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Low-functioning pinhead...
July 30, 2004
The President Answers his Critics
Thanks to the fevered imagining of Will Ferrell, we have a coherent response from Crawford Texas to all the calumnies emanating from the Democratic National Convention in Boston:
There’s certain Liberal agitators out there who’d like you to believe my Administration’s not doing such a good job. Those are people such as Howard Stern, Richard Clark and the News.
…
There’s people out there who’d like you to believe the Economy isn’t doing so well. To that I answer, “Hey, for the two million jobs we’ve lost, that means two million unemployed people sitting at home, watching reruns of quality television such as The Jeffersons or Facts of Life. And that just means more ad revenue to Radio and TV stations.”
…
So stick with Bush. Don’t vote, and don’t listen to Liberals, Democrats or other Republican who make fun of me, or read the News or watch the News, except for Fox.
Thank you, and God bless.
Brought to you by the good folks from ACT.
July 29, 2004
Blackhole Production
One (unlikely, but) exciting prospect is that, if there are large extra dimensions and TeV-scale gravity, one would produce blackholes at the LHC. Giddings and Eardley’s calculation of semiclassical blackhole formation in high energy scattering leads one to believe that, if the fundamental Planck scale is low enough (say, ) and the number, , of large extra dimensions large enough, one ought to see copious numbers of blackholes (/year for , and a luminosity of ) at the LHC.
This would be incredibly dramatic. But, as Yoshino and Nambu showed, there’s an important effect which needs to be included, namely that some significant fraction of the center-of-mass energy is radiated away before the blackhole forms. When you include this inefficiency, the production rate goes way down (because we need to be further out on the tail of the parton distribution function).
Anchordoqui, Feng, Goldberg and Shapere have looked at the numbers for the LHC, and compared them with what can be seen from cosmic ray observatories. In cosmic ray searches, one can look for quasi-horizontal showers produced by cosmic ray neutrinos. If the TeV-scale gravity story is correct, blackhole formation dominates the total cross section at high energies. Looking only at quasi-horizontal showers minimizes the background from other sources, so this provides a fairly clean, dramatic signal.
Cosmic ray neutrinos have some advantages over the LHC.
- You pay the price of only one parton distribution function, instead of two.
- AGASA is already taking data, and Auger will be soon.
By the time LHC turns on, they will have accumulated several more years of data. Either they will have seen blackholes, or they will have set limits which leave the LHC only a very narrow window for discovery.
The discovery reaches for the LHC (solid) for 3 different integrated luminosities and extra dimensions. Also shown is the region of parameter space which can be excluded at 95% CL if no neutrino showers mediated by BHs are observed in 5 years at the PAO. The shaded (cross-hatched) region assumes 2 SM neutrino + 0 (10) hadronic background events. is the fundamental Planck scale, and is the mass (in units of ) of the smallest mass blackhole which can be cleanly distinguished. Ten years of the LHC running at represents an integrated luminosity of 1000 fb-1 (from Anchordoqui et al).
There is, at least, one loophole in this analysis. Yoshino and Nambu only computed a lower bound on . But it seems unlikely that the actual ratio will be significantly larger than this and — in any case — this affects both cosmic ray and LHC searches. Also, the flux of cosmic ray neutrinos isn’t really known but it is, at least, bounded below by that due to pion production from cosmic ray protons.
Now, I don’t think either Auger or LHC will see blackholes. But I was surprised to learn how competitive high energy cosmic ray observatories have become in doing particle physics.
July 22, 2004
No Information Lost Here!
The blogosphere — or, at least, that little corner of it that I pay attention to — is all a-twitter about Hawking’s announcement that he has solved the blackhole information problem (30 years after posing it). Normally, I try not to devote attention to such things, but the flurry of discussion prompted me to take a look at the transcript of Hawking’s talk.
He starts off with the amusing comment,
I adopt the Euclidean approach, the only sane way to do quantum gravity non-perturbative [sic].
Of course, among its other defects, the Euclidean path-integral he wishes to do is horribly infrared divergent. So his first step is to introduce an infrared regulator, in the form of a small negative cosmological constant. This is not merely a technicality. None of the subsequent arguments make any sense without it.
Anyone who hasn’t been asleep for the past 6 years knows that quantum gravity in asymptotically anti-de Sitter space has unitary time evolution. Blackholes may form and evaporate in interior, but the overall evolution is unitary and is holographically dual to the evolution in a gauge theory on the boundary.
With the large accumulation of evidence for AdS/CFT, I doubt there are many hold-outs left who doubt that the above statement holds, not just in the semiclassical limit that Hawking considers, but in the full nonperturbative theory.
Nonetheless, a “bulk” explanation of what is going on is desirable, and Hawking claims to provide one. Hawking devotes a long discussion to the point that trivial topology dominates the Euclidean path-integral (at zero temperature). Since the trivial topology can be foliated by spacelike surfaces, one can straightforwardly Wick-rotate and it follows that Minkowski-signature time evolution is unitary. Presumably, Hawking is aware of, but neglected to mention Witten’s old paper, which not only show the dominance of the trivial topology at low temperature, but shows that, at high temperature, the path integral is dominated by the Hawking-Page instanton (the analytic continuation of the AdS blackhole) and that, moreover, the phase transition which separates these two regimes (which Hawking and Page argued for, in the context of semiclassical gravity in AdS) is related to the confinement/deconfinement transition in the large-N gauge theory.
All of these are true facts, well-known to anyone familiar with AdS/CFT. But the latter goes well beyond the semiclassical approximation that Hawking uses. No one (at least, no one I talk to) has the slightest doubt that quantum gravity has unitary time evolution in asymptotically AdS space. The blackhole information paradox is solved in AdS, and it was “solved” long ago.
However, most people agree that the extrapolation to zero cosmological constant is not straightforward. There still room to doubt that time evolution in asymptotically flat space is unitary. On the thorny issue of extrapolating to zero cosmological constant, Hawking is silent.
July 20, 2004
Hair of the Dog
Longtime readers of this blog know that it started by accident. A bicycling accident