## Horava-Lifshitz Gravity April 28, 2009

Posted by keithkchan in Journal club.

Recently, there are a lot of papers on the a new quantum gravity model, now dubbed Horava-Lifshitz gravity. It has been published in Physical Review. This model appears to be very exciting since it claims to be renormalizable. Many authors have immediately started calculating various things: inflation in this model, bouncing model, Schwarzschild solution, non-Gaussianity, blah, blah, blah.

Beside being renormalizable, another thing that is particularly exciting is that this model seems to be able to generate scale invariant spectrum without inflation. That sounds very cool!

Because of these seemingly attractive features in this model, in last week’s journal club, people in another journal club tried to read this paper. Since the model is novel and complicated, the smart people in our centre failed to understand the model. Gruzinov was very excited, and asked Kleban to talk about it in next week. He even criticized that the high energy people in our centre for their ignoring this interesting model.

So this week, Kleban presented this model. Before the start of the talk, Gruzinov asked Porrati for his comments. It turns out Porrati often makes funny comments. He said that I haven’t really read it, but if it was right, I would change my career to study swine flu. I find his comment very funny.

After all these prelude, I now briefly describe the model. Frankly speaking, I don’t really understand it, so I am not going to comment on it. The model is motivated by the Lifshitz point, which is used to model the triple point in condensed matter physics. By the way, this is another example that ideas in high energy physics originate from condensed matter physics. In this model there is critical exponent z, if it is 1, we get relativistic dispersion relation. This model is z=3 in the UV, and it becomes z=1 in the IR, and Lorentz symmetry is recovered as an accidental symmetry. The model is power counting renormalizable. In this model, space and time scales in differently, so that the the theory is rendered power counting renormalizable. Note that power counting renormalizable is just a first test of renormalizability. One really needs to check in details if it is indeed renormalizable. During the talk, some of the audience, in particular Gruzinov was increasingly impatient as the Lagrangian introduced was rather ad hoc. I don’t really understand it so I am not going to comment on that, just to say that the terms added are mostly spatial, and of high derivatives. Needly to say, it is very different from the usual GR and field theory. Gruzinov even said that life was short, why do we spend time on such a theory. Kleban reminded him that he should blame himself for asking him to present the paper. People were worried about about a parameter introduced in the theory, which start as 1/3 in UV, it has to flow to 1 in order to agree with GR in the IR regime. After browsing the whole paper, Horava did not say (or know ) how to realize this flow. Porrati said that it was almost impossible for any perturbative effect drive the parameter to flow from 1/3 to 1. At the end, nobody was happy about this paper. Gruzinov seemed to be pissed off. I personally find the talk rather amusing. The comments by various people, in particular Porrati and Gruzinov very interesting and illuminating.

So it was pretty funny that last week everybody was excited about this paper and this week everybody loses interest in it. We have to wait for another breakthrough…

Update: I would like to add a few comments on it. Nobody really knows how to come up a workable quantum theory of gravity. So anybody can take strong opinion in this regard. But anybody that asks the right question is much much more likely to succeed than others. So Porrati and Gruzinov think that Horava’s approach is not going to work. Fine.

But shouldn’t it be obvious that anything that is obvious is not going to work. Anything that is going to work is not going to be obvious. The reason that those smart people fail to come up with a sensible theory of quantum gravity maybe because they are too strongly biased. I defend this theory does not mean that I believe that this the right theory. But I certainly think that this is an interesting and novel idea. OK, Horava did not lay out the model in details. But I think he has set up a proposal that is interesting enough to pursue it further. One of the reason that so many people have jumped into this theory because they think that it is interesting and worth trying. So my philosophy to these kind of difficult open questions is that we should keep mind opened.

Up-update: Apparently, this post is controversial and stimulating. Lubos Motl has written a post about this post. And he tries to dissect each comments I said. So I should feel honored since nobody has taken my words so seriously before. But I am going to de-comment since I got much more publicity than I want. You check out his comments to see if it is justified.

I got so much publicity that I may need to put up a stupid disclaimer that the contents of this blog has nothing to do with NYU, don’t sue the director of graduate studies if you don’t like this post. OK, anyway I now shut up.

1. Luboš Motl - April 30, 2009

Dear Keith,
for some criticism of your approach to research, see

Best wishes
Lubos

2. Zephir - April 30, 2009

If ideas in high energy physics originate from condensed matter physics, why not to consider the vacum as a compressible fluid of high energy and mass density? Such environment can be modeled in particle simulations on computers with no physicist involved.

3. keithkchan - April 30, 2009

Lubos
Thanks for your criticism. Apparently, this post is sufficient to make me famous, much more likely infamous. I am not going to defend it any more, the post stands for itself. It is up to the readers to decide.

Keith

4. keithkchan - April 30, 2009

Zephir
Higgs mechanism originates from Anderson’s insight about superconductivity.

OK. Enough is enough. I don’t want to add more spice to this post any more. I now shut up.

Keith

5. last post - May 1, 2009

aha, when you said the same words as Lubos, he will praise you, however, you said so many words not support motls, so, ……

6. Zephir - May 1, 2009

The Higgs mechanism for giving mass to particles was actually first proposed in the context of solid state physics to explain how particle-like structures in metals can act as if they had an effective mass. In solid state physics Higgs mechanism is responsible for gaining effective electron mass and conductivity in transition metals and semiconductors. Of course this mechanism is a quite different from the way, in which condensate droplets of every particle environment are obtaining mass. And it has nothing to do with superconductivity also, as it takes place at room temperature, too. In my opinion, scientists are just mixing various concepts and mechanisms mutually.

7. Artful Codger - May 1, 2009
8. Jocker - May 7, 2009

There is a phenomena “the arXiv dark side”: As people use citation indices (as the “Hirsh index”) to evaluate the reseachers, some undestood that they have to write as many crappy papers that will collect the largest citation numbers. People forget that the subject was crappy and retain the citation index. For instance here are some examples of such phenomenon:

1- The non-commutative geometry had as output that the theory is not defined on the right background (UV-IR mixing) but on the way generated a huge number of useless papers.

2- With the unparticles: the 45th paper cites the previous 44 and is cited by the 46th and none has anything useful to say as they do not even understand how to define unparticles, couple them … Asia entered the silly game at that point.

4- With Horava-Lifshitz we reach the top: people explain early universe with a theory they did not show to be consistent with the observed facts as does GR. The equation of motion of the Horava-Lifshitz theory states “No time to think, you need to publish and get cited”

The whole game became “write something that sounds smart and sexy about crap very quickly such that one gets cited by the big number of people that come after”. After a while, he same people are gonna write about some other “great idea”, and forget that the previous “great idea” was crap stamped “great” by …

One should create special journals for this crap: They should be just electronic journals such that we do not destroy forests for such things.

9. keithkchan - May 14, 2009

Zephir
What I mean by Higgs mechanism in superconductivity is that suppose some composite field acquires nonzero vacuum expectation value, and the electromagnetic field acquire mass and $A_{\mu}$ is given by $\partial_{\mu} \phi$. From this assumption, one can go pretty far. One can get Meissner effect, flux quantization, Josephson effect. Of course I did not cook it up, in fact I follow Weinberg II Chapter 21. It has nothing to do with temperature.

” In my opinion, scientists are just mixing various concepts and mechanisms mutually.”
I disagree with your opinion.

10. keithkchan - May 14, 2009

Jocker
I agree with you that these days a lot of papers have pretty low signal to noise ratio. Those people who write a lot of papers get a lot of citations. Others are forced to follow suit if they want to get a job. I heard of a joke: the archive in Physical Review has increased in size faster than speed of light in last couple of decades, but it doesn’t violate causality since no information can be extracted from it.

People are tired of old things and rush to do when interesting ideas come out. They are tired of the old ideas because they are not perfect.

All the ideas you mention may turn out be nonsense. Some of them may have gone to the hell. It seems to me nobody talks about unparticle any more. However, a priori it is not clear that those ideas are crap. It become clear only after thoroughly investigated. That’s the purpose of the follow-up papers. Some of the ideas may have some undesirable features at the beginning. But it may be cured, it is not clear at the beginning, at least not to everyone.

11. Zephir - May 18, 2009

keithkchan, IMO Higgs mechanism is the same product of information noise, as Jocker has explained in previous post.

What I know is Higgs-Andersson mechanism, which is responsible for effective mass of electron. If some other effect is related to background field in superconductivity, it’s OK – but such effect shouldn’t be called a Higgs mechanism anymore – or we simply get two different kinds of Higgs bosons. Until we aren’t perfectly sure, Higgs mechanism responsible for mass of bosons in vacuum is conceptually the same thing like both effects above noted, we should’t called it a Higgs mechanisms – or we get another kind of Higgs boson.

And so on..

12. Zephir - May 18, 2009

Phenomenologically, main concept of Horava-Lifshitz Gravity appears rather close to AWT, which considers Universe as a systems of nested massive fluctuations (unparticles). The requirement of minimal energy propagation speed leads to triple, quadruple, etc… point condition of the Aether gas, so I can agree with H-L model in this point.

Unfortunately I cannot understand, how H-L gravity is supposed to work at the formal level, i.e. how relativity and quantum mechanics equations are connected mutually and how they can be renormalized. In my opinion it’s virtually impossible to seamlessly connect pair of theories with inconsistent postulate set – so if some theory claims the opposite, it must contain some approximation, which could change the meaning of the whole theory in less or more distant perspective.

13. keithkchan - May 23, 2009

Zephir
It is my fault that I called it Higgs mechanism. The mechanism only qualifys to be called Spontaneously Symmetry Breaking of the electromagnetic gauge symmetry. Since it is a global symmetry breaking, there is a Goldstone mode, and no such “Higgs field” to eat it.