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They’re not unrelated: they’re the same experiment, looking for an aether — because spacetime is an aether theory. When the original aether theory failed, they

They’re not unrelated: they’re the same experiment, looking for an aether — because spacetime is an aether theory. When the original aether theory failed, they pointed out that you squish when moving in the aether, and so they detected aether waves rather than aether motion. With an upgraded version of the same device.

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].
- 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.
tsimionescu 2y ago
Only in a very vague sense. The aether theories were looking to explain two things: 1, how a transverse wave, like light, can propagate in a vacuum; and 2, why does the speed of light appear to not respect Galilean relativity? The solution for both was posited to be a physical medium permeating space that had certain special properties, including very low interactions with other matter (explaining why we didn't directly detect it). Light would only propagate in this medium, and since this medium interacted very little with other matter, it explained why it wouldn't be dragged along by, say, a moving train. The speed of light would be mostly determined by the speed of this universal medium, explaining the constancy of the speed of light. Current physics has nothing to do with these theories. The medium for light's propagation is the electric field, which is not a material medium at all. And Galilean relativity doesn't agree with Maxwell's equation because it is wrong - we have Einstein's special relativity as the successor explanation, and this doesn't require any kind of medium, and doesn't say anything about space-time waves. General relativity does see space-time as something that looks more like a medium, except that, like the electric field and unlike aether, it is still not a material medium. And gravity waves have absolutely nothing to do with the aether wind. Gravity waves are specific phenomena triggered by specific events. They have a source and a direction of propagation like any other wave, and differenet gravity waves move in different directions. The aether wind was a fixed universal thing: all of the aether in the entire universe moved in a single direction, and had no source. If you want to look at something that's actually closer to a modern aether, the quantum models of the void are actually much closer. Per quantum mechanics, all of space-time is permeated by fields and random fluctuations in those fields ensure that there exist virtual particles getting created and destroyed at every point in spacetime. So, in QM, there actually exists a physical medium that does permeate all of spacetime (but it doesn't have any of the other properties of the aether).
- zmgsabst 2y ago
  > The solution for both was posited to be a physical medium permeating space that had certain special properties, Light is an excitation of the EM aether — or if you prefer, “field”. (Which you then explain later, yourself.) We failed to detect the aether with MM because we’re made of aether-stuff and so we squish in a way that cancels out what they were trying to detect with the original experiment. > Current physics has nothing to do with these theories. > [explanation of how fields are aethers] Wilczek also points out modern theories are aether theories. LIGO works because it detects waves in the aether — which do not suffer from the same squishing problem as in the original experiment. But that’s why a scaled up version of the original experiment worked: we’re fundamentally detecting the same thing. Aether theories are more legitimate than dark matter theories: the MM test for the aether failed, but then they reformulated the theory and successfully observed an effect; dark matter has yet to find a working reformulation that is observable.

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].

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.

tsimionescu 2y ago

Only in a very vague sense. The aether theories were looking to explain two things: 1, how a transverse wave, like light, can propagate in a vacuum; and 2, why does the speed of light appear to not respect Galilean relativity? The solution for both was posited to be a physical medium permeating space that had certain special properties, including very low interactions with other matter (explaining why we didn't directly detect it). Light would only propagate in this medium, and since this medium interacted very little with other matter, it explained why it wouldn't be dragged along by, say, a moving train. The speed of light would be mostly determined by the speed of this universal medium, explaining the constancy of the speed of light. Current physics has nothing to do with these theories. The medium for light's propagation is the electric field, which is not a material medium at all. And Galilean relativity doesn't agree with Maxwell's equation because it is wrong - we have Einstein's special relativity as the successor explanation, and this doesn't require any kind of medium, and doesn't say anything about space-time waves. General relativity does see space-time as something that looks more like a medium, except that, like the electric field and unlike aether, it is still not a material medium. And gravity waves have absolutely nothing to do with the aether wind. Gravity waves are specific phenomena triggered by specific events. They have a source and a direction of propagation like any other wave, and differenet gravity waves move in different directions. The aether wind was a fixed universal thing: all of the aether in the entire universe moved in a single direction, and had no source. If you want to look at something that's actually closer to a modern aether, the quantum models of the void are actually much closer. Per quantum mechanics, all of space-time is permeated by fields and random fluctuations in those fields ensure that there exist virtual particles getting created and destroyed at every point in spacetime. So, in QM, there actually exists a physical medium that does permeate all of spacetime (but it doesn't have any of the other properties of the aether).

zmgsabst 2y ago
> The solution for both was posited to be a physical medium permeating space that had certain special properties, Light is an excitation of the EM aether — or if you prefer, “field”. (Which you then explain later, yourself.) We failed to detect the aether with MM because we’re made of aether-stuff and so we squish in a way that cancels out what they were trying to detect with the original experiment. > Current physics has nothing to do with these theories. > [explanation of how fields are aethers] Wilczek also points out modern theories are aether theories. LIGO works because it detects waves in the aether — which do not suffer from the same squishing problem as in the original experiment. But that’s why a scaled up version of the original experiment worked: we’re fundamentally detecting the same thing. Aether theories are more legitimate than dark matter theories: the MM test for the aether failed, but then they reformulated the theory and successfully observed an effect; dark matter has yet to find a working reformulation that is observable.