### Talking to ’t Hooft about tossing TVs

#### Posted by Urs Schreiber

Today Prof. Gerard ’t Hooft gave a talk at University Duisburg-Essen on *Black Holes in Elementary Particle Physics*. Maybe due to the media hype about Hawking’s announcement of his new idea about the black hole information ‘paradox’, ’t Hooft decided to throw his TV set away, and not only his but lots of them, in fact enough that they would form a spherical shell collapsing to a black hole.

Using this picture to emphasize the process in which ‘known physics’, represented by well understood TV sets, passes the horizon and hence a border beyond which all kinds of apparent paradoxes lurk, he talked about some standard facts of high energy physics and then briefly mentioned some of his intriguing observations and speculations concerning physics of the stretched horizon, the collision of infalling particles with outgoing Hawking radiation as well as the possibility of a deterministic hidden variable model of quantum theory, which, as he says, he develops as a hobby.

After the talk we went to a nearby *Biergarten* and I had the chance to ask some more detailed questions.

I have to admit that I haven’t read any of ’t Hooft’s papers concerning the above mentioned issues, so I learned for the first time about his calculation which indicates that, somehow, the scattering of Hawking radiation at infalling matter (one form - even though not the only one relevant I’d think - of *back reaction* which is not usually taken into account in related discussions, but which certainly should be) has some surprising resemblance to string scattering amplitudes - well, except for the curious fact that the analogy requires a *imaginary* string tension.

Very interesting are also his ideas about the foundations of quantum mechanics, holography and string theory.

He says that he expects that there is a deterministic and local (yes, local) hidden variable theory behind it all, which would be apparent if only we knew the correct degrees of freedom of nature. Since we don’t, we only see a statistical average of this deterministic process, and this translates in a *non*-local way to the quantum mechanical wavefunction, roughly.

To me this philosophy sounded a lot like approaches by Lee Smolin to get quantum mechanical dynamics from the classical statistics of ensembles of large matrices that encode the deterministic interrelation of all particles (well, probably, if at all, of all D0 branes) in the universe. But when I asked Prof. ’t Hooft about this he said he wasn’t fully familiar with Smolin’s approach.

Anyway, ’t Hooft’s idea now is that the full deterministic theory has no information loss, but that on the ‘coarse grained’ level of familiar quantum theory information is lost all the time in virtual black holes that are abundant in vacuum fluctuations. The point is that, he says, this way information about degrees of freedom in the *bulk* diasappears. The only information left is that at some holographic *boundary*! This way, I think, he tries to give a ‘dynamical’ explanation of holography.

I asked if and how he sees string theory fit into this picture, and he said that he thinks that since in string theory essentially only the S-matrix is a well defined observable, and since this means that only on-shell information at the ‘boundary’ is available while *local physics in the bulk* is fundamentally out of reach of present day string theory, this fits in perfectly with the above picture, where ordinary quantum mechanics is kind of an ‘effective theory’ on the boundary while the true bulk theory is a deterministic hidden-variable thingy.

I have to say that when first confronted with speculations like this some alarm bells go off - but then I realize that when ’t Hooft discovered holography a while back this idea must have sounded - before Maldacena came along and gave an explicit realization - just as weird, and now it is widely accepted and even standard lore.

So maybe in this little chat over a glass of beer I was actually shown a glimpse of the big physics picture of the future, without my poor mind being able to fully grasp it.

On the other hand, when asked what he thinks about how his ideas about string/gauge duality and holography have come to life in string theory, he answered, humbly and jokingly, that he almost fails to recognize his original ideas.

There was much more discussion, but that’s all I am going to report here. It was a big pleasure to talk to such an outstanding person as ’t Hooft is, and I have some things to think about now. First of all, I’ll toss away my TV set…

Posted at July 19, 2004 9:31 PM UTC
## Re: Talking to ’t Hooft about tossing TVs

Hello Urs,

thanks for this interesting report.

For German reading people perhaps my popular review article about information loss in black holes might be interesting:

Vaas, R. (2002): Finales Fiasko. bild der wissenschaft Nr. 9, pp. 60-65.

(unfortunately not online)

It contains an interview with Gerard ‘t Hooft.

Regarding Lee Smolin’s work: Lee and Fotini are developing an interesting idea for a hidden variable quantum theory from quantum gravity at the moment, see

http://arxiv.org/abs/gr-qc/0311059

Sketchy, but worth looking.

Best wishes,

Rudy