The Quantum World

Quantum Physics for Everyone

Kenneth W. Ford

• Éditeur: Harvard University Press
• Année: 2005
• ISBN:
  9780674263550
• Sujet: Ciencias biológicas
• Sous-sujet: Ciencias en general

As Kenneth W. Ford shows us in The Quantum World, the laws governing the very small and the very swift defy common sense and stretch our minds to the limit. Drawing on a deep familiarity with the discoveries of the twentieth century, Ford gives an appealing account of quantum physics that will help the serious reader make sense of a science that, for all its successes, remains mysterious. In order to make the book even more suitable for classroom use, the author, assisted by Diane Goldstein, has included a new section of Quantum Questions at the back of the book. A separate answer manual to these 300+ questions is available; visit The Quantum World website for ordering information.There is also a cloth edition of this book, which does not include the "Quantum Questions" included in this paperback edition.

chapter 1

Beneath the Surface of Things

Knock on wood. It seems solid. It is solid. But probe more deeply and you encounter other worlds. You probably learned in school that solid matter is made of atoms and that atoms are mostly empty space. Actually, they are empty in the same sense that the disk carved out by a whirling propeller blade is empty. It’s easy for something small and fast to pass through an atom or a spinning propeller, but next to impossible for something large and slow to do so.

By most measures, atoms are small.* Yet to some scientists, they are gargantuan. These scientists—nuclear and particle physicists—are concerned with what goes on in bits of space much smaller than atoms, smaller even than the tiny nuclei that sit at the centers of atoms. We call their realm of study the subatomic world. That’s the world I want to explore in this book.

In the twentieth century we learned that nature in the subatomic world behaves in weird and wonderful ways unknown in the ordinary world around us. When we look at the smallest specks of space and the tiniest ticks of time, we see what can only be called fireworks. Myriad new particles pop into existence, some long-lived, most short-lived, each of them interacting in some way with every other, and each capable of being destroyed as well as created. In this world, we come face to face with a speed limit in nature, find space and time stirred together, and learn that mass can change to energy and energy to mass. The strange rules of the game in this world stretch the minds of scientists and nonscientists alike.

These rules are the product of two great revolutions in twentieth-century physics, called quantum mechanics (roughly, the physics of the very small) and special relativity (roughly, the physics of the very fast).

In this book I want to explain how these two revolutionary theories—and especially quantum mechanics—have changed the way we view the world. To illustrate the ideas, I’ll enlist the help of the subatomic particles (henceforth I’ll just call them particles), the “things” that are governed by the quantum rules. After a look at “how small is small” and “how fast is fast,” I’ll introduce you to quite a few members of the particle families. Then I’ll turn to the marvelous array of ideas that physicists have developed to account for the behavior of the particles and all that is built of particles.

In 1926, the year I was born, the only known inhabitants of the subatomic world were the electron and the proton. The electron is a speck of negative electricity that zips around inside atoms and migrates along wires carrying electric current. Nowadays, it is also steered magnetically toward the pixels on the screens of cathode-ray tubes that serve as computer monitors and television tubes, causing them to shine in words and pictures. The proton, nearly 2,000 times more massive than the electron and carrying a positive charge, sits alone at the center of the lightest hydrogen atom, pulling on the electron that circles it. Protons were, in the 1920s, assumed to exist in heavier atomic nuclei as well, and we know now that indeed they do. They also come barreling toward Earth in great numbers and with great energy from outer space, constituting what we call primary cosmic radiation.

The photon—the particle of light—was also known in 1926, but it wasn’t considered a “real” particle. It had no mass; it couldn’t be slowed down or corralled; and it was all too easily created and annihilated (emitted and absorbed). It wasn’t a reliable, stable chunk of matter like the electron or the proton. So, even though the photon surely behaved like a particle in some respects, physicists fudged by calling it a “corpuscle” of light. Only a few years later it gained full and equal status as a real particle, when physicists realized that electrons could be created and annihilated every bit as easily as photons, that the wave properties of electrons and the wave properties of photons were all pretty much of a piece, and that a particle with no mass was, after all, a perfectly ordinary particle.

The year 1926 was right in the middle of a golden age of physics. In a brief few years, 1924 to 1928, physicists came up with some of the most important—and startling—ideas that science has ever known. These included the discovery that matter, not just light, has wave properties; the realization that the fundamental laws of nature are laws of probability, not laws of certainty; the understanding that there is a limit in principle to the accuracy with which certain properties of matter can be measured; the discovery that electrons spin about an axis that can point in only two possible directions, “up” and “down”; the prediction that for every particle there is a companion antiparticle; the insight that a single electron or photon can be moving in two or more different ways at the same time (as if you could be driving due north and due west simultaneously, or window shopping in New York and Boston simultaneously); and the principle that no two electrons can be in the same state of motion at the same time (they are rather like marchers who, try as they might, can’t march in step).

These are among the “big ideas” that are central to this book. Using the subatomic particles as illustrations will help to bring these ideas to life. Particles (and, to some extent, whole atoms) are the entities most conspicuously influenced by the laws governing the very tiny and the very speedy.

I should note right here that in quantum physics, separating what is (the particles) from what happens (the laws) is not so easy. In the “classical” physics developed in the three centuries before the twentieth, the separation between what is and what happens is pretty clean. The Earth (what is) follows an orbital path (what happens) about the Sun in response to laws of force and motion. What the Earth is made of, whether it contains life or doesn’t contain life, whether it spews out lava or lies dormant—these are features that have nothing to do with how the Earth moves around the Sun. To take another example: An oscillating electric charge generates electromagnetic radiation. The radiation “cares” not in the least whether the charge is carried by electrons, protons, ionized atoms, or tennis balls. It “knows” that some electrified thing is oscillating in a certain way, but doesn’t “know” or need to know what that thing might be. The nature of the oscillating object (what is) has nothing to do with the radiation that is emitted (what happens).

For the particles, things are not so simple. What the particles are and what they do are intertwined. That’s all part of the strangeness of the subatomic world. So, in the following chapters, you (and I) will have to keep an eye out for times when the properties of the particles get mixed up with the actions of the particles.

Let me also pause to ask why the subatomic world is so strange, why it is so weird and wonderful. Why do the laws governing the very small and the very swift defy common sense? Why do they stretch our minds to the limit? Their strangeness could not have been predicted. The classical scientists (pre-1900) assumed, rather naturally, that ordinary conceptions from the world around us, the world of our senses, would continue to serve us as knowledge accumulated about realms of nature beyond the range of our senses—about things too small to touch and too fast to glimpse. On the other hand, those classical scientists had no way of knowing that the rules would stay the same. How could they be sure—how can anyone be sure—that the “common sense” derived from ordinary observations will serve to account for phenomena that can’t be seen, heard, or touched?

In fact, the physics of the past hundred years has taught us that common sense is a poor guide in the new realms of knowledge. No one could have predicted this outcome, but no one should be surprised by it. Everyday experience shapes your opinions about matter and motion and space and time. Common sense says that solid matter is solid, that all accurate watches keep the same time, that the mass of material after a collision is the same as it was before, and that nature is predictable: sufficiently accurate input information yields reliable prediction of outcomes. But when science moves outside the range of ordinary experience—into the subatomic world, for instance—things prove to be very different. Solid matter is mostly empty space; time is relative; mass is gained or lost in a collision; and no matter how complete the input information, the outcome is uncertain.

Why is this? We don’t know why. Common sense could have extended beyond the range of our senses, but it didn’t. Our everyday worldview, it turns out, is a limited one, based on what we directly perceive. We can only echo the parting words of the respected old TV news anchor Walter Cronkite: “That’s the way it is.” You can be enchanted, you can be amazed, you can be befuddled, but you shouldn’t be surprised.

By the time I was fifty, in 1976, the known subatomic particles numbered in the hundreds. A few were added in the 1930s, a few more in the 1940s, then a flood in the 1950s and 1960s. Physicists had stopped calling the particles “elementary” or “fundamental.” There were just too many of them for that. Yet just as the number of particles seemed to be getting out of hand, physicists were coming up with a simplifying scheme. A manageably small number of particles appeared to be truly fundamental (including quarks, which, to this day, no one has seen directly). Most of the known particles, including the old familiar proton, were composite—that is, built from combinations of the fundamental particles.

We can see an analogy here to what had happened decades earlier with our understanding of atoms and nuclei. By the time the neutron (an uncharged, or neutral, sibling of the proton) was discovered in 1932, the number of known atomic nuclei had grown to several hundred. Each was characterized by a mass and a positive charge. The charge determined the atomic number, or place in the periodic table. In other words, it defined the element (an element is a substance with unique chemical properties). The hydrogen nucleus had a charge of one unit, the helium nucleus a charge of two units, the oxygen nucleus a charge of eight units, the uranium nucleus a charge of ninety-two units, and so on. Some nuclei with the same charge (therefore nuclei of the same element) had different masses. Atoms built around these nuclei were called isotopes. Scientists felt sure that these several hundred nuclear types, of ninety or so elements averaging two or three isotopes each, were built of a smaller number of more fundamental constituents; but prior to the discovery of the neutron, they couldn’t be sure exactly what those constituents were. The neutron made it all clear (though later it, too, was discovered to be composite). Nuclei were constructed of just two particles, the proton and the neutron. The protons provided the charge, and the protons and neutrons together provided the mass. Whizzing around the nucleus in the much larger volume of the whole atom were electrons. So just three basic particles accounted for the structure of hundreds of distinct atoms.

For the subatomic particles, the “discovery” of quarks played a role similar to the discovery of the neutron for atoms. I put “discovery” in quotation marks because what Murray Gell-Mann and George Zweig, both at Caltech, did independently in 1964 was to postulate the existence of quarks, not prove their existence through observation (the name “quark” we owe to Gell-Mann). The evidence for quarks, though still indirect, is by now overwhelming. Today, quarks are recognized as the constituents of protons, neutrons, and a whole host of other particles.

Physicists then invented the standard model of the subatomic particles. In this model there are twenty-four fundamental particles, including the electron, the photon, and half a dozen quarks, accounting for all observed particles and their interactions.* Twenty-four is not as pleasingly small a number as three (the number of particles known in 1926), but so far these twenty-four remain stubbornly “fundamental.” No one has found any of them to be made of other, more fundamental entities. But if the superstring theorists have it right (I’ll discuss their ideas later), there may be smaller, simpler structures that await discovery.

Murray Gell-Mann (b. 1929), 1959. Photo by Harvey of Pasadena; courtesy of AIP Emilio Segrè Visual Archives.

Some of the fundamental particles are called leptons, some are called quarks, and some are called force carriers. Before I introduce you to them, let’s have a look at quantities and magnitudes typically used to describe what goes on in the subatomic world.

 

 

* How small? Ten million atoms in a row stretch less than a tenth of an inch. The scanning tunneling microscope, invented in 1981, showed for the first time the outlines of individual atoms. As late as 1900, many scientists even doubted the existence of atoms.

* These twenty-four do not include the graviton—the hypothetical particle of gravity—or another hypothetical entity, the Higgs particle (the only member of the particle zoo named after a person). Also excluded from the count are the antiparticles.

chapter 2

How Small Is Small?

How Fast Is Fast?

How big—or how small—are the things we measure in the subatomic arena? You probably know that atoms are tiny and subatomic things even tinier, that light moves at enormous speed, that particles fly nearly as fast, and that the blink of an eye is vastly longer than the typical lifetime of a particle. It’s easier to say these things than to visualize them. My purpose in this chapter is to help you “see” the subatomic realm, so that you begin to feel comfortable with the small sizes, the high speeds, and the brief time spans.

It turns out that most of the concepts needed to describe particles aren’t strange at all; they are merely different in scale. Length, speed, time, mass, energy, charge, and spin can be used to describe a bowling ball as well as an electron. In the subatomic realm, the questions are: How big are these quantities? How do we know? What are convenient units in which to measure them?

To deal with the large and the small, we need a simple noncumbersome notation. Many readers may know the notation already. One thousand is 1,000, or 10³. One million is 1,000,000, or 10⁶. One billion is 1,000,000,000, or 10⁹. It looks simple. The power of ten is the number of zeros when the number is written out. But it’s better to think of the power of ten as shorthand for the number of places the decimal point is moved. Thus 243 million, or 243,000,000, become 2.43 × 10⁸. From 2.43 to 243,000,000, the decimal point is moved 8 places to the right. The shorthand involving powers of ten is called exponential notation (or, often, scientific notation).

The rules for numbers smaller than 1 are similar (in fact, they are really the same). One thousandth is 0.001, or 10⁻³ (if you start with 1 and move the decimal point three places to the left, you get 0.001). Suppose a large molecule has a length of 2.2 bill...