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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

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.
        thx-2718 3y ago
        No. Original thought experiment said matter you're talking about total energy in your response. We ought to take matter to mean ordinary every day matter, mass being the property that all regular ordinary matter posses. Since that's normal everyday language. Yes in a closed universe the net energy is 0, nothing in; nothing out, so you wouldn't be able to magically 'pop' mass into one place without taking from somewhere else. Now if mass A is 100 meters away from mass B it would take X amount of energy to push/pull them together. If they are now 150 meters apart now the energy required to bring them together is now higher. So if galaxies are pulling away and away from each other over time then overtime the energy you would need to bring them closer together again increases.
    - faangsticle 3y ago
      Now take off your nitpick hat and ignore that detail. Its what makes it a thought experiment.
      - pdonis 3y ago
        You can't ignore it because it's not a "detail"--it's a crucial feature of the thought experiment that doesn't work for the universe as a whole. What you're suggesting is like saying, in my thought experiment I assume that 2 + 2 = 5, just ignore the fact that 2 + 2 is actually 4.
        faangsticle 3y ago
        Yes, that's what a thought experiment is. A cat can't be alive and death at the same time, either.
        pdonis 3y ago
        > that's what a thought experiment is. No. A thought experiment cannot be based on a contradiction. > A cat can't be alive and death at the same time, either. And the Schrodinger's Cat thought experiment does not claim that it is, even though many pop science discussions try to claim otherwise. The Schrodinger's Cat thought experiment is based on the math of QM, i.e., on a consistent underlying model. It simply points out consequences of that model that might not be obvious to many people. The "thought experiment" I have been objecting to in this thread, by contrast, is not based on any consistent mathematical model. The operation it is proposing to do on the universe as a whole is inconsistent with GR, which is the only consistent model we have of the universe as a whole. That means it's not a valid thought experiment. "Thought experiment" does not mean you can make up whatever you want.
        faangsticle 3y ago
        > "Thought experiment" does not mean you can make up whatever you want. The only thing we can agree on is that "thought agreement" means that you can't make up whatever you want :-)
- 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.

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.
        thx-2718 3y ago
        No. Original thought experiment said matter you're talking about total energy in your response. We ought to take matter to mean ordinary every day matter, mass being the property that all regular ordinary matter posses. Since that's normal everyday language. Yes in a closed universe the net energy is 0, nothing in; nothing out, so you wouldn't be able to magically 'pop' mass into one place without taking from somewhere else. Now if mass A is 100 meters away from mass B it would take X amount of energy to push/pull them together. If they are now 150 meters apart now the energy required to bring them together is now higher. So if galaxies are pulling away and away from each other over time then overtime the energy you would need to bring them closer together again increases.
  - faangsticle 3y ago
    Now take off your nitpick hat and ignore that detail. Its what makes it a thought experiment.
    - pdonis 3y ago
      You can't ignore it because it's not a "detail"--it's a crucial feature of the thought experiment that doesn't work for the universe as a whole. What you're suggesting is like saying, in my thought experiment I assume that 2 + 2 = 5, just ignore the fact that 2 + 2 is actually 4.
      - faangsticle 3y ago
        Yes, that's what a thought experiment is. A cat can't be alive and death at the same time, either.
        pdonis 3y ago
        > that's what a thought experiment is. No. A thought experiment cannot be based on a contradiction. > A cat can't be alive and death at the same time, either. And the Schrodinger's Cat thought experiment does not claim that it is, even though many pop science discussions try to claim otherwise. The Schrodinger's Cat thought experiment is based on the math of QM, i.e., on a consistent underlying model. It simply points out consequences of that model that might not be obvious to many people. The "thought experiment" I have been objecting to in this thread, by contrast, is not based on any consistent mathematical model. The operation it is proposing to do on the universe as a whole is inconsistent with GR, which is the only consistent model we have of the universe as a whole. That means it's not a valid thought experiment. "Thought experiment" does not mean you can make up whatever you want.
        faangsticle 3y ago
        > "Thought experiment" does not mean you can make up whatever you want. The only thing we can agree on is that "thought agreement" means that you can't make up whatever you want :-)
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.