# New to QM, with questions relating to GR

1. Jun 18, 2005

### εllipse

excuse all the questions, but i've just recently started reading about quantum mechanics and want to know about what quantum physicists currently believe about GR..

How sure are physicists that a graviton exists?

Is there such a thing as curved space-time in quantum mechanics, and if so is the curvature quantized? If not is all geometry Euclidean?

Is it known from experimental evidence that the general theory of relativity has flaws, or is it just that GR doesn't make sense in the context of QM? If so, could it be that revisions need to be made to QM to make it work with GR or is it necessary that GR is flawed and a graviton must be found?

Is it impossible to describe the other forces as curvatures in space-time?

Are there any known flaws with SR? I would guess the thing where one particle's spin automatically changes another particle's spin (entanglement?) points out a flaw in SR, but I don't know. If so, is it certain that the problem here is with SR and it's not QM that needs to be fixed on this issue?

And can anyone recommend a good popularized book on quantum mechanics with very little math? I'd like to learn the math as well, but first I'd like to be a little familiar with the ideas behind QM.

thanks.

Last edited: Jun 18, 2005
2. Jun 18, 2005

Staff Emeritus
The graviton is a prediction of string theory. String theory has not been tested against experiment; it's proponents believe in it; others have grave doubts.

There are research programs on doing quantum mechanics in curved spacetime, but the standard model of particle physics exists in flat Minkowsi space, and so do all the quantum field theories that are actually used to predict particle data. The geometry of Minkowsi space is not Euclidian; it has three spatial dimension, call them x, y, and z, and a time dimension, call it t, and the fundamental form is $$t^2 - x^2 - y^2 - z^2$$. Note the minus signs; in Euclidean geometry they would be plus signs.

GR has so far met every experimental test, and is one of the most successful theories in history. Physicists would like to quantize it, but the approaches to doing this have previously failed. There are several research programs called collectively background independent quantum gravity (BIQG) that are trying new ways to quantize GR or build quantum theories that reduce to GR in the classical limit. Two of these BIQG programs you may see mentioned are Loop Quantum Gravity (LQG) and Causal Dynamic Triangulations (CDT).

Kalusza and Klein, back in 1919 - 1940, defined a theory that explained electromagnetism along with gravity as curvatures in a FIVE dimensional spacetime. It had an extra space dimension, which Klein theorized was curled up in a tiny circle, and the geometry of this augmented spacetime naturally gave GR gravity and classical EM. Einstein was initially enthusiastic, but then decided it was too artificial. Other physicists were not interested in a theory of classical EM, because they were already into quantizing EM.

There is no problem here. QM predicts what happens, and it is not true that one particle affects the other over a distance. But there is, according to QM a correlation between the observed properties of entangled particles, and SR has no problem with this. In order to find out what the two spins are, experimenters still have to communicate at the speed of light, and it is only after doing that that they can observe the correlation.

Two good books are Heinz Pagels, The Quantum Code and Nick Herbert's Quantum Reality. Avoid Taking the Quantum Leap and {i]The Dancing Wu Li Masters[/i] which will steer you wrong on what it all means. After you've read a book on the ideas of QM you might waant to try John Gribbens In Search of Schroedinger's Kittens which details a bunch of mindblowing QM experiments.

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3. Jun 18, 2005

### εllipse

Thank you very much, selfAdjoint. I am actually currently reading John Gribbens' In Search of Schroedinger's Cat, so I'm glad to see you mention another of his books because at least I know he's not a quack . I'll add the ones you recommended as the next on my list.

4. Jun 18, 2005

### danAlwyn

I thought the theory behind the graviton pre-dated the development of modern string theory. Certainly the idea of a mediating particle for gravitational interactions is as old as the other elementary bosons. Are all gravitons now derived from string theory? (It might be true, the PDG certainly doesn't talk about the possbility of finding a graviton in the near future).

5. Jun 18, 2005

Staff Emeritus

The idea that there should be a graviton is probably as old as modern QM, say the 1920s. But until string physics in the 1980s, there was never a cogent theory that PRODUCED gravitons. And that's still AFAIK the only game in town for gravitons as such.

6. Jun 19, 2005

### Ratzinger

Two good books are Heinz Pagels, The Quantum Code and Nick Herbert's Quantum Reality.
SelfAdjoint, you are the man. Not only you are always taking the time and the effort to explain things to us mere mortals, now you are recommending two of my all-time favorite books. Ellipse and everybody else go get them. These books should be mandatory reading for every QM course.
(Heinz Pagel also had written a marvelous book on complexity, sadly this fine man died at a mountaineering accident few years ago, I believe.)

"If you can't explain something to a first year student, that you haven't really understood it", a Feynman quote, if I'm correct. Self-Adjoint, you are truly a physicist/ intellectual in that Feynman spirit.

7. Jun 19, 2005

### ZapperZ

Staff Emeritus
But before you use that quote as the gospel of physics, why don't you find out how many "first year students" actually survived and stayed around till the bitter end of Feynman's first intro physics course. And we're talking about the calibre Caltech students here. Feynman has been known to NOT do what he says. Respect him and other giants in physics for what they have accomplished, but don't use them as if they are the prophets of physics whose words should be revered.

You can seldom explain ALL area of physics to first year students. You may be able to explain ABOUT physics, but this is a different beast. As I like to quote Integral's famous quote, there is a difference between learning physics, and learning ABOUT physics. One should never fool oneself that in reading these pop-science books that one has understood that subject of physics. All one has understood is the description about that physics. There is a distinct difference here.

Zz.

8. Jun 19, 2005

### Ratzinger

Schrödinger was also a trained philospher

You are totally right. Doing something or talking about something, two different things. Implicit/ learning- by-doing knowledge is often essential for true understanding. Also, translating physics and mathematics into ordinary language often results in giving an incomplete picture or is sometimes simply impossible.

Also, this quote is flawed, because there have been many great thinkers that were miserable teachers.

But I like to mention too, that there a quite some people that learned physics, but never learned ABOUT physics. For those, and everybody else, these two above-mentioned books are highly recommended.

Last edited: Jun 19, 2005
9. Jun 20, 2005

Staff Emeritus
Amen to these posts. I do not pretend to be a working physicist. I have clear ideas about a few things, and I do think they are correct in the view of the physics community (I am always opeen to correction if not!).

10. Jun 21, 2005

### straycat

There are people who are trying. Myron Evans has recently (2003-now) claimed to have incorporated EM into the equations of general relativity, much the same way as Einstein did for gravity. He's published lots of papers on the subject, but honestly I haven't seen much in the way of reaction by the rest of the physics community. So I am NOT saying he has succeeded ... merely that there are people who are trying ...

If you'd like to learn (about) relativity, I'd recommend Kip Thorne's _Black Holes and Time Warps: Einstein's Outrageous Legacy_ as an excellent pop-sci book.

David