Okay, so is Mach's principle true, or not?

[Paige_Turner]

TL;DR

It seems crazy to me

You don't need another object in the universe in order to rotate and feel centrifugal force.

You establish an inertial frame just by sitting there and existing. When you rotate, you rotate relative to that frame, even though the only mass in it is rotating.

Or am I, like, WAY off?

If so, then what problem does it solve? If we suddenly don't believe it, what can we suddenly not explain?

 

[Delta2]

Summary:: It seems crazy to me

Or am I, like, WAY off?

Absolutely yes!

 

[Ibix]

You don't need another object in the universe in order to rotate and feel centrifugal force.

That's not a testable statement. You can't empty the universe of everything except a test particle and start it rotating to see what happens. And assuming physical law derived from observations in a universe full of matter apply to a universe empty of matter is assuming your answer.

The point about Mach's Principle is that Mach wondered why it was that you cannot measure position or speed without reference to some external object, but you can measure proper acceleration in a closed box. He argued that this fact needed some physical explanation, and Mach's Principle, IMO, boils down to "there ought to be a reason for that". In particular when we talk about rotation it's fairly common to talk about "rotating with respect to the fixed stars", and it's difficult to imagine anything I do affecting the stars, so they make sense as a reference. On the other hand, proposing that all that matter has some effect (edit: to clarify before we're swarmed by astrologers, I mean some simple physical effect) on me isn't totally crazy.

I think the problem with the whole topic is that it's quite hard to nail down a precise testable version of Mach's Principle, and harder still to get other physicists and philosophers to agree that it us the testable version. People still argue about it, and I suppose something may come of it one day. But I wouldn't hold my breath while waiting.

 

[Dale]

I think the problem with the whole topic is that it's quite hard to nail down a precise testable version of Mach's Principle, and harder still to get other physicists and philosophers to agree that it us the testable version.

I agree with that assessment. Also, in the rare cases where a researcher claims to have made a testable Machian theory (e.g. Brans Dicke gravity) experimental measurements seem to indicate that the universe does not follow Mach’s principle (as formulated by Brans and Dicke)

 

[Dale]

Absolutely yes!

I don’t think it is way off. As @Ibix says it is hard to pin down testable statements of Mach’s principle. So doubting it is perfectly reasonable.

 

[gmax137]

Mach wondered why it was that you cannot measure position or speed without reference to some external object, but you can measure proper acceleration in a closed box. He argued that this fact needed some physical explanation, and Mach's Principle, IMO, boils down to "there ought to be a reason for that".

When posed as a "why" question it seems pretty clear what the issue is.

in the rare cases where a researcher claims to have made a testable Machian theory (e.g. Brans Dicke gravity) experimental measurements seem to indicate that the universe does not follow Mach’s principle (as formulated by Brans and Dicke)

Thanks, this looks interesting.

EDIT: and it gets deep quickly. The Brans Dicke Gravity wiki page was beyond me. I never studied GR.

 

[vanhees71]

I also do not think that the statements in #1 are way off. Of course, you need to define a frame of reference and check by observation, whether it is an inertial frame or not. This has nothing to do with far-distant masses in the universe but with local equipment establishing the reference frame.

Then the question, I could never answer satisfactorily for myself, indeed is, what is the precise statement of Mach's principle, i.e., how can it be tested by observation. It's vague statement seems to be that somehow the inertia of some matter "here" is due to all the far-distant matter "out there". I know it's a long debate among philophers whether General Relativity is a Machian theory. At least I don't see, in which sense any of the usual spacetime models (i.e., Newtonian, Minkowski=special relativistic, and the most comprehensive of all, which is the general-relativistic spacetime model) are somehow realizing Mach's idea that the inertia of matter is due to the presence of all the other far-distant matter.