The Big Physics Bus Tour

My eyes adjusted to the dim tunnel a couple of beats after I stepped inside from the brilliant midsummer California sky. A brief glance ahead found a tangle of ductwork and instrumentation before me. Then my eyes zoomed off down the endless corridor of the longest, straightest building in the world, built snugly into the dry grasslands here at the Stanford Linear Accelerator Center (SLAC). The view was a perfect illustration of one-point perspective, straight out of a drawing textbook: Vertical supports, ceiling, and floor all blurred into diagonal lines converging at a distant vanishing point.

Of course, the only vanishing point particle physicists care about is where particles and their antimatter twins collide – in a high energy explosion that can help them unlock the mysteries of our universe. Well, technically that would be more of an annihilation point, but who’s counting?

My SLAC tour group was standing in the Klystron Gallery, named for the 240 potbellied microwave amplifiers that accelerate electrons and their antimatter twins, positrons, to near light speed. Two miles down the line the beams of particles collide inside a 1,200-ton, 10-meter-tall, $110 million detector. Every day, billions of collisions are monitored and analyzed by a team of more than 600 particle physicists from 10 countries. The researchers, all part of the BaBar experiment, are seeking those few colorful collisions that might give clues as to why we have no antimatter in the visible universe. The Big Bang theoretically should have birthed equal amounts of anti- and regular matter when it blew 13.7 billion years ago.

It’s an enormous question, but then particle physics is science taken to the extreme. Extreme amounts of data: BaBar produces 700 CDs or half a terabyte worth of collision data per day (unfortunately, the interesting ones are exceedingly rare). Extreme velocities and energies. Extreme money – a multimillion dollar yearly electricity bill, above and beyond nine-digit construction costs.

Yet for the geeky tourist, the only goods on display are extremely large equipment and the empty spaces, such as the linear accelerator, or linac, we are standing in. All of the real action is hidden deep within the beam line and detectors, humming with radiation. The visitor center, where our tour began, offered safer samples of the equipment – klystrons, magnets, detectors and a section of the actual beamline tube – cut open for close-up viewing.

Upon emerging into the daylight, I heard the faint whoosh of cars zipping along Interstate 280, which straddles the linac tunnel. Aerial photos of the linac from the 1960s show that same overpass hovering alone and unconnected, a strange square of highway floating in space. This pre-made construction was done so that the sensitive machinery wouldn’t get knocked around when the rest of the highway was built years later.

The linac kept up its end of the bargain, contributing to three different Nobel prize-worthy results obtained within the first decade after the beam line was first turned on in 1966. The prizes are on display at the visitor center: one for clarifying how protons and neutrons are made up of quarks, and two for the discoveries of new subatomic particles, the charm quark and the tau lepton.

These achievements seem kind of, well, esoteric, even to a former physicist like myself. Let’s just say it’s hard to get excited about a tau lepton or a quark, no matter how charming. Particle physics is not always a science of the people. But SLAC is about to begin a new chapter in its history. Once BaBar shuts down, the linac will stop solving grand mysteries and instead become a big laser. No really. The last third of the beam line will generate radiation for the Linac Coherent Light Source (LCLS), the first hard X-ray laser in the world, scheduled to open in 2009. The LCLS will produce a beam of light 10 billion times brighter than any X-ray source on earth, capable of, for example, viewing chemical reactions at femtosecond (one millionth of a nanosecond) slices.

Already an increasing amount of astrophysics, chemistry and biology research goes on at SLAC, our tour guide told us, owing to the ultrahigh-powered imaging technology that runs off the accelerators. Just a few months after my visit, the 2006 Nobel prize in chemistry went to Stanford professor Roger Kornberg for determining how RNA converts the cell’s DNA code into proteins. Much of his research was conducted at the Stanford Synchrotron Radiation Laboratory at SLAC.

After ogling the linac, the second leg of our bus trek took us to the future site of the LCLS building. This square of parched Californian hillside will house the research station where scientists will set up experiments using the X-ray laser beam. Though ground has not yet been broken, dirt tracks indicate that activity has already begun. With luck, the construction will not bother the red-tailed hawks, mule deer, gopher snakes and occasional mountain lions that call this 426-acre campus their home. I suppose they are used to it, anyways.

Our last bus stop was the Collider Experimental Hall where the SLAC Large Detector followed electron-positron collisions until 1998. Now being dismantled, the immense space is still filigreed with catwalks, ductwork and giant metal panels. The great concrete hall dipped about four floors below ground, rose as many stories above it and approached the length of a football field. It was like a spectacular industrial cathedral, and the stillness made it seem even larger and emptier. “It looks like they just walked away on the last day of the experiment,” one man said.

But despite its vast, dusty decrepitude, there was no room for tears: all particle physics experiments have finite life spans. And if particle physicists don’t understand the importance of creative destruction, then who does?

If you go:

SLAC is located about 40 miles south of San Francisco, California, just off Interstate 280 at the Sand Hill Road exit. All of the information needed to arrange a tour is available at SLAC’s website. Visitors must be 12 years or older, and tours are free. Visits must be arranged in advance, and currently public tours are offered about once a week, though they can be less frequent at times.