tag:blogger.com,1999:blog-8148573551417578681.post5995814557420403340..comments2020-02-18T23:15:04.706-08:00Comments on Dark Buzz: New physics prizeRogerhttp://www.blogger.com/profile/03474078324293158376noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-8148573551417578681.post-31284067957314501552012-08-08T12:07:29.954-07:002012-08-08T12:07:29.954-07:00General relativity describes a massless spin-2 fie...General relativity describes a massless spin-2 field. Conversely, if you have a theory that describes a massless spin-2 field, you can show that it couples to matter in exactly the same way as the gravitational field, and so it gives rise to a force that it indistinguishable from gravity. The statement that physicists discovered a massless spin-2 field in string theory is therefore equivalent to the statement that they discovered a theory of gravity. If you want to learn more, this is all explained very nicely in Zee's QFT textbook.Bob Jonesnoreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-66913718035141021252012-08-08T09:30:28.106-07:002012-08-08T09:30:28.106-07:00Thanks for the detailed response, following the ot...Thanks for the detailed response, following the <a href="http://blog.darkbuzz.com/2012/08/what-bleep-do-we-know.html" rel="nofollow">other thread</a>. Maybe you can explain this. Yes, I've read that string theory was proposed for the strong force, and then when a spin-2 field was found, they assumed that it was gravity. But it is my understanding that there are general arguments that any spin-2 field would look like gravity. So the argument that the string theory spin-2 field looks like gravity doesn't have anything to do with string theory, except that string theory has a spin-2 field.Rogerhttps://www.blogger.com/profile/03474078324293158376noreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-40380470873077571172012-08-08T00:04:45.897-07:002012-08-08T00:04:45.897-07:00Did you mean to post this in the earlier thread? N...Did you mean to post this in the earlier thread? No matter, I'll continue the discussion here.<br /><br />(1) I am not claiming that gravitons are observable with present day technology. I'm just saying that there are good experimental reasons for believing in gravitational waves, and if you accept the existence of gravitational waves, a very simple argument in quantum mechanics shows that gravitational radiation must be quantized. Although he's not my preferred source of information, Lubos Motl has written an elegant little proof of this fact over on his blog:<br /><br />http://motls.blogspot.com/2012/01/why-semiclassical-gravity-isnt-self.html<br /><br />(The argument starts near the end of the article.)<br /><br />(2), (3) Again, I am not an expert on the subject of finiteness in string theory, but I can look up references as easily as you can. These references are not proofs in the mathematician's sense, but that's obviously too much to hope for since string theory (and for that matter, quantum field theory) do not have any rigorous mathematical foundation. For perturbative finiteness, the main reference seems to be<br /><br />http://www.sciencedirect.com/science/article/pii/0550321388903227<br /><br />The expert on this topic seems to be this guy at Berkeley:<br /><br />http://www.physics.berkeley.edu/research/faculty/mandelstam.html<br /><br />To prove that string theory is non-perturbatively consistent, I suppose you just need some non-perturbative definition of the theory. For example, the AdS/CFT correspondence tells you that string theory on AdS5 x S^5 is equivalent to N=4 super Yang-Mills theory, so the consistency and finiteness of the latter makes the consistency and finiteness of the former completely manifest.<br /><br />(4) About Ricci-flatness, let me say first of all that it makes no sense to say that spacetime is flat in a theory where the gravitational field is quantized. In such a theory, you have a sort of quantum superposition of different spacetime geometries. I think what you mean to say is that perturbative string theory requires a choice of background. This means we're choosing a metric to start with (for example the Minkowski metric, which is flat) and we're considering perturbations of that metric. When you're doing quantum gravity in this way, it's reasonable to ask whether the theory depends on this choice of background. (If we look at perturbations of something other than the Minkowski metric, will our calculations give the same results?)<br /><br />In string theory, this issue is resolved by the various approaches to defining string theory non-perturbatively. The most successful of these is the AdS/CFT correspondence in which string theory in D+1 dimensions is equivalent to a quantum field theory on the D-dimensional boundary of the spacetime. So, to answer your question, if you want to define string theory in a non-perturbative, background-independent way, you can just define it to be the theory which is dual to a conformal field theory on the boundary of spacetime.<br /><br />I suppose the canonical reference on the AdS/CFT correspondence is<br /><br />http://arxiv.org/abs/hep-th/9905111<br /><br />If you're looking for something more accessible, I guess you could look at<br /><br />http://www.sns.ias.edu/~malda/sciam-maldacena-3a.pdf<br /><br />In addition to flatness of the background, you're also asking whether general relativity is somehow an assumption of string theory. As I explained in my last post, the answer is definitely "no". Indeed, string theory was originally a theory of the strong force, and only later was it discovered that it contains a massless spin-2 particle that can serve as a graviton. I don't think I need to give you a reference for this; you can read about it in any history of the subject.Bob Jonesnoreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-32750883482007814282012-08-07T19:41:02.961-07:002012-08-07T19:41:02.961-07:00I don't think you are right, but please post t...I don't think you are right, but please post the links to prove me wrong. In particular, I would like to see papers that (1) give circumstances where a graviton is observable; (2) prove that string theory is perturbatively finite; (3) prove that string theory is consistent non-perturbatively; and (4) define a string theory model that does not assume Ricci-flatness or a spin-2 field or some other general relativity assumption.Rogerhttps://www.blogger.com/profile/03474078324293158376noreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-62420903274290938902012-08-07T13:36:23.938-07:002012-08-07T13:36:23.938-07:00Good point. And David Gross already has a Nobel Pr...Good point. And <a href="http://en.wikipedia.org/wiki/David_Gross" rel="nofollow">David Gross</a> already has a Nobel Prize for work done before he converted to string theory.Rogerhttps://www.blogger.com/profile/03474078324293158376noreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-49522694389071519742012-08-07T13:19:02.594-07:002012-08-07T13:19:02.594-07:00I agree that the Nobel prize won't be awarded ...I agree that the Nobel prize won't be awarded for string theory anytime soon, but a string theorist could certainly win the prize. For example, Witten could easily win it for his work on the mass of the eta-prime:<br /><br />Nucl. Phys. B156 (1979) 269-283Bob Jonesnoreply@blogger.comtag:blogger.com,1999:blog-8148573551417578681.post-64031494057981637122012-08-05T10:47:13.689-07:002012-08-05T10:47:13.689-07:00"Physics has lost its way."
I agree - t..."Physics has lost its way."<br /><br />I agree - this is not likely to be positive for physics.A K Haarthttps://www.blogger.com/profile/05897490979828603179noreply@blogger.com