Physics Q&A #2: The Fundamental Forces (part 1.5)

(If you haven't read the first post on this subject, check it out here.)

Before I get into the latter two of the fundamental forces, I realized that one aspect of physics I take for granted probably isn't well understood by most who haven't done much with particle physics. This aspect is the fundamental particles of the universe; the basic building blocks of everything. I'm making an interlude post here to explain that to everyone, since the knowledge of it is critical to understanding the strong and weak nuclear forces.

There are many dividing lines between the fundamental particles, and I'll go through them one at a time to best sort them out. The first dividing line is between what are known as "Bosons" and "Fermions." Fermions are like solid matter, in that you can never put two of them into the same place (or quantum state). Bosons, on the other hand, can be stacked up without limit in the same place/state. Fermions tend to make up the bulk of matter, while bosons work to mediate forces. There also tend to be conservation laws about the number of Fermions, so you can't just go and create or destroy them. Bosons, however, have no such conservation laws.

Bosons

First, I'll go over the Bosons seeing as there are fewer of them and no more divisions between them (among the fundamental ones, that is. Many bound groups of particles can also be bosons, and there are extra divisions here). We know that the following fundamental bosons exist:

• Photon: Photons are little quantum pieces of light, and mediate the electromagnetic force. They have no mass and never decay, so the electromagnetic force has unlimited range. Photons are all their own antiparticles.
• W⁺, W^-, and Z: The W and Z particles mediate the weak nuclear force. They have very high mass (each ways about as much as an iron atom), and short lifetimes, leading to the weak force having only short range. The W⁺ and W^- are each other's antiparticle, and the Z is its own antiparticle.
• Gluons: Gluons mediate the strong nuclear force. They have zero mass and don't decay, but the strong nuclear force is still limited to short range for reasons I'll cover in the next part of this mini-series. Gluons are electrically neutral, but have "color charge" which determines attraction from the the strong nuclear force.

There are also two other theoretical bosons that deserve a mention here. The first is the Higgs particle (H), which is a massive uncharged particle predicted to exist by the Standard Model and which is currently being tested for. You might have heard that the Higgs particle is what "gives other particles mass," but this is a common misinterpretation of the theory. It's actually the Higgs field which gives particles mass. The W⁺, W^-, Z, and H particles are all quanta (fundamental pieces, like photons are of light) of this field. If the H particle exists, we might expect to see it interacting through the weak force in a similar way to the Z boson, except working at different energies due to its different mass.

The other theoretical boson is known as the graviton, and it was predicted to mediate the gravitational force similar to how photons mediate the electromagnetic force (see my previous post for exactly how). However, no direct evidence of its existence could be found, and Einstein's model of General Relativity did a much better job of explaining the how and why of gravity, so the graviton has generally been given up on.

Fermions

Fundamental fermions, unlike bosons, come with another dividing line into two more families: leptons and quarks. The difference here is that quarks feel the strong nuclear force while leptons don't. There also appears to be a nice symmetry between leptons and quarks, as they each have six particles which can be further divided up into three pairs (called "generations," an important concept for the weak nuclear force). For convenience of envisioning how the forces work, we then introduce a property called "isospin" which is equal to +1/2 for one particle of each pair and -1/2 for the other.

Additionally, each of these fermions has a distinct antimatter counterpart to it. These antiparticles have the same mass as the normal particles, but opposite isospin, charge, and color.

Leptons

Among the leptons is one you've likely heard of: The electron. It also has a light, neutrally charged particle called a neutrino which corresponds to it. There's also the muon and the tau, plus their corresponding neutrinos. However, if your only goal is to understand how the weak force works, you really only need to worry about the electron and its neutrino. A quick fact sheet on them:

Generation 1

Electron
Isospin: -1/2
Charge: -1
Mass: Very Low

Electron-neutrino
Isospin: +1/2
Charge: 0
Mass: Extremely low

Generation 2

Muon
Isospin: -1/2
Charge: -1
Mass: Medium

Muon-neutrino
Isospin: +1/2
Charge: 0
Mass: Extremely low

Generation 3

Tau
Isospin: -1/2
Charge: -1
Mass: High

Tau-neutrino
Isospin: +1/2
Charge: 0
Mass: Extremely low

Quarks

Quarks are the building blocks of protons, neutrons, and other similar particles. They bind in groups of 2 or 3 through the strong nuclear force to form these particles. Like leptons, there are 6 types of quarks, but to understand the weak and strong forces, you only need to know about the first two (subsequent generations work essentially the same way).

Quarks are affected by the strong nuclear force because they carry what's known as "color charge." Quarks can have one of three colors: red, green, and blue. Quarks of different colors are then attracted to each other. However, there appears to be a law of nature that no particle can exist alone unless it's color-neutral, so you can never see individual quarks. Instead, you see them either in groups of three with one of each color (known as "baryons") or in pairs of one normal quark and one antiquark (antiquarks have opposite colors of regular quarks, anti-red, anti-green, and anti-blue, and these alone cancel out the regular color). Also, unlike electric charge, color charge isn't innate to the type of particle; each quark can have any color (and antiquarks any anticolor), and even oscillate through different colors.

A quick fact sheet on quarks:

Generation 1

Down Quark
Isospin: -1/2
Charge: -1/3
Mass: Low

Up Quark
Isospin: +1/2
Charge: +2/3
Mass: Low

Generation 2

Strange Quark
Isospin: -1/2
Charge: -1/3
Mass: Medium

Charm Quark
Isospin: +1/2
Charge: +2/3
Mass: High

Generation 3

Bottom Quark
Isospin: -1/2
Charge: -1/3
Mass: High

Top Quark
Isospin: +1/2
Charge: +2/3
Mass: Very High (in fact, the highest of any particle known; about the mass of a gold atom)

If you want a convenient chart of these particles, there's one very popular poster you might have seen in your high school physics room which Wikipedia has gotten permission to host online for their article on the Standard Model. It's a bit dated and doesn't mention the Higgs particle, but other than that it's a great reference on the basic properties of the fundamental particles and forces (at least until you're at the point where you've got it memorized).

As before, if you have any questions on any of this, please ask. I'm doing this purely to help your understanding, so be sure to let me know of anything I haven't explained well enough.