5 ms·

Not familiar with this take. Is the idea that curved space time is a semi-intuitive model or output of something more fundamental?

Filligree 2y ago
The usual idea is that there’s something more fundamental. Threads to weave the fabric out of, to extend the analogy. Nobody has yet created a fully convincing model, but it certainly would be elegant if spacetime itself arises from something that doesn’t need four-dimensional semi-hyperbolic space to be assumed.
taylodl 2y ago
Curved spacetime is a mathematical model Einstein used to model gravity. Einstein advised to be careful to not confuse the model with reality. To wit, GR doesn't explain how mass/energy curves spacetime, it merely(!) provides the tensor needed to explain gravity as the result of such a curvature. Consider this: you have two massless particles that are motionless with respect to one another. Suddenly both particles acquire mass at the same time (let's set aside this is physically impossible - this is a thought experiment). How does curved spacetime explain the sudden gravitational attraction between those two particles and how they're accelerating toward one another? It doesn't. These problems are how we know GR is wrong, the problem is it's not flat-out wrong! Einstein's field equations are better than what Newton had provided and explains more observed phenomena - they even predict phenomena we have since observed! GR is a very good theory (model) for gravity, but it's not the whole picture - and we know it. We simply don't know how to improve upon it and I personally think that believing curved spacetime is actual "reality" is a big part of why we haven't made much progress. Well, that and some other issues QFT has when attempting to model a graviton, but this comment is already long! EDIT: merely(!) - this isn't to understate Einstein's contribution to the matter which was quite considerable! But when the old genius himself is warning you to not get too caught up in the model, I suggest we take heed. EDIT 2: QFT has issues modeling a graviton, not a gluon.
- 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.

Filligree 2y ago

The usual idea is that there’s something more fundamental. Threads to weave the fabric out of, to extend the analogy. Nobody has yet created a fully convincing model, but it certainly would be elegant if spacetime itself arises from something that doesn’t need four-dimensional semi-hyperbolic space to be assumed.

taylodl 2y ago

Curved spacetime is a mathematical model Einstein used to model gravity. Einstein advised to be careful to not confuse the model with reality. To wit, GR doesn't explain how mass/energy curves spacetime, it merely(!) provides the tensor needed to explain gravity as the result of such a curvature. Consider this: you have two massless particles that are motionless with respect to one another. Suddenly both particles acquire mass at the same time (let's set aside this is physically impossible - this is a thought experiment). How does curved spacetime explain the sudden gravitational attraction between those two particles and how they're accelerating toward one another? It doesn't. These problems are how we know GR is wrong, the problem is it's not flat-out wrong! Einstein's field equations are better than what Newton had provided and explains more observed phenomena - they even predict phenomena we have since observed! GR is a very good theory (model) for gravity, but it's not the whole picture - and we know it. We simply don't know how to improve upon it and I personally think that believing curved spacetime is actual "reality" is a big part of why we haven't made much progress. Well, that and some other issues QFT has when attempting to model a graviton, but this comment is already long! EDIT: merely(!) - this isn't to understate Einstein's contribution to the matter which was quite considerable! But when the old genius himself is warning you to not get too caught up in the model, I suggest we take heed. EDIT 2: QFT has issues modeling a graviton, not a gluon.

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.