These Ain’t Democritus’s Particles

Physicists talk a lot about fundamental particles. But what do we mean by fundamental?

The Ancient Greek philosopher Democritus thought the world was composed of fundamental indivisible objects, constantly in motion. He called these objects “atoms”, and believed they could never be created or destroyed, with every other phenomenon explained by different types of interlocking atoms.

The things we call atoms today aren’t really like this, as you probably know. Atoms aren’t indivisible: their electrons can be split from their nuclei, and with more energy their nuclei can be split into protons and neutrons. More energy yet, and protons and neutrons can in turn be split into quarks. Still, at this point you might wonder: could quarks be Democritus’s atoms?

In a word, no. Nonetheless, quarks are, as far as we know, fundamental particles. As it turns out, our “fundamental” is very different from Democritus’s. Our fundamental particles can transform.

Think about beta decay. You might be used to thinking of it in terms of protons and neutrons: an unstable neutron decays, becoming a proton, an electron, and an (electron-anti-)neutrino. You might think that when the neutron decays, it literally “decays”, falling apart into smaller pieces.

But when you look at the quarks, the neutron’s smallest pieces, that isn’t the picture at all. In beta decay, a down quark in the neutron changes, turning into an up quark and an unstable W boson. The W boson then decays into an electron and a neutrino, while the up quark becomes part of the new proton. Even looking at the most fundamental particles we know, Democritus’s picture of unchanging atoms just isn’t true.

Could there be some even lower level of reality that works the way Democritus imagined? It’s not impossible. But the key insight of modern particle physics is that there doesn’t need to be.

As far as we know, up quarks and down quarks are both fundamental. Neither is “made of” the other, or “made of” anything else. But they also aren’t little round indestructible balls. They’re manifestations of quantum fields, “ripples” that slosh from one sort to another in complicated ways.

When we ask which particles are fundamental, we’re asking what quantum fields we need to describe reality. We’re asking for the simplest explanation, the simplest mathematical model, that’s consistent with everything we could observe. So “fundamental” doesn’t end up meaning indivisible, or unchanging. It’s fundamental like an axiom: used to derive the rest.

Want to Make Something New? Just Turn on the Lights.

Isn’t it weird that you can collide two protons, and get something else?

It wouldn’t be so weird if you collided two protons, and out popped a quark. After all, protons are made of quarks. But how, if you collide two protons together, do you get a tau, or the Higgs boson: things that not only aren’t “part of” protons, but are more massive than a proton by themselves?

It seems weird…but in a way, it’s not. When a particle releases another particle that wasn’t inside it to begin with, it’s actually not doing anything more special than an everyday light bulb.

Eureka!

How does a light bulb work?

You probably know the basics: when an electrical current enters the bulb, the electrons in the filament start to move. They heat the filament up, releasing light.

That probably seems perfectly ordinary. But ask yourself for a moment: where did the light come from?

Light is made up of photons, elementary particles in their own right. When you flip a light switch, where do the photons come from? Were they stored in the light bulb?

Silly question, right? You don’t need to “store” light in a light bulb: light bulbs transform one type of energy (electrical, or the movement of electrons) into another type of energy (light, or photons).

Here’s the thing, though: mass is just another type of energy.

I like to describe mass as “energy we haven’t met yet”. Einstein’s equation, $E=mc^2$, relates a particle’s mass to its “rest energy”, the energy it would have if it stopped moving around and sit still. Even when a particle seems to be sitting still from the outside, there’s still a lot going on, though. “Composite” particles like protons have powerful forces between their internal quarks, while particles like electrons interact with the Higgs field. These processes give the particle energy, even when it’s not moving, so from our perspective on the outside they’re giving the particle mass.

What does that mean for the protons at the LHC?

The protons at the LHC have a lot of kinetic energy: they’re going 99.9999991% of the speed of light! When they collide, all that energy has to go somewhere. Just like in a light bulb, the fast-moving particles will release their energy in another form. And while that some of that energy will add to the speed of the fragments, much of it will go into the mass and energy of new particles. Some of these particles will be photons, some will be tau leptons, or Higgs bosons…pretty much anything that the protons have enough energy to create.

So if you want to understand how to create new particles, you don’t need a deep understanding of the mysteries of quantum field theory. Just turn on the lights.

What are colliders for, anyway?

Above is a thoroughly famous photo from ATLAS, one of six different particle detectors that sit around the ring of the Large Hadron Collider (or LHC for short). Forming a 26 kilometer ring spanning a chunk of southern France and Switzerland, the LHC is the biggest experiment of its kind, with the machine alone costing around 4 billion dollars.

But what is “its kind”? And why does it need to be so huge?

Aesthetics, clearly.

Explaining what a particle collider like the LHC does is actually fairly simple, if you’re prepared for some rather extreme mental images: using incredibly strong magnetic fields, the LHC accelerates protons until they’re moving at 99.9999991% of the speed of light, then lets them smash into each other in the middle of sophisticated detectors designed to observe and track everything that comes out of the collision.

That’s all well and awesome, but why do the protons need to be moving so fast? Are they really really hard to crack open, or something?

This gets at a common misunderstanding of particle physics, which I’d like to correct here.

When most people imagine what a particle collider does, they picture it smashing particles together like hollow shells, revealing the smaller particles trapped inside. You may have even heard particle colliders referred to as “atom smashers”, and if you’re used to hearing about scientists “splitting the atom”, this all makes sense: with lots of energy, atoms can be broken apart into protons and neutrons, which is what they are made of. Protons are made of quarks, and quarks were discovered using particle colliders, so the story seems to check out, right?

The thing is, lots of things have been discovered using particle colliders that definitely aren’t part of protons and neutrons. Relatives of the electron like muons and tau particles, new varieties of neutrinos, heavier quarks…pretty much the only particles that are part of protons or neutrons are the three lightest quarks (and that’s leaving aside the fact that what is or is not “part of” a proton is a complicated question in its own right).

So where do the extra particles come from? How do you crash two protons together and get something out that wasn’t in either of them?

You…throw Einstein at them?

E equals m c squared. This equation, famous to the point of cliché, is often misinterpreted. One useful way to think about it is that it describes mass as a type of energy, and clarifies how to convert between units of mass and units of energy. Then E in the equation is merely the contribution to the energy of a particle from its mass, while the full energy also includes kinetic energy, the energy of motion.

Energy is conserved, that is, cannot be created or destroyed. Mass, on the other hand, being merely one type of energy, is not necessarily conserved. The reason why mass seems to be conserved in day to day life is because it takes a huge amount of energy to make any appreciable mass: the c in m c squared is the speed of light, after all. That’s why if you’ve got a radioactive atom it will decay into lighter elements, never heavier ones.

However, this changes with enough kinetic energy. If you get something like a proton accelerated to up near the speed of light, its kinetic energy will be comparable to (or even much higher than) its mass. With that much “spare” energy, energy can transform from one form into another: from kinetic energy into mass!

Of course, it’s not quite that simple. Energy isn’t the only thing that’s conserved: so is charge, and not just electric charge, but other sorts of charge too, like the colors of quarks.  All in all, the sorts of particles that are allowed to be created are governed by the ways particles can interact. So you need not just one high energy particle, but two high energy particles interacting in order to discover new particles.

And that, in essence, is what a particle collider is all about. By sending two particles hurtling towards each other at almost the speed of light you are allowing two high energy particles to interact. The bigger the machine, the faster those particles can go, and thus the more kinetic energy is free to transform into mass. Thus the more powerful you make your particle collider, the more likely you are to see rare, highly massive particles that if left alone in nature would transform unseen into less massive particles in order to release their copious energy. By producing these massive particles inside a particle collider we can make sure they are created inside of sophisticated particle detectors, letting us observe what they turn into with precision and extrapolate what the original particles were. That’s how we found the Higgs, and it’s how we’re trying to find superpartners. It’s one of the only ways we have to answer questions about the fundamental rules that govern the universe.

Yang-Mills: Plays Well With Itself

Part Two of a Series on N=4 Super Yang-Mills Theory

This is the second in a series of articles that will explain N=4 super Yang-Mills theory. In this series I take that phrase apart bit by bit, explaining as I go. Because I’m perverse and out to confuse you, I started with the last bit here, and now I’m working my way up.

N=4 Super Yang-Mills Theory

So first these physicists expect us to accept a nonsense word like quark, and now they’re calling their theory Yang-Mills? What silly word are they going to foist on us next?

Umm…Yang and Mills are people.

Chen Ning Yang and Robert Mills were two physicists, famous for being very well treated by the Chinese government and for not being the father of nineteenth century Utilitarianism, respectively.

Has a wife 56 years younger than him

Did not design the Panopticon

In the 1950’s, Yang and Mills were faced with a problem: how to describe the strong nuclear force, the force that holds protons and neutrons in the nuclei of atoms together. At the time, the nature of this force was very mysterious. Nuclear experiments were uncovering new insight about the behavior of the strong force, but those experiments showed that the strong force didn’t behave like the well-understood force of electricity and magnetism. In particular, the strong force seemed to treat neutrons and protons in a related way, almost as if they were two sides of the same particle.

In 1954, Yang and Mills proposed a solution to this problem. In order to do so, they had to suggest something novel: a force that interacts with itself. To understand what that means and why that’s special, let’s discuss a bit about forces.

Each fundamental force can be thought of in terms of a field extending across space and time. The direction and strength of this field in each place determines which way the force pushes. When this field ripples, things that we observe as particles are created, the result of waves in the field. Particles of light, or photons, are waves in the field of the fundamental force of electricity and magnetism.

The electric force attracts charges with opposite sign, and repels charges when they have the same sign. Photons, however, have no charge, so they pass right through electric and magnetic fields. This is what I mean when I say that electricity and magnetism is a force that doesn’t interact with itself.

The strong force is different. Yang and Mills didn’t know this at the time, but we know now that the strong force acts on fundamental particles inside protons and neutrons called quarks, and that quarks come in three colors, unimaginatively named red, blue, and green, while their antiparticles are classified as antired, antiblue, or antigreen. Like all other forces, the strong force gives rise to a particle, in this case called a gluon. Unlike photons, gluons are not neutral! While they have no electric charge, they are affected by the strong force. Each gluon has a color and an anti-color: red/anti-green, blue/anti-red, etc. This means that while the strong force binds quarks together, it also binds itself together as well, keeping it from reaching outside of atoms and affecting the everyday world like electricity does.

Quarks and Gluons in a Proton

Yang and Mills’ description wasn’t perfect for the strong force (they had two types of charge rather than three) but it was fairly close to how the weak force worked, as other physicists realized in 1956. It was realized much later (in the 70’s) that a modification of Yang and Mills’ proposal worked for the strong force as well. In recognition of their insight, today the names Yang and Mills are attached to any force that interacts with itself.

A Yang-Mills theory, then, is a theory that contains a fundamental force that can interact with itself. This force generates particles (often called force-carrying bosons) which have something like charge or color with respect to the Yang-Mills force. If you remember the definition of a theory, you’ll see that we have everything we need: we have specified a particle (the force-carrying boson) and the ways in which it can interact (specifically, with itself).

Tune in next week when I explain the rest of the phrase, in a brief primer on the superheroic land of supersymmetry.