Unifying Quantum Mechanics and General Relativity: A Debate on Strings vs LQG

[marcus]

By this remark einstein meant that the criticism of his work by german scientists had nothing to do with science and was simply an act of anti-semitism, general relativity being derided at that period in germany's history as "jewish" physics.

I agree with jeff's inference as to what he meant. His comment also
points up their servile intellectual conformity----political correctness by the standards of the day.

 

[marcus]

http://arxiv.org/abs/gr-qc/0409045
Yes, but LQG can incorporate the other interactions. In the paper above, is proposed an unification of LQG with Yang-Mills theory. QCD and electroweak theory, the 2 pillars of the Standard model, are both Yang-Mills theories. If this path becomes fruitful then LQG will also become a TOE

If LQG is correct, then I hope it's possible to turn it into a TOE. I like the paper meteor linked to.

thanks meteor, pat777, I had not registered the existence of this new paper by Gambini, Pullin, and somebody else. I'll put the title and abstract here to make it clear what meteor his talking about and go have a look. It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case.

Unified model of loop quantum gravity and matter
R. Gambini, S. Jay Olson, J. Pullin
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
http://arxiv.org/abs/gr-qc/0409045

"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldan, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."

 

[pat777]

Marcus, Is Thiemann's unification of LQG and ST a TOE? I'm just a layperson who's interested in TOE.

 

[selfAdjoint]

thanks meteor, pat777, I had not registered the existence of this new paper by Gambini, Pullin, and somebody else. I'll put the title and abstract here to make it clear what meteor his talking about and go have a look. It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case.

Unified model of loop quantum gravity and matter
R. Gambini, S. Jay Olson, J. Pullin
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
http://arxiv.org/abs/gr-qc/0409045

"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldan, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."

Note the last sentence. And Thiemann's methods are the same ones he applied to quantize the closed string, which have been slammed by string physicists, and in that case seem not to work. Not to say this isn't good work, but to continually cry TOE over it is as silly as the stuff Peter Woit mocks in string physics.

 

[marcus]

... Not to say this isn't good work, but to continually cry TOE over it is as silly as [the string hype that Woit mocks.]

that's a pretty harsh accusation of silliness because some of the string hype is pretty dreadful

but what I said, selfAdjoint, was
"It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case."

I do not believe anyone here is "continually crying TOE"----I usually make a point of contrasting the narrow objective of quantizing gravity with the broader more inclusive aims of stringy thought. And I thought Meteor was reserving judgement when he qualified his notice of the Gambini et al paper by saying "if the approach is fruitful"

So who are you suggesting is indulging in silly hype (if that is the suggestion). Thomas Thiemann? Robert Helling? Gambini and Pullin?
Or is it actually me, or meteor?

That is not meant rhetorically, your point just needs clarification. My personal suspicion is that Hermann Nicolai has a reason for encouraging people like Thiemann and Helling to build bridges between Loop and String,

even if the bridges do not work perfectly and can be "slammed" as you say, and bearing in mind that even individually Loop and String are not perfect either.

and even if the aim of Loop theorizing is not to explain all fields and interactions (not the same as String's)

I am not sure why Hermann Nicolai encourages this kind of crossover research---maybe it is partly an instinctive effort to break down the sense of division into "camps"---or a hunch that it will shed some unexpected light on one or both research lines.

 

[pat777]

I too think that LQG and Strings are part of the same theory- but that LQG provides a more fundamental glimpse at this theory- and the basic conjecture of connectionist relationships out of a matrix/graph/lattice generating spacetime and matter fields [and strings] is a solid model that resembles the nature of the Causality that we observe-

Although I think LQG underlies string theory, I believe M-theory underlies LQG and ST if ST and LQG are both true. Loops are probably made from zero branes.

 

[selfAdjoint]

Although Thomas Larsson's criticism of LQG is only one man's opinion (see https://www.physicsforums.com/showthread.php?p=334373#post334373 for the latest iteration), it seems to be well founded. There is a deep aspect of generating a physical hamiltonian based theory that LQG hasn't addressed and seems to have missed entirely. It has to do with an anomaly that is NECESSARY for unitarity. Larsson says LQG as currentlly constructed is anomaly-free, and hence non-unitary, a physical no-no.

It was Thiemann's LQG quantization of the closed string that brought these methods to the attention of the non-LQG theorist community (not only string theorists!), and the criticism that resulted seems now to have been justified. This does not mean that LQG is sunk; it is currently in a period of changing it's inner methods (chunkymorphisms and all) and may yet arise justified. But we have to wait for that event.

 

[marcus]

... anomaly-free, and hence non-unitary, a physical no-no.
...

but is non-unitary evolution a no-no or is it, instead, a logical requirement for successfully quantizing GR (a physical "yes-yes" in other words)?

Rovelli addressed the non-unitary issue one year ago in
hep-th/0310077. We have come full circle back to October 2003, it seems

---exerpt from page 18 of "Dialog"---
Simp - ... Is loop gravity unitary?
Sal – No, as far as I understand.
Simp – This is devastating.
Sal – Why?
Simp – Because unitarity is needed for consistency.
Sal – Why?
Simp – Because without unitarity probability is not conserved.
Sal – Conserved in what?
Simp – In time.
Sal – Which time?
Simp – What do you mean “which time?”. Time.
Sal – There isn’t a unique notion of time in GR.
Simp – There is no coordinate t?
Sal – There is, but any observable is invariant under change of t, therefore everything is constant in this t just by gauge invariance.
Simp – I am confused.
Sal – I know, it is always confusing. . . Nonperturbative GR is quite different from physics on Minkowski . . .
Simp – Do we really need to get in the conceptual complications of GR?
Sal – Well, if we are discussing the theory that is supposed to merge GR and QM . . .
---end exerpt---

BTW, just a side comment. This spring Gambini and Pullin posted a paper which quantified a necessary rate of non-unitariness using any real clock and produced an argument to show that perfect unitariness is a no-no, namely it is non-physical. they used what I thought was a clever thought experiment to bound how good a realistic quantum clock could be.
and proceeded to resolve the black hole information paradox (which is based on the questionable assumption of perfect unitariness). I thought Gambini et al was neat---I will get the link

 

[marcus]

this was a series of 4 papers that appeared in February through August of this year, the authors were Rodolfo Gambini, Rafael Porto, Jorge Pullin

A relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence
http://arxiv.org/gr-qc/0402118

No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260

Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
http://arxiv.org/gr-qc/0408050

they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time). maybe I should quote some of what they say in the abstracts, have to go out at the moment but will get back to this in a few hours

 

[pat777]

I think a good argument for string theory is that it predicts(or should I say postdict)that we can only observe 3 spatial dimensions. Strings and anti-strings are likely to collide in 3 spatial dimensions or less. Thus, it expands the 3 dimensions. Strings and anti-strings are very unlikely to collide in 4 spatial dimensions.

 

[selfAdjoint]

this was a series of 4 papers that appeared in February through August of this year, the authors were Rodolfo Gambini, Rafael Porto, Jorge Pullin

A relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence
http://arxiv.org/gr-qc/0402118

No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260

Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
http://arxiv.org/gr-qc/0408050

they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time). maybe I should quote some of what they say in the abstracts, have to go out at the moment but will get back to this in a few hours

Correct me if I'm wrong, but it appears these authors are proposing (yet) another way to quantize gravity. They "promote all the variables to quantum operators" before establishing one of them as their time operator. Does this help LQG? Many people have tried to produce a quantum time operator, and maybe this one will work. We'll see. It is brave of them to simply accept the self destruction of pure states, even if they can show the decay to be very slow.

 

[marcus]

Correct me if I'm wrong, but it appears these authors are proposing (yet) another way to quantize gravity.

Hi selfAdjoint. Gambini and Pullin are major LQG figures, have done loop quantum gravity for quite some time. Pullin, as I think you know, does Matters of Gravity the APS newsletter on gravity and he has occasionally dropped in here at PF under the name Jorge. I think non-unitary knows them.

I am glad your interest was stimulated in the Gambini Pullin work. Here is another recent paper of theirs, to give a more complete perspective.

R. Gambini, S. Jay Olson, J. Pullin
Unified model of loop quantum gravity and matter
http://arxiv.org/abs/gr-qc/0409045
( 4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday)

Gambini's approach diverges from that of Ashtekar and others but it is hard to say how far the divergence goes. Do Gambini et al constitute a separate species? In my opinion not. They give talks at the same conferences as the others, and cite a common core of Loop Gravity references. Maybe the taxonomical term is "variety".

 

[marcus]

... Rodolfo Gambini, Rafael Porto, Jorge Pullin
...
No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260
...
they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time)...

selfAdjoint, you correct me if I am wrong but I think the arguments in these two papers are not meant to apply narrowly to one quantum theory or another.
They made them generally to apply to any quantum theory of gravity and not just gravity (it seems to me) but to any quantum theory at all. To the extent that the theory pretends that time evolution is perfectly unitary, the theory is unphysical.

this is because time must have an operational meaning.

 

[RingoKid]

for what it's worth I'm going for strings cos I like the simple idea that they can tie everything together...

...so what if it means a fixed background there are always hidden dimensions to explore

 

[pat777]

http://arxiv.org/PS_cache/hep-th/pdf/9208/9208027.pdf
Ed Witten wrote a paper about getting a background independent string theory. Yes, I know it's an old paper.