Computations from String Theory to the Large Hadron Collider

Yet the results Witten obtained led to the Britto, Cachazo, and Feng papers, including the one written with Witten himself, which managed to pull from the string theory developments some key insights that were needed for more general calculations.  From there we follow the results to BlackHat, whose leaders all did some amount of string theory early in their careers but who turned by choice to practical calculations and invented many new techniques themselves.  They’re exactly the sort of people you’d expect to dismiss these efforts by string theorists as naive.  But no. They credit Britto et al. prominently for a key insight that makes BlackHat possible.  And finally we arrive back at the dinner table, with me listening to Joe Incandela, who, fresh from the completion of the CMS paper on the observation of the Higgs-like particle, is praising BlackHat for its contribution to searches for new phenomena in CMS data.  It’s only a few steps from Incandela to Witten — from experiment to the most apparently-useless edge of string theory.

This is a really insightful article describing the place of String Theory in physics today. If you are at all interested in the combination of physics and computation, you’ll learn something about current high energy physics research, the place of computation in understanding physical theories, and the worth of String Theory as a sub-discipline in physics.

Highly recommended. Read the whole thing.


Dark Matter, now we see evidence in local space.

Currently, the most solid evidence of dark matter comes from  analyzing the Cosmic Microwave Background, and from the observed flatness of the galactic rotation curves. It is less known than in our galaxy the support for dark matter comes from studying the rotation curves at distances of 20 kpc or more from the galactic center. In the immediate neighborhood of the Sun (8 kpc from ground zero), the presence of dark matter is more difficult to deduce.

The recent paper by Moni Bidinattempted to measure the local Dark Matter density by observing gravitational effects on stellar motion near the Sun. The basic idea was to look at the orbits of stars above the galactic plane; if there is a Dark Matter halo around our galaxy, the matter inferred from gravitational effects on the orbits of visible stars will continue to grow, even as we look at stars farther above the galactic plane. The Moni Bidin paper found that stellar orbits implied only effects from the visible matter in the galactic plane. However…


A new paper by Bovy and Tremaine examine an assumption of the Moni Bidin paper and find it is in error. Correcting for this error restores consistency between the observed data and the Dark Matter halo hypothesis. The idea behind the Moni Bidin paper is sound, and in fact provides as far as I know the first observational evidence for local Dark Matter. In any case, we should soon have new results by examining other surveys of stellar motion.

Distributed production of radioactive medical isotope Tc 99 may solve longstanding problem.

We are currently using a centralized production model for this isotope with just a six hour half-life. This model involves just a handful of dedicated, government-funded research reactors, producing molybdenum-99 from highly enriched uranium (which is another issue for another time). Moly, as we’ve come to affectionately call it, decays via beta emission to technetium, and when packaged into alumina columns, is sterilized, and encased in a hundred pounds of lead. It is then shipped by the thousands to hospitals around the world. The result: the world has come to accept Tc-99m, which is used in 85% of the 20 to 40 million patient scans every year as an isotope available from a small, 100 pound cylinder that was replaced every week or so, without question, without worry. Moly and her daughter were always there…but in 2007 and again in 2009, suddenly they weren’t. The world had come to realize that something must be done.

Distributed solutions can be more robust than centralized solutions, especially if coordination problems plague the central model. Here is a great example of how distributed production of Tc 99 may be superior to centralized manufacturing.

The great weakness of distributed models centers around communication, and when the task being distributed requires little to no communication, distributed solutions usually scale very well (e.g., image processing).

Cosmologists Try to Explain a Universe Springing From Nothing –

There is a deeper nothing in which even the laws of physics are absent. Where do the laws come from? Are they born with the universe, or is the universe born in accordance with them? Here Dr. Krauss, unhappily in my view, resorts to the newest and most controversial toy in the cosmologist’s toolbox: the multiverse, a nearly infinite assemblage of universes, each with its own randomly determined rules, particles and forces, that represent solutions to the basic equations of string theory — the alleged theory of everything, or perhaps, as wags say, anything.

This is deeply unsatisfying, for reasons I’ll get to in a minute. Dr. Krauss is following in the tradition of Pierre-Simon Laplace:

Laplace went in state to Napoleon to present a copy of his work, and the following account of the interview is well authenticated, and so characteristic of all the parties concerned that I quote it in full. Someone had told Napoleon that the book contained no mention of the name of God; Napoleon, who was fond of putting embarrassing questions, received it with the remark, ‘M. Laplace, they tell me you have written this large book on the system of the universe, and have never even mentioned its Creator.’ Laplace, who, though the most supple of politicians, was as stiff as a martyr on every point of his philosophy, drew himself up and answered bluntly, Je n’avais pas besoin de cette hypothèse-là. (“I had no need of that hypothesis.”) Napoleon, greatly amused, told this reply to Lagrange, who exclaimed, Ah! c’est une belle hypothèse; ça explique beaucoup de choses. (“Ah, it is a fine hypothesis; it explains many things.”)

The deeply unsatisfying part is hinted at in the NYT article: “But even the multiverse is not totally lawless, as Dr. Krauss acknowledged. We are not quite there yet. At the very least, there would still be the string equations and those quantum principles that undergird them.

Ah yes, because if we have nothing at all, we have nothing at all. Nothing that can fluctuate, nothing that describes strings, not even the abstract entities of mathematics, such as randomness. We just have… nothing. No existence of any kind, abstract or concrete. How can existence emerge from nonexistence? This puzzle is in fact a riddle wrapped in an mystery inside an enigma. Lengthening the chain from the Multiverse of today to Nothing At All doesn’t remove the paradox.

Why Skepticism in Science isn’t just Politics

The reasons for the intense scepticism about OPERA are both general and specific.  The general reasons stem from the track record of experiments on the frontiers of science, which is pretty dismal.  This is not because experimentalists are careless or foolhardy (well, occasionally this happens) but because doing first-of-a-kind experiments, using new and clever methods and the latest technology, is extremely difficult, and prone to unforeseen problems.  And statistical flukes can always happen, too.  Everyone who has worked in high-energy physics for a while knows that the vast majority of exciting results, even from the best experimentalists, simply don’t hold up over time.  I made an informal list over the weekend of false alarms that have occurred during the nearly 30 years that I’ve been following or actually doing high-energy physics, and came up with nearly two dozen separate incidents — and I keep thinking of new ones.  [I may do some writing later this week about how some of these “discoveries” went awry.]  Meanwhile I can think of only three actual discoveries that survived, one of which (the top quark) was expected, one of which (neutrino oscillations) was pretty exciting but not unexpected, and only one of which really violated the prejudices of my field.  The last — the only real shocker to occur during my career — won this year’s Nobel Prize: the discovery that the universe’s expansion is accelerating instead of decelerating.

First, read the whole article by Dr. Strassler. I’ll wait.

OK. Prof. Strassler is exactly correct; whenever interesting results come out in physics, it pays to be skeptical. This isn’t because physicists want to protect the current paradigm, but a response born of long experience. Most interesting results have a good chance of being wrong. Nature always has the last say, and if an interesting result can be replicated, well, everyone wants to be part of a physics revolution. But if a result cannot be reproduced… it doesn’t matter how beautiful the math or how much explanatory power a theory has, at the end of the day, we can only accept those explanations that match up with the behavior of Nature. Feynman said it best: “It doesn’t matter how beautiful your theory is or how smart you are, if it doesn’t agree with experiment, it is wrong.”

Natural science has this wonderful property that an objective standard exists for judging the correctness of explanations. The behavior of Nature cannot be dismissed.

The 2011 Nobel Prize in Physics

For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the Universe will end in ice.

The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma – perhaps the greatest in physics today. What is known is that dark energy constitutes about three quarters of the Universe. Therefore the findings of the 2011 Nobel Laureates in Physics have helped to unveil a Universe that to a large extent is unknown to science. And everything is possible again.

This year’s Nobel in Physics goes to Saul Perlmutter, Brian Schmidt, and Adam Riess for their discovery that the expansion of the Universe is accelerating, not decelerating as expected. This result came from an examination of distant supernova, and has been confirmed by an examination of the cosmic microwave background and the dynamics of galaxy clusters.

Dr. Perlmutter led a team studying distant type Ia supernova occurring in binary star systems in which a white dwarf accretes matter from the evolving companion. As the white dwarf’s mass grows, it becomes unstable, and eventually destroys itself in a huge blaze of energy which can be seen across the Universe. These explosions are amazingly uniform and consistent in energy, which enable astronomers to use them as “standard candles”, or objects of known absolute brightness. By measuring the observed peak brightness, the distance to the supernova can be computed.

Drs. Schmidt and Riess started a second search for high-z, or distant, supernova a few years later. Both teams came to similar but astonishing conclusions; the Universe’s expansion is not decelerating, but rather is speeding up over time.

Solar neutrinos and flavor Physics

By the 1960s physicists thought they had a pretty good understanding of the nuclear reactions that caused the sun to shine. One of the many predictions of their model is the number of neutrinos emitted by the sun.

A scientific model is only as good as its the experimental verification of its predictions, so the next step was to actually count these solar neutrinos. Of course, our readers already know that neutrinos are very weakly interacting and this makes them very hard to detect.

A nice, nontechnical explanation of the solar neutrino problem. I especially like the way Flip tells us where the simple explanation is misleading. So if you are curious about how we know neutrinos oscillate between three different states, enjoy!

Ultra High Energy Cosmic Rays

The origin of ultra-high-energy cosmic rays is a question that makes it onto many top-unsolved-problems-in-physics lists.

The math says that these particles, which carry in excess of 1019 electron volts, or eV,  from somewhere in outer space, are far too energetic not to have interacted themselves out of existence before reaching the earth. And yet every year scientists see evidence in the earth’s atmosphere of a handful of these particles, which have several million times the energy of the protons being collided at the Large Hadron Collider.

The TAUWER experiment looks at particle showers that originate from tau neutrinos that take short paths through the earth (inset). An ideal site to observe these events is a mountain bowl. Image courtesy of Maurizio Iori.


Cosmic rays with these amazing energies are an existence proof that the LHC won’t trigger unusually dangerous physics at energies it can reach. The big question is how these particles obtain their energy, and what keeps them intact till they reach Earth.