~ July, 2012 ~


Why I’m not excited about the Higgs boson

A friend at work (@obviouscorp) asked me why the Higgs boson was such a big deal. And as nearly as I can tell, the answer is “It’s not.”

Now don’t get me wrong. I think the discovery of a particle we have been hunting down for 50 years is a big deal. But I’m still not excited.


I happen to have a PhD in physics, but I left the field right after getting my degree almost 20 years ago. So I’m basically a layman at this point, but I do have a background in some of this stuff.

I’ve done a good amount of digging into articles, forums, etc., and I find that a lot of the information out there is confusing. This is my current understanding of what the Higgs actually is. It think it’s basically right, but there is a chance that I have some of it wrong. If anyone who is current on the research can help correct any misperceptions I have, that would be awesome.

Different kinds of mass

Roughly speaking, scientists are interested in the Higgs field because it helps explain where mass comes from. But the word “mass” can mean many things.

The term “inertial mass” is the idea of how hard it is to push something. If something is massive, you need to apply more force in order to move it.

The term “gravitational mass” is the idea of how strongly a piece of matter attracts other things with mass. If something is massive, it pulls you toward itself.

As it turns out, these two concepts of mass are identical. In other words, if something is twice as heavy in terms of “hard to push”, its gravity also pulls twice as hard.

The confusing molasses analogy

Many science writers talk about the Higgs field giving mass to particles by “slowing them down” like marbles traveling through molasses. This is obviously just a colorful metaphor, but I took this to mean that the Higgs field explained the concept of inertial mass, i.e., why massive things are hard to push.

If that were true, it would be super exciting, because it’s such a fundamental concept. But I couldn’t see how it could possibly be true, for a bunch of reasons.

As it turns out, that’s not what scientists are saying. To understand what scientists are actually saying, you have to know a little bit about our current view of the world of subatomic particles.

Mass, vs. other properties

As far as we know, the “stuff” in the world is made up of quarks and leptons. An electron is an example of a lepton. There are six leptons total (or 12, if you count anti-particles). There are also six kinds of quarks (or 12, if you count anti-quarks). Finally, there are four force mediating particles (e.g, photons, gluons, etc.).

All of these particles have properties, like “charge”. An electron has a -1 charge. An up quark has a +2/3 charge.

These properties tend to come in exact increments. For example, the spin of a particle can be 1/2, 1, -1/2, etc. Nothing in between.

Mass is different. The masses of particles have totally bizarre values. Photons have zero mass. Neutrinos (probably) have mass, but it is such a tiny, tiny amount of mass that we have trouble detecting their existence. Meanwhile, the top quark has as mass of 170GeV or so. That’s something like 100 billion times heavier than a neutrino.

So scientists look at that and say “why is mass so weird?”

Rest mass vs. energy

The masses of particles I referred to above is more properly called “rest mass”. We use that specific term because energy also counts toward “mass”. If an electron is at rest, it has .5 MeV of mass. But if it’s moving really fast, that electron has more mass because of the energy of its motion.

All kinds of energy contributes to mass. For example, let’s look at protons.

A proton is composed of two up quarks and a down quark. But if you add up the “rest mass” of two up quarks and a down quark, you end up with much, much less mass than the mass of a proton. That’s because 99% of the mass of a proton comes from the energy that binds all three quarks together. Weird, right? 99% of the mass is just energy. 1% comes from the rest mass of the stuff inside.

No more rest mass

Ok… with all that in mind, here’s my current understanding of what it means for the Higgs field to be the source of mass.

The Higgs field lets you replace the rest mass of particles with another energy term, which means that all particles are massless, and there is no such thing as “rest mass”. That makes our equations cleaner, and gets rid of a messy concept (rest mass).

That’s pretty fundamental, so I guess I understand why people are excited.

Why do you keep saying “Higgs field” instead of “Higgs boson”?

In the standard model, all fields are mediated by force-carrying particles. The electromagnetic force is mediated by photons, and the strong force is mediated by gluons. It’s kind of complicated, but the force and the particle are kind of the same thing.

When you talk about stuff that comes out of particle accelerators, you tend to talk about the particles (Higgs boson). When you talk about how they affect things in the world, you tend to talk about fields or forces (the Higgs field).

But it’s all the same thing.

Why I’m not excited

Ok. So we found something that looks like the Higgs boson, which gives more weight to this theory that the Higgs field is the source of the rest mass of fundamental particles.

We still have the question of why the masses of particles are so different from one another. Before, we said “we have no idea what mass is, and why the values are so weird.” Now, we say “rest mass comes from the Higgs field, and we have no idea why the coupling constants are so weird.” (coupling constant is just a fancy way of saying “how much each kind of particle is affected by the Higgs field”)

Now the normal answer to the question of why the coupling constants got to be the way they are is “spontaneous symmetry breaking”, which is a fancy way of saying “it happened a long time ago, and was basically random”.

I guess that’s progress, but it’s not super satisfying.

Another way to read the news is something like this: “Our model for how the universe works is still basically correct”.

I guess that’s news, but I would have been more excited by the opposite result.