the outside. How is this not an obvious difference?
      - wyager 3y ago
        If you fix a sub-volume of the universe where the boundaries of the volume are subject to the expansion of space, you can calculate the energy in the volume. The question upthread is clearly "does the energy in this volume increase due to expansion?". I'm not sure why you're so focused on integrating over the entire universe; that wasn't an important part of the question upthread. You are being very vague. If you have a coherent mathematical objection that you are trying to explain indirectly, please just say the mathematical objection.
        pdonis 3y ago
        > If you fix a sub-volume of the universe Then you are not talking about the thought experiment that I was responding to, but about a different one. I have no objection to talking about the different thought experiment that you propose (and I'll do that below), but nothing in any such discussion is relevant to the objection I made to the original thought experiment, which was about the entire universe, not just some portion of it. > you can calculate the energy in the volume Actually, no, you can't. There is no known invariant in GR that corresponds to "the total energy inside this volume" for an expanding universe. There are only two cases in GR where we have known invariants that correspond to "the total energy inside this volume": (1) an asymptotically flat spacetime, where we can define the ADM energy and the Bondi energy; and a stationary spacetime, where we can define the Komar energy. An expanding universe does not fall into either of these categories. You will find claims in the literature that a "total energy" for cases like an expanding universe can be calculated using so-called "pseudo tensors". However, such claims are not accepted by many physicists, and even physicists who do accept that "pseudo-tensors" are physically meaningful don't all agree on which pseudo-tensors those are. You can, of course, choose some set of coordinates (such as the standard FRW coordinates used in cosmology), and integrate energy density over some spatial volume in a 3-surface of constant coordinate time. (It is not clear that this is a correct way to get "total energy", because in GR the source of gravity is the total stress-energy tensor, which includes momentum, pressure, and stresses as well as energy density, but we'll leave that aside for now.) But the result of any such computation is not an invariant; it depends on your choice of coordinates. The energies I referred to above (ADM, Bondi, Komar) do not. That is why they are accepted as physically meaningful by all physicists. > The question upthread is clearly "does the energy in this volume increase due to expansion?" It's not at all clear to me that that is the question being asked upthread (for one thing, that poster, in another subthread, has explicitly said the "energy" they are thinking of adding comes from outside the universe). But even if we assume it is, the question is still meaningless because it assumes there is such a thing as "the energy in this volume", which, as above, there isn't.
wyager 3y ago
Why is it not meaningful? "Isolated systems" seems meaningless - there is no objective cutoff where a gravitational system becomes "isolated", except perhaps in the sense of "non-intersecting light cones".
- pdonis 3y ago
  See my response to thx-2718 downthread, about having to add energy from outside the system.
  - wyager 3y ago
    I have read all of your comments and not one of them actually says anything concrete. It's the exact same vague objection repeated over and over again. Please explain exactly why you think calculating the total gravitational potential energy of the entire universe or a well-defined sub-volume of it is intractable. Feel free to use arbitrarily technical mathematical or physics language, just please stop being vague.
    - pdonis 3y ago
      I responded to this in the other subthread where we are having this discussion.
deleted 3y ago
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by pdonis 3y ago

thx-2718 3y ago

Wouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? Which that amount should be increasing as space-time expands. Thought experiment, if you could place a mass of an arbitrary amount at any one point in space, how much mass would you need such that all the mass of the universe is now falling towards it. Or could you bend space-time to a point that all mass falls into it.

pdonis 3y ago
> Wouldn't there be meaning in saying it would take this much energy to push all the matter in the universe to one place? No. The universe is not an isolated system that we can operate on from the outside. You can't treat it as though it is. So your thought experiments aren't meaningful.
- thx-2718 3y ago
  Obviously the thought experiment requires energy that doesn't 'exist' or doesn't have meaning in the sense that it could happen literally. It's a what-if and that does have a number and that does have meaning. So there is meaning to the previous persons question which is what the thought experiments were meant to show but obviously that's something you can't imagine.
  - pdonis 3y ago
    > Obviously the thought experiment requires energy that doesn't 'exist' No, it requires energy to be added to the system from outside the system. Which is precisely what you cannot do with the universe as a whole. That's what makes such thought experiments meaningless for the universe as a whole.
    - thx-2718 3y ago
      > it requires energy to be added to the system from outside the system. Yes that is exactly what energy that doesn't 'exist' means.
      - pdonis 3y ago
        But you can't do that for the universe as a whole. Asking what would be the case if you could is meaningless; it's like asking what would be the case if 2 + 2 were 5. No consistent model exists of such a situation, so the question is meaningless.
        thx-2718 3y ago
        > you can't do that for the universe as a whole. Yes correct, that's why that energy doesn't 'exist' because it's outside of the universe.
        pdonis 3y ago
        There is no such thing as "outside of the universe". If you disagree, please show me the consistent model on which your scenario is based.
        thx-2718 3y ago
        > There is no such thing as "outside of the universe". Yes that's correct in the general accepted sense for the term 'universe'. Edit: question if the initial thought experiment was posed as 'observable universe' would it make a difference to you?
        pdonis 3y ago
        > if the initial thought experiment was posed as 'observable universe' would it make a difference to you? It would address my "can't operate on it from the outside" objection, yes. But it still wouldn't make the thought experiment meaningful, for the reasons I gave in response to wyager elsewhere in the thread.
- wyager 3y ago
  How is your objection not a useless and unfalsifiable catch-all that applies to calculating any quantity over a volume?
  - pdonis 3y ago
    Um, what? I can operate on an ordinary volume (say a beaker in my lab or a planet that I am in a distant orbit around) from the outside. I can't operate on the universe as a whole from the outside. How is this not an obvious difference?
    - wyager 3y ago
      If you fix a sub-volume of the universe where the boundaries of the volume are subject to the expansion of space, you can calculate the energy in the volume. The question upthread is clearly "does the energy in this volume increase due to expansion?". I'm not sure why you're so focused on integrating over the entire universe; that wasn't an important part of the question upthread. You are being very vague. If you have a coherent mathematical objection that you are trying to explain indirectly, please just say the mathematical objection.
      - pdonis 3y ago
        > If you fix a sub-volume of the universe Then you are not talking about the thought experiment that I was responding to, but about a different one. I have no objection to talking about the different thought experiment that you propose (and I'll do that below), but nothing in any such discussion is relevant to the objection I made to the original thought experiment, which was about the entire universe, not just some portion of it. > you can calculate the energy in the volume Actually, no, you can't. There is no known invariant in GR that corresponds to "the total energy inside this volume" for an expanding universe. There are only two cases in GR where we have known invariants that correspond to "the total energy inside this volume": (1) an asymptotically flat spacetime, where we can define the ADM energy and the Bondi energy; and a stationary spacetime, where we can define the Komar energy. An expanding universe does not fall into either of these categories. You will find claims in the literature that a "total energy" for cases like an expanding universe can be calculated using so-called "pseudo tensors". However, such claims are not accepted by many physicists, and even physicists who do accept that "pseudo-tensors" are physically meaningful don't all agree on which pseudo-tensors those are. You can, of course, choose some set of coordinates (such as the standard FRW coordinates used in cosmology), and integrate energy density over some spatial volume in a 3-surface of constant coordinate time. (It is not clear that this is a correct way to get "total energy", because in GR the source of gravity is the total stress-energy tensor, which includes momentum, pressure, and stresses as well as energy density, but we'll leave that aside for now.) But the result of any such computation is not an invariant; it depends on your choice of coordinates. The energies I referred to above (ADM, Bondi, Komar) do not. That is why they are accepted as physically meaningful by all physicists. > The question upthread is clearly "does the energy in this volume increase due to expansion?" It's not at all clear to me that that is the question being asked upthread (for one thing, that poster, in another subthread, has explicitly said the "energy" they are thinking of adding comes from outside the universe). But even if we assume it is, the question is still meaningless because it assumes there is such a thing as "the energy in this volume", which, as above, there isn't.

wyager 3y ago

Why is it not meaningful? "Isolated systems" seems meaningless - there is no objective cutoff where a gravitational system becomes "isolated", except perhaps in the sense of "non-intersecting light cones".

pdonis 3y ago
See my response to thx-2718 downthread, about having to add energy from outside the system.
- wyager 3y ago
  I have read all of your comments and not one of them actually says anything concrete. It's the exact same vague objection repeated over and over again. Please explain exactly why you think calculating the total gravitational potential energy of the entire universe or a well-defined sub-volume of it is intractable. Feel free to use arbitrarily technical mathematical or physics language, just please stop being vague.
  - pdonis 3y ago
    I responded to this in the other subthread where we are having this discussion.

deleted 3y ago

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