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Taking a gravity and then performing an impossible physical modification so you can make conjectures on the model is nonsense. You can't take an model, apply a

by Clent 2y ago

Taking a gravity and then performing an impossible physical modification so you can make conjectures on the model is nonsense. You can't take an model, apply an impossible scenario and then claim the model is invalid because it can't account for this scenario that you admit is impossible.

taylodl 2y ago
> You can't take an model, apply an impossible scenario and then claim the model is invalid because it can't account for this scenario that you admit is impossible. That's not what's being said. The fact is GR cannot explain why these two particles will suddenly start moving toward one another. All I've done in this simple thought experiment is eliminate every other externality. The key point here is GR doesn't explain why curved spacetime causes objects to move. It only says that the movement can be modeled by curved spacetime. This thought experiment was just a way of expressing that. Which is all to say GR doesn't explain what gravity is, it provides us a (complicated!) set of equations for determining an object's motion in a gravitational field. In my mind that's a fundamental shortcoming of GR as it was a fundamental shortcoming of Newton. NEITHER of their theories even tried to explain what gravity was, they provided a mathematical model for describing the effects of gravity. Newton's was good, Einstein's is better - but neither tell us much about gravity itself. OTOH, quantum gravity tries to tell us what gravity is, but that has run into issues. Not only is it going to take someone of the caliber of Newton and Einstein to figure this out and people like that only come around every few centuries, but we have to have the means to test the theory. We have several theories, but we don't have the ability to test which theory is correct. We simply don't know which way to go. Anyway, all that is to say GR is incomplete - and we know it.
- woopsn 2y ago
  I am not a physicist, my sense though as a layman is that GR explains the motions with which it is concerned more effectively than QM or QFT do. As in, with Schrodinger's cat, what causes the poison's release? And why does that happen? The "shut up and calculate" thing I'd assumed referred to quantum theory.
  - taylodl 2y ago
    :) - yes, the "shut up and calculate" was a reference to quantum theory applied to GR, since people neglect to mention that GR doesn't explain what gravity actually is. And really, if you "shut up and calculate" using Einstein's Field Equations, you'll get results that largely align with observations, more so than Newton's equations, anyway. But there are observations that can't be explained by GR and there are simple thought experiments that can't be explained by GR. As I keep saying, we know GR is not the final word on gravity. That doesn't mean GR hasn't been incredibly useful - especially if you just "shut up and calculate!"
- deleted 2y ago
  [deleted]
- drdeca 2y ago
  The thing you are asking for, the "why does mass cause spacetime to bend" seems... perhaps impossible to answer? Or like, I don't see why there should necessarily be anything that satisfies what seems to be your reason for dissatisfaction. Imagine we explained it as "Mass causes X, and because X causes spacetime to be curved, we have that, mass (indirectly) causes spacetime to be curved." . But then, why would you be any more satisfied with this? Why would you not then ask, "Well, why does X cause spacetime to be curved?" ? (Or, "why does mass cause X?") Now, perhaps there is such an X, and perhaps it will be found. But, I don't see why this would satisfy you any more than "Mass causes spacetime to curve" satisfies you. If you keep asking "why", you either eventually end up in a loop, have an infinite regress, or stop getting an answer. A non-empty directed graph either has a cycle, an infinite outgoing path, or a vertex without an edge away from it. (And also, either a cycle, an infinite incoming path, or a vertex without an edge going to it.) Many are content to allow the laws of physics to just be, without explanation, others may say that the laws of physics are explained by God, who is without explanation. Personally, I go with the latter, but, unless one wants to go with infinite regress or a cyclic explanation, one has to allow that something is without explanation. For your complaint to be compelling, I think you should give criteria for, what conditions would something have to satisfy, in order for it to satisfy you? (Also, you seem to assume that if the two particles suddenly acquired mass, that they would immediately begin to feel a force between them. While the hypothetical is presumably impossible, still, what I imagine happening would instead by that there would be a light-speed delay between when they gained mass and when they began to accelerate towards one-another. Though, I'm not sure if there is a fact of the matter as to "what would happen in this impossible hypothetical".)

by Clent 2y ago

taylodl 2y ago

> You can't take an model, apply an impossible scenario and then claim the model is invalid because it can't account for this scenario that you admit is impossible. That's not what's being said. The fact is GR cannot explain why these two particles will suddenly start moving toward one another. All I've done in this simple thought experiment is eliminate every other externality. The key point here is GR doesn't explain why curved spacetime causes objects to move. It only says that the movement can be modeled by curved spacetime. This thought experiment was just a way of expressing that. Which is all to say GR doesn't explain what gravity is, it provides us a (complicated!) set of equations for determining an object's motion in a gravitational field. In my mind that's a fundamental shortcoming of GR as it was a fundamental shortcoming of Newton. NEITHER of their theories even tried to explain what gravity was, they provided a mathematical model for describing the effects of gravity. Newton's was good, Einstein's is better - but neither tell us much about gravity itself. OTOH, quantum gravity tries to tell us what gravity is, but that has run into issues. Not only is it going to take someone of the caliber of Newton and Einstein to figure this out and people like that only come around every few centuries, but we have to have the means to test the theory. We have several theories, but we don't have the ability to test which theory is correct. We simply don't know which way to go. Anyway, all that is to say GR is incomplete - and we know it.

woopsn 2y ago
I am not a physicist, my sense though as a layman is that GR explains the motions with which it is concerned more effectively than QM or QFT do. As in, with Schrodinger's cat, what causes the poison's release? And why does that happen? The "shut up and calculate" thing I'd assumed referred to quantum theory.
- taylodl 2y ago
  :) - yes, the "shut up and calculate" was a reference to quantum theory applied to GR, since people neglect to mention that GR doesn't explain what gravity actually is. And really, if you "shut up and calculate" using Einstein's Field Equations, you'll get results that largely align with observations, more so than Newton's equations, anyway. But there are observations that can't be explained by GR and there are simple thought experiments that can't be explained by GR. As I keep saying, we know GR is not the final word on gravity. That doesn't mean GR hasn't been incredibly useful - especially if you just "shut up and calculate!"
deleted 2y ago
[deleted]
drdeca 2y ago
The thing you are asking for, the "why does mass cause spacetime to bend" seems... perhaps impossible to answer? Or like, I don't see why there should necessarily be anything that satisfies what seems to be your reason for dissatisfaction. Imagine we explained it as "Mass causes X, and because X causes spacetime to be curved, we have that, mass (indirectly) causes spacetime to be curved." . But then, why would you be any more satisfied with this? Why would you not then ask, "Well, why does X cause spacetime to be curved?" ? (Or, "why does mass cause X?") Now, perhaps there is such an X, and perhaps it will be found. But, I don't see why this would satisfy you any more than "Mass causes spacetime to curve" satisfies you. If you keep asking "why", you either eventually end up in a loop, have an infinite regress, or stop getting an answer. A non-empty directed graph either has a cycle, an infinite outgoing path, or a vertex without an edge away from it. (And also, either a cycle, an infinite incoming path, or a vertex without an edge going to it.) Many are content to allow the laws of physics to just be, without explanation, others may say that the laws of physics are explained by God, who is without explanation. Personally, I go with the latter, but, unless one wants to go with infinite regress or a cyclic explanation, one has to allow that something is without explanation. For your complaint to be compelling, I think you should give criteria for, what conditions would something have to satisfy, in order for it to satisfy you? (Also, you seem to assume that if the two particles suddenly acquired mass, that they would immediately begin to feel a force between them. While the hypothetical is presumably impossible, still, what I imagine happening would instead by that there would be a light-speed delay between when they gained mass and when they began to accelerate towards one-another. Though, I'm not sure if there is a fact of the matter as to "what would happen in this impossible hypothetical".)