4

Clearly, if I'm spinning, I'll feel my arms lift away from my torso. But what sets the preferred angular momentum? Is there a preferred angular momentum in a vacuum? In Newtonian mechanics, I imagine one would appeal to the ether, so I suspect the answer lies in general relativity.

Qmechanic
  • 201,751
  • 2
    I don't understand your question, you need the angular momentum to get spinning, independent of air or vacuum around you. This has nothing to do with general or special relativity. – trula Aug 19 '20 at 15:49

2 Answers2

0

If you're spinning you'll have intrinsic angular momentum, it doesn't need to have anything to do with extrinsic objects. If you're spinning You'll feel the force proportional to the distance from the axis of rotation pulling each part of your body away from the axis of rotation, and this force will be felt everywhere except along the axis, thus also uniquely fixing it.

Kugutsu-o
  • 856
  • But what would you feel if you were not spinning but the rest of the universe was spinning around you ? This might seem improbable - but suppose the universe only consisted of two objects ... – gandalf61 Aug 19 '20 at 18:32
  • Than you're not spinning.. – Kugutsu-o Aug 20 '20 at 16:16
  • My point is that if you can tell the difference between these two scenarios, in which the relative motions of all objects are the same, then this implies there must be some absolute frame of reference for rotation, fixed in space. – gandalf61 Aug 20 '20 at 16:35
  • You're saying that if one can tell the difference between two motions equated by the relativity principle, then there us an absolute frame of reference? – Kugutsu-o Aug 20 '20 at 16:53
  • Except, that is a difference between extrinsic and intrinsic angular momentum – Kugutsu-o Aug 20 '20 at 16:56
  • IAM is invariant regardless of the frame – Kugutsu-o Aug 20 '20 at 16:56
-2

Einstein thought about this and coined the term Mach’s principle. The basic idea behind Mach’s principle is that very distant stars and galaxies (or maybe, in modern cosmology, the Cosmic Microwave Background Radiation) define a preferred reference frame against which absolute rotation can be measured.

gandalf61
  • 52,505
  • I'm not sure if I agree. The Cosmic Microwave Background radiation is literally just microwaves. It isn't a magical force that defines all inertia in the universe. You could say the same thing about planets, or stars, or the interstellar medium. – Palbitt Aug 19 '20 at 17:47
  • Additionally, browsing [https://physics.stackexchange.com/q/5483/], it seems that the general gist of this discussion is that the original 1890's description is definitely wrong. However, there some phenomena that are similar-ish to Mach's ideas, such as frame dragging. Unfortunately, there is no evidence for the mysterious, superluminal inertia-defining force Mach was originally talking about. – Palbitt Aug 19 '20 at 18:07