String Gravitons yield GR. NOT

[selfAdjoint]

This paper does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.

 

[Tom McCurdy]

Sorry for asking but what are the ramifications of this discovery for string theory.

 

[marcus]

This paper does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.

Sorry for asking but what are the ramifications of this discovery for string theory.

I printed it out and read all of it I could. It looks like a careful paper, not at all polemical.
I'm tongue-tied. Nothing to say except the obvious---have to wait
and see how things sort out. Paper of this class will get discussion by people of, say, Hermann Nicolai stature at some conference. We may not have to wait all that long. Things never as simple as they seem. Very exciting.

Ganesh is the god with the elephant's trunk for a nose. He has many hands. He is the god of luck. Padmanabhan lives on Ganesh Street

 

[marcus]

Trying to see who Thanu Padmanabhan is. here is a picture of him and an interview
http://www.esi-topics.com/nhp/2004/january-04-ThanuPadmanabhan.html

He had a hot paper, on tachyons and the expansion of the universe, that he published in 2002 and in January 2004 something called "Essential Science Indicators" picked up on that this paper was getting a lot of citations, so they interviewed him.

He may have several highly-cited papers. I will check.

He used to be at Cambridge and then at Tata Institute, now at IUCAA Pune (center for astron. and astroph.) He has published over 100 papers of which 9 were "hot" (over 50 citations). these are listed here
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=find+author+padmanabhan+and+topcite+50%2B&FORMAT=WWW&SEQUENCE=

the hot papers go from present back to 1987 when he was at Cambridge
the numbers of citations received are, most recent paper first:
142, 155, 61, 88 120, 67, 61, 54, 57

the most-cited of his papers is
COSMOLOGICAL CONSTANT: THE WEIGHT OF THE VACUUM.
By T. Padmanabhan (IUCAA, Pune),. Dec 2002. 112pp.
http://arxiv.org/hep-th/0212290

Padmanabhan has published string, or string-inspired, papers for example
he got 120 citations on his
ACCELERATED EXPANSION OF THE UNIVERSE DRIVEN BY TACHYONIC MATTER.
By T. Padmanabhan (IUCAA, Pune),. IUCAA-16-2002, Apr 2002. 4pp.
http://arxiv.org/hep-th/0204150

Thanu Padmanabhan has never written any LQG papers---not a supporter of some rival to string--- may be assumed disinterested, or so I reckon.

Here is his full list of published papers---183 papers going back to 1981.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+author+padmanabhan&SKIP=150

 

[selfAdjoint]

Toward the end of the paper Padmanabhan says that the graviton produces a lagrangian with four extra terms besides the Einstein ones. It might be interesting to see if this theory can be compared with GR by experiment or astronomical observations.

 

[meteor]

What's the Palatini form? It's some differential form appearing in the Palatini formulation of GR?

 

[selfAdjoint]

It's a form of 1/R gravity (I believe that's a class of extra term versions of GR proposed to account for things like dark energy).

See for example, http://www.iop.org/EJ/abstract/0264-9381/21/15/N01.

 

[jgraber]

I too was rather shook when I spotted this paper on ArXiv and read it.
I just want to make two preliminary comments:
First off, over on SPS Lubos has a post entitled "Diff Invariance ..."
in which he says he disagrees with this paper.

Second, in his second paragraph, Padmanabhan (P) makes a distinction between
the Gupta Feynman Deser approach, and the closely related
Weinberg Boulware Deser Grischuk Wald approach,
which reaches similar conclusions. (See P's footnote 6.)
P says his criticism is only aimed at the first approach, not the second.
Also in his acknowledgments on page 15, P says he corresponded with Deser
and they reached agreement on some but not all points.

I may or may not have further substantive comments after I have read and thought some more,
but my first take is if this paper truly upsets these long accepted results, it is revolutionary.
Even if it only opens a small loophole, it is important. But at the moment I am still sceptical.

Jim Graber

 

[naunzer]

Padmanabhan finds so to say the "correct rule" (Eqn (15)) to define the transition from the flat metric to a curvilinear one (in order to be able to use functional derivation) which leads to the correct definition of the energy-momentum tensor of a spin 2 field. Correct means here that when one couples it to itself things lead to GR.
But he gets this rule by linearizing GR and then reading off the tensor which couples to the perturbation metric and identifying it then with the energy-momentum tensor of a spin 2 field.This procedure, of course, requires the knowledge of GR!
But this is similar to the things that Deser, Feynman,... did when they put general covariance into get it out as Padmanabhan complains.
Ok, he seemingly got the "correct" definition of the energy-momentum tensor for a spin 2 field but what is of even greater importance is to DERIVE its form from fundamental principles, I think.

 

[selfAdjoint]

He is criticising there work, so he is free to use the strategies that they do. You seem to miss the point that he is not trying to derive the energy momentum tensor but attacking the "common wisdom" that says you can do this is flat space with no problems.

 

[naunzer]

Yes, I agree that he is free to use their strategies and I didn't want to complain too much about that.
I just wanted to state that now that we apparently know a (the?) correct form of the energy-momentum tensor further investigations about it's (the definitions) origin are needed.
Another thing that came into my mind was if his S_ab is the only one which yields GR for as there a many other choices possible (cf. eqn (10)).

Do you mean by, "...you can do this (?) in flat space...", finding a procedure of developing GR from a spin 2 field by selfcoupling?

 

[selfAdjoint]

Yes that is what I meant. The fact that the tensor S_ab works, and is not T_ab, shows that the attempts made were futile.

 

[marcus]

Thanu Padmanabhan has just posted a very good perspective on
cosmology and dark energy
astro-ph/0411044

I am getting a more rounded-out picture of him. My respect is growing.

 

[selfAdjoint]

Very impressive, and his cites to his work with colleagues look good too. He should only learn to spell Boojum.

 

[marcus]

You are right. He should correct the spelling.
I have taken the liberty and arranged for him to be
informed of this forthwith.

 

[Curious6]

Question: this paper should indeed be revolutionary like a member pointed out, but has it actually been seriously accepted by other string theorists and physicists?? Does it really provide definite proof that string theory does not yield gravitons? Why haven't there been any other papers reacting against this one? Just some questions.

 

[marcus]

Question: this paper should indeed be revolutionary like a member pointed out, but has it actually been seriously accepted by other string theorists and physicists?? Does it really provide definite proof that string theory does not yield gravitons? Why haven't there been any other papers reacting against this one? Just some questions.

hi Curious, I can't answer the main thrust of your questions
but I will try to add to the discussion by a little reinterpretation

I think string theory will (if and when it finally gets developed as a complete, testable theory) yield gravitons.

I don't think there is any question about the string approach being able to crank out gravitons. Padma doesn't challenge this.

what padma's paper brings into question is whether yielding gravitons is the same as yielding gravity----the gravity we know and love from Gen Rel.

It could be that gravitons don't really exist in nature and that they are just a (not completely satisfactory) man-made mathematical approximation.

It could be that gravitons are a mathematical tool which sometimes, in some situations, is appropriate to use in describing how the gravitational field works.

what I have said here is only the beginning---something we should recognize at the outset. Padma goes much farther than this and indicates that gravitons (a la string) actually give a faulty answer about gravity.

I don't think this is settled.

I think that until it is settled, String people should not say that their theories include gravity. this is presumptious, or wishful thinking.
but maybe it will be settled in the future---and then they can say either that it does or that it does not. To say it does, now, would seem to me to be jumping the gun.

There will surely be more papers pursuing this line of investigation that Padma initiated. But it is deep technical stuff and I would not expect followups to be real soon. A couple of months is IMO still a little short time to expect String theorists to respond to the questions raised.

Just to have the link handy
http://arxiv.org/abs/gr-qc/0409089
From Gravitons to Gravity: Myths and Reality

(as you can see it was just September it came out, not time yet for string folk to respond. I hope they do, as I expect you too)

BTW has Padma corrected the spelling of Boojum in his latest preprint?
http://arxiv.org/abs/astro-ph/0411044
Dark Energy: the Cosmological Challenge of the Millennium

 

[Curious6]

OK, thanks again marcus for your insight! I see what is being discussed, and although I don't have a lot to add, I believe that this will as you say be a very technical issue and will take quite a while to be settled.

 

[marcus]

multiple-graviton states constructed in LQG

...
I don't think there is any question about the string approach being able to crank out gravitons. Padma doesn't challenge this.

what padma's paper brings into question is whether yielding gravitons is the same as yielding gravity----the gravity we know and love from Gen Rel.

It could be that gravitons don't really exist in nature and that they are just a (not completely satisfactory) man-made mathematical approximation.

It could be that gravitons are a mathematical tool which sometimes, in some situations, is appropriate to use in describing how the gravitational field works.

what I have said here is only the beginning---something we should recognize at the outset. Padma goes much farther than this and indicates that gravitons (a la string) actually give a faulty answer about gravity.

Just to have the link handy
http://arxiv.org/abs/gr-qc/0409089
From Gravitons to Gravity: Myths and Reality

(as you can see it was just September it came out, not time yet for string folk to respond. I hope they do, as I expect you too)
...

Stringy theories are not the only approaches to Quantum Gravity in which one can find gravitons, of course. The emphasis in LQG is on describing gravity through the quantum geometry of space, not by means of gravitons running around on a fixed rigid space. But one CAN construct gravitons in LQG.

This was recently done, for example, in a paper by Florian Conrady, who is at the Albert Einstein Institute in Golm (part of the Max Planck Institute for Gravitation Physics outside Berlin)

Free Vacuum for Loop Quantum Gravity
http://arxiv.org/gr-qc/0409036

"We linearize extended ADM-gravity around the flat torus, and use the associated Fock vacuum to construct a state that could play the role of a free vacuum in loop quantum gravity. The state we obtain is an element of the gauge-invariant kinematic Hilbert space and restricted to a cutoff graph, as a natural consequence of the momentum cutoff of the original Fock state. It has the form of a Gaussian superposition of spin networks. We show that the peak of the Gaussian lies at weave-like states and derive a relation between the coloring of the weaves and the cutoff scale. Our analysis indicates that the peak weaves become independent of the cutoff length when the latter is much smaller than the Planck length. By the same method, we also construct multiple-graviton states. We discuss the possible use of these states for deriving a perturbation series in loop quantum gravity."

by free vacuum is meant perfectly flat, not curved by matter, and static. this the kind of idealization used in perturbative approaches to QG---often e.g. in string contexts. So it can only be an idealized approximation.
However the perturbation series approximation is a useful tool in many
kinds of field theory---basic to QED for instance. At least it gets you started calculating!

So I guess it was thought that, although LQG is mainly a NONperturbative approach that does not need a static flat vacuum on which to calculate perturbations, and does not need gravitons as a tool in that kind of analysis, nevertheless the OPTION of resorting to perturbative methods could be handy.

Hence Conrady's recent paper.
In an earlier paper along these lines, Conrady co-authored with Rovelli, who is evidently interested in this himself and keeping abreast of it.
Minkowski Vacuum in Background Independent Quantum Gravity
http://arxiv.org/gr-qc/0307118

In effect, one can construct gravitons, and they may eventually come in handy as analytical constructs, but if one judges from the paper by Thanu Padmanabhan which this thread is about, then merely to find gravitons in a theory is not, by itself, ENOUGH so that one can say the theory contains gravity!

For this, see
T. Padmanabhan
From Gravitons to Gravity: Myths and Reality
http://arxiv.org/abs/gr-qc/0409089

 

[jeff]

Stringy theories are not the only approaches to Quantum Gravity in which one can find gravitons, of course. The emphasis in LQG is on describing gravity through the quantum geometry of space, not by means of gravitons running around on a fixed rigid space. But one CAN construct gravitons in LQG.

I'm sorry marcus, but as everyone here knows, you've stated quite plainly numerous times that you don't believe in the idea of gravitons. Of course you said this because you thought that lqg - your religion - is inconsistent with the idea of the graviton so that this idea must be unholy.

You've changed your tune now because yesterday I pointed out that carlo rovelli states in the introductory chapter of his new book that...

"What we need is not just a technique for computing, say, graviton-graviton scattering amplitudes (although we certainly want to be able to do so, eventually)"

Of course I've tried to explain this to you many times.

 

[Chronos]

I'm sorry marcus, but as everyone here knows, you've stated quite plainly numerous times that you don't believe in the idea of gravitons.

Marcus is in pretty good company. There are some fairly well regarded researchers who share that opinion.

Of course you said this because you thought that lqg - your religion - is inconsistent with the idea of the graviton so that this idea must be unholy.

I assume that assertion is a private joke.

You've changed your tune now because yesterday I pointed out that carlo rovelli states in the introductory chapter of his new book that...
"What we need is not just a technique for computing, say, graviton-graviton scattering amplitudes (although we certainly want to be able to do so, eventually)"
Of course I've tried to explain this to you many times.

Different interpretations of that statement by Rovelli are possible.

 

[jeff]

Marcus is in pretty good company. There are some fairly well regarded researchers who share that opinion.

Name two.

Different interpretations of that statement by Rovelli are possible.

Name a plausible one.

 

[marcus]

Jeff this is thread is not about my personality and whatever opinions you say I have expressed in the past, nor about your personality and related issues, or about Carlo Rovelli and the new book by him which apparently concerns you. Please start another thread if you feel there are not enough about those topics.

Let's focus this thread on T. Padmanabhan around the article of his that selfAdjoint started the thread by citing, namely:
From Gravitons to Gravity: Myths and Reality
http://arxiv.org/abs/gr-qc/0409089

My experience has been that some forms of participation always end up diverting a thread from the main topic and redirecting it into discussing personality issues. Overall the best way to handle this may be is not to fight it (because fighting it just leads to more vituperation) but just ignore it, and maybe start a separate thread for talking persons and their animosities and what this one said and that one said.

 

[jeff]

Jeff this is thread is not about my personality and whatever opinions you say I have expressed in the past, nor about your personality and related issues, or about Carlo Rovelli and the new book by him which apparently concerns you. Please start another thread if you feel there are not enough about those topics.

Let's focus this thread on T. Padmanabhan around the article of his that selfAdjoint started the thread by citing, namely:
From Gravitons to Gravity: Myths and Reality
http://arxiv.org/abs/gr-qc/0409089

My experience has been that some forms of participation always end up diverting a thread from the main topic and redirecting it into discussing personality issues. Overall the best way to handle this may be is not to fight it (because fighting it just leads to more vituperation) but just ignore it, and maybe start a separate thread for talking persons and their animosities and what this one said and that one said.

You've got a lot of nerve to reprimand me for going off topic, especially since you do so all the time. In fact my remarks where well within the range of what is on topic in this post, namely gravitons.

As I've said, the reason you've made it a hobby to dismiss the notion of gravitons is that you mistakenly thought they have no foundation in lqg. Carlo's statements in his book clearly show that he disagrees with you.

 

[ohwilleke]

Thanu Padmanabhan has never written any LQG papers---not a supporter of some rival to string--- may be assumed disinterested, or so I reckon.

This would be an incorrect assumption. Padmanabhan is coming from the direction of the most credible part of the alternative cosmology community, and thus has an interest is showing that mainstream GR-SM Cosmology is flawed. He needs to defeat this paradigmn to establish himself as an important scientific figure, and to the extent that string theory would bolster conventional GR cosmology, this is a problem for him.

Padmanabhan has co-authored a couple of papers with J.V. Narlikar, his institutional colleague, on Creation Field Cosmology and alternative cosmology in general. See, e.g., Creation-field cosmology: A possible solution to singularity, horizon, and flatness problems, J. V. Narlikar and T. Padmanabhan, Phys. Rev. D 32, 1928–1934 (1985) http://prola.aps.org/abstract/PRD/v32/i8/p1928_1, and Gravity, Gauge Theories and Quantum Cosmology, J.V. Narlikar, T. Padmanabhan (1986).

J.V. Narlikar is a favorite of Halton Arp, who is the leading non-standard cosmologist working today, and something of a pariah in the cosmology community, despite a legitimate scientific background, for arguing the Quasar redshift is intrinsic, that Quasars are much closer than they appear, and that they represent phenomena related to the birth of new galaxies, set out, for example, in his recent book "Seeing Red" (a review pointing out the connection between Arp and Narlikar can be found here: http://www.quackgrass.com/roots/arp.html). J.V. Narlikar, by the way, is no crackpot either. He is one of India's premier astronomers and has won a major French award for accomplishments in astronomy by a non-French citizen. See here: http://www.indianembassy.org/i_digest/2004/apr/french_astronomer.htm

Arp has been particularly criticized in his non-alternative cosmological approaches for relying on some rather weak statistical arguments on the probability of finding quasars near galaxies and apparent banding of galactic size, which other researchers don't see, and for a fairly radical version of his Quasar evolution scenario involving widespread mass formation in galactic cores ("white holes"). Arp's favorite theory of Narlikar, which is that mases increase over time, is also not widely held. (My personal opinion is that Arp is correct that Quasars have intrinsic redshift which makes them look more distant than they really are, but that his statistical argument is not the best proof of the theory and that his matter creation mechanism is wrong).

Narlikar himself is a multiple co-author with Hoyle and Burbidge, famous anti-Big Bang theorists who are known particularly for the steady state and quasi-steady state cosmological models which are currently not in favor in the cosmological community. See here for a wikisource overview: http://encyclopedia.thefreedictionary.com/Steady state theory and here for some of the co-authored articles: http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=a+Narlikar,+J+V A fairly readable exposition of the current state of the art quasi-steady state theory as applied to recent Ia supernova data can be found here: http://arxiv.org/PS_cache/astro-ph/pdf/0205/0205064.pdf

 

[marcus]

This would be an incorrect assumption. Padmanabhan is coming from the direction of the most credible part of the alternative cosmology community,...

that is fascinating. thanks for point it out.
what do you think of the "Gravitons to Gravity" paper on its own merits?

 

[ohwilleke]

that is fascinating. thanks for point it out.
what do you think of the "Gravitons to Gravity" paper on its own merits?

The basic argument, which is that a fairly straightforward approach to quantum gravity using a graviton which produces a close approximation of GR can only be achieved if the form of the stress energy tensor differs from that standard T form of GR and instead must have the form of their Second Order S tensor which is not covariant in its self-coupling is heartening. Why?

1. If quantum gravity is identical to GR in all respect, proving that it exists is dreadfully difficult. We would expect that a quantum as opposed to a continuous view of things would have some effect.

2. The basic problem in string based quantum gravity has been that the graviton self-coupling term is not renormalizable. This provides some hope that this might be because we were using the GR stress energy tensor believing it to flow from first principals when it actually does not.

3. General Relatvity models spacetime as generally covariant which means that it has scale-independent symmetry under all smooth coordinate transformations. This S tensor is not generally covariant in graviton self-coupling. This is fine. Scale-independence is precisely the sort of thing that should not exist in a quantum gravity theory. If there is quantum gravity, and not just a continuous space-time, then it should act differently as one approaches the quantum scale.

Put another way, this appears to argue that one can have something that closely approximates GR in Minkowski space so long as one is willing to have some quantum effects. This in turn, would allow for a non-geometric interpretation of GR which is at the heart of what non-LQG approaches to quantum gravity are about. Conceptually, this would make string based QG far more "natural" to think about without questioning our basic intuitions about time and space, than GR is.

4. The article doesn't say it, but I infer that non-generally covariant self-interacting gravitational fields, which is what this article is talking about, could produce MOND similar to the non-abelian field supposition using a QCD analogy I read somewhere. If MOND fell naturallly out of the most workable way to approximate GR from from gravitons this would be serendipitous indeed.

There are no real obvious flaws, although the point that quantum gravity theories are made with the end in mind, isn't as serious a critcism as it is made out to be in my mind. The bigger problem he points out with conventional derivations is circularity. Y follows from X only because something equivalent to Y is assumed.

 

[Rothiemurchus]

I am not so sure that virtual gravitons are viable theoretical entities.
In electromagnetism,virtual photons and real photons are absorbed by matter.But
real gravitons (which are in gravitational waves) are not absorbed whereas virtual gravitons are.What is the explanation for this difference.As far as I'm concerned it is simpler to assume virtual gravitons do not exist and that gravity is caused when real gravitons are,sometimes,absorbed.