# Why does Stephen Hawking omit all mention Loop Gravity?

1. Jul 9, 2009

### 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.

2. Jul 9, 2009

### 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.

3. Jul 9, 2009

### 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.

Last edited: Jul 9, 2009
4. Jul 9, 2009

### atyy

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.

5. Jul 9, 2009

### marcus

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.

6. Jul 9, 2009

### MTd2

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.

7. Jul 9, 2009

### marcus

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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8. Jul 9, 2009

### 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.)

9. Jul 9, 2009

### 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 [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.

Last edited by a moderator: May 4, 2017
10. Jul 10, 2009

### Civilized

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.

Last edited by a moderator: May 4, 2017
11. Jul 10, 2009

### 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.

12. Jul 10, 2009

### humanino

Are you aware that Hawking now partly works at PI ?

13. Jul 10, 2009

### suprised

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.

14. Jul 10, 2009

### MTd2

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...

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.

Last edited by a moderator: May 4, 2017
15. Jul 10, 2009

### humanino

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 ?

16. Jul 10, 2009

### atyy

17. Jul 10, 2009

### 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.

18. Jul 10, 2009

### Naty1

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:

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?

19. Jul 10, 2009

### Coin

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.

20. Jul 10, 2009