Plato was all about the invisible. He believed that reality in its most exquisite state was unknowable, that the most perfect things were too perfect to see. The world of ordinary sensation? That was a crude lie, a dishonest distraction. What Plato wanted to do was lead us out of this dimly lit cave so we could know more than the shadows dancing on the wall.
At some point in the early 20th century, the philosopher king became a theoretical physicist. It turned out that Plato’s pure forms — those unseen things that gave rise to everything else — were made out of subatomic particles, a surreal collection of electrons, neutrinos, gluons, and quarks of all directions. Of course, we can’t actually see these fundamental substances. Instead, scientists have been forced to detect the particles indirectly, as ghostly insinuations in accelerators. This year, on a 27-kilometer track beneath the Swiss and French countrysides, the world’s most powerful particle accelerator went online, enabling the hunt for the most elusive particle yet. The Higgs boson is of ten cal led the “God particle” (to the dismay of most physicists) since it’s thought to compose part of a unifying field pervading all of space. If the Higgs is real, then even the emptiest corners of the universe are filled with an omnipresent field that acts a bit like cosmic glue, giving mass to particles that otherwise wouldn’t have any.
But how do physicists know what to look for? How do you hunt for a figment of a theory? The answer reveals an essential aspect of science, the way the known is leveraged against the unknown — even the deepest mysteries are slowly undressed with empirical facts. Although modern physics often flirts with the surreal — Salvador Dali would have loved string theory — its claims depend on the precision of its predictions. A beautiful equation is merely beautiful until it has been tested. Only then can it be true.
Over the years, at least 16 different fundamental particles have been discovered. While this list of cosmic crumbs is impressive, it remains incomplete. We know that we don’t know everything, if only because the quantum equations still can’t account for gravity, the extreme conditions of a black hole, or even the nature of the universe at the moment it began. The biggest empirical void belongs to the Higgs, which, unfortunately for physicists, often sounds like a paradox perfectly designed to confuse them: It gives mass to matter, yet it remains practically weightless. It is indivisible and pointlike, yet so ephemeral that to say it exists requires one to redefine existence. Can such a thing ever be found? Or is this the one particle that will stay invisible to the end?
That’s the great riddle for the Large Hadron Collider, an instrument whose name is very literal: It is very, very large and will try to collide protons — a type of hadron comprising two up quarks and one down — at a velocity approaching the speed of light. (The particles tend to travel in a straight line, so they need to be guided by superconducting magnets operating at a temperature of -271 ˚C, so cold it makes deep space seem warm.) The hope is that these shards of matter will be transformed into flashes of energy, which will then re-form as slightly heavier shards. (According to the estimates, a Higgs should weigh about 150 times more than a proton.) Most of these collisions will lead to meaningless litter, the usual detritus of smashed particles. But every once in a while, the hadrons should miraculously assemble into something more. It’s like breaking a champagne flute and getting a stained-glass window.
The question, of course, is how to see something that lasts less than 10-24 seconds, this is where the detective work begins. Although the collider contains a vast array of sensory equipment, these sensors are blind to the Higgs. So physicists must search for the particle by looking for the signature of its decay, the collection of subatomic species that are left behind after the Higgs disappears.
Standard theory suggests that Higgs particles will fall apart in a well-choreographed sequence of events. Unless the wreck of hadrons is exactly right — and 99.999 percent of all collisions are wrong — the data is instantly erased. However, when the proton collision manages to create a certain pattern of decay, the detectors record the destruction. The debris is then carefully analyzed for the progeny of the Higgs. The trail of empirical evidence is followed backward towards the crux of theory.
It’s easy to take such epic experiments for granted. But think about how strangely wonderful this is. When confronted with the limits of our science and our senses — the Higgs is defined by its elusiveness — we still find ways to scour the darkness. We build an $8 billion underground microscope so that a set of abstruse equations will finally make sense. We gather specks of nearnothingness and then smash them together to re-create the very origins of the universe. We look at those shadows on the wall and can infer the forms that cast them.
Originally published October 15, 2008