Thursday, September 27, 2018

Another Problem With String Theory

Sabine Hossenfelder explains a potential phenomenological test of string theory involving information loss from black holes at their horizons via Hawking radiation: "if the string theory calculations were correct then the information should leak out of the black hole."

She notes that a superfluid analog of a black hole tested in an laboratory does not show the properties predicted to exist in string theory for black holes, although admittedly, the margins of error in the experimental black hole analog are large.

The superfluid analog is notable because it is described by the same equations as GR mathematically, even though the quantities in the respective equations refer to different things.


Marnie said...

Thanks for this. I never was a big fan of string theory, and when it emerged, I was kind of horrified.

Mitchell said...

A comment I just posted at Sabine's:

The firewall paradox isn't specific to string theory, right? It's a potential problem for any theory describing the late stages of black hole evaporation?

I believe the actual situation in string theory is that there isn't any clear description of black hole evaporation proceeding to its end. The black holes that *are* understood microscopically are eternal black holes that are in equilibrium with a surrounding gas.

So within string theory, the "firewall debate" consists of a clash of ideas regarding how the theory *might* describe the late stages of evaporation. But only a minority of string theorists believe in an actual firewall. That view got promoted because Joseph Polchinski advocated it. Others think the paradox is avoided differently, e.g. through subtleties of the fuzzball description, or through an algebraic subtlety regarding operators behind the horizon.

andrew said...

The next shoe that I'm hoping to see drop, which Mitchell alludes to, is developing a broader consensus that string theory and SUSY aren't exclusive solutions to the unsolved problems in physics.

I think Sabine's work to undermine the view that problems like the hierarchy problem, naturalness, the strong CP problem, and the baryon asymmetry of the universe are failures of the universe to live up to the aesthetic expectations of arrogant and presumptuous physicists, rather than true "problems" in fundamental physics will help move that emerging consensus along.

There are genuine problems in fundamental physics that don't simply involve it being ugly.

We need "new physics" of some kind to explain dark matter phenomena.

We need some sort of quantum gravity theory to reconcile GR and the SM.

We can't have even an extremely ugly comprehensive theory of everything that isn't internally inconsistent or in conflict with the empirical evidence without those two pieces (we have a solution to dark energy, called the cosmological constant, which is consistent with the data, simple, and competitive with the alternatives in its fits to the data, although transporting that solution from GR to quantum gravity might require new physics).

We need to figure out anomalies like muon g-2, the muonic hydrogen proton radius problem, and the 25% shortage of muons relative to electrons in a couple kinds of B meson decays that cast doubt on lepton universality (all of which, I strongly suspect will go away with more experimental effort and more theoretical attention, unless we are very lucky).

We would really like to have a deeper theory that provides a source for the constants of the SM and GR from fewer experimentally measured constants via a few more relationships that we haven't worked out yet, and we would similarly really like to have a better way to do the insane calculations of QCD and a hypothetical quantum gravity in complex systems. But, ultimately, that is optional.

I think that it is possible to do both what we need to do, and some of the wish list want to do priorities without string theory, but it would sure be nice to have a really influential physicist write a paper that purports to prove that this is the case refuting 'no go" theorems with sometimes dubious assumptions or requirements.