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I don’t see any good reason to be attached to using the word “aether” to describe [actual things that have been found].

by drdeca 2y ago

zmgsabst 2y ago
Well, because they’re aethers - pervasive “stuff” which is the substance of the traveling wave, eg those detected by LIGO. Wilczek for instance points out that modern theories are aether theories.
- drdeca 2y ago
  What makes it "stuff"? Stuff/substances, as I think of it, has/have a velocity, etc. . In this sense, spacetime isn't a "substance"/isn't "stuff". Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
  - zmgsabst 2y ago
    There’s waves in it (gravitational waves) and objects could be made from it (blackholes). That sounds like “stuff” is there. > Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time. In what sense can you do this with a liquid but not spacetime?
    - drdeca 2y ago
      Say you have flat (Minkowski) spacetime. Suppose you could talk about a chunk of space/spacetime in such a way. Take some frame, and in that reference, pick a region of space. Where is it a bit later? Ok. So, you consider it to have not moved? Now consider the system again but in a different reference frame, where things at rest in the first frame are in motion in the second. So, where is the chunk a bit later in the second frame? Things that are Lorentz invariant are unlike fluids.
      - zmgsabst 2y ago
        Your example fails at Galilean relativity: the water you claim is static from your frame on an anchored boat is moving from my frame on an airplane above it — and so isn’t a counter example. We don’t need Lorentz invariance at all. (Though, your example of non-moving water fails Einstein relativity too.) So you’ve failed to show a difference between spacetime and a fluid. And to address your point directly: Gravitational waves leave a wake that perturbs the medium. https://www.sciencealert.com/gravitational-waves-could-be-leaving-some-weird-lasting-effects-in-their-wake https://www.sciencealert.com/gravitational-waves-could-be-le... We can also coherently talk about a chunk moving, eg subluminal warp drives. https://phys.org/news/2024-05-subluminal-warp.html https://phys.org/news/2024-05-subluminal-warp.html So we have two ways “where you can talk about a chunk of it, and say where it is at some later time”.
        drdeca 2y ago
        Sorry, I was unclear. The contradiction was that if you could take chunks like that it would be moving in the other frame, but Lorentz invariance says that flat spacetime has the same structure in different frames, and so wouldn’t be moving. Fluids aren’t Lorentz invariant.
        zmgsabst 2y ago
        But if the spacetime is moving, there’s local curvature. As in the two examples I gave. So I’m not following. Perhaps you could explain how either of those operates without being able to identify chunks of spacetime as moving.
        drdeca 2y ago
        You say that “if the spacetime is moving, there’s local curvature”? Then, in flat spacetime, as there is no curvature, then (by the contrapositive) there is no “movement of spacetime”? But if some stuff is not moving in one inertial frame, then in another inertial frame it is moving. But that implies that, if spacetime is stuff and such that, if it is moving then there is local curvature, it follows that in a different inertial frame, the spacetime is not flat. But, whether spacetime is flat doesn’t depend on the choice of an inertial reference frame. As for how to describe those things without talking about spacetime moving, How spacetime works has the general coordinate invariance or covariance or whatever. You have a manifold with a pseudo-Riemannian metric tensor. You can talk about like, geodesics and such. For a warp bubble (suppose a sub-luminal one) you could talk about a non-accelerating time-like path that remains within the bubble, and talk about how the spacetime interval along it relates to the spacetime interval of some time-like path that starts in the bubble, leaves it, “travels besides it”, and finally re-joins the original path. (Of course, in some reference frame, the bubble would not move.) I don’t think it really makes sense to talk about spacetime moving, even in the case of a warp bubble. A position doesn’t have a position, or a time a time, but rather a position is a position, and a time is a time. You can talk about how, in some particular coordinate system, with some particular foliation into space-like slices, some aspects/pattern of the metric tensor at some position and time coordinate values, correspond to those at a later time coordinate at a different position coordinate value, but this is relative to a choice of coordinate system.

I don’t see any good reason to be attached to using the word “aether” to describe [actual things that have been found].

by drdeca 2y ago

I don’t see any good reason to be attached to using the word “aether” to describe [actual things that have been found].

Well, because they’re aethers - pervasive “stuff” which is the substance of the traveling wave, eg those detected by LIGO. Wilczek for instance points out that modern theories are aether theories.

drdeca 2y ago
What makes it "stuff"? Stuff/substances, as I think of it, has/have a velocity, etc. . In this sense, spacetime isn't a "substance"/isn't "stuff". Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
- zmgsabst 2y ago
  There’s waves in it (gravitational waves) and objects could be made from it (blackholes). That sounds like “stuff” is there. > Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time. In what sense can you do this with a liquid but not spacetime?
  - drdeca 2y ago
    Say you have flat (Minkowski) spacetime. Suppose you could talk about a chunk of space/spacetime in such a way. Take some frame, and in that reference, pick a region of space. Where is it a bit later? Ok. So, you consider it to have not moved? Now consider the system again but in a different reference frame, where things at rest in the first frame are in motion in the second. So, where is the chunk a bit later in the second frame? Things that are Lorentz invariant are unlike fluids.
    - zmgsabst 2y ago
      Your example fails at Galilean relativity: the water you claim is static from your frame on an anchored boat is moving from my frame on an airplane above it — and so isn’t a counter example. We don’t need Lorentz invariance at all. (Though, your example of non-moving water fails Einstein relativity too.) So you’ve failed to show a difference between spacetime and a fluid. And to address your point directly: Gravitational waves leave a wake that perturbs the medium. https://www.sciencealert.com/gravitational-waves-could-be-leaving-some-weird-lasting-effects-in-their-wake https://www.sciencealert.com/gravitational-waves-could-be-le... We can also coherently talk about a chunk moving, eg subluminal warp drives. https://phys.org/news/2024-05-subluminal-warp.html https://phys.org/news/2024-05-subluminal-warp.html So we have two ways “where you can talk about a chunk of it, and say where it is at some later time”.
      - drdeca 2y ago
        Sorry, I was unclear. The contradiction was that if you could take chunks like that it would be moving in the other frame, but Lorentz invariance says that flat spacetime has the same structure in different frames, and so wouldn’t be moving. Fluids aren’t Lorentz invariant.
        zmgsabst 2y ago
        But if the spacetime is moving, there’s local curvature. As in the two examples I gave. So I’m not following. Perhaps you could explain how either of those operates without being able to identify chunks of spacetime as moving.
        drdeca 2y ago
        You say that “if the spacetime is moving, there’s local curvature”? Then, in flat spacetime, as there is no curvature, then (by the contrapositive) there is no “movement of spacetime”? But if some stuff is not moving in one inertial frame, then in another inertial frame it is moving. But that implies that, if spacetime is stuff and such that, if it is moving then there is local curvature, it follows that in a different inertial frame, the spacetime is not flat. But, whether spacetime is flat doesn’t depend on the choice of an inertial reference frame. As for how to describe those things without talking about spacetime moving, How spacetime works has the general coordinate invariance or covariance or whatever. You have a manifold with a pseudo-Riemannian metric tensor. You can talk about like, geodesics and such. For a warp bubble (suppose a sub-luminal one) you could talk about a non-accelerating time-like path that remains within the bubble, and talk about how the spacetime interval along it relates to the spacetime interval of some time-like path that starts in the bubble, leaves it, “travels besides it”, and finally re-joins the original path. (Of course, in some reference frame, the bubble would not move.) I don’t think it really makes sense to talk about spacetime moving, even in the case of a warp bubble. A position doesn’t have a position, or a time a time, but rather a position is a position, and a time is a time. You can talk about how, in some particular coordinate system, with some particular foliation into space-like slices, some aspects/pattern of the metric tensor at some position and time coordinate values, correspond to those at a later time coordinate at a different position coordinate value, but this is relative to a choice of coordinate system.