Chapter 7: Quantum Physics

What is Quantum Physics?

As if Einstein’s theories weren’t mind-bending enough, things get even weirder in the microscopic world of atoms and subatomic particles. At the end of chapter 5, we took inventory of the various types of particles (at least, the known ones) that comprise the physical world; but I said very little about the behaviors of these particles. In this chapter, we’ll encounter some perplexing discoveries about microphysical systems—tiny particles or collections of particles, such as electrons, atoms, and molecules.

Some of the theories concocted to explain these discoveries have profound philosophical and theological implications, so a word of caution is in order. The ways in which tiny pieces of matter behave and interact with each other are deeply mysterious, and there is little agreement (even among physicists,Physicists generally agree on the mathematical formalism of quantum mechanics, but the interpretation of that formalism is another matter. We’ll discuss a variety of (radically different) interpretations of quantum mechanics later in this chapter. let alone philosophersAs a professional philosopher myself, I am happy to admit that we philosophers seldom agree on anything. Challenging assumptions, examining arguments, and defending ideas pretty much sums up the job description of a philosopher. It’s what we do.) about how we should understand what is really going on at the most basic level of physical reality. We’ll consider a number of different “interpretations” of quantum physics in this chapter, but I won’t attempt to survey all of the diverse ways in which physicists have tried to make sense of the microphysical world.

To set the stage for quantum physics, we’ll start with a little bit of history. As Einstein developed his innovative theories of relativity, another revolution was beginning in physics. In 1905, the year he proposed his special theory of relativity, Einstein also published a groundbreaking paper on the photoelectric effect—the fact that electrons are emitted when light shines on a piece of metal. (The effect happens with other materials too, but the resulting electric current is most easily detectable with conductive metals.)

Physicists had already figured out that this occurs because electrons absorb energy from light, and if the electrons absorb too much energy, they’ll escape the pull of the nucleus and fly out of the atoms to which they were attached. However, there was something puzzling about the photoelectric effect. Remember that light consists of electromagnetic waves, according to Maxwell’s theory. Since waves ordinarily transfer energy by a gradual, continuous process, Maxwell’s theory seemed to imply that low-energy (low-frequency, long wavelength) light should eventually transfer enough energy to free electrons from any atom, though it might take longer with low-energy light than with high-energy light. In experimenting with the photoelectric effect, however, physicists made two surprising discoveries:

  1. No electrons were emitted when the energy of the light was below a certain threshold (that is, when the frequency of the light was too low), regardless of how long the light shone on the metal.
  2. If the energy of the light was above the threshold needed to free the electrons, increasing the energy even further didn’t make any difference: the electrons weren’t emitted any faster.

To explain these surprising observations, Einstein hypothesized that light waves aren’t really waves in the ordinary sense. Light waves aren’t continuous, but act as discrete (non-continuous) packets of energy, which he called quanta:

The energy of a light wave … is not spread continuously over ever larger volumes, but consists of a finite number of energy quanta that are spatially localized at points of space, move without dividing and are absorbed or generated only as a whole.Quotation from this English translation of Einstein’s 1905 paper “On a Heuristic Point of View about the Creation and Conversion of Light.”

In other words, light doesn’t always act like a wave; it also behaves in some respects as though it consists of little particles, or packets of energy. (These quanta or “particles” of light were later dubbed photons.) Einstein’s hypothesis provided a tidy explanation for the two observations mentioned above. If a quantum of light has enough energy to free an electron from an atom, the electron will be emitted instantly. Otherwise, the electron won’t be emitted at all, regardless of how many low-energy quanta hit it.

A few years later, in 1913, Niels Bohr proposed a similar hypothesis regarding electrons. As you may recall from chapter 4, Bohr’s Model of atoms suggested that electrons aren’t really particles—at least, they’re not particles in the familiar sense. They’re not like tiny grains of sand, or pieces of dust, or anything like that. Electrons behave as particles in some ways, yet they also act like waves.

As it turns out, photons and electrons aren’t the only things that behave as both waves and particles. Other fundamental particles, entire atoms, and even large molecules also exhibit wave-like properties in certain kinds of experiments (e.g. double-slit experiments, which will be discussed on the next page). At sufficiently small scales, practically everything exhibits this dual nature, known as wave-particle duality.

The curious behaviors of photons, electrons, and other microphysical objects could not be adequately described in terms of classical physics—Newton’s laws and Maxwell’s equations. Nor could they be explained using Einstein’s special and general theories of relativity, which give essentially the same predictions as classical physics for things that aren’t either very fast or very massive. A new theory was needed to explain how things work at the fundamental (most basic) level. Thus, in the years following Bohr’s proposal, a new theory called quantum mechanics gradually took shape. (Mechanics is the branch of physics that studies the motions of material objects, as you may recall from chapter 2.) In contrast to Einstein’s theories, quantum mechanics was not the brainchild of any individual person, but was pieced together by many scientists over a period of decades. The earliest complete formulations of quantum mechanics were developed in the 1920s, but these versions of the theory were inconsistent with Einstein’s theories of relativity.

Eventually, early versions of quantum mechanics were superseded by quantum field theories, which regard fields rather than particles as the fundamental components of physical reality. Quantum field theories form the basis of the contemporary Standard Model of particle physics, which was briefly introduced in chapter 5. These theories are consistent with special relativity, but not with general relativity. To date, there is no theory that can account for the predictions of general relativity and the predictions of quantum mechanics. Hopefully, someday, both of those theories will be superseded by an even better theory—a “theory of everything” as physicists like to call it.

We’ll talk about that later in this chapter. First, let’s examine some of the surprising—no, shocking, maybe even spooky—discoveries that led to the admittedly incomplete theories of quantum physics used by physicists today. Whatever the truly fundamental laws of physics turn out to be, they will have to be quite strange indeed.