Einstein suspected that if the whole universe was rotating around you while
you stood still, you would feel a centrifugal force from the incoming
gravitational waves, corresponding exactly to the centripetal force of
spinning your arms while the universe stood still around you. So you could
construct the laws of physics in an accelerating or even rotating frame of
reference, and end up observing the same laws—again freeing us of the
specter of absolute space.
(I do not think this has been verified exactly [emphasis arundelo’s],
in terms of how much matter is out there, what kind of gravitational wave it
would generate by rotating around us, et cetera. Einstein did verify that a
shell of matter, spinning around a central point, ought to generate a
gravitational equivalent of the Coriolis force that would e.g. cause a
pendulum to precess. [Wow!] Remember that, by the basic principle of
gravity as curved spacetime, this is indistinguishable in principle from a
rotating inertial reference frame.)
Edit: You are correct from a classical physics standpoint that if you are
in a windowless room on a merry-go-round, you can tell whether the
merry-go-round is standing still versus spinning at a constant speed. (For
instance, you could shoot a billiard ball and see whether its path is straight
or curved.) This contrasts with the analogous situation in a windowless train
car, where you cannot tell whether the train is standing still versus moving
with a constant velocity.
Right, that (a small portion of it) was what I quoted first, one exchange upthread, and satt still held to the intuition that there are rotational stresses in the absence of the universe’s background matter. So I went back/up/down[1] a level to the basic question of when you can rule out a certain “absolute” in nature: when the simplest laws stop requiring it.
The point I was trying to make (which I should have been more specific on) was that, just as the Galilean observation set sufficed to rule out “special” velocities and leave only relative ones, our observation set now has, as an optimal description, laws that give no privilege to any non-relative motion, including higher derivatives of velocity.
[1] whichever preposition would be least offensive
[EY quote on the covariance of physical law for a spinning body]
Edit: You are correct from a classical physics standpoint that if you are in a windowless room on a merry-go-round, you can tell whether the merry-go-round is standing still versus spinning at a constant speed.
As far as I can tell, what I’m saying holds even for non-spinning accelerating objects, and under quantum physics. According to QFT, a sufficiently sensitive thermometer accelerating through a vacuum detects a higher temperature than a non-accelerating thermometer would. This appears to be a way for a thermometer to tell whether it’s accelerating without having to “look” at distant stars & such.
Hm, I’m not sure the thermometer can conclude that it’s accelerating from seeing the black body radiation. I think it’s equivalent to there being an event horizon behind it emitting hawking radiation (this happens when you accelerate at a constant rate). The thermometer can’t tell if it’s next to a black hole or if it’s accelerating. Could be wrong though, but I vaguely remember something along these lines.
I don’t see anything incorrect in what you say. (Sounds to me like a direct consequence of the equivalence principle, although I’m no GR expert.) But I’m assuming away the possibility of rogue black holes in this hypothetical, since I’m wondering whether a sufficiently sensitive sensor could detect its own acceleration even inside an otherwise empty universe (or at least without reference to the rest of the cosmos).
I think I misunderstood what you and Silas were talking about. (Note though that my train thought experiment was about a train with a constant velocity. The billiard ball technique works to detect acceleration of the train even if no rotation is involved.)
Yes, all acceleration is absolute, not relative. You don’t need hypothetical esoteric effects to detect it, a usual weighing scale will do. Gravity throws a bit of a quirk in it, of course.
I’m simultaneously reassured (that my intuition’s correct) & confused (about SilasBarta & Eliezer’s remarks, since they read to me like they contradict my intuition). Maybe I should post a comment on the Sequences post rather than continuing to press the point here, though.
[Edit: originally linked the wrong Sequences post, fixed that.]
Silas is talking about this:
Edit: You are correct from a classical physics standpoint that if you are in a windowless room on a merry-go-round, you can tell whether the merry-go-round is standing still versus spinning at a constant speed. (For instance, you could shoot a billiard ball and see whether its path is straight or curved.) This contrasts with the analogous situation in a windowless train car, where you cannot tell whether the train is standing still versus moving with a constant velocity.
Right, that (a small portion of it) was what I quoted first, one exchange upthread, and satt still held to the intuition that there are rotational stresses in the absence of the universe’s background matter. So I went back/up/down[1] a level to the basic question of when you can rule out a certain “absolute” in nature: when the simplest laws stop requiring it.
The point I was trying to make (which I should have been more specific on) was that, just as the Galilean observation set sufficed to rule out “special” velocities and leave only relative ones, our observation set now has, as an optimal description, laws that give no privilege to any non-relative motion, including higher derivatives of velocity.
[1] whichever preposition would be least offensive
Ah, sorry. Upthread reading fail on my part.
As far as I can tell, what I’m saying holds even for non-spinning accelerating objects, and under quantum physics. According to QFT, a sufficiently sensitive thermometer accelerating through a vacuum detects a higher temperature than a non-accelerating thermometer would. This appears to be a way for a thermometer to tell whether it’s accelerating without having to “look” at distant stars & such.
Hm, I’m not sure the thermometer can conclude that it’s accelerating from seeing the black body radiation. I think it’s equivalent to there being an event horizon behind it emitting hawking radiation (this happens when you accelerate at a constant rate). The thermometer can’t tell if it’s next to a black hole or if it’s accelerating. Could be wrong though, but I vaguely remember something along these lines.
I don’t see anything incorrect in what you say. (Sounds to me like a direct consequence of the equivalence principle, although I’m no GR expert.) But I’m assuming away the possibility of rogue black holes in this hypothetical, since I’m wondering whether a sufficiently sensitive sensor could detect its own acceleration even inside an otherwise empty universe (or at least without reference to the rest of the cosmos).
I think I misunderstood what you and Silas were talking about. (Note though that my train thought experiment was about a train with a constant velocity. The billiard ball technique works to detect acceleration of the train even if no rotation is involved.)
Yes, all acceleration is absolute, not relative. You don’t need hypothetical esoteric effects to detect it, a usual weighing scale will do. Gravity throws a bit of a quirk in it, of course.
I’m simultaneously reassured (that my intuition’s correct) & confused (about SilasBarta & Eliezer’s remarks, since they read to me like they contradict my intuition). Maybe I should post a comment on the Sequences post rather than continuing to press the point here, though.
[Edit: originally linked the wrong Sequences post, fixed that.]