5 ms·

Um, I must be missing something here, because feel like I have a pretty simple solution to why there's only a single rotational reference frame, and this is a p

Um, I must be missing something here, because feel like I have a pretty simple solution to why there's only a single rotational reference frame, and this is a problem that apparently eluded Einstein, if I'm to believe your link. Imagine a spinning glass ball with a 1cm radius doing one full rotation per second. If we draw a dot on its outer edge, at the end of 1 second that dot will have done a full turn. Given that its movement was a circle with radius 1, the distance the dot traveled was 2pi cm, for a speed of 2pi cm/s. Now imagine that instead of drawing the dot on the outer edge, we drew it halfway between the center and the outer edge (somehow). This would place it 0.5cm from the center. It would still complete a full turn in 1 second, but now its distance traveled would be pi cm, for a speed of pi cm/s. All this is to show that when the ball is spinning, the different particles it's composed of are moving at different speeds. On opposite sides of the ball, the particles are even moving in different directions. The faster the ball spins, the greater the difference in velocity of the different parts of the ball. Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute". At least, that's what it seems like to me.

westoncb 9y ago
That's an interesting idea. Check out the second answer from 'Vesselin Petkov' which talks about a geometric case for the difference between the two. His answer overall seemed to me more interesting than the most upvoted one there.
- librexpr 9y ago
  Whoa. I kept reading past that, and the third answer by Logan R. Kearsley is almost exactly what I said, but better. I should have kept scrolling the first time I opened the link! It ends by saying that in general relativity, "rotation can be considered as existing only relative to a certain choice of coordinates after all", though, so there seems to be more to it.
PeachPlum 9y ago
Is the particle in the centre moving at all? Are the particles point mass?
- librexpr 9y ago
  I don't think a point can rotate. What would it mean if it did? There would be no observable change. So no, I don't think the absolute center point moves. From a physical standpoint, though, if you wanted to talk about the absolute center point, you'd have to talk about the lowest level of particles, which gets into the weirdness of quantum mechanics (or maybe a lower level exists?). At that point, I really couldn't say what happens. And I don't think it matters if the particles are point mass or not, except possibly for the absolute center. The point is that the rotating object is not just one thing, but made up of smaller things that have different velocities and acceleration. These differences in velocities and acceleration are what rotation is. If there exists something that is not made up of smaller things which also rotates, then that would prove me wrong. Of course, all the above is just speculation by someone who's hasn't had a physics class since high school, so take it with a grain of salt.
  - randallsquared 9y ago
    The most fundamental particle can't rotate, because that would imply that parts of it had different velocities from other parts... but it doesn't have parts at all, by initial definition. So, Democritus' atoms imply that rotation is not a fundamental motion. I think every attempt to visualize this which imagines a single object rotating is doomed. Rotation inherently implies a difference in velocities, and some attractive force keeping those velocities changing to maintain a distance. So, the existence of any attractive force, together with inertia, implies rotation and the stickiness of direction of a gyroscope. But inertia itself already implied stickiness of direction, given that a change in direction would require acceleration...
    - philipov 9y ago
      Okay, then explain Spin. Angular momentum is defined by how a particle responds to fields, not by the relation of its parts.
Ended 9y ago
>Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute". Imagine the ball is floating in space, and you are watching things through a camera fixed to the ball. To you, the two dots will always appear stationary with zero relative velocity. So there is no way to determine your absolute reference frame. Now suppose you are watching through an external camera, and suppose you observe the dots having a relative velocity. Is the ball spinning, or is the camera orbiting around the ball? Again, there is no way to tell.
- lmm 9y ago
  But if you were the camera you'd feel a force, or not, right? It's not like being in an elevator in free-fall, where you can't tell whether you're accelerating downwards or sitting still in flat space.
  - Ended 9y ago
    True, but the GP suggested that you can find the absolute reference frame by looking at the relative velocities of the dots (independent of any force measurements).
  - JoBrad 9y ago
    How would you tell whether that force is gravitational or centrifugal?
    - lmm 9y ago
      If it were gravitational there would have to be a mass in the right place to cause it. Maybe you could look at how the force changed as you moved around?
  - philipov 9y ago
    The elevator analogy breaks down if you're larger than a point-particle in a point-elevator. Gravity's force varies with distance, causing tidal forces on your body, allowing something large/sensitive enough to feel the difference. Gravity stretches you whereas uniform acceleration does not, and non-uniform acceleration compresses you. In other words, you can determine that you're in a gravitational field by measuring the difference in force at different locations in the elevator.

westoncb 9y ago

That's an interesting idea. Check out the second answer from 'Vesselin Petkov' which talks about a geometric case for the difference between the two. His answer overall seemed to me more interesting than the most upvoted one there.

librexpr 9y ago
Whoa. I kept reading past that, and the third answer by Logan R. Kearsley is almost exactly what I said, but better. I should have kept scrolling the first time I opened the link! It ends by saying that in general relativity, "rotation can be considered as existing only relative to a certain choice of coordinates after all", though, so there seems to be more to it.

PeachPlum 9y ago

Is the particle in the centre moving at all? Are the particles point mass?

librexpr 9y ago
I don't think a point can rotate. What would it mean if it did? There would be no observable change. So no, I don't think the absolute center point moves. From a physical standpoint, though, if you wanted to talk about the absolute center point, you'd have to talk about the lowest level of particles, which gets into the weirdness of quantum mechanics (or maybe a lower level exists?). At that point, I really couldn't say what happens. And I don't think it matters if the particles are point mass or not, except possibly for the absolute center. The point is that the rotating object is not just one thing, but made up of smaller things that have different velocities and acceleration. These differences in velocities and acceleration are what rotation is. If there exists something that is not made up of smaller things which also rotates, then that would prove me wrong. Of course, all the above is just speculation by someone who's hasn't had a physics class since high school, so take it with a grain of salt.
- randallsquared 9y ago
  The most fundamental particle can't rotate, because that would imply that parts of it had different velocities from other parts... but it doesn't have parts at all, by initial definition. So, Democritus' atoms imply that rotation is not a fundamental motion. I think every attempt to visualize this which imagines a single object rotating is doomed. Rotation inherently implies a difference in velocities, and some attractive force keeping those velocities changing to maintain a distance. So, the existence of any attractive force, together with inertia, implies rotation and the stickiness of direction of a gyroscope. But inertia itself already implied stickiness of direction, given that a change in direction would require acceleration...
  - philipov 9y ago
    Okay, then explain Spin. Angular momentum is defined by how a particle responds to fields, not by the relation of its parts.

Ended 9y ago

>Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute". Imagine the ball is floating in space, and you are watching things through a camera fixed to the ball. To you, the two dots will always appear stationary with zero relative velocity. So there is no way to determine your absolute reference frame. Now suppose you are watching through an external camera, and suppose you observe the dots having a relative velocity. Is the ball spinning, or is the camera orbiting around the ball? Again, there is no way to tell.

lmm 9y ago
But if you were the camera you'd feel a force, or not, right? It's not like being in an elevator in free-fall, where you can't tell whether you're accelerating downwards or sitting still in flat space.
- Ended 9y ago
  True, but the GP suggested that you can find the absolute reference frame by looking at the relative velocities of the dots (independent of any force measurements).
- JoBrad 9y ago
  How would you tell whether that force is gravitational or centrifugal?
  - lmm 9y ago
    If it were gravitational there would have to be a mass in the right place to cause it. Maybe you could look at how the force changed as you moved around?
- philipov 9y ago
  The elevator analogy breaks down if you're larger than a point-particle in a point-elevator. Gravity's force varies with distance, causing tidal forces on your body, allowing something large/sensitive enough to feel the difference. Gravity stretches you whereas uniform acceleration does not, and non-uniform acceleration compresses you. In other words, you can determine that you're in a gravitational field by measuring the difference in force at different locations in the elevator.