Is there more to gravity than gravitons?
This is an intriguing paper by Padmanabhan:
From Gravitons to Gravity: Myths and Reality
I saw this paper back in 2004, when it was submitted, but failed to see reactions to it at that time. In the comments section of a post about string theory over at Cosmic Variance, the issue of gravitons came about and made me wonder again about that “semi-classical” work by Padmanabhan. I wonder whether the issues raised by Padmanabhan are indeed correct and if so, how would they be interpreted in string theory.
May 28, 2007 at 4:44 am
I vote for “nothing more than gravitons”, if by that the questioner asks whether gravity is a simple particle force (like the others), as opposed to a complicated curved space thing. That gravitons don’t match GR is not a problem for a graviton theory, as experiments cannot distinguish the theories.
I think that there has been way too much effort at unifying gravity and quantum mechanics while trying to preserve both theories unchanged. Instead, one should allow both theories to be modified, or replaced, within the constraints of experiment.
May 28, 2007 at 3:55 pm
I will attempt to briefly address some of the specific points of From Gravitons to Gravity: Myths and Reality by Professor T Padmanabhan, (hereafter PTP, with deepest respect.)
I also saw this paper when it came out on the ArXiv in 2004 and have wondered about it since then.
However, I will not attempt to consider any relation to String theory, (ST), as I am not qualified to discuss string theory.
A few preliminary comments: PTP acknowledges a six month private exchange with Deser, not ending in full agreement. Also, he refers to a further work in progress by himself that has not yet appeared, to my knowledge. These two items make me cautious.
PTP acknowledges in the paper itself that it goes against conventional wisdom, still another reason to be cautious.
Nevertheless I will plunge forward: To oversimplify, I see two points in this paper. One is that the standard derivations of GR from a spin2 field contain hidden or unstated assumptions. The second is that if the derivation is done carefully, you end up with âSâ rather than âTâ.
On the hidden assumptions point, this paper seems to say that one such assumption is symmetry. This seems to imply that nonsymmetrical generalizations of GR are possible.
This reminds me of the recent work by Moffat.
On the S versus T, PTP seems to say that S is mathematically identical to T for most ordinary forms of matter, so of course, no difference could be measured. Also, PTP says the principle of equivalence is conserved. If I understand correctly it is only the energy of the spin two field itself that differs between S and T. To me that sounds similar to the old Field stress energy Pseudotensor concept
As I understand it, PTP says S is equal to T in all three physically important cases. Hence, I conclude these two theories are not measurably different, and hence are equivalent FAPP.
As I understand this paper, it is on the strictly classical field theory level and does not involve any quantum techniques, although it investigates the logical prerequisites for certain quantization approaches. Or is a spin 2 field necessarily a quantum concept?
But I infer that PTP thinks this S versus T difference may be more important once the theory is quantized. I have probably extended my speculations here too far.
Perhaps someone better qualified than I will address some of these points.
May 29, 2007 at 3:41 pm
Hi Christine & All…
I have followed this interesting thread from Cosmic Variance to here, with particular interest in Pad’s conjecture, which goes against the common Feynman folklore.
It appears that this is not an open/shut case. Norbert Straumann, argues that a field-theoretic formulation of GR in flat Minkowski space-time, leads to the conclusion that in fact Minkowski S-T is a kind of “unobservable aether”. In other words Feynman’s derivation of GR, based upon the massless spin-2 particle in Mink.space is in fact based upon a curved metric, apriori.
As I understand it, this notion of a massless spin-2 particle underlying GR is the bedrock (along w/SUSY) of superstring theory being a theory of quantum gravity. This notion is both funny & ironic, since Feynman did interviews in which he expressed extreme doubts about string theory’s veracity. Hence, would it not be worthwhile for people to reassess this notion, & not just quote Feynman ? Here is the cite: astro-ph/0006423.
Jimbo