Rovelli in effect concedes that lqg is wrong, but still worthwhile

[jeff]

Let me begin by saying that I won't be the first to post inappropriate responses in this second attempt to treat the above subject.

I previously posted the following:

In the forward to rovelli's new book "Quantum Gravity", james bjorken states quite plainly that the effective field theory approach to quantum gravity correctly taught us that GR must be viewed as just an effective field theory, and in fact this is the universally shared view.


The problem for lqg is that the central construct in lqg, spin networks, only makes sense if GR is in fact exactly correct. I don't see why it would make sense for an author to allow a forward to be written by someone else, that contradicts the basic premise of the book.

In fact, rovelli defends lqg by stating...

"But the modification of the notions of space and time has to do with the diffeomorphism invariance and the background independence of the action, not with it's specific form."

In other words, it is inaccurate to view lqg as a genuine candidate quantum gravity theory, and thus as a rival of string theory. Rather, lqg is just a toy theory serving as a laboratory to explore a small number of fundamental issues in quantum gravity.

I'll respond on two levels. One is on the specific physics of lqg, and the other, on the plausibility of alternative interpretations of these statements.

For example, someone may have a physics reason for not believing that lqg requires that GR be treated as if the einstein-hilbert action remains uncorrected at arbitrarily high energies.

Another example would be that someone may believe that it's plausible that rovelli does in fact believe that GR is exactly correct. In this case the best thing to do is just to email him, which I've done and am waiting for his response. Of course, nobody is stopping anyone from doing the same thing.

 

[setAI]

it's funny- if LQG were actually wrong- that would still make it better than String Theory- because at least LQG would BE WRONG!

that's got to be tough for you critics- if you win- you lose- and if you lose you lose-

 

[jeff]

it's funny- if LQG were actually wrong- that would still make it better than String Theory- because at least LQG would BE WRONG!

The adage that "an idea is so stupid that it's not even wrong" is due to wolfgang pauli. His point was that only self-consistent theories can be judged to be logically false.

In the absence of empirical data, we must rely quite a bit on self-consistency checks as a way to discriminate between good and bad ideas.

Part of the reason strings is so popular is not only that it has never failed a self-consistency check, but that these checks have been numerous and spectacular.

It is precisely the opposite with lqg. Two good examples are it's failure to reproduce correctly the black hole area-entropy law, and that it's degrees of freedom are manifestly volume-extensive (represented by the nodes in spin networks) rather than area-extensive, as we know is required of correct quantum theories of gravity on the basis of theory-independent general principles. The idea is that scattering at asymptotic energies are almost certainly dominated by black hole production, so since the information in black holes is stored holographically, and, according to the theory of the renormalization group, all of the fundamental degrees of freedom can be assumed to occur at asymptotic energies, self-consistent and correct quantum theories of gravity must be holographic, and clearly, lqg is not.

 

[selfAdjoint]

Jeff, let me try to address your primary point, which I take to be this (correct me if I'm wrong): GR is agreed to be an effective theory, and therefore it is from a final point of view a wrong theory. But spin foams are constructed from an assumed GR manifold, and therefore they must be fundamentally wrong themselves. Is that it?

Now quantum theory is replete with classical theories that are wrong in and of themselves but become better, though still perhaps provisional, by being quantized. Schroedinger theory, Dirac equation, even the string worldsheet with its classical action and the analytical results that flow from that. None of them is satisfactory until quantization is imposed.

And similarly people have been trying for decades to quantize gravity. They have assumed that that means quantizing the classical theory GR. Spin foams is just one of the more recent ways to go about this. I think that if you want to criticize spin foams for being based on GR you have to go deeper and show HOW quantizing GR is not the right way to procede.

Note that one of the great talking points of string physics has always been that the graviton that comes naturally out of string theory SIMULATES THE PHYSICS OF GR. What is sauce for the goose is perhaps sauce for the gander?