## Archive for **August 2008**

## Obama 08

His acceptance speech tonight was good, but the best speech of the convention was John Kerry’s. See here for more discussion.

Never in modern history has an administration squandered American power so recklessly. Never has strategy been so replaced by ideology. Never has extremism so crowded out common sense and fundamental American values. Never has short-term partisan politics so depleted the strength of America’s bipartisan foreign policy. …

To those who still believe in the myth of a maverick instead of the reality of a politician, I say, let’s compare Senator McCain to candidate McCain. Candidate McCain now supports the wartime tax cuts that Senator McCain once denounced as immoral. Candidate McCain criticizes Senator McCain’s own climate change bill. Candidate McCain says he would now vote against the immigration bill that Senator McCain wrote. Are you kidding? Talk about being for it before you’re against it. …

How insulting to suggest that those who question the mission, question the troops. How pathetic to suggest that those who question a failed policy, doubt America itself.

How desperate to tell the son of a single mother who chose community service over money and privilege that he doesn’t put America first.…

## A warning from von Neumann

I can’t resist reproducing this quote from John von Neumann, which I think applies well to certain branches of particle theory today. Thank goodness the LHC is coming on line soon…

As a mathematical discipline travels far from its empirical source… it is beset with very grave dangers. It becomes more and more purely aestheticizing, more and more purely

l’art pour l’art. …In other words, at a great distance from its empirical source, or after much “abstract” inbreeding, a mathematical subject is in danger of degeneration.

From the opening material of the book John von Neumann and Modern Economics. If you can get a copy of this book, I highly recommend the chapter by Paul Samuelson.

## ECT in Trento

I’m off soon to the following meeting at ECT: European Center for Theoretical Studies in Nuclear Physics, which is located in the mountain town of Trento, in the Italian Alps. The picture above was taken just north of Trento. I’m excited to see the dolomiti!

I’m flying in and out of Venice — any tips on what to do there or in Trento would be appreciated 🙂

**Meeting: The statistical model of hadron formation and the nature of the QCD hadronization process**

## Dense nuclear matter: intuition fails!

I usually don’t get into detailed physics exposition on this blog, but I thought I would make an exception with regard to the paper 0808.2987 which I recently wrote with my student David Reeb. (See earlier blog post here.)

In the paper we conjectured that there might be regions in the QCD phase diagram where the sign problem does not prevent monte carlo evaluation of the Euclidean functional integral. We rewrite the partition function as

Z = Z+ – Z-

where Z+ and Z- are sums with positive weights, and each define independent statistical ensembles. Defining Z+ = exp( – V F+ ), and similarly with Z-, so that F+ and F- are the (piecewise) analytic free energies of the two ensembles, we conjectured that

F+ < F-

is the generic situation. Note Z > 0 so F+ > F- is not possible, but they can be exactly equal: F+ = F- , which is where the sign problem is most severe (see below). Since the F’s are analytic except at phase boundaries, we reasoned that if they are equal in a region they must be equal everywhere within that phase region. At mu = 0 we know Z- = 0, so we assumed that there would be a region of small mu where F+ < F- and that this region would extend into the mu-T plane.

It turns out this last assumption is probably wrong! We were unaware of results which strongly suggest that even at arbitrarily small (but positive) mu and small T, Z+ does not dominate Z. That is, in the thermodynamic limit F+ = F- exactly even at small nonzero mu. The order of limits matters: taking V to infinity for fixed nonzero mu (no matter how small) leads to large phase fluctuations. The only way to avoid it is to take mu to zero before taking V to infinity. (See 0709.2218 by Splittorff and Verbaaschot for more details. Note their results rely on chiral perturbation theory, so don’t apply to the whole plane.)

It is quite strange to me that zero density QCD can only be reached in this way. The case we are most familiar with turns out to be the oddball.

To make a long story short, our conjecture is probably incorrect: what we thought would be “exceptional” regions in the phase diagram are the typical ones, and vice versa — at least as far as anyone knows.

Note to experts: we used the term “sign problem” a bit differently than apparently it is used in the lattice community. We refer to dense QCD as having a sign problem even though we don’t know for sure (i.e., for all mu and T) that Z is exponentially small in V due to cancellations (i.e., a “severe sign problem”). Our usage probably translates to “potential sign problem” — the functional measure isn’t positive, so potentially such cancellations can occur, although we do not know if in fact they do. We got a lot of emails from people who thought we were claiming to have a method for dealing with severe sign problems, but in fact we were claiming something else entirely: that there should be regions in the phase diagram where the sign problem is *not* severe.

## Dense nuclear matter

New paper! Probably too technical to go into here, but it relates to our current inability to directly simulate dense nuclear matter (QCD at nonzero baryon density). When the number of quarks and antiquarks is equal, the functional integral representation of the partition function Z has good positivity properties and can be evaluated using importance sampling (lattice Monte Carlo methods). That is no longer true when the system has nonzero baryon number, as would be the case inside a neutron star or in nuclear matter.

We rewrite Z = Z+ – Z- , where Z+ and Z- have good positivity properties, and conjecture, based on arguments using the analytic properties of the free energy, that at most points of the phase diagram Z+ dominates Z-. At such points one can simulate the theory using Monte Carlo.

http://arxiv.org/abs/0808.2987 (paper available after 5 pm pacific 8.24.08)

Sign problem? No problem — a conjectureStephen D.H. Hsu, David Reeb

We investigate the Euclidean path integral formulation of QCD at finite baryon density. We show that the partition function Z can be written as the difference between two sums, each of which defines a partition function with positive weights. We argue that at most points on the phase diagram one will give an exponentially larger contribution than the other. At such points Z can be replaced by a more tractable path integral with positive definite measure, allowing for lattice simulation as well as the application of QCD inequalities. We also propose a test to control the accuracy of approximation in actual Monte Carlo simulations. Our analysis may be applicable to other systems with a sign problem, such as chiral gauge theory.

## The cost of gold

What did all those medals cost China? Billions of dollars and the effort and sacrifice of countless young athletes, most of whom came nowhere near the Olympics, let alone a medal. There are an estimated 400k kids in specialized sports schools in China.

WSJ: …In the eight years to 2006, the latest set of full-year figures available, China’s spending on sports increased 149% to 9.2 billion yuan ($1.35 billion at today’s exchange rates), compared with a 36% increase to 7.1 billion yuan in natural-disaster relief. Adding to the budgetary pressure is the need to rebuild after this year’s Sichuan earthquake, which will be financed by a 5% reduction in all other government spending. On top of all that, China’s Olympic building spree has left the government with 31 new and refurbished stadiums that it now must maintain.

…Because of China’s population-control policies that allow most families only one child, they are increasingly reluctant to turn over their offspring to the state sports academies. Much of China’s athletic success has been built on vast numbers of athletes from peasant stock who were willing to chi ku — to “eat bitterness” — to grind through the state sports system and have a shot at success.

Despite such heavy spending, China didn’t make up very much ground in track and field. Liu Xiang’s 110m hurdles gold in Athens was described as “the heaviest” and “the one with the most gold content” of all of China’s medals. (Note, cf. Phelps, that they didn’t say this about a swimming medal. They have a pretty realistic idea of which sports are the most competitive 🙂 Liu’s failure to compete in Beijing due to injury may have been the single biggest Olympics story in the Chinese media.

On the other hand, Jamaica (population less than 3 million) had a great Olympics on the track: three golds and three world records for superman Bolt, and a medal sweep of the women’s 100m.

## Bolt is the greatest of all time

19.30 in the 200m, breaking Michael Johnson’s record! Two world records, two gold medals. He led by 10m at the finish, and this time there was no show boating.

I remember watching Johnson set the record in Atlanta (on television). I couldn’t imagine when — if ever — it would be broken.

In the semifinal heat yesterday Bolt looked like he was jogging at the finish, yet placed ahead of Shawn Crawford, the defending gold medalist from Athens who finished third in the final. Bolt destroyed his competition effortlessly. He is a superman among boys.

Hopefully the silly US media will forget about Phelps and focus on the real story in Beijing. **Note added**: Bolt donated $50k to earthquake relief in Sichuan province. He said he was moved to tears on the night of winning the 200m race, when more than 90,000 spectators in the Bird’s Nest sang “happy birthday” for him. Unfortunately, not of interest to NBC.

NYTimes: The margin of victory seemed almost impossible. His finishing time, a sport-shattering moment. Just days after Usain Bolt electrified track and field with a world-record run for the ages in the 100 meters, he might have outdone himself in the 200.

Jamaica’s wunderkind surged so far ahead of a stellar Olympic final field Wednesday night that the final 50 meters inspired sheer awe. Running hard through the finish, Bolt not only ran 19.30, breaking the world record by two-hundredths of a second less than two hours before his 22nd birthday, but he seemed to set new parameters on what humans can achieve.

This time, unlike in the 100 meters, Bolt ran hard the entire race, clearly wanting to show what he can do when he is serious. In the 100, he essentially stopped racing with about 10 meters to go, threw out his arms and slapped his chest before he crossed the finish line. That made his time of 9.69 — .03 better than the world record — that much more astounding because it could have been even lower.

In the 200, Bolt overpowered the field in the turn, entering the straighaway with the only question left being how much would he win by and would he break the world record. That was 19.32 seconds, set by the American Michael Johnson at the 1996 Atlanta Games. Before Wednesday night, Johnson’s record run was the only performance under 19.62. Bolt’s previous personal best was 19.67.

“I didn’t think I’d see under .30 in my lifetime,” said Renaldo Nehemiah, a former gold medalist in the 100 hurdles. “He’s a freak of nature. He did it at 14 and he did it at 17. Most people aren’t surprised he did it. They might be surprised he did it here, but it was inevitable.”

More from George Vecsey of the Times; Michael Johnson agrees with me on the singular nature of Bolt’s performance:

NYTimes: …“It was the most impressive athletic performance I have ever seen in my life,” Johnson said Wednesday, before the next one. “It was amazing to watch — especially since I didn’t have to watch from behind.”

…Usain Bolt has made this a two-athlete Olympics. In two bursts of speed, he has matched much of the buzz for Michael Phelps, who won eight medals in the pool. Bolt’s two gold medals were won out in the open, on two feet. The first one was play. The second was work. But both are records. Kiss the old ones goodbye.