# Why does Stephen Hawking omit all mention Loop Gravity?

## Main Question or Discussion Point

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.

Related Beyond the Standard Model News on Phys.org
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.

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.

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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
Gold Member
Dearly Missed
... 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
Gold Member
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
Gold Member
Dearly Missed
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.

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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
Gold Member
Dearly Missed
"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 [Broken]

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 [Broken]

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.

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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:

It links to a blog post which links to a recent article co-authored by Lee Smolin: http://arxiv.org/abs/0906.3731" [Broken]. 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.

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MathematicalPhysicist
Gold Member
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.

Are you aware that Hawking now partly works at PI ?

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
Gold Member
Are you aware that Hawking now partly works at PI ?
PI also funds string theorists...
http://www.perimeterinstitute.ca/en/Scientific/Research/Superstring_Theory/ [Broken]

But I am very suspecious of calling them string theorists. One of them is a young researcher, Freddy Cachazo... But I wouldnt really call him a string theorist. He doesnt 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" [Broken]. 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 vaccum permeability. It is like the vaccum became "thicker" at higher speed. I guess this is why they talk about birefringence.

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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 ?

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.

"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?

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.

What is Hawking's opinion on LQG?
Unless hoping you'll have him show up to answer, why do you care ?

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" [Broken] 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.

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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.

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atyy
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.