Monday, February 15, 2010

Applied String Theory!?

Now here's surprising twist in the string theory story, to say the least...

I blogged a little bit the other day about the "Ads/CFT" correspondence, which relates string theory in certain spaces to non-string theories on the surface of those spaces. This bizarre dimension-shifting idea is 13 years old now but its ramifications continue to expand. Juan Maldacena's paper proposing the idea was, as of last year, the second most-cited paper of all time in the Spires high-energy physics database, and will certainly hit number one soon. (I have disqualified an unfair review paper which actually sits at number one).

When first conceived, it seemed like a novel way to figure out things about string theory and therefore, perhaps, about quantum gravity. It seemed like one more bit of cool but ultimately arcane mathematics coming out of string theory.

But in the last few years that logic has being turned on its head and physicists have found it very fruitful to go the other way - to use string theory to understand the surface theories, which are "quantum field theories" quite a bit like the one believed to describe quarks in atomic nuclei.

Now, the quark theory ("QCD") is very hard, because it is "strongly interacting". However, strongly interacting theories are precisely the ones with good "Ads/CFT" dual descriptions. So we have the bizarre phenomenon of actual observable properties of colliding nuclei - messy, hot globs of quarks and gluons - being described in terms of 5-dimensional gravity, strings, membranes, and black holes! I don't think, 15 years ago, that anyone in their wildest thoughts had imagined that black hole physics could be relevant in any way to nuclear interactions; let alone black hole physics in 5 dimensions!

And, more speculatively, some condensed matter systems (e.g. high temperature superconductors) at the temperatures of their phase transitions, also can be connected to a dual gravity description. This, I believe, is still much more tentative than the quark connection.

Note, nobody is saying that actual black holes or other quantum gravity effects are created in nuclear collisions or high-temperature superconductors. The string theory and gravity here are just a "dual description", or equivalent way of looking at them. What's acting like a "string" in the quark-gluon soup would actually be a chain of gluons or something like that. What's acting like the "5th dimension" would actually the energy scale of the reaction. And now I am getting out of my depth and cannot comment in further detail.

For those of you who have read about this elsewhere in the media, I am sorry to probably not add much more. For those who haven't, I hope you find this development as remarkable as I do! I mean seriously, black holes in nuclear physics, of all places!

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