I'd like to put the subatomic world in perspective for you. Molecules are huge compared to atoms and the particles that make them up. And using normal numbering would take too long to write their values out. For example, a typical nuclear diameter may be 0.000000000000006 meters. There had to be an easier way to write this. That's where something called scientific notation comes in. This is a method that was developed to write large numbers in powers of ten. So the number above would be written as 6 × 10-14 meters in scientific notation. Using this method, let's look at the relative size of these three subatomic particles.
To give you an idea of how strong the force is that holds a nucleus together, let's scale up the size of the proton. If we made it a foot in diameter, with the center of the protons about 18 inches apart, how large would this force be? Well, if we embedded these two oversized protons in the strongest metal alloy known, the electrical repulsion would be so strong it would tear this metal apart as if it were tissue paper. So this force has to be strong enough to overcome these powerful forces that seek to push apart.
The strong force is one of four fundamental forces under which the universe operates. It is also known as the strong interaction; it operates within the nucleus of the atom, keeping the nucleus together and overcoming the tendency of the positive electric charge to blow it apart. The strong force is about 100 times stronger than electromagnetism (over the size of the nucleus), which is why there are roughly 100 protons in the largest stable nuclei.
To relate this to a more understandable perspective, let's increase the size of the nucleus to about a foot, maybe the size of a bowling ball or medium-sized melon. How big would the atom be? Close to twenty miles in diameter. So if the nucleus of our atom were as large as a bowling ball, the rest of the atom would consist of ten pea-sized electrons scattered around a sphere twenty miles across with the bowling ball at the center. Imagine putting a bowling ball in the center of a city and then scattering ten peas throughout the rest of the city and you'll have some idea of how empty an atom really is.
As you can see, most of the matter in the atom is contained in the nucleus. We also know that the nucleus has a positive electrical charge. But wait a second, don't like charges repel each other? If the nucleus is all protons and neutrons (which as you know are neutral in charge), what prevents the protons from repelling each other and the nucleus from flying apart? Obviously there must be some kind of cosmological glue that keeps the whole thing together. The nature of this force is rather mysterious. It has to be a force that can overcome the repulsive force of an electric charge.
In every reaction involving elementary particles, the total electrical charge is the same before and after the reaction. This called the conservation of electrical charge. Conservation laws play an extremely important role in physics. For example, there are laws that tell us that the energy of a system has to be the same before and after every reaction, which is the bass for Einstein's famous formula, E = mc2, which in turn shows the equivalence of energy and mass. And there are other laws that tell us the same thing about momentum.
The repulsive electrical force is so powerful that this force must be many orders of magnitude stronger to keep the nucleus together. It is actually more powerful than any force we deal with at the everyday level. Physicists have named this the strong force. And part of the development of particle physics has been an attempt to understand what the strong force is and how it is generated. You'll find out about this a little further into the section.
As mentioned in the previous section, the mass of the neutron is pretty close to that of the proton, just a little more. In fact the mass is very close to the combined masses of a proton and an electron. But the neutron exhibits a property not found in the proton or electron—instability. If we could put a neutron on a table and observe it, it wouldn't be there for very long; it would disappear. And in its place you would find a proton, an electron, and another type of particle, which I will get to shortly.
In the language of particle physics, a neutron outside of a nucleus, “decays,” a term you've already been introduced to. This process of decay is what it means for a particle to be unstable. As we get further into looking at the whole slew of elementary particles you'll see that virtually all of them share this characteristic of instability. And if you remember back in “The Relative Nature of Space and Time,” where I discussed the carbon-14 dating method, we saw that this was exactly the form of decay that allowed scientists to date the age of any material.
One way of gauging the neutrino's lack of interaction with other matter is to give you a good example. If you were to make a lead plate neither meters thick, nor kilometers thick, but light-years thick, you might stand a chance of getting the neutrino to interact with an atom in it. Even if you were to run a lead tube from the Earth to our nearest star, Alpha Centauri, and started a neutrino down the tube today, four years from now it would emerge without disturbing a single atom of lead in the tube.
Let's take a closer look at neutron decay. When the decay occurs, two charged particles are created—a proton and an electron. This means the total electrical charge after the final decay is 0, which is exactly the charge on the neutron. The total electrical charge of the process has been conserved, or in other words, remains the same as before the decay took place. However, upon very close measurement, physicists found that the amount of electrical charge left after the decay was slightly different than before. Well, they couldn't throw out the laws of conservation, so there had to be something else that could account for the missing amount of energy.
In 1934, Italian physicist Enrico Fermi (the man who built the first nuclear reactor at the University of Chicago) put together the first successful theory of decay that explained where the missing energy went. He theorized that there was another particle that must be electrically neutral (otherwise it would have been detected earlier) that he named the neutrino. If such a particle existed, it would carry away the missing energy of the neutron decay and the laws of conservation would be preserved.
The weak force or weak interaction occurs during a process known as beta decay. Beta decay is just another term used by physicists for the breakdown or decay of a neutron. This occurs when a neutron, either in the nucleus of an atom or as a free neutron, transforms through this weak force into an electron, a proton, and a neutrino (in actuality it's called an electron antineutrino, but let's not confuse the issue). It is the second of the four fundamental forces operating in our universe.
In particle physics the only way for any particle to be detected is for it to interact with something and cause a change. Well, the neutrino posed certain difficulties for physicists because the neutrino, like a photon, has no mass. You can't detect an interaction when the particle has no mass to interact with anything. And the neutrino doesn't readily interact with other matter at all.
Eventually in 1956, almost 20 years after Fermi first theorized the neutrino's existence, they were finally detected in an experiment with a nuclear reactor. The neutrino is now a fully accepted particle routinely produced for experimental purposes in many large accelerators around the world. Reactions in which the neutrino is created through particle decay occur on a relatively slow time scale and are given the name weak force or weak interaction.
Let's pause for a moment and tally our sheet of forces and particles. So far we've looked at two of the four fundamental interactions of nature—the strong force and the weak force. We've also examined the electron, proton, neutron, and neutrino. In addition to these four particles, we should also add a fifth, the photon. As you know, the photon is a particle of light and therefore also a constituent of radiation. Almost all of the particles that you'll meet from here on will be unstable, only the electron and the proton can exist by themselves for indefinite periods of time. All other particles decay into some combination of photons, neutrinos, protons, and electrons. And just to clarify things, we have two forces and five particles so far. Before we move into all of the other families of particles and interactions, let's complete this section with a look at the last two of the four fundamental forces.
Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.