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...