Twenty years ago this past week, John Baez posted the first of his "This Week's Finds in Mathematical Physics" to the sci.physics newsgroups, inaugurating internet blogging about Mathematical Physics, many years before anyone even knew what a blog was. For his first posting, try looking at
and for all the rest of them see
There's a huge amount of interesting material in John's TWF postings, and the amount of effort that he has put into providing detailed, clear explanations on all sorts of topics is kind of staggering. While I often try to emulate what John has done (and "This Week's Hype" of course is a sort of homage, one he may not appreciate...), I feel I'm doing well if I can manage to put together a few sentences of comments about a "Find", with John's much more useful detailed expository work something beyond my capabilities.
For a "Find" from this past week in mathematical physics, I can recommend Hermann Nicolai's "Quantum Gravity: the view from particle physics" (arXiv:1301.5481) a write-up of his lecture this past summer at a conference in Prague. He makes a point about quantum gravity that I very much agree with: the problem with the subject is not so much that of finding a consistent quantum gravity, but of finding one that fits together with the SM and tells us something new that we can check. He writes:
Being exposed to many talks from the different ‘quantum gravity camps’ I am invariably struck by the success stories I keep hearing, and the implicit or explicit claims that ‘we are almost there’. I, for one, would much prefer to hear once in a while that something does not work, and to see some indications of inconsistencies that might enable us to discriminate between a rapidly growing number of diverging ideas on quantum gravity [27, 28]. If, however, the plethora of theory ambiguities were to stay with us I would conclude that our search for an ultimate explanation, and with it the search for quantum gravity, may come to an ignominious end (like in Breughel’s painting).
To conclude let me restate my main worry. In one form or another the existing approaches to quantum gravity suffer from a very large number of ambiguities, so far preventing any kind of prediction with which the theory will stand or fall. Even at the risk of sounding polemical, I would put this ambiguity at 10^500 (or even more) – in any case a number too large to cut down for any conceivable kind of experimental or observational advance.
Included in his talk are various more specific comments about these issues, well worth pondering. If I were John Baez, I'd have the energy to describe them in detail and explain clearly exactly what is going on, but for this I fear someone will have to get John more interested in quantum gravity again...
Update: I should mention other tributes to TWF here, here, and here.
What? I can’t really read font.
John Baez, Chris Hillman and Dr Kevin Brown were the three main figures from the 1990s on sci.math that I fondly remember.
Dr Kevin Brown still hosts Math Pages http://mathpages.com/home/index.htm
whereas Chris Hillman mysteriously deleted his Relativity on the Net page in 2007.
Any model of quantum gravity can be tested only against one fact: the entropy of black holes. A good friend who works in quantum gravity once summarized the situation in a simple way: there are zillions of models that reproduce the entropy of black holes and gravity, but there is no model that explains the other interactions.
It is very unlikely, therefore, that a good theory of everything will appear through quantum gravity research.
I think Nikolai is too pessimistic and on the other hand too demanding.
A suitable candidate for a theory of evererything (as a working project ) should
1. be able to reproduce in principle the theoretical content of the low energy effective world i.e. GR+QFT
2. incoproprate natuarally all the ingridients needed for the derivation of SM as we know it (Fields, gauge groups etc).
3. be able to treat Gravity quantum mechanically
4. exhibit mathematical and theoretical consistency
5. be consistent with current experimental data
String theory (and String theory only BTW) fulfills the above criteria and so the theory should be taken seriously as a base of research for a unified theory.
At some point of development the theory should be able to offer theoretical solution to known problems of GR+SM e.g. singularities , EW scale stabilization, fate of spacetime at planck scale, resolution of black hole information paradox, dark matter candidate, the CC problem etc
String theory has made considerable progress answering some of these questions.
Finally it would be great if the theory could make defintive testable, predictions at low energies but we don’t know if this is possible even for the correct theory.
Thus all in all it looks good for String theory in my opinion.
“Any model of quantum gravity can be tested only against one fact: the entropy of black holes.”
No, a theory of quantum gravity definitely cannot be tested using black holes because we will probably never be able to measure the entropy or temperature of a black hole. There are, however, other ways in which we could potentially observe quantum gravitational effects. For example, it is conceivable that quantum gravitational effects in the early universe could lead to testable predictions about the structure of the present universe. In addition, many people believe that the strong and electroweak interactions will be unified at a length scale which is only a few orders of magnitude larger than the Planck scale. Thus, it is possible that quantum gravity could play an important role in the unification of the other forces.
“there are zillions of models that reproduce the entropy of black holes and gravity”
Not really. The only models that successfully reproduce the semiclassical result for black hole entropy are models based on the holographic principle, including string theory. In other approaches, like loop quantum gravity, the situation is not at all satisfactory because the theory has a free parameter (the Immirzi parameter) which needs to be adjusted to a certain value in order to get the correct result. In fact, this is why string theory is such an important tool for studying quantum gravity. It provides a concrete realization of the holographic principle and a paradigm for doing microscopic calculations of black hole entropy.
Enough with the tedious hype about string theory and how it will someday magically be testable. I wrote about Nicolai’s talk because it’s clear he’s given up on such nonsense and is trying to get beyond it, which is interesting. How about reading what he has to say and thinking about it instead of repeating BS?
The argument over black hole entropy calculations wasn’t interesting when it was news, and there are probably now students here at the university who weren’t even born when these calculations were done. Enough.
I’m not sure if you’re talking to me, but just to be clear, I am not suggesting that string theory will become “magically testable” in the near future. I did mention that quantum gravity could play a role in the unification of the forces and that this could lead to testable predictions from quantum gravity, but it sounds like you would agree with me about that.
anyone know what happend to chris hillman? is he in acadamia He had such mastery over GR ?
Well, after the Byrds, and The Flying Burrito Brothers, he bounced around and is currently touring with the Desert Rose band… 🙂
Chris Hillman got upset about the way the web was developing (too many cranks), posted negative opinions about wikipedia and the like (he had initially contributed many articles), and decided to delete his site and go offline. But you can still access his old relativity site (hosted by John Baez) on the internet archive here
He contributed to physicsforums until the end of 2007, posted comments on some math related blogs (including this one) in 2008 and then seems to have disappeared completely?
Nicolai’s article doesn’t harbor any surprises. It certainly captures the current attitude of particle theorists, who seem to consider quantum gravity (QG) as some kind of panacea, or merely a synonym for what is currently unknown. Unfortunately, the state of knowledge reflected therein is most often of particle physics folklore, rather than the technical state of the art. One could nit pick at any number “issues” addressed by Nicolai as either requiring QG for their resulution or providing a clue as to how QG should be and in turn argue that they are not necessarily problems at all or have little to do with gravity.
There is one particularly opaque and unfortunately inaccurate statement at the end of section 6 on Anomalies. He correctly points out that anomalies are a serious worry in quantization, especially for gauge theories. This is followed by a remark about the possible anomalies occurring in the diffeomorphism algebra in 2d theories (incidentally, of which the string worldsheet theory is an example). Unfortunately, that remark is followed up by another one, namely that we do not have a classification of diffeomorphism algebra anomalies in higher dimensions, which is actually incorrect. The Belgian group of Henneaux, Barnich and Brandt worked out in the mid-to-late 90’s that, in in most dimensions including 4d, the SM+GR does not have any new anomalies compared to just the SM. Similar information can also be found in volume 2 of Weinberg’s QFT treatise.
Having been in the audience, I drew attention to this point in the question period after Nicolai’s talk. However, my remark does not seem to have had much impact.
Nor sure if this is the painting he is referring to, but fits the bill
How about The Tower Of Babel. This image was used in Carlo Rovelli’s book or maybe Laurent Nottale’s, not sure.
The font was a nice touch
I think it’s Breugel’s “The Fall of Icarus”, the subject of a famous poem by Auden.
“He makes a point about quantum gravity that I very much agree with: the problem with the subject is not so much that of finding a consistent quantum gravity, but of finding one that fits together with the SM and tells us something new that we can check.”
Exactly, that I think is what is often overlooked: Quantum gravity is not primarily the unification of q.m. & gravity BUT the SM + gravity, the former bringing in a lot of interesting geometry.
I just watched this video, where this is motivated very nicely
Thanks! For a second I thought that font was the result of some error.
Chris Hillman began his internet career as a relentless scourge of crackpots, but then became increasingly scared of them, probably due to threats he received. He then tried to expunge every trace of himself, for example demanding that I remove his page Relativity on the World-Wide Web from my website.
He doesn’t have an academic career, and when I last checked he was unemployed and ill. He originally had some connection to the math department at the University of Washington, and had an email account there. Later he lost that. More recently all communications I’ve received from have been via pseudonymous email accounts.
One moral is that if you plan to spend a lot of time debunking crackpots, you should expect some of them to eventually spend a lot of time attacking you. So, think a bit about whether you have the stomach for this. I decided a while ago that there are better things to do.
Bob Jones wrote:
That was certainly true of the old calculations I was involved with, and I agree that this was a frustrating situation. But anyone who is interested should look at the more recent papers by Eugenio Bianchi, especially Entropy of non-extremal black holes from loop gravity.
“anyone who is interested should look at the more recent papers by Eugenio Bianchi”
John, what about the criticism that Bianchi isn’t even counting black hole microstates, he’s just reproducing Hawking’s semiclassical calculation? Hossenfelder, Motl, and others all say this.