Is rotation a relative property?

[Lazzini]

I was recently trying to explain to a grandchild the relative nature of velocity (the different paths of a coin dropped by a passenger on a train, as seen by the passenger on one hand and a trackside observer on the other), and the invalidity of the concept of absolute velocity.

For some reason my thoughts turned to the Earth's rotation, and I tried, in my head, to relate linear motion to rotation, ending up in utter confusion.

I imagined a reverse rotation, of the same magnitude as its present rotation but in the opposite direction, being applied to the Earth. It would then, presumably, have a rotation of zero. But with respect to what? If we speak of zero velocity, it's zero with reference to something else. But what could zero rotation mean? Are there an infinity of states of zero rotation, just as there must be an infinite number of "zero" velocities? I can visualise one situation, but not the other, and I find that it all seems to conjure up the notion of absolute rotation. It leaves me completely baffled. Can someone explain?

 

[Ibix]

Rotation produces locally detectable effects (see Foucault's pendulum, for example), so is not relative in the sense that linear velocity is. There is an absolute state of "not rotating" - the one in which Foucault's pendulum does not turn.

Edit: just to add - acceleration is, in general, a direct observable and "am I accelerating or not" is a question with an absolute answer (although there are a few technical caveats around that). When you are going round in a circle, you are accelerating, so it's directly measurable whether you are rotating or not.

 

[PeroK]

Summary: Is rotation a relative property?

I was recently trying to explain to a grandchild the relative nature of velocity (the different paths of a coin dropped by a passenger on a train, as seen by the passenger on one hand and a trackside observer on the other), and the invalidity of the concept of absolute velocity.

For some reason my thoughts turned to the Earth's rotation, and I tried, in my head, to relate linear motion to rotation, ending up in utter confusion.

I imagined a reverse rotation, of the same magnitude as its present rotation but in the opposite direction, being applied to the Earth. It would then, presumably, have a rotation of zero. But with respect to what? If we speak of zero velocity, it's zero with reference to something else. But what could zero rotation mean? Are there an infinity of states of zero rotation, just as there must be an infinite number of "zero" velocities? I can visualise one situation, but not the other, and I find that it all seems to conjure up the notion of absolute rotation. It leaves me completely baffled. Can someone explain?

Do not read this!

https://en.wikipedia.org/wiki/Mach's_principle

 

[gmax137]

Do not read this!

... Too late...

 

[vanhees71]

Also watch this with your grandchild:

 

[Nugatory]

Edit: just to add - acceleration is, in general, a direct observable and "am I accelerating or not" is a question with an absolute answer (although there are a few technical caveats around that).

To be precise, proper acceleration - the thing that an accelerometer measures - is a direct observable that is happening or not in an absolute sense. Coordinate acceleration - "the speed is changing" - is relative because speed itself is relative.

(Ibix already knows this, of course. This comment is for others following the thread)