# Mach's Principle

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How come when I spin around, the universe doesn't fly apart as if its rotating around me? Why is my frame of reference that I am at rest and its the universe spinning around me not valid?

jbriggs444
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How come when I spin around, the universe doesn't fly apart as if its rotating around me? Why is my frame of reference that I am at rest and its the universe spinning around me not valid?
In the rotating frame where you are at rest and the universe is spinning, there is a centrifugal force on the rest of the universe trying to tear it apart. But there is also a Coriolis force that is twice as large. That Coriolis force is strong enough to cancel the centrifugal (that's one factor of one) and enough to provide the centripetal acceleration to keep the contents of the universe in orbit around your head (that's the other factor of one).

So classically, the frame of reference in which you are at rest and the universe is spinning is perfectly valid. It just features some inertial forces that are not present in more mundane "inertial" frames.

Dale
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How come when I spin around, the universe doesn't fly apart as if its rotating around me? Why is my frame of reference that I am at rest and its the universe spinning around me not valid?
As @jbriggs444 mentioned, it is a perfectly valid frame, it is just non-inertial. If you are using the right kind of mathematical objects, called "tensors", then you can write the laws of physics the exact same way in inertial and in non-inertial frames. However, most of the high-school and undergraduate physics that you will see does not use tensors. So in those math classes the laws of physics look quite a bit different in inertial and non-inertial frames.

What defines the inertial frame?

Dale
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What defines the inertial frame?
Roughly speaking, that accelerometers at rest measure 0 acceleration. To get more detailed would probably go beyond a B thread.

Can something be massive enough that when it rotates it takes some of the inertial frame with it?

Dale
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I am not sure what you mean by that.

I assumed that mass defines this inertial frame. I assume the same assumption as mach. That's the way I interpreted Mach's principle anyway.

Dale
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Well, Mach's principle is not very precise. One theory that tried to make it precise is called Brans-Dicke gravity. But if you accept their interpretation, then it turns out that the experimental evidence indicates that the universe is not very Machian.

It seems to me from an apriori perspective that Mach's Principle is so strong. That's why I have a strong affinity to it. When I model a machian universe I see things that look like dark matter and dark energy, but it doesn't make sense when I apply it to our universe. In order for that phenomenon to exist the moment of inertia of the outside universe has to be comparable to that of a single rotating galaxy. And besides it doesn't account for observed gravitational lens. Still it's interesting.

Dale
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It seems to me from an apriori perspective that Mach's Principle is so strong. That's why I have a strong affinity to it.
That is the way most people feel, but Nature doesn't care about our apriori strong affinities.

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I can make a prediction based on this inertial frame dragging hypothesis that doesn't fit any known models that I am aware of. Hypervelocity stars far from the galactic center are accelerating away. If that isn't the case then I am wrong and Mach's principle is bogus.

Dale
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Since this is turning into a personal theory development, the thread is closed.