Theories of the Universe: Forces, Particles, and Some Cosmological Glue

Forces, Particles, and Some Cosmological Glue

We're now going to spend a section discovering the world of particle physics. From atoms to quarks and fermions to bosons, we'll delve into the subatomic universe and stare these microcosmic inhabitants in the face. Linear accelerators and cyclotrons will split particles into smaller and smaller pieces, with the hope of some day finding the tiniest form of matter. By the time we're through, you'll know that strange and charming are terms that can apply to particles as well as people. So strap yourself to the nearest super collider and let's go find the standard model of this microscopic cosmos.

An Atomic Review

Do you know the first particle that was discovered? We've talked about it a few times already … no, not the atom—that's too big. It was the electron. However, I don't want to spend a lot of time talking about the atom and its parts—there are too many pieces in this puzzle to put together—but that's a great place to start. The key to the structure of the atom, what it's composed of, why the nucleus stays together, and the mass of each of its parts are all very important. It's where particle physics began. So let's just spend a little time reviewing what we know about the atom and take it from there; everything after the atom just gets smaller and smaller.

In 1897, J.J. Thompson, a young English physicist, had been performing a number of experiments with cathode rays, trying to find out if they were really particles. He built a simple apparatus in which the cathode rays were directed across a region between two electrically charged plates, and in this region there was also a magnetic field. The important result that Thompson obtained was that the cathode rays were particles and that they were attracted toward a positively charged plate. This meant that the particles carried a negative electrical charge (and I know you know that opposite charges attract, I just wanted to remind you).

This particle that Thompson discovered is now known as the electron. It has a mass of 9.1 × 10^-28 gram. That's extremely small. And once it was properly identified and labeled, it was realized that the electron was a very important particle. Every electrical current, whether it's a man-made circuit or a nerve in your body, is simply a flow of electrons. And where else could the electron come from but the interior of what was once thought of as the indivisible atom? The existence of a negatively charged particle that could be taken from the atom implied that there must also be a positively charged segment left behind, and this in turn implied that the atom must have structure. If this was so, there must be a type of matter more fundamental than the atom. The electron was the first example of matter from the subatomic realms.

All this may seem a little after the fact since we've already talked about electrons and the structure of the atom. But for our purposes, it's good to start at the beginning so that we can trace the chain of events and ideas that developed into the science of particle physics.

The Nucleus and the Proton

In the early 1900s, the ideas current at the time envisioned the atom as a large, diffuse, positively charged chunk of material in which the electrons were embedded like raisins in a bun. Thanks to Ernest Rutherford, whom you met in “The Dual Nature of Light,” the atom took on the familiar shape of a planetary system that was later adopted by Neils Bohr. Once the existence of the nucleus was established, scientists began to question its composition. After a few more experiments in which Rutherford observed the emission of certain particles from nuclear collisions between helium and hydrogen nuclei, he discovered and named the new particle the proton, which in Greek means “the first one.”

The proton is a particle that has a positive electrical charge. The magnitude of the charge is precisely equal to the magnitude of the negative charge of the electron. However its mass is about 1,836 times that of the electron. An interesting point about protons is that the number of protons in the nucleus of an atom also reflects its atomic number, or its numerical order, in the periodic table of elements. One other insight that Rutherford had was that there was probably another constituent to the heavier nuclei. In other words, he thought there was an electrically neutral particle as massive as the proton in the nucleus as well. He arrived at this conclusion by noting that most atoms apparently weighed about twice as much as you would expect them to if you added up the masses of the protons and electrons. He called this hypothetical particle the neutron, which was eventually discovered in 1932.