For a while I have been trying to get my head around Mach's Principle.
I know that when some principle in science seems wrong to me then it is just a case that I have misunderstood it. It is hardly likely that I have some insight that has somehow evaded the thousands of people who do physics for a reason.
But Mach's Principle seems to be one of those things on the periphery. I don't find it in the textbooks I am reading. In fact it is not even clear to me what Mach's Principle is.
Wikipedia illustrates it with an anecdote from Weinberg:
Quote:
You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?
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To me it seems perfectly obvious why the arms are pulled away. Wikipedia continues:
Quote:
Mach's principle says that this is not a coincidence—that there is a physical law that relates the motion of the distant stars to the local inertial frame. If you see all the stars whirling around you, Mach suggests that there is some physical law which would make it so you would feel a centrifugal force. There are a number of rival formulations of the principle. It is often stated in vague ways, like "mass out there influences inertia here". A very general statement of Mach's principle is "local physical laws are determined by the large-scale structure of the universe".
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This mystifies me. Centrifugal force is not any kind of fundamental force, it is emergent from any system where things move in straight lines and are constrained by some bond, like the passengers in a car that makes a fast turn.
If I model a system of particles moving in straight lines and constrained by bonds, it exhibits centrifugal force without me having to program it in - it is an inevitable result of the geometry.
So it seems to me that centrifugal force is simply a higher level description of how matter acts in an almost flat space and only depends on the local geometry, not the large scale structure of the universe.
