by Gary Zukav
This theory is called quantum field theory. Some major cornerstones of the theory were laid in 1928 by the English physicist Paul Dirac. Quantum field theory has been highly successful in predicting new types of particles and in explaining existing particles in terms of field interactions. According to this theory, a separate field is associated with each type of particle. Since only three types of particles were known in 1928, only three different fields were required to explain them. The problem today, however, is that there are over one hundred known particles, which, according to quantum field theory, require over one hundred different fields. This abundance of theoretical fields is somewhat awkward, not to mention embarrassing, to physicists whose goal is to simplify nature. Therefore, most physicists have given up the idea of a separate field existing for each type of particle.
Nevertheless, quantum field theory is still an important theory not only because it works, but also because it was the first theory to merge quantum mechanics and relativity, albeit in a limited way. All physical theories, including quantum theory, must satisfy the requirement of relativity theory that the laws of physics be independent of the state of motion of the observer. Attempts to integrate the theory of relativity with quantum theory, however, have been generally unsuccessful. Nonetheless, both relativity and quantum theory are required, and routinely used, in the understanding of particle physics. Their forced relationship is best described as strained but necessary. In this regard, one of the most successful integrations of the two is quantum field theory, although it covers only a relatively small range of phenomena.*
Quantum field theory is an ad hoc theory. That means that, like Bohr’s famous specific-orbits-only model of the atom, quantum field theory is a practical but conceptually inconsistent scheme. Some parts of it don’t fit together mathematically. It is a working model designed around the available data to give physicists a place to stand in the exploration of subatomic phenomena. The reason that it has been around so long is that it works so well. (Some physicists think that it may work too well. They fear that the pragmatic success of quantum field theory impedes the development of a consistent theory.)
Even with these well-known shortcomings, the fact is that quantum field theory is a successful physical theory, and it is premised on the assumption that physical reality is essentially nonsubstantial. According to quantum field theory, fields alone are real. They are the substance of the universe and not “matter.” Matter (particles) is simply the momentary manifestations of interacting fields which, intangible and insubstantial as they are, are the only real things in the universe. Their interactions seem particle-like because fields interact very abruptly and in very minute regions of space.
“Quantum field theory” is, of course, an outrageous contradiction in terms. A quantum is an indivisible whole. It is a small piece of something, while a field is a whole area of something. A “quantum field” is the juxtaposition of two irreconcilable concepts. In other words, it is a paradox. It defies our categorical imperative that something be either this or that, but not both.
The major contribution of quantum mechanics to western thought, and there are many, may be its impact on the artificial categories by which we structure our perceptions, since ossified structures of perception are the prisons in which we unknowingly become prisoners. Quantum theory boldly states that something can be this and that (a wave and a particle).* It makes no sense to ask which of these is really the true description. Both of them are required for a complete understanding.
In 1922, Werner Heisenberg, as a student, asked his professor and friend-to-be, Niels Bohr, “If the inner structure of the atom is as closed to descriptive accounts as you say, if we really lack a language for dealing with it, how can we ever hope to understand atoms?”
Bohr hesitated for a moment and then said, “I think we may yet be able to do so. But in the process we may have to learn what the word ‘understanding’ really means.”5
In human terms, it means that the same person can be good and evil, bold and timid, a lion and a lamb.
All of the above notwithstanding, particle physicists of necessity analyze subatomic particles as if they were like little baseballs that fly through space and collide with each other. When a particle physicist studies a track on a bubble-chamber photograph of a particle interaction, he assumes that it was made by a little moving object, and that the other tracks on the photograph likewise were made by small moving objects. In fact, particle interactions are analyzed in much the same terms that can be applied to the collision of billiard balls. Some particles collide (and are annihilated in the process) and other newly created particles come flying out of the collision area. In short, particle interactions are analyzed essentially in terms of masses, velocities, and momenta. These are the concepts of Newtonian physics and they also apply to automobiles and streetcars.
Physicists do this because they have to use these concepts if they are to communicate at all. What is available to them is usually a black photograph with white lines on it. They know that (1) according to quantum theory, subatomic particles have no independent existence of their own, (2) subatomic particles have wave-like characteristics as well as particle-like characteristics, and (3) subatomic particles actually may be manifestations of interacting fields. Nonetheless, these white lines (more patterns) lend themselves to analysis in classical terms, and so that is how particle physicists analyze them.
This dilemma, the dilemma of having to talk in classical terms about phenomena which cannot be described in classical concepts is the basic paradox of quantum mechanics. It pervades every part of it. It is like trying to explain an LSD experience. We try to use familiar concepts as points of departure, but beyond that, the familiar concepts do not fit the phenomena. The alternative is to say nothing at all.
“Physicists who deal with the quantum theory,” wrote Heisenberg,
are also compelled to use a language taken from ordinary life. We act as if there really were such a thing as an electric current [or a particle] because, if we forbade all physicists to speak of electric current [or particles] they could no longer express their thoughts.6
Therefore, physicists talk about subatomic particles as if they were real little objects that leave tracks in bubble chambers and have an independent (“objective”) existence. This convention has been extremely productive. Over the last forty years almost one hundred particles have been discovered. They constitute what Kenneth Ford calls the particle zoo.*
The first thing to know about the particle zoo is that every particle of the same species looks exactly alike. Every electron looks exactly like every other electron. If you’ve seen one, you’ve seen them all. Likewise, every proton looks exactly like every other proton, every neutron looks exactly like every other neutron, and so on. Subatomic particles of the same type are absolutely indistinguishable.
Subatomic particles of different types, however, can be recognized by their distinguishing characteristics (properties). The first distinguishing characteristic of a subatomic particle is its mass. A proton, for example, has about 1800 times more mass than an electron. (This does not necessarily mean that a proton is 1800 times larger than an electron since mass and size are not the same thing—a pound of lead and a pound of feathers have the same mass).
When physicists refer to the mass of a particle, unless they indicate otherwise, they are referring to the mass of the particle when it is at rest. The mass of a particle at rest is called its rest mass. Any mass other than a rest mass is called a relativistic mass. Since the mass of a particle increases with velocity, a particle can have any number of relativistic masses. The size of a particle’s relativistic mass depends upon its velocity. For example, at 99 percent of the speed of light a particle’s mass is seven times larger than it is when the particle is at rest.
At velocities above 99 percent of the speed of light particle masses increase dramatically. When the former electron accelerator at Cambridge, Massachusetts, was in operation, it received ele
ctrons from a small feeder accelerator. The electrons from the feeder accelerator were fed into the main accelerator at .99986 the velocity of light. The main accelerator then increased the velocity of these electrons to .999999996 the speed of light. This increase in velocity may look significant, but actually it is negligible. The difference between the initial velocity of the accelerated electrons and the final velocity of the accelerated electrons is the same as the difference in velocity between one automobile that can make a given trip in two hours and a faster automobile that can make the same trip in one hour fifty-nine minutes and fifty-nine seconds.7
The mass of each electron, however, increased from 60 times to as much as 11,800 times the electron rest mass! In other words, particle accelerators are misnamed. They do not increase the velocities of subatomic particles (the definition of “acceleration”) as much as they increase their mass. Particle accelerators are actually particle enlargers (massifiers?).
The masses of particles, whether at rest or in motion, are measured in electron volts. An electron volt has nothing to do with electrons. An electron volt is a unit of energy. (It is the energy acquired by any particle with one unit of charge falling through a potential difference of one volt.) The point is that to measure something in terms of electron volts is to measure its energy, yet this is precisely the unit of measurement that particle physicists use to measure a particle’s mass. For example, the rest mass of an electron is .51 million electron volts (Mev) and the rest mass of a proton is 938.2 million electron volts. The transformation of mass into energy and energy into mass is such a routine phenomenon in particle physics that particle physicists employ units of energy to designate a particle’s mass.
Mass is only one particular form of energy, the energy of being. If a particle is moving it not only has energy of being (its mass) but it also has energy of motion (kinetic energy). Both types of energy can be used to create new particles in a particle collision.*
Often it is easier to compare a particle’s mass with the lightest massive particle, the electron, instead of referring to the number of electron volts it contains. This arrangement makes the mass of an electron one and the mass of a proton, for example, 1836.12. Using this system, the mass of any particle tells immediately how much heavier it is than an electron. This is the system that is used in the table at the back of the book.
When physicists listed all the known particles by the order of their masses, from the lightest to the heaviest, they discovered that subatomic particles fall into roughly three categories: the light-weight particles, the medium-weight particles, and the heavy-weight particles. When it came to naming these categories, however, they unaccountably lapsed into Greek again. The group of light-weight particles they called “leptons,” which is Greek for “the light ones.” The group of medium-weight particles they called “mesons” (maze’ons), which is Greek for “the medium-sized ones.” The group of heavy-weight particles they called “baryons” (bary’ons), which is Greek for “the heavy ones.” Why physicists did not call these new groups “light,” “medium,” and “heavy” is one of the unanswerable questions of physics.*
Since the electron is the lightest material particle, it is, of course, a lepton. The proton is a heavy-weight particle (a baryon), although it is the lightest of the heavy-weight particles. Most subatomic particles are classified in this way, but not all of them, which brings us to a phenomenon of particle physics which, like much of quantum mechanics, escapes the bounds of concept. A few particles do not fit into the lepton-meson-baryon framework. Some of them are well known (like the photon) and others have been theorized but not discovered yet (like the graviton). All of them have in common the fact that they are massless particles.
“Wait a minute,” we exclaim. “What is a massless particle?”
“A massless particle,” says Jim de Wit, who has studied this phenomenon, “is a particle that has zero rest mass. All of its energy is energy of motion. When a photon is created, it instantly is traveling at the speed of light. It cannot be slowed down (it has no mass to slow) and it cannot be speeded up (nothing can travel faster than the speed of light).”
“Massless particle” is an awkward translation from mathematics to English. Physicists know exactly what they mean by a massless particle. A “massless particle” is the name they give to an element in a mathematical structure. What that element represents in the real world, however, is not so easy to describe. In fact, it is impossible because the definition of an object (like a “particle”) is something that has mass.
Zen Buddhists have developed a technique called the koan which, along with meditation, produces changes in our perceptions and understanding. A koan is a puzzle which cannot be answered in ordinary ways because it is paradoxical. “What is the sound of one hand clapping?” is a Zen koan. Zen students are told to think unceasingly about a particular koan until they know the answer. There is no single correct answer to a koan. It depends upon the psychological state of the student.
Paradoxes are common in Buddhist literature. Paradoxes are the places where our rational mind bumps into its own limitations. According to eastern philosophy in general, opposites, such as good-bad, beautiful-ugly, birth-death, and so on, are “false distinctions.” One cannot exist without the other. They are mental structures which we have created. These self-made and self-maintained illusions are the sole cause of paradoxes. To escape the bonds of conceptual limitation is to hear the sound of one hand clapping.
Physics is replete with koans, i.e., “picture a massless particle.” Is it a coincidence that Buddhists exploring “internal” reality a millennium ago and physicists exploring “external” reality a millennium later both discovered that “understanding” involves passing the barrier of paradox?
The second characteristic of a subatomic particle is its charge. Every subatomic particle has a positive, a negative, or a neutral charge. Its charge determines how the particle will behave in the presence of other particles. If a particle has a neutral charge, it is utterly indifferent to other particles, regardless of what charge they may have. Particles with positive and negative charges, however, behave quite differently toward each other. Positively and negatively charged particles are attracted to particles with the opposite sign and repelled by particles with the same sign. Two positively charged particles, for example, find each other’s company quite repulsive and immediately put as much distance between themselves as possible. The same is true of two negatively charged particles. A negatively charged particle and a positively charged particle, on the other hand, are irresistibly attracted to each other, and they immediately move toward one another if they are able to do so.
This dance of attraction and repulsion between charged particles is called the electromagnetic force. It enables atoms to join together to form molecules and it keeps negatively charged electrons in orbit around positively charged nuclei. At the atomic and molecular level it is the fundamental glue of the universe.
Electric charge comes only in one fixed amount. A subatomic particle can have no electrical charge (neutral), or one unit of electrical charge (either positive or negative), or, in certain instances, two units of electrical charge, but nothing in between. There is no such thing as a particle with one and one fourth units of electrical charge, or a particle with 1.7 units of electrical charge. Every subatomic particle has either one whole unit of electrical charge, two whole units of electrical charge, or no electrical charge at all. In other words, like energy (Planck’s discovery) electrical charge is “quantized.” It comes in chunks. In the case of electrical charge, all of the chunks are the same size. Why this is so is one of THE unanswered questions in physics.*
When the characteristic of charge is added to the characteristic of mass, a particle personality, so to speak, begins to emerge. An “electron,” for example, is the only subatomic particle with a rest mass of .51 million electron volts and a negative charge. With this information a particle physicist knows not only how massive an electron is,
he also knows how it will interact with other particles.
The third characteristic of a subatomic particle is its spin. Subatomic particles spin about a theoretical kind of axis like a spinning top. One big difference between a spinning top and a spinning particle, however, is that a top can spin either faster or slower, but a subatomic particle always spins at exactly the same rate. Every electron, for example, always spins at exactly the same rate as every other electron.
The rate of spin is such a fundamental characteristic of a subatomic particle that if it is altered, the particle itself is destroyed. That is, if the spin of a particle is altered, the particle in question is changed so fundamentally that it no longer can be considered an electron, or a proton, or whatever it was before we altered its spin. This makes us wonder whether all of the different “particles” might be just different states of motion of some underlying structure or substance. This is the basic question of particle physics.
Every phenomenon in quantum mechanics has a quantum aspect which makes it “discontinuous.” This is also true of spin. Spin is quantized just like energy and charge. It comes in chunks. Like charge, all of the chunks are the same size. In other words, when a spinning top slows down, its rotation does not diminish smoothly and continuously, but in a series of tiny steps. These steps are so small and close together that it is impossible to observe them. The top appears to spin more and more slowly until it stops spinning altogether, but actually, the process is very jerky.