Quantum Gravity

See http://en.wikipedia.org/wiki/Quantum_gravity

I'd propose to cut the page right here and the rest MoveItElsewhere, for obvious reasons. If RK has anything to say (or to shout ) about quantum physics, he certainly can try to do that elsewhere.

QuantumGravity refers to the unification of General Relativity with QuantumTheory. Unlike GrandUnifiedTheory, which only seeks to unify two theories that are already quantized, QuantumGravity requires first the quantization of one of its theories. Quantizing gravity is difficult since GR is non-renormalizable.

In current research, QuantumGravity refers to M-Theory, the theory that used to be known as superstrings. Loop quantum gravity has too many problems and isn't considered a serious candidate by anyone (eg, it is artificial), not even its proponents.

LeeSmolin seems to consider it a serious candidate. See http://arxiv.org/format/hep-th/0303185 . He certainly counts as a proponent, and as I understand it he's done work on M-theory too.

LQG isn't a unified theory. It doesn't take into account the strongest 3 of the 4 forces in the universe. LQG isn't dead, it's stillborn. What Smolin and others are trying to do isn't to build an alternative to M-Theory but show that an alternate, and very uninteresting, way to quantize gravity is possible. Most people in the field seem to think this kind of limited effort isn't worth wasting time on.

As for Lee, I am way out of my field here so can't say with any authority what people think in general, but he works up the street these days I have been at talks (his and others) and roundtables with him several times in the last few months. It certainly isn't my impression that he or others in the room consider LQG stillborn.

If Lee Smolin doesn't consider LQG stillborn, that's because he doesn't aim for what everyone else in the field does; a complete and natural theory, not an artificial construct as a proof of principle.

Fair enough, and it is a noble goal. I certainly can't speak for Lee or others, it is was my impression only. However, again from my perception as an outsider looking in occasionally, nobody seems to be particularly close to this goal, be it LQG or M-theory or what have you. Lord knows there is need for a natural, let alone natural and complete theory in the field, but I don't see it hiding around the corner. This could just be my superficial understanding missing something, of course.

Do you consider M-theory not descriptive of our universe? Because the consensus is that it is both natural and complete. Another way to express this is that M-theory is real physics whether or not it turns out to describe our universe.

Brian Greene, Professor of Physics and Mathematics at Columbia University, is one of the world's leading string theorists. Pulitzer finalist. In his new book "Fabric of the Cosmos" he claims String/M-Theory and LQG are approaching the same question from different angles and the strengths of each complements the weaknesses of the other. He sees LQG helping string theory progress, not detracting from it. Re It doesn't take into account the strongest 3 of the 4 forces in the universe" JohnBaez? also claims Loop Quantum approaches have been used for other forces besides gravity (though it's main focus is gravity).

Loop Quantum Gravity is an honest attempt to unify Quantum Mechanics and General Relativity. It is based on the most important lesson from Einsteinian philosophy of spacetime, which is completely ignored by string or M theory. That lesson is -- Spacetime is Dynamic. This is a very subtle and profound statement which is very little understood and in fact misunderstood by people working on quantum gravity. The statement for example implies that black hole is not in a spacetime but it is a dynamical spacetime itself. String theory is based on treating spacetime as static and that's why they don't have background independence. Most of the people do not even realise what it means. Loop Quantum Gravity is a background independent non-perturbative quantization of spacetime. Unlike M theory it is predictable but true that is far less ambitious. It only aims to unify quantum and gravity. By the way people who wiaste their time in making unfair attacks on Loop quantum Gravity should sit back and think how far M theory has come. There is no theory and there are no predictions. Loop uantum Graviy has predicted discrete apectrum for geometrical operators, has obtained results for Black Hole thermodynamics and more recently has solved various cosmological singularity problems. If still someone is blind to it, then he deserves to remain so.

Superstring theory has had a background independent formulation for a bloody long time. It made a prediction before it was even realized what the theory was about. It's made more predictions since then. LQG on the other hand has made no predictions that weren't manually put into it by LeeSmolin and his ilk.

Background independent? I don't understand this. What I find is:

Extremely widely cited (part I was 1991): "On the Background Independence of String Field Theory: III. Explicit Field Redefinitions", Sen, Ashoke, Nucl. Phys. B391 (1993) 550.

He merely says, in the paper's final paragraph:

"A complete quantum string field theory has been constructed using the Batalin-Vilkovisky (BV) formalism, which requires adding new terms to the string field theory action at the loop level. A proof of background independence of the complete quantum string field theory action will then involve showing that the actions of BV quantized string field theories formulated around dirent backgrounds are related to each other by appropriate field redefinitions, after taking into account the change of path integral measure due to this field redefinition. We hope to return to this question in the near future."

So he didn't consider it a closed issue at the time.

More recently, looking around at work involving the Batalin-Vilkovisky formalism that he is saying was the promising one, I find for example: "Proposal for background independent Berkovits' superstring field theory", Josef Kluson 2001 J. High Energy Phys. JHEP07(2001)039, http://www.iop.org/EJ/abstract/1126-6708/2001/07/039

That abstract says "In this paper we would like to propose the background independent formulation of Berkovits' superstring field theory. Then we will show that the solution of equation of motion of this theory leads to the Berkovits' superstring field theory formulated around particular CFT background"

All of this just makes it sound like all these string theorists think background independence is a good thing to strive for, but that it's not a settled issue in string theory, and they're still working on it along with other issues.

I don't know much about this. Is there a better place I should be looking to find out why you're so sure that background independence was a settled issue from the beginning?

There's a difference between knowing a thing and proving it. And with superstrings, there's a difference between the theory itself and its formulation.

Superstrings was background independent from its inception since every change of background corresponded exactly to the addition or substraction of a dynamic field. If you want to work in a different kind of spacetime, you can either assume it or you could add gravitons in the right places until you got that spacetime. It makes no difference.

The moment you had a theory that explained all fields, you should have known that it was background independent. Proving it is a different matter entirely. And coming up with a background independent formulation is something else yet again.

What it all boils down to is the fact that "theory" refers to a piece of mathematics that may or may not be known to any human beings. Saying that superstrings has been background independent since the beginning just means that there is no alternative, no choice involved in the matter. That you can't come up with a background dependent version of superstrings even if you try.

Yes...But this sort of thing is a huge part of the work that these string theorists work on, of course. They're sure that it has to be background independent (amongst many other qualities), but they have to work hard to make those known-good qualities come true in any particular piece of research, and proofs are important to them, even though people like me just want to hear whether or not they've pulled it off.

Proofs are not nearly as important in physics as in mathematics. Or alternatively, what constitutes a proof is far weaker. The only sensible option is background independence.

Well, Doug, you can tell better if my advice was any good, but I was reponding to the anonymous contributor, who probably unknowingly managed to ask RK for directions. I'm sure whatever is the current situation on sci.physics.research, it just can't be worse than coming to Wiki and trying to conduct a sensible discussion on the subject with RK around.

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