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Thursday, January 17, 2013

Reading List: The Particle at the End of the Universe

Carroll, Sean. The Particle at the End of the Universe. New York: Dutton, 2012. ISBN 978-0-525-95359-3.
I believe human civilisation is presently in a little-perceived race between sinking into an entropic collapse, extinguishing liberty and individual initiative, and a technological singularity which will simply transcend all of the problems we presently find so daunting and intractable. If things end badly, our descendants may look upon our age as one of extravagance, where vast resources were expended in a quest for pure knowledge without any likelihood of practical applications.

Thus, the last decade has seen the construction of what is arguably the largest and most complicated machine ever built by our species, the Large Hadron Collider (LHC), to search for and determine the properties of elementary particles: the most fundamental constituents of the universe we inhabit. This book, accessible to the intelligent layman, recounts the history of the quest for the components from which everything in the universe is made, the ever more complex and expensive machines we've constructed to explore them, and the intricate interplay between theory and experiment which this enterprise has entailed.

At centre stage in this narrative is the Higgs particle, first proposed in 1964 as accounting for the broken symmetry in the electroweak sector (as we'd now say), which gives mass to the W and Z bosons, accounting for the short range of the weak interaction and the mass of the electron. (It is often sloppily said that the Higgs mechanism explains the origin of mass. In fact, as Frank Wilczek explains in The Lightness of Being [March 2009], around 95% of all hadronic mass in the universe is pure E=mc² wiggling of quarks and gluons within particles in the nucleus.) Still, the Higgs is important—if it didn't exist the particles we're made of would all be massless, travel at the speed of light, and never aggregate into stars, planets, physicists, or most importantly, computer programmers. On the other hand, there wouldn't be any politicians.

The LHC accelerates protons (the nuclei of hydrogen, which delightfully come from a little cylinder of hydrogen gas shown on p. 310, which contains enough to supply the LHC with protons for about a billion years) to energies so great that these particles, when they collide, have about the same energy as a flying mosquito. You might wonder why the LHC collides protons with protons rather than with antiprotons as the Tevatron did. While colliding protons with antiprotons allows more of the collision energy to go into creating new particles, the LHC's strategy of very high luminosity (rate of collisions) would require creation of far more antiprotons than its support facilities could produce, hence the choice of proton-proton collisions. While the energy of individual particles accelerated by the LHC is modest from our macroscopic perspective, the total energy of the beam circulating around the accelerator is intimidating: a full beam dump would suffice to melt a ton of copper. Be sure to step aside should this happen.

Has the LHC found the Higgs? Probably—the announcement on July 4th, 2012 by the two detector teams reported evidence for a particle with properties just as expected for the Higgs, so if it turned out to be something else, it would be a big surprise (but then Nature never signed a contract with scientists not to perplex them with misdirection). Unlike many popular accounts, this book looks beneath the hood and explores just how difficult it is to tease evidence for a new particle from the vast spray of debris that issues from particle collisions. It isn't like a little ball with an “h” pops out and goes “bing” in the detector: in fact, a newly produced Higgs particle decays in about 10−22 seconds, even faster than assets entrusted to the management of Goldman Sachs. The debris which emerges from the demise of a Higgs particle isn't all that different from that produced by many other standard model events, so the evidence for the Higgs is essentially a “bump” in the rate of production of certain decay signatures over that expected from the standard model background (sources expected to occur in the absence of the Higgs). These, in turn, require a tremendous amount of theoretical and experimental input, as well as massive computer calculations to evaluate; once you begin to understand this, you'll appreciate that the distinction between theory and experiment in particle physics is more fluid than you might have imagined.

This book is a superb example of popular science writing, and its author has distinguished himself as a master of the genre. He doesn't pull any punches: after reading this book you'll understand, at least at a conceptual level, broken symmetries, scalar fields, particles as excitations of fields, and the essence of quantum mechanics (as given by Aatish Bhatia on Twitter), “Don't look: waves. Look: particles.”

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