# B Curve Space-time from Spinning disc?

#### lucas_

Please see attached illustration (from Discover September 2004. Einstein 100 years special issue).

Is it correct that one key to Einstein's thinking is to analyze a spnning disk. That "Since the rim of the disk travels faster than the center of the disk, the theory of relativity states that the rim is compressed more than the center. If so, the disk must be distorted (its circumference is no longer pi times its diameter). So The surface of the disc, is in fact, curved?"?

Related Special and General Relativity News on Phys.org

#### PeterDonis

Mentor
Please see attached illustration (from Discover September 2004. Einstein 100 years special issue).
This is a good example of why it's not a good idea to learn science from pop science articles. The article's description is highly misleading. See below.

Is it correct that one key to Einstein's thinking is to analyze a spnning disk.
It's possible that it played a role, but I don't think the connection the article is claiming is valid.

First, a key distinction: the article discusses curved space, but space is not the same as spacetime, and the curvature of the "space" of a spinning disk has nothing whatever to do with spacetime curvature. Spacetime in the case of the spinning disk is flat.

Second, the "space" of the spinning disk isn't even a 3-dimensional spatial "slice" cut out of 4-dimensional spacetime. It's an abstract "space" that you get by performing a particular mathematical operation (the technical term is "quotient space"), and doesn't correspond to any actual 3-dimensional space at all.

Third, the "rubber sheet" model that the article describes is not an analogue of the curved "space" of the spinning disk. Why? Because the curved "rubber sheet" space is an actual 3-dimensional spatial "slice" cut out of the 4-dimensional spacetime of a static gravitating mass. However, this "space" is curved because of the particular way it is cut from the spacetime; there are other ways of cutting such slices that make the spatial slices flat. So once again, the curvature of the "rubber sheet" space is not the same as the curvature of the spacetime; the latter is there no matter how we cut spatial slices out of the spacetime.

#### FactChecker

Gold Member
2018 Award
Einstein himself wrote some books that motivate his theory. You should look there. One should be careful not to take too literally any description that is meant to help visualize the concepts but is not valid physics.

#### lucas_

This is a good example of why it's not a good idea to learn science from pop science articles. The article's description is highly misleading. See below.

It's possible that it played a role, but I don't think the connection the article is claiming is valid.

First, a key distinction: the article discusses curved space, but space is not the same as spacetime, and the curvature of the "space" of a spinning disk has nothing whatever to do with spacetime curvature. Spacetime in the case of the spinning disk is flat.

Second, the "space" of the spinning disk isn't even a 3-dimensional spatial "slice" cut out of 4-dimensional spacetime. It's an abstract "space" that you get by performing a particular mathematical operation (the technical term is "quotient space"), and doesn't correspond to any actual 3-dimensional space at all.

Third, the "rubber sheet" model that the article describes is not an analogue of the curved "space" of the spinning disk. Why? Because the curved "rubber sheet" space is an actual 3-dimensional spatial "slice" cut out of the 4-dimensional spacetime of a static gravitating mass. However, this "space" is curved because of the particular way it is cut from the spacetime; there are other ways of cutting such slices that make the spatial slices flat. So once again, the curvature of the "rubber sheet" space is not the same as the curvature of the spacetime; the latter is there no matter how we cut spatial slices out of the spacetime.
Thank you.

How about the analogy of the moving clock up and down explaining time dilation. Is it real or incorrect? Watch this video (it's explained towards the end) which is winner of the 2017 Breakthrough Junior Challenge where 1 video out of 11,000 entries around the world was selected (winner takes home $400,000). Is there any flaw in of the explanation of the winning entry "Relativity and the Equivalence of Reference Frames"? (just click on the 2017 winner entry) #### lucas_ Thank you. How about the analogy of the moving clock up and down explaining time dilation. Is it real or incorrect? Watch this video (it's explained towards the end) which is winner of the 2017 Breakthrough Junior Challenge where 1 video out of 11,000 entries around the world was selected (winner takes home$400,000). Is there any flaw in of the explanation of the winning entry "Relativity and the Equivalence of Reference Frames"?

(just click on the 2017 winner entry)
The Light clock with 2 mirrors at the end and moving seems to be valid:

Why can't this be done in actual experiment where laser light bounces in mirror and it is moving. Then observer or detector in the moving device compares the time to the stationary observer?

#### PeterDonis

Mentor
The Light clock with 2 mirrors at the end and moving seems to be valid
Sure, the light clock is a well-known thought experiment that is often used to teach SR.

Why can't this be done in actual experiment where laser light bounces in mirror and it is moving
It could, but probably nobody has thought it necessary to go to the trouble since there is so much other evidence already that confirms SR.

However, I don't see what any of this has to do with the subject of this thread.

#### FactChecker

Gold Member
2018 Award
The video just gives a brief hint at what is going on. There is also length contraction, the relativity of simultaneity, and other effects. The effect on time is roughly correct, but the other effects complicate things. By using the Doppler effect as an example, the video also implies that people in the same inertial reference frame might disagree with each other about the relativity effects, but they would not.

#### lucas_

The video just gives a brief hint at what is going on. There is also length contraction, the relativity of simultaneity, and other effects. The effect on time is roughly correct, but the other effects complicate things. By using the Doppler effect as an example, the video also implies that people in the same inertial reference frame might disagree with each other about the relativity effects, but they would not.
Can you give an example about this thing where people "
in the same inertial reference frame might disagree with each other about the relativity effects, but they would not."?

#### FactChecker

Gold Member
2018 Award
People in the same inertial reference frame do observe different frequencies due to the Doppler effect. But their observations about the time and length changes in a fast-moving object are the same.

#### lucas_

Sure, the light clock is a well-known thought experiment that is often used to teach SR.

It could, but probably nobody has thought it necessary to go to the trouble since there is so much other evidence already that confirms SR.

However, I don't see what any of this has to do with the subject of this thread.
The Spinning disc example in Discover was written by Michiu Kiku. Someone as knowledgeable as Einstein. So if it's misleading. Then I just fathomed whether the light clock was another common misleading example. So the spinning disc is not commonly used?

Also is it not General Relativity can be derived from Special Relativity. Or is the concept separate. So a universe could have special relativity without general relativity. Or does the former implies the latter?

#### PeterDonis

Mentor
The Spinning disc example in Discover was written by Michiu Kiku. Someone as knowledgeable as Einstein.
But it's still a pop science article. It's not peer reviewed. Nobody is telling the author that what he writes is misleading. He's just writing what sounds good to him and works to sell articles and magazines and books. Whereas in a textbook or peer-reviewed paper, there are other experts involved who can keep the author from writing misleading things.

Then I just fathomed where the light clock is another common misleading example.
Why? The fact that one particular video happened to say something misleading while discussing the light clock, does not mean the light clock itself is misleading. You just need to stop looking at pop science articles and videos and start looking at textbooks and peer-reviewed papers.

So the spinning disc is not commonly used?
Commonly used for what? As a pedagogical example in textbooks and peer-reviewed papers? Sure, it is.

is it not General Relativity can be derived from Special Relativity
No, it's the other way around. Special relativity is a special case of general relativity, in which spacetime curvature is zero.

So a universe could have special relativity without general relativity.
Only in the sense that a universe in which spacetime was flat everywhere could be described using SR without having to use the full machinery of GR. But such a universe could not contain any matter or energy, which means it could not contain any people.

#### lucas_

But it's still a pop science article. It's not peer reviewed. Nobody is telling the author that what he writes is misleading. He's just writing what sounds good to him and works to sell articles and magazines and books. Whereas in a textbook or peer-reviewed paper, there are other experts involved who can keep the author from writing misleading things.

Why? The fact that one particular video happened to say something misleading while discussing the light clock, does not mean the light clock itself is misleading. You just need to stop looking at pop science articles and videos and start looking at textbooks and peer-reviewed papers.

Commonly used for what? As a pedagogical example in textbooks and peer-reviewed papers? Sure, it is.

No, it's the other way around. Special relativity is a special case of general relativity, in which spacetime curvature is zero.

Only in the sense that a universe in which spacetime was flat everywhere could be described using SR without having to use the full machinery of GR. But such a universe could not contain any matter or energy, which means it could not contain any people.
Ok. Last question about Michiu Kiku before I donate the Discover magazine. Is it true that without relativity, our bodies' molecules would fall apart? I don't think it's because of something moving at relativistic speed. But without relativity, there is no spin too? So can one say relativity doesn't only work for relativistic speed?

#### lucas_

But it's still a pop science article. It's not peer reviewed. Nobody is telling the author that what he writes is misleading. He's just writing what sounds good to him and works to sell articles and magazines and books. Whereas in a textbook or peer-reviewed paper, there are other experts involved who can keep the author from writing misleading things.

Why? The fact that one particular video happened to say something misleading while discussing the light clock, does not mean the light clock itself is misleading. You just need to stop looking at pop science articles and videos and start looking at textbooks and peer-reviewed papers.
I meant since the spinning disc was misleading, I wondered whether light clock in general was misleading too. But you referred to that particular video concerning the light clock as misleading. Which part of it? It won \$400,000. Almost 1/3 the Nobel Prize out of 11,000 choices. So I ought to know why was its light clock example misleading?

Don't worry. I'd start looking at textbooks next month at library (if I can understand it). So I just need to know these basic for bird eye views.

Commonly used for what? As a pedagogical example in textbooks and peer-reviewed papers? Sure, it is.

No, it's the other way around. Special relativity is a special case of general relativity, in which spacetime curvature is zero.

Only in the sense that a universe in which spacetime was flat everywhere could be described using SR without having to use the full machinery of GR. But such a universe could not contain any matter or energy, which means it could not contain any people.

#### haushofer

Your paper claims that quantum mechanical spin is demanded by relativity. I don't see how. Spin is an inherent property of particles enforced upon us by experiments. There is no physical law which says that matter should have spin, let alone spin 1/2. It follows from considerations that this inherent property explains the zeeman-effect, the periodic table etc. So I don't understand that claim.

#### PeterDonis

Mentor
Is it true that without relativity, our bodies' molecules would fall apart?
The argument given in the image you show can't be right, since the statement "relativity requires that all particles spin like tops" is false; spin zero particles exist and are perfectly compatible with relativity. So the argument seems bogus to me.

It is true that you can't have atoms as we know them without fermions (particles that obey the Pauli exclusion principle, which says that no two fermions can be in the same state). But I don't see any way to argue from relativity that fermions must exist.

#### PeterDonis

Mentor
you referred to that particular video concerning the light clock as misleading.
No, I didn't. I haven't even watched the video. I think you are confusing me with @FactChecker here.

#### lucas_

Your paper claims that quantum mechanical spin is demanded by relativity. I don't see how. Spin is an inherent property of particles enforced upon us by experiments. There is no physical law which says that matter should have spin, let alone spin 1/2. It follows from considerations that this inherent property explains the zeeman-effect, the periodic table etc. So I don't understand that claim.
Whoa. If it was written by Tom and Gerry. We could doubt it. But not when it was written by a highly regarded esteemed physicist of highest caliber (Michio Kaku). This is the full context. I won't share other images, just this for scrutiny:

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#### lucas_

If Michio third claim was false. How about his first and second claim (see illustration above):

1. "Moreover, since relativity governs the properties of electricity and magnetism, all modern electronics would come to a halt, including generators, computers, radios, and TV.". Relativity really governs the
properties of electricity and magnetism?

2. "Without relativity, earth would freeze solid", "The nuclear furnace that drives the sun and stars would shut down without relativity. If there were no E=mc^2, the universe would suddenly become dark and cold, making life impossible". True?

#### jbriggs444

Homework Helper
Argument from authority does not fly. And Kaku's pop science presentations are infamous here.

#### Nugatory

Mentor
How about his first and second claim (see illustration above):
Both claims have it backwards. We develop and use physical theories to describe how the universe behaves. Relativity does not “govern the properties of electricity and magnetism” - the properties of electricity and magnetism compel us to accept relativity as an accurate description of the universe.

#### Nugatory

Mentor
Whoa. If it was written by Tom and Gerry. We could doubt it. But not when it was written by a highly regarded esteemed physicist of highest caliber (Michio Kaku).
You want to be careful here. Doing physics and describing physics are different skills; many bad popularizations have been written by good physicists.

If you want understanding as opposed to entertainment, there is no substitute for the real thing: peer-reviewed publication and serious textbooks. It’s more work, but infinitely more rewarding.

#### lucas_

The argument given in the image you show can't be right, since the statement "relativity requires that all particles spin like tops" is false; spin zero particles exist and are perfectly compatible with relativity. So the argument seems bogus to me.

It is true that you can't have atoms as we know them without fermions (particles that obey the Pauli exclusion principle, which says that no two fermions can be in the same state). But I don't see any way to argue from relativity that fermions must exist.
But is it not spin occurs when special relativity is added to Schrodinger equation which produced Dirac equation that included spin? So doesn't this mean spin and relativity is connected. So Michio illustration may be just a simplified statement that if there was no relativity, there was no Dirac equation that can make particles execute the Dirac equation so no spin can be possible (you don't have spin in the non-relativistic Schrodinger Equation)?

#### PeterDonis

Mentor
If it was written by Tom and Gerry. We could doubt it. But not when it was written by a highly regarded esteemed physicist of highest caliber (Michio Kaku).
Go back and read my post #11 again. Your confidence in pop science articles is misplaced. And if you're going to refuse to believe us here when we explain to you why your confidence in pop science articles is misplaced, and instead keep arguing from authority, why are you even bothering to ask the question?

#### PeterDonis

Mentor
is it not spin occurs when special relativity is added to Schrodinger equation which produced Dirac equation that included spin?
No. There were non-relativistic treatments of spin before the Dirac equation was discovered.

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