Why does Stephen Hawking omit all mention Loop Gravity?

[ensabah6]

Why does Stephen Hawking omit all mention Loop Gravity?


I have read his popular books from Brief history of time to Universe in a nuthsell to "Stephen Hawking and the Theory of Everything" Stephen Hawking's Universe A Briefer History of Time

From what I recall, he absolutely spends time "starting over" in each one on supergravity, GUT's, wormholes, higher dimensions, strings, superstrings, M-theory, p-branes. He more or less reintroduces these topics in each book, as if re-cycling a wikipedia article, and speculates on how they solve everything and black hole entropy, etc.

I don't recall he spends a single sentence on loop gravity (and others such as emergent scenarios) anywhere or any time, either in those books or in the popular press, or considers the possibility that GUT is simply the wrong approach.

 

[humanino]

The content of his books reflects his own work and you may check his publications to contemplate to obvious correlation. I believe the fact that he does not touch upon LQG is independent of his personal opinion on the matter. Because somebody does not contribute to a specific field does not necessarily imply they have any specific opinion on the matter.

 

[Civilized]

Stephen Hawking is a mainstream physicist, and LQG is not a mainstream theory. Even though supergravity is obviously incomplete, it is predictive enough that we know that it rules out LQG (the big problem with supergravity is that it is non-renormalizable, which means it requires an infinite number of free parameters, but even without knowing the values of these parameters we know that supergravity obeys lorentz symmetry, which LQG doesn't). Therefore mainstream quantum gravity theorists who think that supergravity (general relativity + QM) is the correct low-energy description of QG saw LQG as a dead end about 5 years after its inception, when they saw it was not consistent with supergravity.

 

[atyy]

Stephen Hawking is a mainstream physicist, and LQG is not a mainstream theory. Even though supergravity is obviously incomplete, it is predictive enough that we know that it rules out LQG (the big problem with supergravity is that it is non-renormalizable, which means it requires an infinite number of free parameters, but even without knowing the values of these parameters we know that supergravity obeys lorentz symmetry, which LQG doesn't). Therefore mainstream quantum gravity theorists who think that supergravity (general relativity + QM) is the correct low-energy description of QG saw LQG as a dead end about 5 years after its inception, when they saw it was not consistent with supergravity.

LQG hasn't been shown to reduce to any known physics to date, but isn't supergravity = GR + QM + supersymmetry? And supersymmetry has not been experimentally detected at present.

 

[marcus]

... And supersymmetry has not been experimentally detected at present.

As I recall that was a point that Rovelli made in his talk on LQG at Strings 2008. LQG is consistent both with SUGRA and with no-SUGRA. He said you could think of this either as a disadvantage or as an advantage. Then he remarked that he personally saw it as an advantage (because LQG is flexible in that way and sugra has not been either proved or disproved.)

There have already been SUGRA-LQG papers, if I remember correctly. Maybe just a couple. It is so far not very important because no evidence. One knows one can handle it, if it shows up.

At Gerard 't Hooft's 60th birthday party Lee Smolin was among friends invited to give talks and Smolin gave a talk on LQG. I glanced at the PDF slide set and one of the first points he made was compatibility with a broad class of diffeo invariant gauge theories including ordinary GR and SUGRA. I'll try to think about this later today when I have some time. Have to run out right now.

 

[MTd2]

supergravity obeys lorentz symmetry, which LQG doesn't.

Marcus insisted so much that LQG obeys lorentz symmetry. I wonder if he changed his mind, because he didn't answer to this part of your post.

 

[marcus]

... because he didn't answer to this part of your post.

MTd2, it was Atyy that I was responding to
There is a lot of rumor-mongering that doesn't need to be answered because it is self-discrediting except with people uncritical enough to believe it.
What I do like to do, though, is keep a list of what the current rumors are. I think one can learn from them.

1. one recent rumor: LQG is not mainstream unless you think the ESF (european science foundation) is mainstream which recently set up the QG network that puts mainstream euros into LQG and allied research lines, which funded the recent Planck Scale conference: about equal balance Loop and String papers. And LQG is not mainstream unless you think George Ellis (of Hawking and Ellis) is mainstream because by the way look who is speaking at the Ellis-fest this summer, his 70th. Great guy

2. another recent rumor: someone here was implying LQG research is not normally published in peer-review journals *yawn*

3. another rumor that apparently gets mongered: LQG is not sugra-friendly.

4. We get the failed-Lorentz canard here at PF from time to time and you may remember that when I happen to notice it I usually give a link to that 2003 paper of Rovelli's. There'v also been some interesting new papers about Lorentz invariance of the post-2006 spin foam model, which is important because that is a re-formulation so that had to be checked. But the basic problem was addressed in 2003.
Canard is a lovely word. I think it means "decoy". A wooden duck.

I think there are a bunch of hostile misinformations floating around and what one needs to do is basically keep track and see if there are any trends.
It doesn't seem to do any harm and provides a kind of extra flavoring to the discussion. The most fascinating question, as I see it, is why would anyone think they needed to sleaze the competition or deny its (real or mainstream) existence? The rational behavior is to instead see what you can learn from the best of rival research.

 

[Coin]

I can't help but notice, Brief History of Time was written in 1988 and the only revised edition was written in 1996 (with an "updated, abridged" version released in 2005). Universe in a Nutshell was written in 2001. LQG can't be said to have existed in even the most rudimentary form before 1990, it was some years after that before it really got any sort of rigor, and I don't think any popular science book discussed its existence before Smolin's 2001 "three roads to quantum gravity". LQG wouldn't have been at the time of those previous books' publications, and an author could reasonably argue still might not still be so in 2005 or today, a mature or large enough research program to justify a mention in a book that's (as if I'm not mistaken Hawkings' books mostly are?) less about "what is happening at the very edge of science" and more about "what is consensus in the scientific community". (Penrose's "Road to Reality", also published in 2005, does spend a good amount of time talking about LQG, but that was a book not only had a strong focus on mathematical physics but also made explicit effort to survey strange or speculative ideas at the edge of quantum physics research at the time-- to say nothing of the fact that Penrose has a bit of a personal connection to LQG himself.)

 

[marcus]

"Why does Hawking fail to mention" this or that is an interesting question, although since he's a mere human and doesn't speak for Nature herself the answers are bound to have a "sociological" element and you need to know some background to understand.

A good way to get the necessary background perspective is to compare pre-1990 quantum cosmology:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+QUANTUM+COSMOLOGY+AND+DATE+%3C+1990&FORMAT=www&SEQUENCE=citecount%28d%29

with post-2006 quantum cosmology:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

These are cite-ranked so you tend to get the most important research first. The pre-1990 list has Hawking in the top 25 and is dominated by people associated with him. The authors of the top 25 are primarily from Cambridge, Tufts, and Santa Barbara. Hawking, Hartle, Vilenkin, Esposito, Gary Horowitz (string), Tom Banks (string), Gabriele Veneziano (string), and so on.

The post-2006 list of top-cited QC is primarily Loop QC, and other QC which explore the same features---the bounce getting rid of the cosmo singularity etc. In the top 25, there is nothing by the the old-guard. Nothing by Hawking, or Hartle, or Vilenkin, or Veneziano, or Banks, or Horowitz, or Linde, etc etc.

Significantly, the post-2006 list is topped by a paper by a postdoc of Ashtekar's named Calcagni which, instead of being about Loop this time is about Horava QC. Most of the top-25 is either Ashtekar (a founder of Loop QG and QC) and postdocs and young faculty whom Ashtekar has brought to PennState, or his PhDs who have gone elsewhere. QC is sort of in the Ashtekar "family" now.

Well I think that since QC used to be in the Cambridge family and now the center of activity has shifted (if we go by publication and citation rates) it is really too much to expect of Hawking, who is now retired, that he be telling us about what is currently going on in contemporary Quantum Gravity and its application to Quantum Cosmology.

So it's not surprising to scope this situation out, but it is pleasantly instructive.

 

[Civilized]

Marcus insisted so much that LQG obeys lorentz symmetry. I wonder if he changed his mind, because he didn't answer to this part of your post.

4. We get the failed-Lorentz canard here at PF from time to time

It s not a canard. Here is a thread you started:

https://www.physicsforums.com/showthread.php?t=322234"

It links to a blog post which links to a recent article co-authored by Lee Smolin: http://arxiv.org/abs/0906.3731" . Equation (2) on page 5 is a blatant statement of Lorentz violation:

m^2 = E^2 - p^2 +\Delta_{qg}(E,p^2;M_{qg})

Leonard Susskind described Lee Smolin as producing ideas which go "glub glub glub straight to the bottom of the sea." How many nobel prize winners have spoken positively about LQG? I know Weinberg doesn't like it, in his QFT book he says that even though sugra is non-renormalizable and so involves an infinite number of free parameters, it is still predictive enough to rule out LQG. Why do nuclear theorists and condensed matter theorists (to say nothing about particle theorists) at my university speak positively about string theory and say nothing about LQG ? Why do professors look uncomfortable when incoming grad students ask them about non-string QG? The answer is pretty harsh, so I won't say it, no one does except Motl, but suffice to say LQG is not in the mainstream of physics.

 

[MathematicalPhysicist]

Well ofcourse, that string people would negate any non string approach, they are fighting for their right to keep working in their fields, and that means keep getting grants, if god forbid there would be more alternatives, the money would be spread out to more people.

 

[humanino]

Are you aware that Hawking now partly works at PI ?

 

[suprised]

Well ofcourse, that string people would negate any non string approach, they are fighting for their right to keep working in their fields, and that means keep getting grants, if god forbid there would be more alternatives, the money would be spread out to more people.

Oh man, how can one get it so wrong. Can't you just realize that there are very simply physics reasons, and not sociological ones why only so few work on those "alternatives"? Already the word alternative expresses a bias, namely some other equally valid (and tacitly assumed superior) possibility, but that's just not the case. If you guys, who are not experts working in this field, all the time reinforce your opinions based on pseudo arguments and neglect the opinions of the experts, then you can't be helped. You completely waste your time and misguide other people.

 

[MTd2]

Are you aware that Hawking now partly works at PI ?

PI also funds string theorists...
http://www.perimeterinstitute.ca/en/Scientific/Research/Superstring_Theory/

But I am very suspecious of calling them string theorists. One of them is a young researcher, Freddy Cachazo... But I wouldn't really call him a string theorist. He doesn't even call as such. If you read the recent slides of talks of his main collaborator, Nima Arkani-Hamed, I get the impression they are trying to come up with a non stringy susy QG...

It links to a blog post which links to a recent article co-authored by Lee Smolin: http://arxiv.org/abs/0906.3731" . Equation (2) on page 5 is a blatant statement of Lorentz violation:

m^2 = E^2 - p^2 +\Delta_{qg}(E,p^2;M_{qg})

Hmm. This DSR is not double speed of light, but deformed. As far as I understand, it is like modifying the way particles sense vacuum permeability. It is like the vacuum became "thicker" at higher speed. I guess this is why they talk about birefringence.

 

[humanino]

How many nobel prize winners have spoken positively about LQG?

Why do all discussions here end up like this ? Why do people always side to this or that ? Is it because after studying one for a couple of days, you think you understood something but you are tired and do not want to study what other people have done ? Would you mind showing some respect towards the research work of people more competent than you ? Who cares that you prefer Weinberg to Rovelli ? Are you confusing horse race bets with science ?

 

[atyy]

Depends on what one means by "mention". Hawking has edited a book with a chapter by Ashtekar about "Quantum Geometry and its ramifications" http://books.google.com/books?id=yLy4b61rfPwC&dq=hawking+AND+loop+quantum&source=gbs_navlinks_s

 

[grosquet]

Civilized asks: "How many nobel prize winners have spoken positively about LQG?"
Gerard t’Hooft, joint winner of the 1999 Nobel Prize for Physics, says in his 1997 book, “In search of the ultimate building blocks”, on page 174: “another approach began with a formal analysis of quantum gravity by Abhay Ashtekar” and “young investigators Lee Smolin and Carlo Rovelli”...”This looks a bit like what we saw in string theory, but this is an altogether different approach;”...”This is an attempt at constructing a new theory that I am following with interest - at last something that looks a bit like I would want.”, etc.

 

[Naty1]

"How many nobel prize winners have spoken positively about LQG?"

maybe Al Gore?? (LOL) what an idiotic criteria...a better question is how many nobel prize winners have been wrong at one time or another.

One schools "canard" is another school's most fondly held theory. And of course funding plays a part in research selections...does anyone think the Obama administration will fund work which contradicts man man global warming, for example...such thinking is currently "out" at least among a lot of politicians...that does not make it wrong...(In 1915 to say 1925, would anyone have thought studying the dynamic nature of space and time was worthwhile? Or that light was really a particle phenomena...to compete with Maxwell's equations which had been "law" for fifty years! Likely you would have been laughed out of the physics community)

Objective competition among schools, faculties and concepts in physics is part of the game. Because the Perimeter Institute, for example, might specialize in one area, Harvard in another and Princeton a third does not mean any of those roads are useless. Fads and funding come and go.


Marcus posts:

I think there are a bunch of hostile misinformations floating around ...It doesn't seem to do any harm [/Band provides a kind of extra flavoring to the discussion. The most fascinating question, as I see it, is why would anyone think they needed to sleaze the competition or deny its (real or mainstream) existence? The rational behavior is to instead see what you can learn from the best of rival research.

There sure are! I believe it DOES harm those who, for example, take advisors "slams" as accurate when they do not know any better. These are supposed to be trusted sources of objective accurate information. Sleazing the competition means you do not have to bother finding the value nor meaning in what they do. Learning IS what it IS all about...well said! Isn't that the purpose of these forums?

 

[Coin]

Civilized asks: "How many nobel prize winners have spoken positively about LQG?"

Even aside from humanino's point, one thing I'd ask is: How many nobel prize winners have been active quantum gravity researchers at all? The only one I can think of is T'Hooft (unless you count Einstein, which I wouldn't). Certainly no one has ever been given a nobel prize for QG work.

 

[ensabah6]

Depends on what one means by "mention". Hawking has edited a book with a chapter by Ashtekar about "Quantum Geometry and its ramifications" http://books.google.com/books?id=yLy4b61rfPwC&dq=hawking+AND+loop+quantum&source=gbs_navlinks_s

What is Hawking's opinion on LQG?

 

[humanino]

What is Hawking's opinion on LQG?

Unless hoping you'll have him show up to answer, why do you care ?

 

[Civilized]

Why do all discussions here end up like this ? Why do people always side to this or that ? Is it because after studying one for a couple of days, you think you understood something but you are tired and do not want to study what other people have done ? Who cares that you prefer Weinberg to Rovelli ?

You presume a lot about me Humanino, and you are attacking me personally. You choose to use personal attacks instead of responding to my evidence that Smolin's LQG violates Lorentz symmetry. I am not an amateur physicist, I am a professional. I am not interested in personal attacks. I do not care about Rovelli or Weinberg as people, I look at their arguments. If I had been active in research in the early 1990s, I would have given LQG a chance just like the rest of the physics community did. But in 1995 LQG turned out to violate lorentz symmetry and and a few years later predicted the wrong value for the entropy of black holes (i.e. the immirzi parameter debacle) and the majority of physicist took these as signs of mathematical and empirical inconsistency and moved on without looking back. The currently existing LQG community consists of diehards who refuse, for whatever reasons, to admit that these inconsistencies are fatal to the theory. I don't respect them because they are working on a theory that has already been debunked, and their proposed fix-ups for the inconsistencies are mathematically bogus.

How many nobel prize winners have been active quantum gravity researchers at all?

t'Hooft is the only one I know of who has worked on non-string QG, I was not aware of his positive comments about LQG in the 1990s, as far as I know he has more recently worked on his own non-string, non-loop, QG that has not received much attention from the broader community.

Last year the nobel prize went to Nambu, namsake of the central-to-string-theory Nambu-Goto action, although it should be pointed out that his nobel winning work was done prior to his work on strings. Other nobel winners turned string theorists include S Weinberg and D Gross, and also K Wilson (although his highly cited work on string theory was done in the 1970s, more so in the context of understanding gauge theories). Basically all the people who have won nobels for particle theory work done in the 1970s have gone on to work on string theory, and t'Hooft is not really an exception. http://www.phys.uu.nl/~thooft/theorist.html" t'Hooft says:

Finally, if you are mad enough that you want to solve those tremendously perplexing problems of reconciling gravitational physics with the quantum world, you end up studying general relativity, superstring theory, M-theory, Calabi-Yau compactification and so on. That's presently the top of the sky scraper.

I have not dishonestly taken the quote out of context (check for yourself); in his guide to students t'Hooft mentions only string theory with utterly no mention of LQG. I would offer the speculation that the 1990s quote from t'Hooft in support of LQG was made before it became clear that LQG was mathematically and empirically inconsistent.

I find it strange that anyone would argue against my calling LQG a non-mainstream theory. It seems that Smolin and Woit agree with me, in their books they complain at length about the marginalization of LQG, why would they be doing that if they did not agree with me that LQG is non-mainstream?

If anyone doubts that LQG is outside the mainstream, then I propose the following fair test. Go to the nearest university and ask the physics faculty about what they think of LQG (warning: you might not like what they say).

The LQG fans should ask themselves honestly, "why does the mainstream of physics avoid LQG?" I have never met anyone who loves strings theory so much that they would continue clinging to it if there was something better. String theorists are not illogical, they want to produce new physics results and understand the universe. The idea that string theorists are too fossilized to retrain from string theory is flawed, because high energy physics research moves so quickly that everyone is constantly learning new things anyway e.g. the rapid response sessions at KITP. The funding conspiracies are also flawed, because the people who get the funding are the people who derive the most results, and if LQG was a better route to results then people would switch to it to have more successful careers.

The true reason that most physicists have no interest in LQG is very simple: it has already been shown to be mathematically and empirically inconsistent, Lorentz violation, wrong value for black hole entropy, vague dynamics, intractable / infinitely many free parameters. Despite what the LQG diehards claim, there have been no viable proposed solutions to these fatal problems; if there were, I guarantee that the broader physics community would take notice and care, they are rational people who want nothing more to work on challenging theories that are ripe for results, not some kind of string-loving cult of priests, theorist will flock to wherever the action is.

 

[humanino]

You keep repeating that violation of Lorentz symmetry as a bad thing. However if LQG predicted Lorentz voilation unambiguously, it would be a falsifiable prediction making it instantly in better scientific shape than string theory. Same story about the Immirzi parameter if we had another constraint apart from just BH entropy.

Indeed I attack you personally, and you have no shame to declare how you lack elementary professional respect, together with misinformed claims, not even deserving debunking. Since you also mention the despicable human being Motl it is barely a surprise.

 

[atyy]

What is Hawking's opinion on LQG?

Hawking's controversial concession of a baseball encyclopedia seems to utilize the conjectured AdS/CFT correspondence, which is a fascinating result from string theory. He also seems to have considered the string landscape.

http://arxiv.org/abs/hep-th/0507171
http://arxiv.org/abs/hep-th/0602091

 

[Civilized]

You keep repeating that violation of Lorentz symmetry as a bad thing.

Lorentz symmetry is one of the core principles of QFT, and all of theoretical physics. Not only is it extremely well-tested, but it is also an extremely productive and deep principle that has guided nearly all of the discoveries in the last 50 years of high energy theoretical physics, we never would have found the standard model without the constraint of lorentz symmetry.

Experimentally, the existence of Lorentz violations is strictly constrained by the non-observation of the hoards of junk that they generate. This is no problem for Smolin et al, since their recent paper I linked to in this thread predicts that the violations are very, very small, so that the junk effects are also small. But the existence of arbitrarily small lorentz violations is non-falsifiable, since if we probe for Smolin's predicted 1 part in 10^{19} lorentz violations and don't find them, he can always say that the violations occur at an even smaller level. The same arguments that Woit and Smolin use to say SUSY is unfalsifiable can also be used to say that LQG's prediction of lorentz violation is unfalsifiable.

Personally, the main reason I favor lorentz symmetry is not because it is tested every day in trillions of collider events to an accuracy up to 10 decimal places, it's because of what I alluded to earlier, that lorentz symmetry is a productive workhorse that we have been relying on for 50 years to make progress in physics. By "rely on" I mean that when we try to create a new correct theory, we have been using lorentz symmetry to constrain the theory and make it true and useful.

Before you say that I am too attached to a tool just because it was useful in the past, let me say that's not it. It's just that whatever detaches me and the rest of physicists from lorentz symmetry, and thus detaches us from QFT, will need to be more compelling by far than results in LQG have been to date.

However if LQG predicted Lorentz voilation unambiguously,

Let there be no doubt that it does, if you read the Smolin paper I linked.

it would be a falsifiable prediction making it instantly in better scientific shape than string theory.

I claim that this is faulty logic. I could make up a theory that definitely predicts green goblins eating pastries at the Planck scale, and it would not be "instantly in better scientific shape than string theory."

Furthermore, through the gauge/string correspondence string theory has already made predictions that agree with experiments done at e.g. the RHIC. For example, using string theory to calculate the angular momentum spectrum for an open string being used through the duality to predict the energy spectrum of mesons at finite (neither low nor high) temperature.

String theory also predicts the existence of gravitons, the quanta of Einstein's gravitational field. This is falsifiable. String theory predicts the existence of extra dimensions which are definitely no smaller than the Planck scale, which is conceivably falsifiable. Similarly, string theory predicts supersymmetry at some energy scale below the Planck scale. Therefore string theory has many definite, falsifiable predictions at remote scales similar to Smolin's 1 part in 10^{19} lorentz violation. Smolin's suggestion to test these remote scales using cosmology also has dozens of counterparts on the string side.

Same story about the Immirzi parameter if we had another constraint apart from just BH entropy.

No, you are remembering the story wrong. LQG made a falsifiable prediction based on the immirzi parameter, and it failed the test, within its own mathematical framework. The bogus solution to these problems are "noiseless subsystems" which make the theory unfalsifiable because they explain-away inconsistencies using free parameters.

and you have no shame to declare how you lack elementary professional respect,

Should I extend "elementary professional respect" to everyone who works on a debunked, mathematically inconsistent, empirically unlikely theory just because they have convinced a few universities and private funding groups to pay them salaries (making them "professionals")? I think the answer is "no", since otherwise I would be extending respect to the 'over-unity' perpetual motion community (incidentally, Penrose has shown how lorentz violation leads to perpetual motion machines).

, together with misinformed claims, not even deserving debunking.

why is it worth it to attack me personally but not worth it to discuss physics? Your claim that I am "misinformed" does not have any weight because you have not yourself rebutted any of my points. Furthermore, your statements that my claims are "not even deserving [of] debunking" suggest that you would rather discuss only personal attacks and vague arguments than anything involving the detailed content of the theory you are supporting.

Since you also mention the despicable human being Motl it is barely a surprise.

You attacked me for a "lack elementary professional respect" and for not "showing some respect towards the research work of people more competent than you" and then you called Lubos Motl, a former Havard professor who has co-authored papers with the likes of Vafa, a "despicable human being." On the basis of these comments, you have shown yourself to be an inconsistent hypocrite.

 

[humanino]

small lorentz violations

UHE cosmic rays.

Personally, the main reason I favor lorentz symmetry is not because it is tested every day in trillions of collider events to an accuracy up to 10 decimal places

Where do the best test of Lorentz symmetry come from ? Colliders !?

Let there be no doubt that it does, if you read the Smolin paper I linked.

Did you read the paper yourself ?

Experts will be aware that the intuitions of theorists on the ultimate fate of Lorentz invariance are diverse, with accomplished theorists expressing views all along the spectrum of expectations from the perfect validity to the complete breaking of Lorentz transformations.
[...]
their scenario should not be viewed as a definite prediction of loop quantum gravity or more generally of other quantum theories of gravity.

Furthermore, through the gauge/string correspondence string theory has already made predictions that agree with experiments done at e.g. the RHIC. For example, using string theory to calculate the angular momentum spectrum for an open string being used through the duality to predict the energy spectrum of mesons at finite (neither low nor high) temperature.

The conjecture was indeed inspired from string theory. But it is certainly untrue that QCD has been derived. Besides, I can use the conjecture to model many things, like the temperature of my fridge. Finally, you have not read what was linked by others earlier. It is clearly explained that the conjecture is a mathematical result. It is not just a statement about Nature. If string theory turned out not to apply to fundamental physics, the conjecture would be untouched.

String theory predicts the existence of extra dimensions which are definitely no smaller than the Planck scale, which is conceivably falsifiable.

That at least is true.

Similarly, string theory predicts supersymmetry

Yes we also agree on this.

you called Lubos Motl despicable human being

Well I apologize to other people about this, although I did not start this atmosphere. I just can not stand this person. I know he is highly qualified, but his misbehavior is not even restricted to LQG. It even applies to gender !

 

[Haelfix]

Umm, Hawking mostly works on the Euclidean path integral. Quite a bit different focus than string theory or LQG. At various times, he's called it the only 'sane way to do quantum gravity'. In recent years, he's obviously accepted holography and has always been interested in super gravity and things like that.

Still he's a cosmologist primarily, not a pure quantum gravity person.

 

[Civilized]

UHE cosmic rays.

UHE cosmic rays could also, in theory, could be used to probe for extra dimensions, SUSY, etc. Therefore your comment "it would be a falsifiable prediction making it instantly in better scientific shape than string theory" shows an ignorance of the fact that string theory predicts SUSY and extra dimensions which could in theory be detected in the same cosmic rays that Smolin is using. I'm not saying this wouldn't be a test for LQG, I'm saying that it does not "instantly make it better...", that is a highly disagreeable statement, especially since ST definitely predicts SUSY and extra dimensions, not a vague having and eating of the cake that Smolin sets up in the passage you quote. Sure Lee, if your dynamics are vaguely constrained / undefined, you can evade explicit proofs of their Lorentz violations, we know you learned that lesson in the 1990s.

Where do the best test of Lorentz symmetry come from ? Colliders !?

Those are the best empirical tests, by I am more concerned with theoretical tests, and you ignored my point about lorentz symmetry being a guiding light in developing the entire standard model.

The conjecture was indeed inspired from string theory. But it is certainly untrue that QCD has been derived. Besides, I can use the conjecture to model many things, like the temperature of my fridge. Finally, you have not read what was linked by others earlier. It is clearly explained that the conjecture is a mathematical result. It is not just a statement about Nature. If string theory turned out not to apply to fundamental physics, the conjecture would be untouched.

How could string theory give a mathematically correct description of nature while simultaneously not applying to fundamental physics? All this talk of whether a theory "really applies" or whether it is a mathematically deep and accurate description, I see as naive philosophical positions.

To a physicist, string theory is not the claim "there are strings in the physical world." There is no textbook on string theory that says these strings actually exist! String theory is the theory of relativistic quantized strings. String theory is used to predict the properties of these objects. The predictions about spectra have been tested using the gauge/gravity correspondence. This is a test of what physicist call "string theory."

The question of whether there are fundamental strings in the world will never be tested by gauge/gravity duality, I am fully aware of that. But this ontological statement is not the content of string theory, the content of string theory is a body of mathematical solutions to equations of motion, and these solutions have been and are being tested!

In contrast, because of the vauge dynamics problem in LQG, there aren't any predictions based on non-trivial calculations, only a series of ontological conjectures. Excessive hang-ups over ontology are a key source of the rift between LQG and mainstream physics; the latter has backed away from that muddy territory in the last century.

Well I apologize to other people about this, although I did not start this atmosphere. I just can not stand this person. I know he is highly qualified, but his misbehavior is not even restricted to LQG. It even applies to gender !

I agree that Lubos is mostly naive in his political opinions, but I like to think that's because he has been a physics geek all his life. In other words, I think he is very intelligent and is unafraid to discuss technical scientific details, and I just filter out the hate-speech and skip the purely political posts. Feynman and many other greats have repeated the sentiment to focus on the ideas and not the person. The bottomline is he does sometimes make valid, honest, technical criticisms of LQG at it is not valid to totally discredit these points because of his opinions about social matters.

 

[humanino]

ignorance of the fact that string theory predicts SUSY and extra dimensions

No, but I never read that UHE where favorable to do so and I still doubt, they don't have the luminosity

you ignored my point about lorentz symmetry

I just did not acknowledge it. It's not a bad point but I was focused on experimental tests.

How could string theory give a mathematically correct description of nature while simultaneously not applying to fundamental physics?

You mentioned that the conjecture is already used as a mathematical tool, and by the way no just for RHIC physics. However, in string theory it applies to SUSY QCD for instance, not QCD. And independently a good chunk of the models have 5 dimensions not 11. The conjecture as any mathematical statement is independent of the specific models making use of it. Another example is the just recent high temperature superconductor calculation. String theory is very rich mathematically I do acknowledge and appreciate very much.

 

[Civilized]

No, but I never read that UHE where favorable to do so and I still doubt, they don't have the luminosity

Because Cosmic rays occur at a higher energy than currently conceived accelerators, it is theoretically possible that cosmic rays could be used to detect SUSY or extra dimensions even if the LHC can't. If such an experiment were constructed and no SUSY was found, then string theorists would say that SUSY occurs at a higher energy towards the Planck scale, thus protecting themselves from falsification.

Similarly, if LQG decides to predict lorentz invariance, and it is not detected at Smolin's proposed scale, they can say that the violations happens on an even smaller scale, protecting themselves from falsification.

You mentioned that the conjecture is already used as a mathematical tool, and by the way no just for RHIC physics.

That's why I said "e.g. at the RHIC", fyi "e.g." has the same meaning as "for example" and so it specifically indicates that the RHIC is only one example of such a test. At my university AdS/CFT makes contact with both nuclear physics and condensed matter, there are a lot of interesting seminars and I am aware of other tests that have been done.

However, in string theory it applies to SUSY QCD for instance, not QCD.

Not even SUSY QCD, but SUSY SU(N) where N is large, both the supersymmetry and the large number of colors are required for conformal symmetry, which is the basis of the modern invigoration of gauge / gravity dualities.

And independently a good chunk of the models have 5 dimensions not 11.

Hmm, the strings live in 10 spacetime dimensions as usual on a (locally) AdS_5 \times S^5 manifold, there are no string theories in 5 dimensions unless by "5 dimensions" you mean "5 uncompactified + 5 compatified dimensions." Maybe there is another subtly I'm missing, links welcome.

The conjecture as any mathematical statement is independent of the specific models making use of it. Another example is the just recent high temperature superconductor calculation. String theory is very rich mathematically I do acknowledge and appreciate very much.

That's good. Of course, instead of "mathematically rich" I would call it "physically rich" because, regardless of whether strings "exist", as long as the emphasis is on calculations rather than rigorous definitions and proofs, string theory resembles physics more so than mathematics. Some people think we should invent a new category for these constructs-which-may-or-may-not-exist that are studied by physicists, but to me and my colleagues that is just theoretical physics.

 

[ensabah6]

Is it a violation of lorentz symmetry if a version of LQG predicts that higher energy photons travel SLOWER than c (w/lower energy photons traveling at c)

Does inflationary cosmology, where the early universe expanded faster than c, violate QFT and lorentz symmetry?

Anyhow aren't there versions of string theory which allow photons to travel faster than c via extra dimensions?

 

[ensabah6]

Unless hoping you'll have him show up to answer, why do you care ?

Since he writes of string theory in his popular books, why not mention some alternatives?

 

[Civilized]

Is it a violation of lorentz symmetry if a version of LQG predicts that higher energy photons travel SLOWER than c (w/lower energy photons traveling at c)

Yes, this would be a violation of special relativity and QED, and it is a violation of lorentz symmetry.

Does inflationary cosmology, where the early universe expanded faster than c, violate QFT and lorentz symmetry?

The calculations of inflationary cosmology are entirely based on QFT in a curved spacetime i.e. QFT + einstein gravity. Here is a classic review paper: http://prola.aps.org/abstract/RMP/v57/i1/p1_1"

The expansion of the universe in general does not violate lorentz symmetry, because once we accept general relativity (which we all do) we are talking about local lorentz symmetry which means the symmetry holds in regions of spacetime which the curvature is locally flat.

Anyhow aren't there versions of string theory which allow photons to travel faster than c via extra dimensions?

No way, lorentz symmetry is one of the main constraints in string theory. This is a critical reason why string theory can be seen as building on the last 50 years of physics without starting anew. For technical reasons gravitons cannot be point particles in QFT, so the next simplest thing would be for them to 1-dimensional (strings) instead of 0-dimensional (point particles). In the modern view there are not different versions of string theory, only different solutions to a single string theory, and all of the solutions contain gravitons and respect lorentz symmetry.

 

[tom.stoer]

There is a subtlety: The modified dispersion relations from DSR violate Lorentz symmetry in an interesting way: Lorentz symmetry is not broken, but it is realized non-linearily, thast means the algebra SO(3,1) is deformed.

There is no (spontaneous symmetry) breaking in the sense of special points or directions.

Anyway: I am not so sure if LQG still insists on predicting these effects. As far as I remember there were no rigorous proofs. I haven't seen papers talking about deriving DSR from LQG for years. Does anybody know about the status?

 

[marcus]

There is a subtlety: The modified dispersion relations from DSR violate Lorentz symmetry in an interesting way: Lorentz symmetry is not broken, but it is realized non-linearily, thast means the algebra SO(3,1) is deformed.

There is no (spontaneous symmetry) breaking in the sense of special points or directions.

Anyway: I am not so sure if LQG still insists on predicting these effects. As far as I remember there were no rigorous proofs. I haven't seen papers talking about deriving DSR from LQG for years. Does anybody know about the status?

So far LQG does not predict Lorentz violation. There was hope back in 2004-2005 that one could prove LQG implies some form of DSR. Because of the subtlety you mentioned, this would not create problems for LQG and would produce a testable prediction about gammaray bursts that could be tested.

There was considerable optimism about this, because the desired result was shown in D=3 case by Laurent Freidel and Etera Livine. For some time people tried to extend to D=4, but as you suggest, they seem to have gotten discouraged.

There may be a renewed effort in this direction if Fermi spacecraft observes dispersion. This would serve as guidance to LQG research, but because there is no outstanding prediction would not constitute a test.

The status is that LQG has been proven compatible with Lorentz invariance (Rovelli 2003) and the rumor that its spacetime is discrete and therefore incompatible is a canard. The underlying spacetime is a continuum---operators corresponding to area and vol measurements have discrete spectrum.

Smolin-Amelino have a recent paper about testing DSR---not about LQG directly although Fermi LAT data will provide guidance.
Rovelli has a recent offering a conjecture as to what the physical basis of DSR might be, if the dispersion effects are observed. It is not a specifically LQG paper, but I think quite interesting. Does anyone want links to any of these? The Freidel-Livine 2005, for example?

 

[nrqed]

Why do all discussions here end up like this ? Why do people always side to this or that ? Is it because after studying one for a couple of days, you think you understood something but you are tired and do not want to study what other people have done ? Would you mind showing some respect towards the research work of people more competent than you ? Who cares that you prefer Weinberg to Rovelli ? Are you confusing horse race bets with science ?

I strongly agree with this point of view!
This is why it is tiresome to see so much cheerleading for LQG whihc has nothing to do whatsoever with the actual science, but more to do with how many papers have been published on this or that, who is working on this or that, who was the student of this or that reseracher, who has given a talk at this or that conference.

I would be delighted to see you express the same opinion with regards tp the LQG cheerleading.

Regards

 

[MTd2]

So far LQG does not predict Lorentz violation. There was hope back in 2004-2005 that one could prove LQG implies some form of DSR.

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

 

[nrqed]

You presume a lot about me Humanino, and you are attacking me personally. You choose to use personal attacks instead of responding to my evidence that Smolin's LQG violates Lorentz symmetry. I am not an amateur physicist, I am a professional. I am not interested in personal attacks. I do not care about Rovelli or Weinberg as people, I look at their arguments.

I, for one, appreciate very much all your insights, Civilized! It's great to see discussiosn on physics instead of discussions on people which is unfortunately the main emphasis on this board (and completely unscientific). I guess it's because it is far easier to find out how many papers a person has published or who's graduate student he/she was than it is to try to understand the physics.

I think your attitude is the correct one. We shoudl not care about who wrote a paper or gave a talk, we shoudl just discuss the physics. II agree that it is hard to avoid complete neutrality, in the sense that I can't help putting more weight on a paper by Witten than on a paper from someone I have never heard of, but we should strive to put as little emphasis on the person as possible and as much emphasis as possible on the science. Unfortunately, this board does the exact opposite.

Than you for your very informative posts and interesting links!

Regards,

Patrick

 

[marcus]

...
I would be delighted to see you express the same opinion with regards tp the LQG cheerleading...

I'm not sure what you mean by cheerleading. It is important to get the facts out. We now have much more interest by string people in what LQG and other nonstring QG research is turning up. People are crossing over. People are attending the same conferences.
So something has to be done to counter the outdated stereotype of string folks sneering and dismissing nonstring QG.
The way to do this is with facts.
Like the Planck scale conference that just occurred. Like the Ellis-fest to be held in Cape Town in August. Like the new money the ESF is pumping into non-string QG via two networks headed by Barrett and by Loll.
The change is more evident in Europe, Canada, Australia than in the USA, where most departments are simply not equiped to handle grad students who want to pursue QG.

There still is a lot of minsinformation of the Motley sort but it has become less important I think. I don't know if it is as necessary to point out inaccuracies as it was in the past.

Anyway in case anyone wanted the links I mentioned in previous post:
Freidel-Livine
http://arxiv.org/abs/hep-th/0512113
http://arxiv.org/abs/hep-th/0502106
(They managed to get DSR in the 3D case.)

Rovelli 2003
http://arxiv.org/abs/gr-qc/0205108
The preprint is 2002 and the publication in Physical Review D was 2003.

 

[MTd2]

This is the apparent lorentz symmetry break, within superstrings:

http://www.ift.uni.wroc.pl/~planckscale/lectures/3-Wednesday/6-Mavromatos.pdf

 

[MTd2]

Like the Planck scale conference that just occurred. Like the Ellis-fest to be held in Cape Town in August. Like the new money the ESF is pumping into non-string QG via two networks headed by Barrett and by Loll

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

 

[Civilized]

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

No, there is no hope of lorentz violations in string theory, unless the theory turns out to be mathematically inconsistent (which seems unlikely after 20+ years of study but could conceivably happen), because lorentz symmetry is one of the primary axiomatic inputs.

This is the apparent lorentz symmetry break, within superstrings:

Thanks for providing a link to your claim. Here is a technical paper by the same author which elaborates on the approach:

http://arxiv.org/abs/gr-qc/9906029"

From page 2:

As commented earlier, one may expect
Lorentz invariance to be broken in a generic theory of
quantum gravity. In the context of string theory, this en-
tails the exploration of non-critical string backgrounds,
since Lorentz invariance is related to the conformal sym-
metry that is a property of critical strings.

Of course, a non-critical string theory is a string theory that is not in the critical dimension, e.g. strings in 3 + 1 d spacetime. These theories can only exist if we relax the constraint of lorentz symmetry.

It may be a matter of semantics Mt2d, and we can agree to disagree, but without any context I think "string theory", "superstrings", etc refers to the mainstream theory worked on by Susskind, Kaku, Greene, Randall, Vafa, Witten, etc which obeys lorentz symmetry without a doubt. It would be fair to say "there are non-critical string theories which violate lorentz symmetry" but since most people on this board won't know the jargon "non-critical" I still think this might be misleading.

Incidentally, some people might want to reflect on the fact that lorentz invariance is considered so important that the mainstream physicists have choosen to preserve this in string theory rather than choosing to preserve d = 4 spacetime.

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

The idea of giving a serious new theory a fair chance is not so new, and I hope everyone who believes in conspiracies about string theorists supressing outside reasearch take this as evidence that they always give a new approach a chance until it is shown to be unviable. There have already been a few papers pointing out problems with Horava gravity, and if that trend continus unabated the mainstream will move on in a year or two.

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

As an aside, why would anyone hope for lorentz violation? As I have said this is one of the deepest principles QFT is based on, and was essential for the development of the standard model, it would be disappointing if this symmetry were violated and it would reduce Einstein's theories and QFT to merely effective approxmations.

I am skeptical of the phrase "quantum foam" applied to string theory. I know Greene uses this phrase in his popularizations, but as far as I can tell that's just his way of communicating the nature of quantum mechanics / quantum field theory. I just want to clarify that spacetime is smooth in the mathematics of ST, even though I think Greene sometimes makes unclear comments about this for the sake of attracting a popular audience.

 

[MTd2]

I read Greene's book in Portuguese many years ago, so I am not aware about his terminology in English. I recall the use of "quatum foam" from Ashok Sen or Sergei Gukov from some colloquim slides from around 2005. I really never read books or articles about strings for the layman. I rather consult Polchinski's book or the older, Superstrings, for a deeper mathematical fundation.

I never talked about violation of lorentz symmetry, but an apparent one, if you just mind the possible effect of having a delay on the arrival time of a photon from a source. QFT and lorentz symmetry, this one even in the case of a "lorentz violation" are just fine from both of the sides of string theory and LQG, specially in the string side. QFT in string theory is effective in the low energy regime of the target space, but it is the natural foundation of both bosonic and susy 1+1 worldsheet...

As I said, in both cases, the delay is caused, in my view, by an increase of the vacuum peameabilty. Not because of fundamental reasons, but as in the same reason as light gets slower in a crystal, that is, it "bumps" more frequently with atoms, or in the other case, with "variations" of the background geometry.

 

[humanino]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

 

[ensabah6]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

Has Hawking worked on string theory since he devotes so much on it in his popular books (and does not appear to regard it as speculative in so much as fact)

 

[Civilized]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

Suppose someone created a thread, "why does Hawking omit all mention of the Planck scale green-goblin pastry theory?"

And the fans of goblin theory said "it's because he hasn't worked on it, and to say anything otherwise you need a reference."

I would ignore that fan because green-goblin pastry theory is so bogus, that although Hawking has never commented on it, it is still possible for educated physicists to suggest probable reasons why he hasn't worked on it. In particular, it's bogus.

You see, when you study physics you learn that "why?" questions like the title of this thread can be answered on multiple levels. The answer "because he has not worked on it" is not a very deep answer, because it immediately begs the question "why hasn't he worked on it?" Some of us work in theoretical physics, which involves using our minds to make logical inferences based on limited information. I would love to see quotes from Hawking about LQG, but if none are availible than the only way to answer the question on a deeper level than "because he didn't" is by make educated guesses into why he didn't.

Hawking's lack of comments about LQG cannot be a good thing for LQG, and this is a reason for LQG fans to end the thread. Afterall, by the trichotomy of "likes" the possibilities are:

(1) Hawking doesn't like LQG.

(2) Hawking doesn't care about LQG.

(3) Hawking likes LQG but never mentions it in his scientific or popular publications.

I think we can all see that (3) is unlikely, therefore LQG fans would like to stop us from discussing the reasons why (1) or (2) might be true. The fans might attack these comments as an invitation to "speculate", but all I am offering are informed inferences based on (a) my personal experiences talking to the professional American high energy physics community and (b) comments by leading mainstream physicists of similar stature as Hawking. I think it is important for us to discuss this because Hawking is a major part of the public image of physics, and the fact that he has ignored LQG my help some students to look at LQG more critically, that in particular it is a non-mainstream theory and it does not compete with string theory in the mainstream.

I would also call on Humanino to provide a reference to the statement in bold:

You mentioned that the conjecture is already used as a mathematical tool, and by the way no just for RHIC physics. However, in string theory it applies to SUSY QCD for instance, not QCD. And independently a good chunk of the models have 5 dimensions not 11.

 

[humanino]

And the fans

Your problem Civilized is that I do not care about LQG. My personal taste is obviously much more inclined towards string theory. My assumption is that my personal tastes are of no interest to anyone. Your assumption is that I am a LQG fan. It is just another wrong opinion of yours.

 

[Civilized]

Your problem Civilized is that I do not care about LQG. My personal taste is obviously much more inclined towards string theory.

Let's review some of your past comments:

You keep repeating that violation of Lorentz symmetry as a bad thing.

How can someone who is "obviously much more inclined towards string theory" not see that violation of lorentz symmetry is a bad thing?

However if LQG predicted Lorentz voilation unambiguously, it would be a falsifiable prediction making it instantly in better scientific shape than string theory.

Why would some who is "obviously much more inclined towards string theory" be unaware that string theory unambiguously predicts SUSY and extra dimensions or be unaware that these predictions of string theory are a least as definite and testable as the lorentz violations Smolin is suggesting? And even if you were unaware of these string predictions, it is still unlikely that someone who is "obviously much more inclined towards string theory" would make such a smug comment as "...would be a falsifiable prediction making it instantly in better scientific shape than string theory", because as I explained, the same faulty logic would make Planck scale green-goblin pastry theory "instantly in better scientific shape than string theory. "

If you are inclined towards strings, then you must not know the first things about them based on the two quotes of your above. Otherwise I would be happy to hear your explanation for those statements, and I am still waiting for you to retract or justfy your statement about (the supposed existence of) gauge/gravity dualities in 5 dimensions.

Lastly, I recognize an argument pattern. Often my opponents attempt to use the same tricks that I use on them, in this case the trick is of the general form "You assume a lot about me, ... , you're wrong." I don't claim to have invented the trick, but I used it on Humanino early in this thread and now Humanino is attempting to use it on me. The difference is that my "assumptions" were actually "inferences" based on evidence i.e. the quotes I have given above. Another common argument pattern is for my opponents responses to become increasingly glib and superficial, ignoring most of my points, not I'm not saying that that's happening here...

 

[suprised]

..There was considerable optimism about this, because the desired result was shown in D=3 case by Laurent Freidel and Etera Livine. For some time people tried to extend to D=4, but as you suggest, they seem to have gotten discouraged.

Right, and for good reason. Namely gravity is special in 3D, in fact in a sense topological and non-dynamical, and the tricks used in 3D just don't work in 4D. AFAIK there has been no proof so far that LQG would generate general relativity at large distances... after 20(?) years of research!

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

By now it should be clear to anybody following the field that there are serious problems with that model. Various recent papers have shown that unphysical degrees of freedom do not decouple, as had been anticipated by many. Even Horava in his recent talk at strings stayed clear from this point as far as I know.

So ... this is _not_ an "alternative" theory, even if some ppl here continue to claim so. And we should be glad about that, namely if all those simple minded modifications/deformations would make sense, we'd have even a bigger mess to figure out which is the right theory. Same applies to Lisi's theory... it is manifestly wrong, conceptually (obvious) and technically (needs some work, see Distler), though it seems that some amateurs are still busy with trying to "assign" elementary particles to roots of E8... though somehow that thread died off over the last year. Just how come?

It seems that the main theme of all of this spin doctoring is "we don't care whether it makes sense, all what we want is that it is NOT string theory". Well, good luck. Let me conclude, again, with a remark Witten made in one of our discussions of Horava-Lifgarbagez (not verbatim but in the sprit): "it is extremely hard to find a modification of a sensible theory that makes sense, and there is a good reason why we should stick to the models we study".

 

[atyy]

As an aside, why would anyone hope for lorentz violation? As I have said this is one of the deepest principles QFT is based on, and was essential for the development of the standard model, it would be disappointing if this symmetry were violated and it would reduce Einstein's theories and QFT to merely effective approxmations.

But isn't that GR and the standard model are just effective theories a motivation of string theory, ie. perturbative calculations indicate that GR is not renormalizable and cannot be a theory with a continuum limit, suggesting the need for a theory in which GR is not fundamental, but which emerges at low energies?

So Asymptotic Safety explores the possibility that GR is non-perturbatively renormalizable (though even AS does not assume that the Einstein-Hilbert action is correct, just that the gravitational field has a geometric interpretation, and could include higher order curvature terms). Whereas string theory, Horava gravity, condensed matter approaches explore what one needs if GR is emergent - in which case they have to deal with the Weinberg-Witten theorem and change dimensions, Lorentz invariance or something.