## Friday, February 14, 2020

### Bojowald: Ashtekar school is fundamentally misguided

Hat tip to @mmanuF at Twitter

Martin Bojowald has been a well-known name in the loop quantum gravity corners and especially in loop quantum cosmology. Aside from other things, while he was working at the Max Planck Institute in Potsdam, he co-authored three papers with Abhay Ashtekar, a guru of loop quantum gravity, in 2003, 2005, 2005.

Amusingly enough, now, when Bojowald and Ashtekar are colleagues at Penn State, he just released a gr-qc paper
Critical evaluation of common claims in loop quantum cosmology
that tries to critically look at some basic aspects of the "Ashtekar school", as he calls it. Bojowald concludes that the Astehkar school fails in all of his nontrivial tests. It makes assumptions about the semiclassical behavior of the metric tensor degrees of freedom while the semiclassical approximation is almost certainly inapplicable in these examples. Also, Bojowald points out that characteristically quantum mechanical effects – such as ambiguities resulting from quantization – are incorrectly assumed not to exist.

Needless to say, loop quantum gravity people may be classified as full-blown or generalized anti-quantum zealots – people who actually think classically and who misunderstand everything in which the classical limit fails to be "basically right".

Much of the paper seems similar to the kind of fundamental criticism of the whole program that I have been promoting for decades; some formulations are different from mine because we're two different people.

Bojowald discusses the foundational misconceptions of the Ashtekar school as well as applications in particular situations, such as the bounces, cosmology, and the "removal" of the singularities. Concerning the latter, the Ashtekar school contradicts the singularity theorems, aside from other settled facts, when it says that it can avoid the singularities by the addition of a new force.

The very idea that singularities (e.g. inside black holes) should be "removed" and it's done by "something like new forces" is nearly a defining misconception among the people who don't really get quantum gravity and string theory at the technical level. Indeed, there is no problem with singularities inside black holes, they just mean a "dead end" in a description that ceases to work there, and there's no need for a consistent theory to "remove" them. Instead, the "survival" of the singularities is nearly guaranteed by the singularity theorems and these theorems are much more robust than the "removers of singularities" want to imagine.

You may start by reading Page 3 of Bojowald's paper where the concentration of the serious criticisms is high (mmanuF's favorite page). Clearly, I've written many of those things before so I mostly agree. It is just a bit unpleasant to have the feeling that it took a very long time (well over 30 years) for a well-known loop quantum gravitist to "rediscover" these critical problems with the Ashtekar school. Also, these men's physical proximity could indicate that the sudden critical attitude by Bojowald could be caused by some not so harmonious personal interactions but it is just a feeling for which I have no strong evidence.

One day earlier, Bojowald wrote a related but less critical paper Non-covariance of the dressed-metric approach in loop quantum cosmology.

It's ironic that these fundamental criticisms of the Ashtekar program by an LQG veteran were posted one week after string theorist Raju and pals (Raju was my TA at Harvard, among other things) wrote a paper that basically parrots some of the basic misconceptions of the Ashtekar program – namely that the semiclassical quantization and semiclassical assumptions about quantum GR do correctly answer statements about quantum gravity including the deep ones such as the information loss paradox. I am convinced that Raju et al. are just wrong just like Ashtekar is. At the level of the naive quantization of gravity, you just don't get holography and the information does get lost in the black holes.

And yes, I have some suspicion – not backed by any substantial evidence for this particular case but justified by some experience in other cases – that Raju's and Ashtekar's Indian birth places could have something to do with Raju's irrational promotion of Ashtekar's misconceptions.

The world is ironic but maybe there is some approximate conservation law for happiness and when some string theorists start to write wrong, LQG-like papers, a LQG theorist starts to see the light instead. ;-)