101 Quantum Questions

There is no definitive list of “big ideas” in quantum physics. But here are a dozen that capture much of the essence of what quantum physics brought to the description of nature. What they all have in common is that they are inconsistent with “common sense”—that is, with how we expect the physical world to behave based on everyday experience.

There is a simple reason for the disconnect between common sense and quantum sense. The world we inhabit is a world in which quantum effects—and relativistic effects—do not directly impinge on our senses. We build our view...

A quantum is a lump, a bundle. In the everyday world around us, there are lots of things that come in “lumps” of a certain size: loaves of bread, quarts of milk, automobiles. But there is no law of nature saying how big the loaf of bread or the bottle of milk or the automobile has to be. The baker could add or subtract a slice or even a crumb (see Figure 1). The dairy could decide to sell milk by the half-liter or by the pound. The car company could make its product just a little larger or smaller,...

Let me start with the second question. Inside the nucleus are protons and neutrons. The proton carries positive electric charge; the neutron is electrically neutral (thus its name). Although very different electrically, these particles have comparable mass and comparable size. The proton’s mass is some 1,836 times that of the electron; the neutron is 1,839 times more massive than the electron. Both measure a bit more than 10⁻¹⁵ m across, a hundred thousand times smaller than an atom but some thousands of times larger than the smallest distances now being probed in particle physics experiments. They are so “big” because...

A nanometer is a billionth of a meter (written as 1 nm, or 10⁻⁹ m), and it is a handy unit in the atomic and molecular realm. Ten hydrogen atoms standing shoulder to shoulder would stretch about 1 nm, and four or five water molecules would cover about the same distance. Tiny circuits or structures of dimension 10 to 100nm are said to be at the nanoscale. It is a scale right at the boundary line between the quantum and classical worlds. Some X-rays have a wavelength of about 1 nm. The light that you see has wavelengths about five...

Any intrinsic property of a particle is, almost by definition, lumpy—that is, quantized. Such properties include the particle’s mass, its charge, and its spin—and other things that I haven’t discussed yet (or have only mentioned), such as lepton number, baryon number, and color charge. Anything that characterizes the particle and can’t be changed without changing entirely the nature of the particle is quantized. Properties associated with the unconfined motion of the particle, on the other hand, are smooth, not quantized. A particle moving freely can have any speed or momentum or kinetic energy. Think of an automobile. Its...

Long before there were photons, there was light. That is to say, scientists knew a great deal about light long before they recognized its quantum nature. In the seventeenth century, Isaac Newton used a prism to separate white light into its constituent colors and called the fan of colors a spectrum. In the century that followed, scientists used prisms to study not only sunlight and candlelight, but also light emitted by specific substances, and they found that the relative intensity of different colors depends on the substance. A flood of discoveries followed in the nineteenth century, beginning with Thomas Young’s...

The ending of the periodic table (at $ {\cal {Z}}=118 $ for all currently known elements, at $ {\cal {Z}}=82 $ for stable elements*) has nothing to do with electrons and everything to do with nuclei—and, specifically, everything to do with the electric repulsion between protons. The strong nuclear force attracts protons to each other, neutrons to each other, and neutrons to protons. In short, it acts to bind all nucleons together. It doesn’t reach out very far, not even as far as from one side to the other side of a large nucleus. It acts only when nucleons are close together. The electric repulsion...

The questions in Sections V and VI moved down the scale of size through atoms to atomic nuclei. In this section and the next, I continue on down that scale to particles. Their names and their properties can come across as a confusing jumble, but they stand as ideal exemplars of the quantum world—first, because they are, at least for now, the ultimate bits of matter and energy that underlie all else, and second, because their properties and behavior illustrate particularly clearly how the governing principles of quantum physics work. That is why I address the particles now, before...

One of the impediments to learning about particles is nomenclature. There is a confusing array of names, some of them whimsical. Here is a glossary of some of the terms used by particle physicists. It should help as a reference point. Some of these terms have appeared in earlier sections; some appear in this and later sections.

Leptons—Fundamental particles of spin ½ that do not feel the strong interaction. There are six leptons: three of charge −1 (electron, muon, and tau), and three that carry no charge (the electron neutrino, muon neutrino, and tau neutrino).

Quarks— Fundamental particles of...

Richard Feynman was a brilliant, playful, endlessly curious, much-admired physicist who earned his doctorate from Princeton University in 1942, made major contributions to the Manhattan Project in World War II while still in his twenties, and, after that war, became a world leader in theoretical particle physics. His 1965 Nobel Prize in Physics (shared with the American Julian Schwinger and the Japanese Sin-Itiro Tomonaga) recognized his contributions to what is called quantum electrodynamics (or QED), the study of the interaction of photons with charged particles, especially electrons and their antiparticles, positrons (see the footnote on page 16). Part of his...

In the world around you, it appears that nothing is static. Clouds shift, leaves flutter, you move from place to place. As Euripides put it in the fifth century b.c., “All is change.” It’s not hard to agree with that sentiment. And indeed, the focus of most scientists looking at nature in the twenty-five centuries since Euripides has been on change. That was Newton’s focus in the seventeenth century when he studied how things move in response to forces. It was Maxwell’s focus in the nineteenth century when he studied the emission of radio waves by oscillating electric charge.

But...

At first thought, it might seem that waves and particles have next to nothing in common. Waves are spread out over space and have nebulous boundaries. You can’t say that a wave is “at that spot.” Particles, by contrast, are little nuggets with well-defined boundaries—or perhaps, if they are leptons or quarks, with no physical extension at all. Particles have mass. We don’t usually assign mass to a wave. Waves are characterized by wavelength, frequency, and amplitude, concepts that have no obvious counterpart for particles. Waves can diffract (bend around corners) and interfere (reinforce or cancel one another), behaviors...

I have referred earlier to the role of probability in quantum physics—for instance, that an electron has a certain chance to be found at various places within an atom, or that a photon passing through a double slit has a certain chance to land at various spots. Something called the wave function controls this probability. A wave function gives an amplitude, a measure of how strongly a wave deviates from a zero value, either positively or negatively. This is analogous to how much water surface deviates, upward or downward, away from its normal level as a wave passes by....

The short answer to the question is “Electrically.” Charged particles can be made to go faster by electric forces.* Magnetic forces change their direction without changing their speed. Cathode ray tubes make use of these facts. Within such a tube, electrons are accelerated to high speed by electric forces, then steered to points on the screen by magnetic forces. Particle accelerators also make use of both kinds of force. In a so-called circular machine (they are not exactly circular), particles—most often protons—are steered around a loop by magnetic forces and pushed to higher and higher energy by pulses...

Black holes were named (by John Wheeler, in 1968), taken seriously, explored theoretically, and finally identified in nature, all without invoking quantum physics. They seemed to be wholly classical objects. Actually, Wheeler at one point hoped that quantum physics would save the world from black holes. He said that he did not want to believe in these strange entities in which matter and energy are crushed to a point (a “singularity”). He hoped that some attribute of quantum physics at the subatomic scale could put a stop to the collapse and prevent it from being total. Perhaps, he thought, a...

Most of the quantities we deal with come with a unit: 6 feet, 3 miles, 2 days, 5 pounds, 40 watts. Change the unit and you change the number. Six feet is 2 yards; 3 miles is 4.83 km; 2 days is 48 hours; and so on. But some numbers are “pure,” or dimensionless. The number of sides on a die is six, no matter in what units you measure the size or the weight of the die. And the ratio of two quantities with the same unit is also dimensionless. If it is fourteen miles to your aunt’s house...