# Q & A: Is rotation absolute?

Q:
Is angular movement absolute? My question stems from the fact that relativity states that there are no absolute coordinates to measure linear movement, all movement is relative to other objects. If two rockets in space move past each other, there is no objective frame of reference to say that one is stationary and the other is moving. It is just as correct to say that rocket no. 1 stationary and ship no. 2 is moving as it is vice versa. Imagine now that the rockets are floating in space, tips of the rockets pointed directly at each other. Imagine, moreover, that one of the rockets is spinning around its long axis. If all angular movement was relative, there should be no way of telling which rocket is stationary and which is spinning. From rocket no. 1 it would appear that the other is spinning and vice versa. However, this cannot be true, as the people inside the rocket that is spinning would feel a centrifugal force pressing their feet against the outer wall of the rocket. In the other rocket, the stationary one, no such effect can be felt and everyone is floating freely. Therefore, it would appear, that the passengers of these rockets can discern objectively (in agreement with each other) which rocket is spinning and which is not by looking at how objects inside the rocket behave: are they pressed against the outer walls by the centrifugal force? If this is true, it would appear that there is an absolute system of stationary coordinates to which angular movement can be compared. Do physicists agree that there is such a stationary system of coordinates that seems to span all of space and to which all observers can compare rotation regardless of their own movement? What is this system of coordinates made of - ether? Why is angular movement so fundamentally different from linear movement? Could rotating objects be used as universal clocks that have an absolute speed of rotation compared to the said coordinates?
- Pekka Tuominen (age 31)
Espoo
A:
The physical effects you describe are real. In general relativistic coordinates, one could perversely choose to say that there are some pseudoforces such that the rocket in which you experience centrifugal forces is actually the one not rotating. It's much cleaner to pick more conventional coordinates and say that's the one rotating. So as you say there's a natural family of coordinate systems lacking that type of large-scale pseudoforce which all agree on who is rotating.

Rotations imply accelerations, which require some sort of explanatory force. Linear motion does not.

I'm not sure what advantage rotational clocks would have over any other sort, e.g. atomic clocks. The gravitational effects on time will persist for any sort of local clock, preventing any lattice of clocks from staying synchronized.

Mike W.

(published on 02/25/2013)