The Dancing Wu Li Masters (29 page)

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.

It is as if the top could spin, by some strange law that nobody understands, only at 100 revolutions per minute, 90 revolutions per minute, 80 revolutions per minute, and so on, with absolutely no exceptions in between. If our hypothetical top wants to spin slower than 100 revolutions per minute, it must jump all the way down to the next slower speed of 90 revolutions per minute. This is analogous to the situation with subatomic particles except that (1) particular types of particles forever spin at the same speed, and (2) the spin of subatomic articles is calculated in terms of angular momentum.

Angular momentum depends upon the mass, size, and rate of rotation of a spinning object. More of any one of these properties increases the angular momentum of the object. In general, angular momentum is the strength of the rotation or, put another way, the effort required to stop the rotation. The more angular momentum an
object has, the more effort is required to stop it from spinning. A spinning top does not have much angular momentum, because it is small and it has little mass. A merry-go-round, in comparison, has an enormous angular momentum, not because it rotates very fast, but because it is large and it has so much mass.

Now that you understand spin, forget everything that you have just learned except the bottom line (angular momentum). Every subatomic particle has a fixed, definite, and known angular momentum, but
nothing is spinning!
If you don’t understand, don’t worry. Physicists don’t understand these words, either. They just use them. (If you try to
understand
them, they become a
koan
).
^*

The angular momentum of a subatomic particle is fixed, definite, and known. “But,” wrote Max Born,

Said another way, the “spin” of a subatomic particle involves “The idea of a spin without the existence of something spinning…”
⁹
Even Born had to admit that this concept is “rather abstruse.”
¹⁰
(Rather!?) Nonetheless, physicists use this concept because subatomic particles
do
behave as if they have angular momentum and that angular momentum has been determined to be fixed and definite in each case. Because of this, in fact, “spin” is one of the major characteristics of subatomic particles.

The angular momentum of a subatomic particle is based upon our old friend, Planck’s constant. Remember that Planck’s constant, which physicists call “the quantum of action,” was the discovery that set into motion the revolution of quantum mechanics. Planck discovered that energy is emitted and absorbed not continuously, but in small packages which he called quanta. Since that initial discovery, Planck’s constant, which represents the quantized nature of energy emission and absorption, has appeared again and again as an essential element in the understanding of subatomic phenomena. Five years after Planck’s discovery, Einstein used Planck’s constant to explain the photoelectric effect, and later still he used it to determine the specific heat of solids, an area far removed from Planck’s original study of black-body radiation. Bohr discovered that the angular momentum of electrons as they orbit atomic nuclei is a function of Planck’s constant, de Broglie used Planck’s constant to calculate the wavelength of matter waves, and it is a central element in Heisenberg’s uncertainty principle.

As important as Planck’s constant is in the realm of subatomic particles, however, it is entirely unobservable in the world at large. This is because the size of the packages by which energy is emitted and absorbed is so small that energy at our gross level appears to be one continuous flow. Similarly, because the indivisible unit of angular momentum is so small, it, too, cannot be observed in the macroscopic world. A spectator swiveling in his chair at a tennis match has 1000000000000000000000000000000000 (10
³³
) times more angular momentum than an electron. Put another way, a change of one penny in the gross national product of the United States is a disturbance more than a
billion billion
times greater than a change by one unit of the spectator’s angular momentum.
¹¹

Instead of writing out the actual angular momentum of a subatomic particle, physicists usually indicate the spin of a subatomic particle by comparing it to the spin of a photon, whose spin they call one. This system has revealed yet another unexplainable pattern of subatomic phenomena. Entire families of particles have similar spin characteristics. The entire family of leptons, the light-weight particles, for example, has a spin of ½, which means that they
all
have an angu
lar momentum which is ½ of a photon’s angular momentum. The same is true for the entire family of baryons, the heavy-weight particles. The mesons also have peculiar spin characteristics. They spin in such a way that their angular momenta is always either 0, 1, 2, 3, etc. in relation to the angular momentum of a photon, but nothing in between (0 = no spin, 1 = the same angular momentum as a photon, 2 = twice the angular momentum of a photon, etc.). The spin characteristics of all of the families and all of the particles are in the table at the back of the book.