Elementary Particles of the Atom’s World
AN ARMY of international detectives is hot on the trail of an elusive quarry—a quarry that is a master of disguise and quick getaway. There are clues aplenty: telltale tracks, a clear “modus operandi” and even photographs. The pursuing team’s technical weapons of detection steadily improve, but the subjects only seem to become more elusive and inscrutable.
They are not after Mr. Big; they are looking for the small fry—in fact, the very smallest. These detectives are atomic scientists, and their case is the search for the elementary particle, the building block of the material universe.
This investigation is at least as old as the fourth century B.C.E. Greek philosophers of that time mused over the result of repeated and successive divisions of matter. They decided it couldn’t be done indefinitely; eventually the result would have to be an indivisible piece of matter. Democritus gets the credit for coining the word “atom” to describe that smallest piece of matter. However, during the 20th century efforts have centered on finding out what makes up the atom itself.
The First “Elementary Particles” Are Found
J. J. Thomson sleuthed out the identity of the electron in 1897. He found that an electric current consists of these particles in great numbers. Electrons are so small that 6,000,000,000,000,000,000 pass through a 100-watt light bulb in one second. Least elusive of all the elementary particles, electrons are like fickle vagabonds that are easily transferred from place to place even by simple friction. When you walk across a carpet, your shoes may pick up billions of them that spread themselves over your body, only to gather and leap pell-mell through the air as a spark from your finger if you reach out to touch a light switch.
In 1911 Ernest Rutherford showed that all the positive charge of the atom and most of its mass resided in a region 1/10000 the size of the atom itself. That gave rise to the popular image most of us have of the atom: a small central core or nucleus surrounded by fast-moving electrons that orbit it like bees around a hive.
By 1932 the nucleus was found to be made up of protons and neutrons. Protons carry the positive charge of the atom—exactly equal in size but opposite in kind to the negative charge on the electron. The proton is about 1,800 times as massive as the electron, about the same ratio as a refrigerator to a biscuit. Only a little more massive than the proton, the neutron carries no charge. By the 1940’s experiments and theory had lifted the curtain on many additional particles playing a role in the nucleus. The scientists’ mental picture of the nucleus was getting much more complex.
Methods of Detection
Physicists “see” particles by examining the residue of their interaction with matter. These interactions could be compared to the path of a mischievous child who disrupts the neighborhood by running through flowerbeds and overturning trash cans. After a time the neighbors can recognize the pattern of evidence and identify the culprit. A charged particle in motion and free of its atom “home” behaves something like a child on a rampage. It bumps into other atoms and dislodges their electrons, leaving a residue of charged atoms.
The cloud chamber was an early detector of particles. Charged particles leave vapor trails in the chamber because of vapor condensation on disturbed atoms in the path, something like the trail of a high-flying jet airplane. More common today are bubble chambers that rely on a near-to-boiling liquid as a medium through which a stream of bubbles marks the path of the particle.
Particles travel at tremendous speeds. The photon shares the all-time speed record with the neutrino and the graviton. All three have no mass and therefore move at the speed of light (300,000 km/sec. or 186,000 mi/sec.), a rate that would take them around the earth more than seven times in one second.
Material particles (those with mass) can get up close to the speed of light but can never quite reach it. In fact, all the electron can muster when circling the nucleus is about one tenth of the speed of light. Its speed compared to that of the fastest particles is like an automobile on a highway compared to a supersonic jet airplane.
The time of existence of a particle is called its lifetime or simply its life. Electrons and protons are stable, which is another way of saying their lifetime is infinite. But most particles “live” only a very short time. For example, the muon, a particle produced by the interaction of cosmic radiation on the upper atmosphere, has an average lifetime of two millionths of a second. When it “dies,” an electron and two neutrinos suddenly appear in its place. This could be compared to a robber taking a few steps out of the bank and miraculously changing into three different people who run off in different directions.
This sudden changing of identity has given scientists no little problem when trying to study short-lived particles. In a few millionths of a second after its formation, a particle may disintegrate into two or more other lesser ones, which may, in turn, change to still other different and smaller particles. The process continues until it produces stable particles. When a particle changes its identity it is said to “decay.” But why are the electron and the proton the only particles with mass that do not decay? Because of what are called conservation laws.
Put simply, a conservation law says that if a conserved quantity is measured before an event, that quantity should total up to the same amount after the event.
To illustrate, imagine a complex of four tennis courts surrounded by a high fence to help keep balls inside. As the players arrive we give each set of players 10 identical balls and tell them not to worry about keeping track of the specific balls they start with. During simultaneous games the balls would likely fly into adjacent courts and get used there too. Some balls may eventually get used by all players. After all games are finished we collect the balls. We expect to get back the same number we handed out. If we have fewer we would conclude that some flew over the fence, were still in the courts, or had gone out with the players. No other explanation makes sense: tennis balls do not disappear into thin air. In this case, ‘tennis balls are conserved.’
Conservation laws rule the physical world. Nothing can happen that violates a conservation law: there are no lawbreakers among the citizens of the world of elementary particles.
The electron is stable because of conservation of mass and electric charge. It is the lightest charged particle. There are lighter particles than the electron but all of them are invariably neutral in electric charge. If the electron were to decay into one of these lighter particles it would have to get rid of its charge but it cannot because that would violate the law of charge conservation. It cannot decay into heavier charged particles because that would violate the law of mass conservation—as impossible as slicing a one-pound loaf of bread and getting slices that weigh two pounds. So the electron cannot decay because there is simply ‘no place to go.’
The proton is stable because it would need to violate a different conservation law to decay. On the other hand, the neutron is stable as long as it has a proton to snuggle up to. Put a neutron into “solitary confinement” and it decays in about 15 minutes.
Kenneth Ford, in his book The World of Elementary Particles, emphasized the importance of conservation laws in this way: “The ‘normal’ thing is for a particle to undergo decay and transmute itself into other lighter particles. For reasons which are not fully understood there are two ‘abnormal’ particles, the proton and the electron, which are prohibited from decaying. According to this larger view of the particles, there are certain rules of nature (conservation laws) which happen to prevent the decay of these two particles. Because of this chance, the construction of a material world is possible.
“Of course, since there is only one Universe, and one set of natural laws, it does not make much sense to say that a particular state of affairs in the world exists by chance. But this view of the multiplicity of particles continues the process, begun by Copernicus, of making man feel more and more humble when facing the design of nature. We and our world exist by the grace of certain conservation laws which stabilize a few particles and permit an orderly structure to be built upon the normal chaos of the submicroscopic world.”
Conservation Laws Predict the “Neutrino”
Early experiments in the study of subatomic particles suggested that the neutron decayed in a nonconserved way. Researchers noticed that as a neutron decayed into a proton and an electron the momentum and energy after the decay was much less than it had been before the decay. Since these were conserved quantities, the conservation laws seemed to be violated in this case. Nuclear physicists could not accept this conclusion.
To save these conservation laws, theorists invented the neutrino and anointed it with all the necessary qualities to make it an indivisible partner in the decay process of the neutron. It could not be “seen,” but the assumption of its existence was a product of the faith of scientists in the conservation laws they had learned to trust.
After 25 years of accepting the neutrino on faith, scientists captured it in 1956. No wonder it was so elusive; it has no charge, no apparent mass, and travels at the speed of light. So rarely do neutrinos interact with matter that most pass completely through the earth as easily as a bullet through tissue paper. In one try to verify the neutrinos’ existence, experimenters sent a calculated 100,000,000,000,000 neutrinos through 44 feet (13 m) of iron to a detection chamber that still recorded the capture of only 29 of them. That is comparable to passing the entire world’s population through a small room containing a bathroom scale, with the result that only one fifth of a pound (1/10 of a kg) is registered.
By 1960 so many particles had surfaced that scientists could feel like a shipwrecked zoologist washed up on an island having a great population of never-before-seen animal life. In an effort to bring some order into the variety of the particle population, physicists classified the particles into groups based on similar properties—similar to the way a zoologist would classify different animals into mammals, reptiles, and so forth.
Heavier particles are called hadrons. Extra-heavy hadrons are called baryons. Baryons (protons, neutrons, and so forth) are the “elephants” of the subatomic-particle zoo. Lighter hadrons are called mesons (pions, kaons, and so forth) and are more “tiger-sized.” Leptons (electrons, muons, neutrinos) are generally the “insects” of the particle world.
The actual system is not based on size and weight but on the likelihood of the members of each class to interact with one another. Elephants interact with other elephants differently from the way they interact with insects. In fact, insect and elephant may not notice each other at all except when the elephant munches a leaf already being dined on by the insect. The elephant-like hadrons interact with each other with what is called the strong force. The insectlike leptons are completely oblivious of the strong force: what does a grasshopper care if two elephants are fighting? But charged leptons are sensitive to the electromagnetic force and they will interact with the hadrons according to the rules of this force, just as both animals must notice if the smaller animal flies into the bigger one’s eyeball.
Is There a More “Elementary” Particle?
About 300 particles, mostly hadrons, have been discovered since man started probing the atom and picking it apart. The leptons seem to be truly “elementary”—that is, they have no discernible size and seem to have no internal structure. Furthermore, there are only six known leptons, a nice small number suggesting simplicity. Hadrons are not so simple. They have a measurable size and number in the hundreds. When a hadron decays, other hadrons come spitting out of the debris.
In the 1960’s Murray Gell-Mann and George Zweig proposed a new particle, the quark. Their theory stated that all hadrons were made up of two or three quarks in some combination. By attributing certain properties to their theoretical quarks Gell-Mann and Zweig could account for all known nuclear particles (hadrons) being constructed from just three different quarks named “up,” “down,” and “strange.” A bonus of the theory was the prediction of the existence of a previously undiscovered particle that was subsequently produced and found to have the anticipated properties. This greatly strengthened the theory’s acceptance. Recent experiments now strongly suggest the presence of three more varieties of quarks dubbed “charmed,” “truth” and “beauty.”
As of this writing, individual quarks have not been conclusively detected; some think they never will be isolated. But quarks are a firm theoretical basis for all particle physicists. As with the neutrino, scientists believe in them without seeing them because they can be used to predict what detectable particles of the atom will do under certain conditions.
Will the number of quarks making up the present theory continue to account for new particles yet to be discovered? Will more quarks be uncloaked? Will a quark ever be isolated? Are the quarks truly the ultimate “elementary particles” of the atom’s nucleus? If not, what is a quark made of?
“What is it made of?” may never be completely answered. Each time the probing of matter advances down a step, the so-called “elementary particle” seems to be made up of something more simple. (Now there’s talk of “gluons.”) Will the search never end? It may be that our curiosity will never be completely satisfied. For some, that prospect seems more tantalizing than discouraging. They feel as did the Christian apostle Paul: “O the depth of God’s riches and wisdom and knowledge! How unsearchable his judgments are and past tracing out his ways are!”—Rom. 11:33.