Mach's Principle: Right or Wrong?

[Jonathan Scott]

If you want to speculate about an empty universe, first, you have to empty it. If we are talking physics and not just doing word-play then you must demonstrate a method to obtain an empty universe from one which is not; or the converse. It's not physics to just 'make it disappear'. There are a few means at hand, but with limitations.

It's not so much the emptiness which matters, but having all the mass in one place.

For example, consider the spherical surface analogy for the universe at a point in time and project the sphere onto a plane touching the north pole using a light situated at the south pole (as for a stereographic projection). In this projection, if you get far enough away from the middle, then the vast majority of the mass is in one direction, and can be considered as a central blob. However, if you consider the local shape of space-time in that projection, you find that the rest of the universe still surrounds that point in a generally isotropic way.

If you simply assume that the mass is all in one place but that known laws of gravity hold, at least approximately, you will probably end up with a similar picture.

 

[Al68]

If you want to speculate about an empty universe, first, you have to empty it. If we are talking physics and not just doing word-play then you must demonstrate a method to obtain an empty universe from one which is not...

Why? This speculation is about an empty universe, not an empty universe that previously contained masses that were removed using a particular method. For this purpose, any proposed method for obtaining an empty universe would be irrelevant, anyway.

It may still be fruitless to speculate about an empty universe, but not for that reason alone.

 

[Passionflower]

Mach's Principle is regarded as a forerunner of Einstein's Theory of Relativity, but is it a legitimate principle in its own right?

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

Does Mach's Principle imply an origin of inertia? - ie. does it mean that a body can only experience inertia if there are other bodies in the universe to pull on it or interact with it?

What happens if a body is just sitting alone in the universe, and it starts rotating? Will it feel any rotational inertia in the absence of other bodies in the universe?

So does inertia originate from a body's interaction with other masses in our universe, or does it originate from a body's interaction with space itself?

In GR a body does not need any other remote objects to experience inertia.

But be careful, a, somewhat silly and unphysical, rotating zero dimensional point body, would even in GR not demonstratively show inertia.

However no atoms inside a (Born) rigid rotating ball will move inertially even if we ignore the mass-energy of the atoms for simplicity's sake, because all undergo a constant proper acceleration in the attempt to keep the object rigid, and each sphere of atom's will have a different rate of time.

 

[Phrak]

Why? This speculation is about an empty universe, not an empty universe that previously contained masses that were removed using a particular method. For this purpose, any proposed method for obtaining an empty universe would be irrelevant, anyway.

It may still be fruitless to speculate about an empty universe, but not for that reason alone.

We don't have any empty universes to experiment with. Without one, we can apply the theory we have, or make up a new one. Any other suggestions?

 

[Phrak]

In GR a body does not need any other remote objects to experience inertia.

But be careful, a, somewhat silly and unphysical, rotating zero dimensional point body, would even in GR not demonstratively show inertia.

Is there any truth to the rumor that general relativity incorrectly handles orbital angular momentum?

 

[Jonathan Scott]

Is there any truth to the rumor that general relativity incorrectly handles orbital angular momentum?

Einstein-Cartan theory is supposed to fix a theoretical problem with spin angular momentum (not orbital angular momentum).

 

[Phrak]

Einstein-Cartan theory is supposed to fix a theoretical problem with spin angular momentum (not orbital angular momentum).

That's what I understand, Jonathan. Though some think corrections to orbital angular momentum are implied from corrections to spin angular momentum.

 

[Al68]

We don't have any empty universes to experiment with. Without one, we can apply the theory we have, or make up a new one. Any other suggestions?

Not for practical experiments, no. But we're in the realm of theoretical physics here, not empirical physics.

Obviously, speculating about an empty universe is very relevant to a discussion of Mach's principle, which is the subject of this thread.

If I knew of a way to empirically prove or disprove Mach's Principle, I wouldn't post it here and let someone else get the Nobel Prize for it, now would I?