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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 th

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.
        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.
- 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 :-)

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.
        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.
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 :-)