An example of simulated data modelled for the Compact Muon Solenoid detector on the Large Hadron Collider (LHC) at CERN. According to the Standard Model of particle physics, the Higgs particle is a boson, a type of particle that allows multiple identical particles to exist in the same place in the same quantum state. The Higgs boson is produced in the collision of two protons.
/ Published August 1, 2012
On July 4, scientists at the Large Hadron Collider (LHC), a particle accelerator in Geneva, Switzerland, announced the most important particle physics discovery in decades. They had found the Higgs boson, often called the “God Particle.” This was the last as-yet-undetected particle predicted in the Standard Model of Particle Physics, our current theory of the 17 particles that fundamentally make up everything in the universe.
So can we now say that we’ve answered the question, “What is everything made of?” Is physics over? And what’s so significant about this particular particle, anyway?
I’m going to address those questions in reverse order, starting with what the Higgs is and what makes it so important. To answer that, let’s go back in time a bit. In the 1960s physicists had theories that could explain almost all of the observed interactions between particles, but these theories predicted that every particle in the universe should have zero mass. That’s easy to refute with observation. Not only would that make it really easy to pick up your suitcases, it would also mean that every particle in the universe would be moving at the speed of light all the time. In fact there would be no suitcases, no people, and no other structures of any kind because it would all fly apart instantly. Everything would behave like the one massless thing we are familiar with, which is light.
Something, then, must be slowing down the particles enough to form structures with mass. There is one important exception to the rule that says massless particles always fly around at the speed of light. To understand that exception, consider the behavior of light itself. A light beam will move at 300 million m/s (which we normally call “the speed of light”) as long as it’s in empty space. Let that light beam enter matter such as glass, water, or air, however, and it will slow down. When it’s inside matter, light acts like it’s made of particles with mass.
To recap: Our theories of particle physics predicted that all particles must be massless. Massless particles move through space at the speed of light, which most observed particles do not. However, massless particles slow down and act like massive particles when they move through something.
All of this together led British theoretical physicist Peter Higgs and several other people to propose a radical new idea about the origins of mass. Perhaps all of space is filled with a uniform field—you can think of it like a fluid—and the particles we see appear to be massive because they are moving through that field. We now call that field the “Higgs field.” When the Higgs field was incorporated into the already-developed theories of particles physics, they perfectly matched what we observe, and the Standard Model was born.
So the LHC set out to observe the Higgs field. The problem is that to observe something, you have to measure a difference from one place to another, but the Higgs field is the same everywhere in space. So to see that the Higgs field is out there they had to make it briefly change in one spot—become a tiny bit stronger or weaker than it is everywhere else—and then measure the effects of that change. Causing that change in the Higgs field required a lot of energy, so they did it by smashing protons together at enormous speeds. The energy of that collision caused the Higgs field to momentarily change. That brief wiggle in the Higgs field—a change that occurs just in one tiny spot—is what we call a “Higgs particle,” or “Higgs boson.”
This year, two different experiments at the LHC independently observed Higgs bosons, thus confirming that everything we are made of is intrinsically massless, and all the structure we see around us is only possible because of this strange field that fills space. With that measurement, every constituent of the Standard Model has now been seen.
So is there nothing left to be discovered? Is physics over? Not by a long shot.
The Standard Model seems to be correct, but it is not a complete description of everything. Here’s a partial list of some of the most important outstanding questions in science today:
- Why does nature contain those particular 17 particles, with those particular properties? Could it be different? Is it different in other parts of the universe so far away we can’t even see them?
- Astronomical measurements reveal that 90 percent of the matter in the universe is in the form of an as-yet-undiscovered particle that we simply call “dark matter.” We don’t know what dark matter is, but we can rule out all of the Standard Model particles, so it must be an additional one that isn’t in the model. What is dark matter?
- Still more astronomical measurements show that two-thirds of the energy in the universe isn’t in the form of matter at all but is instead some kind of radiation that is not part of the Standard Model. Since we have no idea what this is either, we call it “dark energy.” What is dark energy?
- The Standard Model explains most of the known interactions between matter, but it does not explain gravity. Every attempt so far to make a self-consistent theory that includes both the Standard Model and gravity has failed. (Our current best candidate is string theory, but nobody has been able to solve the equations to figure out what it actually predicts.) What’s the overarching theory that explains all of the interactions in nature?
I could certainly make a longer list than that, but the key point is that we are still in the midst of a long journey. Finding the Higgs and thus confirming the Standard Model is a big step, and we should rightly celebrate it, but what we don’t know is still much more than what we do. That’s the most exciting result.
The Higgs Boson Has Been Found...So What Is It and Who Cares? Gary Felder describes particle physics, the Standard Model and the Higgs boson to an audience of high school students.
Science at the Center: What Is the Universe Made Of? Science at the Center is a series of 10-minute informal lunch-time talks given by Smith College faculty.
Gary Felder is an associate professor of physics at Smith College.