Correspondent's diary: Act three, scene one

Aug 5th 2010, 19:54 by G.C. | STANFORD

THERE are, F. Scott Fitzgerald once suggested, no second acts in American life. Not true in Stanford, though. Here, there has been not only a second act, but a third, for a piece of kit that many would have taken off to the knackers’ yard years ago.

The Stanford Linear Accelerator Centre, SLAC, is almost 50 years old. It was the site of Nobel-prize-winning work in the 1970s. In those days the Linac was to particle physics what the Large Hadron Collider is today – the edge that most precisely cut reality into its component parts. Using it, Burton Richter co-discovered (with Sam Ting, of the Brookhaven laboratory on the other side of the country) the first particle containing a charm quark, and Martin Lewis Perl discovered the tau lepton, a sort of heavy electron. Together, these findings of new fundamental particles unleashed a revolution that led to the modern Standard Model of reality, which explains – at least in a hand-waving way – all of the fundamental particles and forces except gravity.

When the caravan moved on to more powerful machines, the Linac might have been abandoned. Instead, it was refitted as a B-meson factory. B mesons are particles that contain yet another fundamental particle, the bottom quark. Theory suggested that B mesons and their antiparticles should decay in different ways, a necessary part of the explanation for why the universe is made of matter, and antimatter is rare. That, too, was confirmed, and the machine became redundant yet again.

But there is life in the old girl yet. The week after next, America’s energy secretary, Steven Chu, himself a Nobel-prize-winning physicist, will formally open the Linac’s latest incarnation, the Linac Coherent Light Source, the world’s most powerful X-ray laser. Your correspondent, who will be back in London by then, has been given a sneak preview.


X-ray specs

The LCLS works by making pulses of electrons that have been accelerated to close to the speed of light by the Linac undulate. This undulation causes the electrons to give off X-rays, which travel alongside them. The X-rays then stimulate the emission of further X-rays (the reason the system is a laser – which is an acronym for “light amplification by the stimulated emission of radiation”). Once this stimulation has made the X-ray beam powerful enough, the electrons are diverted away from it by magnets and the X-rays go on into various experimental chambers, where they are put to use.

At the moment, the LCLS is looking at the changes which occur in individual molecules during chemical reactions. Such reactions take place in a few femtoseconds (a femtosecond is millionth of a billionth of a second), but the beam’s pulses themselves are only ten femtoseconds long, so they act like the movement-freezing flash of a more conventional camera. Soon, though, the real business will begin, and the X-rays will be used to study the structure of individual protein molecules.

Many proteins have had their structures revealed by X-ray crystallography. This works out the arrangement of atoms in a molecule by seeing how a crystal of that molecule refracts X-rays. Not all proteins will form large enough crystals for conventional crystallography, though, and to do crystallography on a “crystal” that is only a few molecules wide – or even, if all goes well, just a single molecule – needs a particularly powerful, focused and short-pulsed beam of X-rays.

That is the prize that the Linac’s latest incarnation is chasing. If it succeeds, the next Nobel prize to come out of SLAC might, ironically, be not for physics, but for medicine.