Hints gravity finite (two recent workshops)

[marcus]

The assumption that gravity in 4D couldn't be treated as a quantum field theory (any attempt would always be overrun by infinities) is being challenged.

Here is the program for a 14-16 April workshop at Bad Honnef
http://quantumgravity.aei.mpg.de/program
See the paper by Kellogg Stelle

K. Stelle: Finiteness (or not) of N=8 supergravity

http://quantumgravity.aei.mpg.de/program/Stelle..pdf

Here's a quote from Stelle's conclusions page 27 of the PDF slide lecture notes.

"...or a series of truly miraculous D=4 cancelations to all
orders? The question remains unresolved, but according to
an old physics tradition, bets have been taken, for bottles
of wine."

Incidentally a bottle of "Stag's Leap" Cabernet Sauvignon is bet against a distinguished Rhineland white called "Diel de Diel".
================

In Kellogg Stelle's talk, the hints seem to point to the case of eightfoldsupersymmetric gravity in 4D. Things seem most hopeful in ordinary 4D spacetime, as one might expect, but using so-called N=8 SUPERGRAVITY, where the graviton has "gravitino" partners.
================

The Bad Honnef workshop had an interesting list of about a dozen speakers including a mix of String and LQG/Spinfoam. Among the non-string folks were:
A. Ashtekar: Loop quantum cosmology: a status report
L. Freidel: Status of spin foam models
M. Reuter: Asymptotically safe quantum gravity
T. Thiemann: Loop quantum gravity
These four represented about 1/3 of the total workshop lineup.
I'd say there seems to have been a decline in camp mentality. String and non-string QG people are interested in talking to and listening to each other. Maybe this was always more the case among European physicists.
=================
Kelly Stelle discussed other N-fold supergravity, for N not equal 8. And he discussed D not equal 4. But the results seemed to point especailly to the D=4 and N=8 case.
=================

MORE ABOUT THIS IN A CURRENT PARIS WORKSHOP 23-28 June
http://ipht.cea.fr/Images/Pisp/pvanhove/Paris08/index.html
"Wonders of Gauge theory and Supergravity"
In particular see the talk by Zvi Bern
http://ipht.cea.fr/Images/Pisp/pvanhove/Paris08/talk_PDF/bern.pdf
Thanks to Peter Woit for calling attention to this. Here is his discussion of it:
http://www.math.columbia.edu/~woit/wordpress/?p=705
Bern's talk is titled
Ultraviolet Properties of N = 8 Supergravity at Three Loops and Beyond
the main "UV property" at issue here is convergence or finiteness. Bern's June talk is close to Stelle's April talk in subject matter.

Stelle was also at the Paris workshop. His paper is here
http://ipht.cea.fr/Images/Pisp/pvanhove/Paris08/talk_PDF/stelle.pdf
Judging by the slide lecture notes, the Paris talk is almost the same as the earlier Bad Honnef. 30 pages instead of 27. Similar conclusion on page 30 that I quoted before. But a new bet! It seems that Stelle is still betting Stag's Leap, but now he has a different opponent, who instead of betting a Rhineland white is betting a bottle of Aloxe-Corton burgundy (Domaine Chevalier).
Same talk, different wine.

 

[Haelfix]

If N=8 D=4 Supergravity is finite, its a big deal in the field. It would be the first example of a theory that has strict nonperturbative emergent geometry. Eg you quantize a naively 4 dimensional theory, and everything looks 4dimensional. But upon looking at the nonperturbative properties, you now are forced by consistency to look at 11dimensional space.

To explain. Its been known for a long time that this type of D=4 supergravity has various pbrane solutions lurking inside upon dimensional reduction of the 11d theory which in turn is a decoupling limit of Mtheory. Whether you like it or not, they have to be there.

So if D=4 is finite, we know exactly the degrees of freedom that are not transparent in perturbation theory (charged black hole solutions, solitonic and magnetic monopole modes, etc). They aren't simply *added* by UV completing the theory.

*But* This is still far off. Theres a lot of controversy about whether or not this actually occurs.

 

[nrqed]

If N=8 D=4 Supergravity is finite, its a big deal in the field.

A quick question: I thought that N>1 SUSY was not phenomenologically viable because the theory cannotbe chiral (the left-handed and right-handed spinors must be treated equally). So is

a) this entire discussion purely formal with no intention to connect with the MSSM?

b) they concentrate on N=8 with the idea that a second stage of the program (if N=8 is indeed finite) to find wasy to break N=8 Susy down to N=1?

c) I am missing completely the point?

It would be the first example of a theory that has strict nonperturbative emergent geometry. Eg you quantize a naively 4 dimensional theory, and everything looks 4dimensional. But upon looking at the nonperturbative properties, you now are forced by consistency to look at 11dimensional space.

To explain. Its been known for a long time that this type of D=4 supergravity has various pbrane solutions lurking inside upon dimensional reduction of the 11d theory which in turn is a decoupling limit of Mtheory. Whether you like it or not, they have to be there.

So if D=4 is finite, we know exactly the degrees of freedom that are not transparent in perturbation theory (charged black hole solutions, solitonic and magnetic monopole modes, etc). They aren't simply *added* by UV completing the theory.

Your comments are very intriguing. Could you elaborate more?

Let's pretend that we have never heard of string theory/M theory at all. Let's pretend that N=8 supergravity is indeed shown to be finite. You are saying that internal consitency of the theory would necessarily lead to the need to see the theory as a dimensional reduction of an 11D theory?

What is the exact line of thoughts here? Ok, let's say that one discovers supersymmetric charged blach holes solutions. You are saying that these will imply the existence of extra degrees of freedom beyond the field of N=8 supergravity, right? (I guess they would be kinds of "branes" although they have nothiing to do with D-branes since there are no strings at all involved). And then you are saying that some consistency of the theory would necessarily lead to the need of seeing the D=4 supergravity theory as the dimensional reduction of an 11D theory?

I am not asking (for now) for the details of each step in the argument, just if I am understanding your comment correctly.

If so, this is new (and very interesting) to me. This would be the first time that I would realize that a point particle theory could lead to the need for extra dimensions through consistency arguments. I thoought this only happened in string theory.

 

[marcus]

I think anyone interested in getting perspective on this might gain by looking at Peter Woit's post about it

Here's a sample
"Perhaps the most remarkable part of this whole story is the mounting evidence that N=8 supergravity amplitudes are finite in perturbation theory. Remember the standard story about how quantum theory and general relativity are incompatible that has dominated discussion of fundamental physics for years now? Well, it turns out that this quite possibly is just simply wrong. See Zvi Bern’s talk on UV properties of N=8 supergravity at 3 loops and beyond for the latest about this. Bern shows that divergences everyone had been expecting to occur at 3 loops aren’t there, and gives evidence that they might also be absent at higher loops. He even sees this as a phenomenon not special to N=8 supergravity,..."

The link, again, is
http://www.math.columbia.edu/~woit/wordpress/?p=705

About the issue of including the Standard Model of particles in with a supergravity finally realized to be finite, Woit has something to say. He goes way out on a limb and makes a bold prediction of what will come about. It contains four separate points, which he runs together and I have spaced so I can focus on each one separately:

"... I’ll predict that sooner or later some variant (”twisted”?) version of N=8 supergravity will be found, which will provide a finite theory of quantum gravity, unified together with the standard model gauge theory

... The problems with trying to fit the standard model into N=8 supergravity are well known, and in any case conventional supersymmetric extensions of the standard model have not been very successful (and I’m guessing that the LHC will kill them off for good). So, some so-far-unknown variant will be needed.

String theory will turn out to play a useful role in providing a dual picture of the theory, useful at strong coupling, but for most of what we still don’t understand about the SM, it is getting the weak coupling story right that matters, and for this quantum fields are the right objects.

The dominance of the subject for more than 20 years by complicated and unsuccessful schemes to somehow extract the SM out of the extra 6 or 7 dimensions of critical string/M-theory will come to be seen as a hard-to-understand embarassment, and the multiverse will revert to the philosophers.

 

[Haelfix]

Hi Nrqed,

N=8 D=4 supergravity is not phenomenologically viable, at least perturbatively. At least no one knows a way to make it so. So this is pure theory work, and understanding the structure of super gauge theories and their nonperturbative completions.

Its possible there are dualities involved somehow to something realistic, but this is not obvious at all.

The blogosphere has already written some excellent discussions on this topic already.

See Motls list of articles:
http://motls.blogspot.com/2008/04/n8-supergravity-lance-dixons-puzzle.html

and Jacques Distler has some related reviews as well

http://golem.ph.utexas.edu/~distler/blog/archives/001235.html

For the more field theorist perspectives, you'd probably have to go into the literature (eg papers by Lance Dixon for instance).

edit: Why should people care? First of all its aesthetically very beautiful. Second of all whenever you learn something nontrivial about nonperturbative field theory, we learn all sorts of things about the types of physical phenomena that we can expect might lurk in realistic theories, but are hidden from us, eg we develop an expectation that something similar might just occur. (see for instance Seiberg-Witten theory and what its told us about supersymmetric theories with coulomb branches)

 

[nrqed]

Hi Nrqed,

N=8 D=4 supergravity is not phenomenologically viable, at least perturbatively. At least no one knows a way to make it so. So this is pure theory work, and understanding the structure of super gauge theories and their nonperturbative completions.

Its possible there are dualities involved somehow to something realistic, but this is not obvious at all.

The blogosphere has already written some excellent discussions on this topic already.

See Motls list of articles:
http://motls.blogspot.com/2008/04/n8-supergravity-lance-dixons-puzzle.html

and Jacques Distler has some related reviews as well

http://golem.ph.utexas.edu/~distler/blog/archives/001235.html

For the more field theorist perspectives, you'd probably have to go into the literature (eg papers by Lance Dixon for instance).

edit: Why should people care? First of all its aesthetically very beautiful. Second of all whenever you learn something nontrivial about nonperturbative field theory, we learn all sorts of things about the types of physical phenomena that we can expect might lurk in realistic theories, but are hidden from us, eg we develop an expectation that something similar might just occur. (see for instance Seiberg-Witten theory and what its told us about supersymmetric theories with coulomb branches)

Oh, I agree completely that it is a very worthwhile endeavour! I did not mean to sound disparaging with my comment. Finding a theory of gravity (without strings) renormalizable in 4 dimensions would certainly be extremely exciting. I really was just checking my own (shaky) understanding that N=8 was not phenomenologically viable.

Going back to my other question, were you implying that some arguments could be made to show that the D=4 N=8 supergravity would have to originiate from an 11D theory (without making any use of string theory argument)?

Thanks for the interesting comments.

 

[Haelfix]

"Going back to my other question, were you implying that some arguments could be made to show that the D=4 N=8 supergravity would have to originiate from an 11D theory (without making any use of string theory argument)?"

Yep. But nonperturbatively. There is a bit of a complicated roundabout way of doing this succintly wiithout string theory that I've seen only passing references too in the literature, but I've seen it done on the blackboard before (including what I thought were a few handwavey steps).

Ideally you really want to have the D=4 theory and knowing the charged black hole solutions are there, to bootstrap your way up to the 11D theory as Distler mentions. This is hard, but some progress can be made doing that.