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One astronaut (the spinning one) will detect a centrifugal force, and the other will not. If the spinning astronaut took a yo-yo out of her pocket, she would o

by tbabb 11y ago

One astronaut (the spinning one) will detect a centrifugal force, and the other will not. If the spinning astronaut took a yo-yo out of her pocket, she would observe it accelerates radially away from her. The stationary astronaut's yo-yo would drift freely with no acceleration. Edit: See: https://en.wikipedia.org/wiki/Bucket_argument https://en.wikipedia.org/wiki/Bucket_argument

danbruc 11y ago
The yo-yo is no different than extending and retracting limbs, it presupposes inertia as we know it. Why should the two yo-yos behave differently if the situation is totally symmetric?
- tbabb 11y ago
  I am not sure what point you are trying to make or what you are asserting is being "presupposed". What is described above are the results of an experiment. That's the way physics works; that's what will happen if you do the experiment. No suppositions needed. The situation is not totally symmetric: The yo-yo behaves differently for the spinning astronaut than it does for the stationary one. That is why the article is wrong; the two situations are measurably different, not equivalent.
  - danbruc 11y ago
    You are missing the point. This is not an experimental fact because we have no empty universe at hand to do it. Two cylinders spinning relative to each other around a common axis in otherwise empty three-dimensional Euclidean space are totally symmetric. Or how would you tell which one is spinning? It is as meaningless to speak about the rotation of one of the cylinders as it is meaningless to speak about the velocity of a single object.
    - tbabb 11y ago
      Ah, I think I see what you are hung up on. In newtonian mechanics, the equivalence principle says that any two inertial coordinate frames look "the same". For example, if you have two spaceships heading toward each other, the picture looks "the same" as if you pretend that one spaceship is stationary and the other is moving twice as fast, or vice versa. This is not true only of geometric relationships, but also for the laws of physics: If you choose to view things in coordinate frame A, and apply the same laws of physics, you will still get the same predicted motion as if you had done the calculation in coordinate frame B. There is no experiment that can tell you which frame is "moving", because you always get the same answer. With your example of two rotating bodies, it is true that the picture looks geometrically "the same" if we pick a different rotating reference frame. The problem is that the laws of physics do not look "the same" under this transformation. If I choose a coordinate frame where the spinning astronaut is stationary, and I try to predict what will happen using the laws of physics of the original frame, I will get wrong results-- the motion of the yo-yo will look "unphysical" for a stationary system. So when you change between rotating and stationary coordinate frames, the laws of physics change too. That means I can tell whether I am in an "absolutely rotating" coordinate frame by doing tests to measure the laws of physics (i.e. are there coriolis and centrifugal forces acting on me?), without any reference to some other, external frame. So the fundamental question is under what circumstances the picture looks "the same". In both cases, geometric relationships are equivalent, but in only one of the cases are the laws of physics also equivalent. The original article lied by saying the laws of physics are the same in the rotating case, and that is simply not true. Does that make sense?
      - danbruc 11y ago
        We are getting closer. You are essentially saying that we can have geometrically indistinguishable states that are physically different. We can have an astronaut with a yo-yo hanging down and one with a yo-yo extending straight away while the world otherwise looks exactly the same to both of them. Both astronauts have to go through the same change in angular velocity just in opposite directions to get to the same state as the other one but one would see the yo-yo dropping down while the other one will see it rising. How does that make sense? What is so special about the one coordinate frame without centrifugal forces? With respect to what is the astronaut not rotating? Note that nothing looks unphysical here, just different but without any obvious thing that justifies the difference. And I am definitely not trying to say I am right and you are wrong, it just doesn't look consistent to me to have centrifugal forces in an empty universe.

by tbabb 11y ago

danbruc 11y ago

The yo-yo is no different than extending and retracting limbs, it presupposes inertia as we know it. Why should the two yo-yos behave differently if the situation is totally symmetric?

tbabb 11y ago
I am not sure what point you are trying to make or what you are asserting is being "presupposed". What is described above are the results of an experiment. That's the way physics works; that's what will happen if you do the experiment. No suppositions needed. The situation is not totally symmetric: The yo-yo behaves differently for the spinning astronaut than it does for the stationary one. That is why the article is wrong; the two situations are measurably different, not equivalent.
- danbruc 11y ago
  You are missing the point. This is not an experimental fact because we have no empty universe at hand to do it. Two cylinders spinning relative to each other around a common axis in otherwise empty three-dimensional Euclidean space are totally symmetric. Or how would you tell which one is spinning? It is as meaningless to speak about the rotation of one of the cylinders as it is meaningless to speak about the velocity of a single object.
  - tbabb 11y ago
    Ah, I think I see what you are hung up on. In newtonian mechanics, the equivalence principle says that any two inertial coordinate frames look "the same". For example, if you have two spaceships heading toward each other, the picture looks "the same" as if you pretend that one spaceship is stationary and the other is moving twice as fast, or vice versa. This is not true only of geometric relationships, but also for the laws of physics: If you choose to view things in coordinate frame A, and apply the same laws of physics, you will still get the same predicted motion as if you had done the calculation in coordinate frame B. There is no experiment that can tell you which frame is "moving", because you always get the same answer. With your example of two rotating bodies, it is true that the picture looks geometrically "the same" if we pick a different rotating reference frame. The problem is that the laws of physics do not look "the same" under this transformation. If I choose a coordinate frame where the spinning astronaut is stationary, and I try to predict what will happen using the laws of physics of the original frame, I will get wrong results-- the motion of the yo-yo will look "unphysical" for a stationary system. So when you change between rotating and stationary coordinate frames, the laws of physics change too. That means I can tell whether I am in an "absolutely rotating" coordinate frame by doing tests to measure the laws of physics (i.e. are there coriolis and centrifugal forces acting on me?), without any reference to some other, external frame. So the fundamental question is under what circumstances the picture looks "the same". In both cases, geometric relationships are equivalent, but in only one of the cases are the laws of physics also equivalent. The original article lied by saying the laws of physics are the same in the rotating case, and that is simply not true. Does that make sense?
    - danbruc 11y ago
      We are getting closer. You are essentially saying that we can have geometrically indistinguishable states that are physically different. We can have an astronaut with a yo-yo hanging down and one with a yo-yo extending straight away while the world otherwise looks exactly the same to both of them. Both astronauts have to go through the same change in angular velocity just in opposite directions to get to the same state as the other one but one would see the yo-yo dropping down while the other one will see it rising. How does that make sense? What is so special about the one coordinate frame without centrifugal forces? With respect to what is the astronaut not rotating? Note that nothing looks unphysical here, just different but without any obvious thing that justifies the difference. And I am definitely not trying to say I am right and you are wrong, it just doesn't look consistent to me to have centrifugal forces in an empty universe.