Tuesday, July 17, 2012

How the Universe got its mass: the Higgs explained (better)

By now, you probably know what the Higgs boson does — it gives everything in the universe mass. What you probably don’t know is why, and you could be forgiven for that given all the terrible explanations out there, including an uncharacteristically confusing and visually displeasing one by the guy that does the Ph.D. comics.

Today, I’m going to explain the Higgs in detail. I’m late to the party, but just because you’re late to the party doesn’t mean you don’t have the best, um, fun. Let’s get started.

What is mass? Mass is energy that slows you down.We usually think of mass as the property of matter that gives it weight, and that’s partly true — things need mass in order to have weight — but mass itself is something more. Rather than giving things weight, what mass really gives things is energy.
Now, there are all kinds of energy. There’s kinetic energy, the sort you have when a ball is bouncing around, and there’s chemical energy, the sort that’s stored in the bonds between atoms. Mass is a special kind of energy, a sort of minimum energy below which you can’t go under any circumstances.

That particle-physics particles have mass has special consequences. Most important, mass prevents them from traveling at the speed of light — only zero-mass particles like photons, which carry energy in the form of light, can do that. Having mass, it turns out, means that not all of a objects energy can go into kinetic energy. In a sense, mass is a special kind of energy that actually slows things down. Keep that in mind.

Just because you have no energy doesn’t make you weak. Now step way back, forget about particles for a minute, and picture a lake with an island in the middle of it. When the weather is very stormy, the water in the lake might go anywhere left, right, forwards, and backwards, and if the storm is strong enough, water can go up and over the island. When the weather is calm, the water in the lake can’t go anywhere it likes — it has to stay in the trough that encircles the island. 

This observation goes by the funny name spontaneous symmetry breaking: when the lake is calm, the water can’t go anywhere it likes, but when the lake is stormy, it’s free to roam, as water should be.

Our lake is a metaphor for particle physics: the water in the lake represents a quantum field, such as the electroweak fields that hold protons and neutrons together. How stormy the lake gets is this field’s energy, and how far the water flows from the center of the island is the field’s strength. 

Now when the weather is most calm, the water in the lake is always some distance away from the center of the island. For quantum fields that act like the water in our lake — for fields that experience spontaneous symmetry breaking — we have an incredible result: these fields remain strong even when they have the least energy. The electromagnetic fields that carry light aren’t like that — you need energy to shine a bright light — but the field that represents the Higgs boson is.

The Higgs boson is a force field that pulls on everything.
Did he just write “the field that represents the Higgs”?

Although you’ve heard a lot of talk about Higgs, the particle, particles and fields are really interchangeable ideas. If it’s convenient, we can talk about a field pulling on a particle. For example, a gravitational field pulls on all kinds of things — bosons, leptons, and you and me — but we can also think of it as a particle called the graviton.

When fields pull on particles and other things, they very often slow them down. Gravity is like that. If you throw a ball in the air, gravity slows its upward progress, eventually turning it around and making it fall back down.

The Higgs is something special. It pulls on pretty much everything, and it pulls in a way that always — always — slows them down. See where this is going?

How the universe got its massWe now have three facts that together explain the origins of mass.
  1. Mass is a special kind of energy that slows things down.
  1. Because of spontaneous symmetry breaking, the Higgs is strong even when it doesn’t have much energy.
  1. The Higgs pulls on everything, and when it pulls on a particle, it always slows them down.

And voila — because the Higgs always pulls hard on other particles and always slows them down, it makes those particles behave as if they had mass — which is to say, they actually do have mass. (In physics, to behave as if you have something is the same as having it. Wouldn’t it be nice if it applied to daily life?)

Detecting the HiggsThe last piece of the puzzle you may be wondering about is why it took so long to detect the Higgs boson, and why those wacky physicists went looking for a particle rather than a field. 

Spontaneous symmetry breaking is responsible once more. At low energies, the Higgs isn’t really itself. It’s constrained, and some of its essential properties are all tied up in giving things mass, which is quite a workout. 

To really see the Higgs as itself, you have to go to really high energies, at which it no longer makes sense to talk about the Higgs as a field. Instead, it makes sense to talk about it as a particle, or rather, as a particle that collides with other particles and eventually breaks up into thousands of other particles — the sorts of particles that after half a century of hoping, we’ve finally seen.