Quantum Diaries

Interesting week last, especially last Monday when I and a theory colleague were asked to chat with a documentary producer for the National Geographic Channel. The producer (who has previously worked on a series about exploring time) is working on a show about black holes, with more focus on what space-time looks like around (and even inside) a black hole. He had heard something about the connections between RHIC physics and black holes and wanted to see whether we had anything to offer. Given our spotty past in explaining these kind of things without freaking out people who would otherwise be interested, my colleague and I actually managed to put together an interesting story (none of which implying any danger to anyone not living in a higher dimensional space). Turns out that there are three almost-distinct pictures of “black holes” being used in connection with high energy nuclear collisions (and at the LHC, it’ll only get higher!):

1. “Real” gravitational black holes – it’s been argued many times over that, for normal gravitational physics, energy densities at RHIC aren’t capable of producing enough matter in a small enough space to induce gravitational collapse, and subsequent decay via Hawking radiation. However, the presence of large extra dimensions has the effect of dramatically lowering the Planck scale and allowing this sort of phenomenon to occur, leading to spectacular isotropic decays of high mass black holes into the kitchen sink of particles from the standard model and beyond. I have relatively little experience bantering about this (i.e. one should check out Backreaction for more details on blackholology), but this is certainly the most “popular” conception of black holes at colliders these days.
2. Hawking-Unruh radiation – my colleague in this interview, Dima Kharzeev, and collaborators have put forth an interesting analogy between “minimum bias” particle production, i.e. the many low energy particles produced in essentially every proton proton collision and, scaled up, every nucleus-nucleus collision. In this scenario, the process of the incoming projectiles “stopping” each other, and thus slowing down, by construction leads to acceleration (well, deceleration in this case). Einstein’s equivalence principle (remember that you can’t tell the difference between an elevator accelerating upwards at 9.8m/s^2 and the Earth pulling you down by gravity at 9.8m/s^2…) allows them to connect this slowing down to the Unruh effect in a gravitational field, which predicts the quantum tunneling of particles with an effective temperature of T=a/2Pi. When numbers are put in, out pops the famous freezeout (or Hagedorn) temperature we measure at RHIC (and in proton-proton collisions for years). So in effect, all strong interactions measured in the laboratory make a “black hole”, but not one resulting from gravitational collapse. As an onlooker, I find this connection curious, but not isolated — over the years I’ve noticed many authors make a variety of connections between gravitational physics and strong interactions, but they always feel mysterious, and thus it’s unclear where to go next.
3. “Dual Black Holes” – this is something I and many others (both amateurs and pros) have found intrinsically exciting for a few years. The famous AdS/CFT conjecture suggests that strong interactions involving strongly-coupled quarks and gluons are really better (and more easily) described as a theory living on the boundary of a 10 dimensional gravity theory, with 5 extended and 5 compact dimensions. In this picture, again, every collision involves a black hole, which controls it’s microscopic properties (e.g. the viscosity), but one that lives in a larger dimensional space, and is thus again not the result of gravitational collapse in 4 dimensions. As people who have followed this thread (e.g. via my various blogs) over the years may be aware, this connection is allowing the development a striking number of techniques relevant to actual heavy ion phenomenology — and carries no risk to the 4-dimensional world (which someone should have told the BBC in early 2005…). Of course, we’re all hoping that the extra dimensions actually have some ontological status beyond being a mere mathematical trick, but time will tell.

Anyway, there we had it: three kinds of black hole physics, all of which are probably connected in some way, and all of which are potentially connected to RHIC or the LHC. (Recheck this post soon for more links…it’s bedtime – ok it’s wednesday, but done!)