Thursday, September 15, 2016

Macro is not like string theory, part II (my personal take)

I wanted to add a few notes to this post (update 16 Sep 2016: which I followed up with some graphics here) -- some things that fell through the cracks.

In my experience as a grad student, I never felt that string theory was some kind of in-group or religious cult. It is true that particle theory group at the University of Washington was definitely more QCD-focused (Stephen Ellis was on my committee). Ann Nelson was the center of beyond the standard model physics, and the only string theorist (Andreas Karch) was hired while I was there (I took his string theory class that was being offered for the first time, with some guest lectures by Joseph Polchinski). This is to say my experience with string theory was mostly through visitors like Polchinski and Edward Witten, reading the literature, and talking to other grad students about it. Maybe it is different at MIT, Harvard, and Princeton.

There was a lot of 'synergy' between the particle theory group and the nuclear theory group regarding QCD (lattice, strings, nonperturbative methods) and the Institute for Nuclear Theory (INT) even put on a great workshop about string theory and QCD (using AdS/CFT and holography to understand non-perturbative QCD) while I was there. I was a grad student in the nuclear theory group (my advisor was Jerry Miller -- who got some press in the NYTimes while I was there), but my thesis touched on lattice QCD and nonperturbative methods like the "large N" expansion of QCD and topological solutions. String theorist Witten wrote the definitive paper on using large N for baryons (which I cited), and it should also be noted that another large N expansion in the SYK model has recently been connected with holography.

I had a lot of respect for string theorists and grad students studying string theory. It is hard stuff. There wasn't a lot of money for particle/high energy theory students so they ended up with higher teaching/grading loads for a longer time. In nuclear theory, I only had to teach classes for the first couple years before I got a full support as a research assistant (and summer support through the INT). But string theory wasn't the only high-status "hard" subject you could use to show you were smart. String theory, esoteric field theories, and general relativity all involved a lot of similar math skills that would fall under the heading of topology and differential geometry of manifolds. That stuff is hard. And that's why black holes and strings have contributed to pure math in those areas.

To bring this back to the comparison between macro and string theory:
  • Cutting edge physics has always been associated with cutting edge math, from Newton and calculus to Einstein and differential geometry. String theory methods were used to prove the Poincare conjecture. Macro math isn't very cutting edge mathematically and it used to be ... more verbose. Therefore anything that references math in an analogy between macro and string theory is way off base.
  • In physics, string theory doesn't have a monopoly on the cutting edge, signalling intelligence, philosophical implications, or being a "rockstar". Most famous physicists are of the "I study black holes" variety (general relativity). You probably had to click the links above to know who Polchinski and Witten were. However I could probably reference Hawking without even a first name and you know who I'm talking about. Even Brian Greene isn't that famous. However as Noah Smith points out, macro really is the "glamour league" of economics. Paul Krugman was primarily in international trade, but he's popular for his macro.
  • There isn't a bright line between string theory and non-string theory. The large N expansion for baryons (N quarks) and the SYK model (N Majorana fermions) have connections to string theory. The conformal field theory in the original AdS/CFT correspondence is very similar to QCD (which is approximately conformal). The differential geometry of string theory and general relativity are directly related. Witten's big contribution was connecting 11-dimensional supersymmetric gravity to M-theory. Is Raphael Bousso a string theorist? Wikipedia says so, but his big contribution (covariant entropy bound) is more on the general relativity side and he studied under Hawking. In contrast in economics, you're either dealing with "the economy" as a whole in macro, or you're not.


  1. Jason, you write:

    "have connections to strin theory"

    but I think you left off the "g."

    1. Thanks Tom. It migrated into the "simgilar" somehow. Fixed.