Mach's Principle: Right or Wrong?

[sanman]

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?

 

[Jonathan Scott]

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?

Mach's Principle as normally interpreted does indeed imply a simple explanation for inertia (see Dennis Sciama's 1953 paper "On the Origin of Inertia", easily found on Google), and conceptually means that if the only body in the universe were rotating, the rotation could not be detected.

(However, I must point out that the shape of a single-body universe is not easy to define and might not look at all like one expects; for example, if you define a single small spherical particle at the origin, then you may well find that from the point of view of a test object in the surrounding space, the "particle" surrounds it as a distant spherical shell in all directions).

General Relativity shows signs of Mach's Principle in both linear and rotational frame-dragging effects, but cannot exactly satisfy the principle because the gravitational constant does not depend on the distribution of masses in the universe.

This means that either Mach's Principle is only loosely true for the actual whole universe (which to me seems odd, given that it is accurate for small local effects) or that GR is inaccurate on the scale of the whole universe.

 

[Buckethead]

Check out the Newton's Bucket thread for a comprehensive discussion about Mach's Principle. I for one think it's a great principle worthy of deeper thought. An empty universe is a strange place indeed, especially with regard to motion, acceleration, and inertia.

 

[starthaus]

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?

Yes, it will , this is what the Michelson-Gale class of experiments detect. This is what the Sagnac effect is all about: detection of rotation from within a closed lab.

 

[Buckethead]

Yes, it will , this is what the Michelson-Gale class of experiments detect. This is what the Sagnac effect is all about: detection of rotation from within a closed lab.

I disagree. Unless I'm missing something here, this experiment simply measures angular velocity of an object within our universe. It cannot be isolated from the mass of the universe (the stars) so it cannot be used to prove the effects of rotation of an object in an otherwise empty universe.

 

[Dale]

Personally, that is why I really dislike Mach's principle. It always seems to come down to completely useless and unverifiable speculation about the behavior of "otherwise empty" universes. Physics is about describing the behavior of this universe, not fantasy universes.

 

[starthaus]

I disagree. Unless I'm missing something here, this experiment simply measures angular velocity of an object within our universe. It cannot be isolated from the mass of the universe (the stars) so it cannot be used to prove the effects of rotation of an object in an otherwise empty universe.

The Sagnac effect is a purely kinematic effect, it has nothing to do with any "mass of the universe". So, it will happen exactly the same way whether the universe is empty or not. The result of the Michelson-Gale experiment has nothing to do with any gravitational effects.

 

[starthaus]

Personally, that is why I really dislike Mach's principle. It always seems to come down to completely useless and unverifiable speculation about the behavior of "otherwise empty" universes. Physics is about describing the behavior of this universe, not fantasy universes.

Seconded.

 

[Buckethead]

Personally, that is why I really dislike Mach's principle. It always seems to come down to completely useless and unverifiable speculation about the behavior of "otherwise empty" universes. Physics is about describing the behavior of this universe, not fantasy universes.

How can you know what a forest is unless you step outside it? Speculating about an empty universe is a path to seeing what our universe actually is and can most definitely lead to experiments in our universe that can lead to expanded understandings of the fabric of space.

 

[Buckethead]

The Sagnac effect is a purely kinematic effect, it has nothing to do with any "mass of the universe". So, it will happen exactly the same way whether the universe is empty or not. The result of the Michelson-Gale experiment has nothing to do with any gravitational effects.

I agree it has nothing to do with gravitational effects, but this is not the question. The question is whether or not the reading in the Michelson-Gale experiment would change if the apparatus were to rotate relative to the stars and then the stars disappeared. This is a very fundamental question with very deep implications. An alternate way to state this is whether or not this apparatus is reading null if it is not rotating relative to the sum of the velocities of all the mass in the universe and if not, what is it not rotating with respect to when it IS null.

 

[Dale]

How can you know what a forest is unless you step outside it?

There is lots that you can learn about a forest without stepping outside of it, and if it is impossible for the forest dwellers to step outside of the forest then none of the "outside perspective" is of any value to the forest dwellers.

 

[Jonathan Scott]

Personally, that is why I really dislike Mach's principle. It always seems to come down to completely useless and unverifiable speculation about the behavior of "otherwise empty" universes. Physics is about describing the behavior of this universe, not fantasy universes.

I agree that it's pointless to talk about "otherwise empty" universes, but that's what Einstein came up with himself when he first pointed out that GR is not completely compatible with Mach's Principle.

I became interested in Machian theories through thinking about the gravitational effect of the whole universe at the current location, which GR doesn't seem to handle very well, not even as an approximation. As Sciama demonstrated in his "Origin of Inertia" paper, if Newtonian gravity is extrapolated in a semi-relativistic way to the whole universe, it gives rise to inertia and rotational effects from relative motion. In contrast, we don't know how to extend GR to a whole universe (except for unrealistically hypothetical universes with special properties such as uniform density).

GR tells us precisely what the frame-dragging effect is due to the motion of a single object, and provided that speeds are non-relativistic and fields are weak, this can be extrapolated reasonably accurately up to almost any scale, if we define an "effective value" of m/r for a distant mass in terms of its contribution to the local potential. The more we do this, the more we see that a test object effectively feels acceleration and rotation caused by frame dragging of other bodies, and it would be logical that if we extend that scheme to all masses, the test object would perceive the overall frame of all the masses to be the rest frame.

With the known values for G and the distribution of the mass in the universe, the result could be (very roughly) around the right order of magnitude to support an exact match, giving the generalized form of the Whitrow-Randall relation:

sum(Gm/rc²) = n

where the sum is for all masses in the universe as seen from any point and n is a simple constant which depends on the specific theory. For the simplest model of linear frame-dragging, n=1. Note that we can't really define the mass and distances for very distant objects, but we can however assume that the effective ratio for a given mass is a constant value (apart from perhaps systematic changes with time).

This expression obviously cannot be true when G is a constant as in GR, because even the variation due to location with respect to a local mass could apparently cause a detectable variation in G.

However, it is possible instead that the local variation in G due to a central mass is part of what we consider to be the gravitational potential, and the effective value of G is given by the above relation for all other masses, in which case it would effectively be "locally constant".

In fact, the Schwarzschild solution to the Einstein Equations can be expressed in terms of this form of "locally constant" G by a coordinate substitution, but the resulting expression can then be simplified so that it only contains the full "variable G" instead, so the solution treats all masses in the universe identically. (The resulting solution only remains a valid solution to the Einstein Field Equations when the "locally constant" G term due to all other masses is really constant).

Even if Machian theory doesn't require the effective value of G to vary in the central mass case, if the Whitrow-Randall relation holds then G would be expected to vary with time and location on a larger scale (although some of the effects might cancel one another out). At present, there are strong experimental constraints from solar system experiments, in particular Lunar Laser Ranging (LLR), which make it unlikely that G could be varying at the moment even as 1/T where T is the age of the universe.

 

[Buckethead]

There is lots that you can learn about a forest without stepping outside of it, and if it is impossible for the forest dwellers to step outside of the forest then none of the "outside perspective" is of any value to the forest dwellers.

There is plenty to learn without stepping outside, but there may also be mistakes made in basic assuptions. For example, if there were no trees would the Earth fall apart and the world explode? By speculating about areas of the planet that might have no trees this question can be addressed. I could give a billion analogies along these lines, but my point is that by imagining what something would be like if something didn't exist is a good way to find out what that somethings purpose in life is. By speculating on whether or not Einsteins principle of equivalence holds in an empty universe we may be able to say something about why it exists at all.

 

[Dale]

There is plenty to learn without stepping outside, but there may also be mistakes made in basic assuptions. ... by imagining what something would be like if something didn't exist is a good way to find out what that somethings purpose in life is. By speculating on whether or not Einsteins principle of equivalence holds in an empty universe we may be able to say something about why it exists at all.

So you are of the opinion that mistaken assumptions can be corrected through imagining completely non-physical and unverifiable situations? What is to say that your speculation is not also mistaken? At least the assumptions can be experimentally challenged and tested, whereas speculation is unfalsifiable (and therefore unscientific).

 

[starthaus]

I agree it has nothing to do with gravitational effects, but this is not the question.

that was precisely the question, you need to re-read the OP.

The question is whether or not the reading in the Michelson-Gale experiment would change if the apparatus were to rotate relative to the stars and then the stars disappeared.

You are trying to change the goal-posts, the presence (or absence) of the stars is totally irrelevant, the M-G experiment would detect the same exact effect in either case.

 

[Buckethead]

(However, I must point out that the shape of a single-body universe is not easy to define and might not look at all like one expects; for example, if you define a single small spherical particle at the origin, then you may well find that from the point of view of a test object in the surrounding space, the "particle" surrounds it as a distant spherical shell in all directions)..

This is particularly interesting to me as I was thinking several months ago about how light from a flashlight would behave in an empty universe with only an observer and a flashlight off in the distance. I suspected that the light beam would not "know" what a straight line was and therefore could propagate in any number of directions. The light could therefore be seen by the observer even if the flashlight was pointing away from the observer. Indeed it might be that the light would "seek out" the single observer and this would define a straight line. A point of light on the other hand could propagate in any number of directions and the result might be this enveloping glope of light you describe. In your opinion, is my reasoning in line with your understanding about why the point of light might show itself as an encapsulation globe?

Just as an aside the reason I felt that light would not know what direction it should propagate is based on my reasoning that centrifugal force depends on a straight line in order to exist (the desired path of the atoms on a rotating wheel for example) and centrifugal force couldn't exist in an empty universe if and only if a straight line could not be defined, or the atoms had no mass (inertia).

General Relativity shows signs of Mach's Principle in both linear and rotational frame-dragging effects, but cannot exactly satisfy the principle because the gravitational constant does not depend on the distribution of masses in the universe.

If inertia did not exist in an empty universe, and if the equivalency principle holds in an empty universe, wouldn't this imply that gravity would not exist either, or that the constant could vary (along with mass) depending on the overall density of the universe?

 

[Buckethead]

that was precisely the question, you need to re-read the OP.





You are trying to change the goal-posts, the presence (or absence) of the stars is totally irrelevant, the M-G experiment would detect the same exact effect in either case.

Respectfully, you need to re-read the OP. It asked:

"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? "

It is not asking if inertia depends on gravity, but rather if it depends on the total mass of the universe or on space itself. The outcome of the M-G experiment depends entirely on the answer to this question.

 

[Buckethead]

So you are of the opinion that mistaken assumptions can be corrected through imagining completely non-physical and unverifiable situations? What is to say that your speculation is not also mistaken? At least the assumptions can be experimentally challenged and tested, whereas speculation is unfalsifiable (and therefore unscientific).

Speculation is not a theory or an assumption. Speculation is a tool that can be used to imagine how something might behave under different or even untestable circumstances. All I can tell you is that I use it and it has helped me in visualizing difficult concepts or in conceptualizing relationships. Even Einstein imagined what it would be like to ride along side of a light beam. This led to SR. So can you really say it's not a useful tool?

 

[Dale]

Not unless it leads to something that is testable from within this universe. Einstein certainly understood that.

 

[Buckethead]

Not unless it leads to something that is testable from within this universe. Einstein certainly understood that.

Yes, of course, I agree completely with this.

 

[starthaus]

Respectfully, you need to re-read the OP. It asked:

"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? "

My answer addresses the other question:

"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?"

The simple answer is that the Sagnac effect in the M-G experiment can be used in order to measure the angular speed \omega and that a proper acceleration equal to r\omega^2 will be measured.

The outcome of the M-G experiment depends entirely on the answer to this question.

Then , you obviously don't understand the Michelson-Gale experiment and the Sagnac effect on which the experiment is based. Neither has anything to do with "inertia', "mass", "universe", etc.

 

[Buckethead]

My answer addresses the other question:

"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?"

The simple answer is that the Sagnac effect in the M-G experiment can be used in order to measure the angular speed \omega and that a proper acceleration equal to r\omega^2 will be measured.

Then , you obviously don't understand the Michelson-Gale experiment and the Sagnac effect on which the experiment is based. Neither has anything to do with "inertia', "mass", "universe", etc.

You are not understanding the real question the poster is asking. The M-G apparatus is able to measure angular velocity in our universe without being affected by gravity, this we agree on, but the point you are missing is that the poster is asking a question that I can restate this way: If the M-G apparatus is at it's null point, in other words, does not seem to be rotating. Is it stationary with respect to the stars in the universe, or more specifically the average velocity and mass of the stars in the universe or is it not rotating with respect to the background empty space.

If the average velocity and mass of the stars happens to be stationary with respect to empty space, then the question has been answered as they are one in the same. However if they are not the same and the null point alignes with the stars and not space, then this indicates that the stars have a meaningful influence on the M-G apparatus independant of gravity. Then the question has to be asked, what is this mechanism by which the M-G apparatus is being affected. This has everything to do with Mach's Principle by the way.

 

[starthaus]

You are not understanding the real question the poster is asking. The M-G apparatus is able to measure angular velocity in our universe without being affected by gravity, this we agree on, but the point you are missing is that the poster is asking a question that I can restate this way: If the M-G apparatus is at it's null point, in other words, does not seem to be rotating.

Firs off, I understand the question very well.
Second off, the M-G experiment measures Earth rotation about its own axis. Precisely what was asked by the OP. Nothing to do with any rotation wrt to any stars in any universe.

Is it stationary with respect to the stars in the universe, or more specifically the average velocity and mass of the stars in the universe or is it not rotating with respect to the background empty space.

You don't understand the basics of the M-G experiment. It measures Earth rotation about its own axis. The distant stars that you are so fond of are irrelevant.

Then the question has to be asked, what is this mechanism by which the M-G apparatus is being affected.

Rotation of Earth about its own axis. Like I told you before, nothing to do with any "Mach principle".

 

[Buckethead]

Firs off, I understand the question very well.

Second off, the M-G experiment measures Earth rotation about its own axis. Precisely what was asked by the OP. Nothing to do with any rotation wrt to any stars in any universe.

I'm sorry but the OP was not asking about the Earth's rotation. It was asking about a rotating bodies inertia Here it is:

"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?"

The M-G experiment only measured the rotational velocity of the Earth on it's axis. Period! It had nothing to say about the inertial effects such a rotating body feels or whether or not it's inertial effects are affected by the presence or absence of the stars in the universe. This is the question being asked.

You don't understand the basics of the M-G experiment. It measures Earth rotation about its own axis. The distant stars that you are so fond of are irrelevant.

I understand the basics of the M-G experiment but it does not address the question. Let me put it to you this way: Let's say the whole universe is rotating 90 degrees every day about the axis of the Earth. The M-G experiment will show that the Earth is rotating once every 24 hours even though in truth it is rotating once every 18 hours relative to an observer outside the universe. In other words, the M-G can say how much the Earth is rotating relative to the universe, but not it's actual rotation relative to an observer outside the universe and the reason is because the M-G apparatus is calabrated to be null relative to the universe, not outside it. So clearly, if the M-G apparatus is calibrated to the movement of the stars, then it will measure the rotation of the Earth relative to the stars, if it is calibrated relative to back ground space, then it will measure the Earth's rotation relative to the background space. It has to be calibrated to one or the other, it can't be calibrated to nothing at all! And this is exactly the question the OP was asking: If it's calibrated to the stars, and the stars disappear, does the Earth in fact feel any rotational inertia, in other words, is it rotating?

 

[starthaus]

I'm sorry but the OP was not asking about the Earth's rotation. It was asking about a rotating bodies inertia Here it is:

"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?"

The answer is given unambigously by experiment, there is a measurable angular speed (\omega) and a measurable acceleration (r\omega^2).

The M-G experiment only measured the rotational velocity of the Earth on it's axis. Period!

I am glad that you are starting to learn.

It had nothing to say about the inertial effects such a rotating body feels

It has everything to do with the detection of rotation. Add to this a dynamometer and you will be able to detect the centripetal acceleration. You may elect to run instead an experiment like Eotvos'.

or whether or not it's inertial effects are affected by the presence or absence of the stars in the universe.

Once again, the stars have no effect whatsoever. Relativity in rotating frames predicts such an effect in the case of isolated rotating bodies. No stars involved.

So clearly, if the M-G apparatus is calibrated to the movement of the stars,

Don't worry, it isn't. If you really understood the Michelson-Gale experiment, you would have known that. Actually, M-G has a very clever self-calibration, one of the most brilliant examples of experimenter's ingenuity.

 

[Buckethead]

The answer is given unambigously by experiment, there is a measurable angular speed (\omega) and a measurable acceleration (r\omega^2).

I have a clear picture now of where our disagreement lies. To me (like you) the definition of rotation is whether or not a body is experiencing rotational acceleration. And this for you is where the story ends. However for me, I have to ask what is a non-rotating body not rotating relative to, is it empty space, is it all of the mass in the universe, is it just a local galactic cluster? If the M-G device shows no rotation, like the Earth it is not showing a rotation relative to something and I feel it is this that the OP is truly asking about. In other words, is Mach's principle valid or not? If the Earth is not rotating (as shown in the M-G apparatus), and it is not rotating relative to all the stars, then what happens if the stars disappear. For you this is a non-sequiter, but for me it is a very relavent question.

 

[starthaus]

I have a clear picture now of where our disagreement lies. To me (like you) the definition of rotation is whether or not a body is experiencing rotational acceleration. And this for you is where the story ends.

Precisely.

However for me, I have to ask what is a non-rotating body not rotating relative to, is it empty space, is it all of the mass in the universe, is it just a local galactic cluster?

None of the above. The Earth is rotating with respect to its own axis.

If the M-G device shows no rotation, like the Earth it is not showing a rotation relative to something and I feel it is this that the OP is truly asking about.

But the M-G shows rotation, so the question makes no sense.

If the Earth is not rotating (as shown in the M-G apparatus),

But the Earth is rotating.

and it is not rotating relative to all the stars, then what happens if the stars disappear. For you this is a non-sequiter

Yep, it is.

 

[Al68]

You don't understand the basics of the M-G experiment. It measures Earth rotation about its own axis. The distant stars that you are so fond of are irrelevant.



Rotation of Earth about its own axis. Like I told you before, nothing to do with any "Mach principle".

Earth's rotation is "about its own axis" not relative to its own axis. Every point of Earth is (approximately) stationary relative to its own axis. Earth's rotation is relative only to other bodies.

Kinematically, there is no relative motion between different points of a rotating body. Rotation simply has no kinematic meaning other than relative to other bodies.

Mach's principle's only reason for existence is the mystery of our ability detect rotation of a body locally, despite the fact that there is no associated relative motion locally. It's obviously relevant to the M-G experiment, since it clearly demonstrates that the rotation of the Earth can be detected locally.

 

[Phrak]

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?

The body's inertial results from interaction with the local shape of spacetime--which really doesn't answer the primary question, but might help in doing so.

Speculations:

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?

Mach's Principle ... means that if the only body in the universe were rotating, the rotation could not be detected.

An empty universe is a strange place indeed, especially with regard to motion, acceleration, and inertia.

So, it will happen exactly the same way whether the universe is empty or not.

...if the apparatus were to rotate relative to the stars and then the stars disappeared.

I agree that it's pointless to talk about "otherwise empty" universes, but that's what Einstein came up with himself when he first pointed out that GR is not completely compatible with Mach's Principle.

By speculating on whether or not Einsteins principle of equivalence holds in an empty universe we may be able to say something about why it exists at all.

...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?"

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.

 

[ajw1]

The problem is that linear movement is always relative (to some reference frame), rotation isn't. The forces in a system are different when an object rotates relative to some other objects, or when these other objects are circling around the first object.

So when an object rotates, does it rotate relative to 'the fabric of spacetime' (possibly induced by the 'far stars', the other mass in the universe)? What is it that makes the object 'feel' it is rotating or not?

 

[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?