Web edition: July 4, 2012
By now, all aficionados of physics news — and quite a few people who don’t know physics from phonics — have heard about the discovery of the Higgs boson. It’s the biggest news in physics ever tweeted. And it came after a long wait. For more than three decades, the Higgs has been physicists’ version of King Arthur’s Holy Grail, Ponce de Leon’s Fountain of Youth, Captain Ahab’s Moby Dick. It’s been an obsession, a fixation, an addiction to an idea that almost every expert believed just had to be true.
But despite years of searching, using the most complex machines ever built on the planet, the Higgs remained as elusive as a World Series ring for a Chicago Cub. Until now. Physicists at the Large Hadron Collider have finally established the existence of a new particle, weighing in at a mass of about 11 dozen protons. Although the official announcement of the new particle was cautiously worded, everybody assumes it’s the Higgs.
Asked why the Higgs boson is so important, most physicists reflexively respond that it’s a piece of the cosmic substance that endows elementary particles with mass. That perhaps, to some, sounds a bit underwhelming — just another culprit to blame for the obesity epidemic. But the Higgs’ importance should be expressed more dramatically.
"We're reaching into the fabric of the universe at a level we've never done before," says Joe Incandela, a physicist at the University of California, Santa Barbara and spokesperson for one of the experimental teams reporting the discovery. "This is telling us something that's a key to the structure of the universe."
In fact, the Higgs is responsible for the structure of the universe as we know it. It's the Higgs that makes physical reality the way it is, with atoms, chemical reactions and life. No Higgs, no molecules. No planets. No people.
Strictly speaking, it’s better to say that without the Higgs, something even more exotic would have to do its job. That job, in physics speak, is “electroweak symmetry breaking.” In the universe’s earliest picoseconds, electromagnetism was a component of a more primordial “electroweak” force, incorporating what’s now called the weak force (known for its role in radioactivity). Equations describing the electroweak force are symmetric — that is, they describe electromagnetism and the weak force as equals. But somehow, the weak force split from electromagnetism. In other words, this mathematical symmetry between electro- and weak forces was “broken.”
Symmetry in nature’s laws is not optional; it ensures that the laws work the same for everybody, no matter where they are or how they move. But real life can get messy if something disrupts the symmetry. That’s what the Higgs does: It puts the universe on course to create reality’s complexities.
“In seeking the agent of electroweak symmetry breaking, we hope to learn why the everyday world is as we find it: why atoms, chemistry, and stable structures can exist,” writes theoretical physicist Chris Quigg of Fermilab.
Mathematically, the Higgs boson is a consequence of equations describing a field of force, the Higgs field. Visually, it’s not so easy to describe. Like the magnetic field around a magnet, the Higgs field exerts its influence without being visible.
Also like a magnetic field, the Higgs field’s strength falls to zero when the temperature is too high. (Heat an iron bar magnet above 770° Celsius, and the magnetism vanishes.) So at the birth of the universe in the Big Bang, when temperatures exceeded a million billion trillion degrees, the Higgs field did not distinguish itself. It and everything else that the universe was destined to contain existed within an undifferentiated primordial fireball of explosive energy.
You could think of the infant cosmos as a huge container of hot steam, so hot that steam was all there was, nothing else. If you watch as steam cools, you’ll eventually see some droplets of water begin to form. And if you wait long enough, sooner or later some ice crystals emerge as well. It is, of course, all the same stuff (H2O), simply boiled into a featureless form. In a similar way, the featureless newborn universe was all the same stuff — in this case, stuff that would become all the species of the standard model of particle physics. Quarks (various flavors and colors), leptons (electrons, neutrinos and their cousins), gluons (for holding quarks together), various bosons (for transporting forces back and forth), everything that makes up everything in the world today was waiting to materialize like ice out of the primordial steam. And, like pure radiation, all these entities possessed only energy, no mass.
As the universe expanded and cooled, the particles of matter — a roster using up most of the letters in the Greek alphabet — began to appear. Quarks, for instance, congealed out of the primordial haze, announcing the arrival of the strong nuclear force, no longer indistinguishable from other forces. But still these particles possessed no mass. Like smooth steel balls rolling over perfectly slick ice, nothing resisted their motion. Resistance to motion is inertia. Inertia is the hallmark of mass. No resistance, no inertia, no mass.
In a world without mass, protons and neutrons would form, but electrons would refuse to orbit them. So atoms and molecules could not exist. None of the features of the familiar world would appear.
Less than a nanotick of the cosmic clock later, though, the grandest event in the universe since its birth changed the game. Higgs stuff condensed into a new form. Just as a sufficiently low temperature permits an iron bar’s magnetism, a sufficiently cool universe turned the Higgs field into something that matter had to contend with. Rather than skimming effortlessly over ice, particles now had to swim through a thick ocean, facing resistance to their motion, thereby acquiring mass. And the universe was never to be the same again.
In essence, the Higgs field split the electroweak force’s personality. Photons, transmitters of electromagnetic influence, were oblivious to the newly palpable Higgs field, and so continued on in their merry massless way, letting there be light. Transmitters of the weak part of the electroweak force, two W particles (one positively charged, one negative) and the neutral Z particle felt the Higgs force dramatically. While the photon remained massless, for the W’s and the Z, flying though space became more like swimming through molasses. Similarly, quarks felt the Higgs’ presence, also acquiring mass (although not in precisely the same way).
Particles have different masses because they interact with the Higgs field to different degrees. At a nontechnical level, physicists sometimes speak of the field as a flock of paparazzi. Massive particles are like Hollywood celebrities — the paparazzi impede their path. The more famous, the more paparazzi get in the way, so the greater the resistance (or the mass). B actors (lightweights) pass through the paparazzi crowds much more quickly. Massless photons cannot even be linked to Kevin Bacon. They are invisible to the paparazzi.
At least, that’s the story physicists had been telling themselves. It’s so compelling, mathematically and aesthetically, that most experts believed that nature had to follow the script. But doubts nagged many who knew history. An imponderable substance filling all of space, responsible for fundamental physical phenomena? A good description of the ether, the 19th century version of the Higgs field. It turned out that the ether didn’t exist. Some feared the same fate for the Higgs.
But this time came success. Smashing protons at more than 99.999999 percent of the speed of light infused the Higgs field with trillions of electron volts of energy, enough to shake loose the field’s signature particle, the Higgs boson. While its life is short, the daughter particles of the Higgs’ decay register their births in the Large Hadron Collider’s detectors, and the Higgs’ brief presence can be deduced, confirming the reality of its field.
Scottish physicist Peter Higgs conceived of such a field in 1964 and predicted the particle’s existence. He wasn’t the only physicist of that era to devise similar mathematical scenarios. Still others showed how the Higgs idea could orchestrate the breaking of electroweak symmetry. All those participants in elucidating the Higgs’ role in reality shared a common prescience, an ability to see deeply into nature through the lens of mathematics.
Their success illustrates a further meaningfulness of the Higgs discovery: It validates the scientific enterprise as a way of knowing nature. Somehow, humans fiddling with squiggles on paper figured out what you would find if you spent billions of dollars on a machine to create temperatures of a million billion degrees. Scientists figured out one of nature’s deepest secrets just by using their heads.
“This is an enormous triumph for mathematical methods to make predictions for things in the real world,” says physicist Brian Greene. “This Higgs particle has been a hypothetical mathematical symbol in our equations for 40 years.”
During that time most physicists came to believe in the Higgs boson’s existence as an article of scientific faith. Without it, something would be desperately wrong with the entire framework of science’s understanding of the universe. Had the Higgs boson not materialized when the Higgs field was properly probed, it would have been as though Voldemort had succeeded in killing Harry Potter.
Harry triumphed, though, and so has the Higgs. Happily, however, the Higgs discovery is not the last chapter in nature’s final book. There will be sequels. Physicists need more particles, not included in the standard set, to explain mysteries like the abundance of dark matter in space and how gravity fits in with the rest of nature’s forces. And the Higgs boson now discovered may merely be one member of a much larger Higgs family, with cousins performing various other important jobs in constructing the universe.
"At some level we're pretty sure that the standard model is not the full picture," says Incandela. "We're on the frontier now, we're on the edge of a new exploration. Maybe we see nothing extraordinary ... or maybe we open up a whole new realm of discovery."
Science News coverage of the search for the Higgs boson: [Go to]