4 ms·

> 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.
        pdonis 3y ago
        > Original thought experiment said matter Your original thought experiment asked about the energy required to push all the matter to one place. But to even try to formulate such a question in GR, "matter" has to mean "whatever stress-energy is present in the universe". And "energy" has to mean the same thing, because stress-energy is what "does stuff" in GR, not "mass". > that's normal everyday language You can't do physics in normal everyday language. > 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. The fact that you can't "magically pop mass into one place" is true in any spacetime in GR, not just a closed universe; it's a consequence of the Einstein Field Equation and the Bianchi identities. You also can't magically "take" mass from some place, for the same reason. > 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. These statements are only valid in the particular cases I have already listed: an asymptotically flat spacetime, or a stationary spacetime. An expanding universe is neither. I understand that you don't get this; that's because you are using ordinary language, but, as I said above, you can't do physics in ordinary language. Your reasoning looks OK to you because you don't understand that, except in the special cases I described, the ordinary language you are using does not correspond to any valid physics. (I have explained why in other subthreads in this discussion.) I know it looks to you like it ought to; but it doesn't.

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.
        pdonis 3y ago
        > Original thought experiment said matter Your original thought experiment asked about the energy required to push all the matter to one place. But to even try to formulate such a question in GR, "matter" has to mean "whatever stress-energy is present in the universe". And "energy" has to mean the same thing, because stress-energy is what "does stuff" in GR, not "mass". > that's normal everyday language You can't do physics in normal everyday language. > 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. The fact that you can't "magically pop mass into one place" is true in any spacetime in GR, not just a closed universe; it's a consequence of the Einstein Field Equation and the Bianchi identities. You also can't magically "take" mass from some place, for the same reason. > 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. These statements are only valid in the particular cases I have already listed: an asymptotically flat spacetime, or a stationary spacetime. An expanding universe is neither. I understand that you don't get this; that's because you are using ordinary language, but, as I said above, you can't do physics in ordinary language. Your reasoning looks OK to you because you don't understand that, except in the special cases I described, the ordinary language you are using does not correspond to any valid physics. (I have explained why in other subthreads in this discussion.) I know it looks to you like it ought to; but it doesn't.