Quantum physics describes the behavior (and misbehavior) of tiny things: atoms, photons, and electrons, to name a few. What electrons lack in size, they make up for in importance. Electrons are the glue in chemical bonds, so quantum physics is used to understand the chemical bonds that hold together metals, plastics, skin, and every other material. Electrons are the lifeblood of computer chips, for instance, so engineers use quantum physics to design faster, smaller devices. Wherever quantum physics is applied, it’s unerringly accurate.
The most amazing feature of quantum physics isn’t its accuracy or its usefulness, but its brazen defiance of our common sense. Quantum physics challenges our basic understanding of reality itself. And yet, quantum physics started off in a very mundane way, seeking explanations for dry, quantitative data.
The most amazing feature of quantum physics isn’t its accuracy or its usefulness, but its brazen defiance of our common sense. Quantum physics challenges our basic understanding of reality itself.
For example, hydrogen gas can emit four colors of visible light: violet, blue, aqua, and red. Physicists had carefully measured the wavelengths of these four colors: 410 nanometers, 434 nanometers, 486 nanometers, and 656 nanometers. Surely there’s a reason for these four specific numbers. But what is the reason? Physicists were scratching their heads. In 1885, a physicist even came up with an equation that fit all four wavelengths, but there was no explanation for the equation. It was a purely empirical equation, with no theory behind it.
Finally, in 1913, Niels Bohr came up with a theory that explained the four wavelengths. He claimed that the electron in the hydrogen atom is constrained to have certain amounts of energy. The electron cannot gain or lose energy smoothly; it can only make “quantum leaps” from one allowed energy level to another. Whenever an electron drops from one energy level to a lower energy level, it releases the energy in the form of light. The light emitted in a single quantum leap is called a photon. A photon is the smallest possible quantity of light with a particular wavelength. More generally, the smallest possible quantity of something is called a quantum.
These new quantum ideas had already solved two other mysteries. Max Planck explained the wavelengths of light emitted by hot objects, and Albert Einstein explained how photons knock electrons off the surface of metals. But even as quantum physics accumulated triumphs and grew in sophistication, it began to hint at deep mysteries in the fundamental nature of reality.
The fundamental quantum equation, which was established by Erwin Schrödinger and then published in 1926, dealt with probabilities: the likelihood of an electron appearing one place or another. Probability was not unfamiliar; the outcomes of coin tosses are also given as probabilities. But once a coin lands, the side that faces up is an objective fact, regardless of whether anybody knows what it is. In contrast with this common understanding of objective facts, the new quantum theory began to hint at a fundamental unknowability or uncertainty in unobserved particles. This conundrum drove Schrödinger to complain about the implications of his own equation.
Schrödinger asks us to imagine a cat trapped in an opaque box with an “infernal machine.” The machine includes a radioactive material that occasionally emits a particle that can be detected by a Geiger counter. If the Geiger counter detects a particle, it triggers the release of a poisonous gas, which kills the sacrificial cat. The radioactive emission is governed by quantum physics. Quantum theory can specify only the probability that a particle will be emitted to trigger the release of poison gas. But unlike a tossed coin, which lands heads or tails up regardless of whether anyone observes it, quantum predictions aren’t so easy to interpret. Quantum theory implies that before a measurement is performed, somehow the particle is neither emitted nor not emitted, or (equivalently?) both emitted and not emitted. In this case, the poison gas is both released and not released, and the cat is both dead and alive. This confusing condition persists until a measurement is performed. But what constitutes a measurement? The intervention of a conscious observer who looks in the box? Or simply the interaction of the emitted particle with the Geiger counter?
Making matters worse, in 1927 Werner Heisenberg showed that the more precisely an electron’s position is known, the more uncertain its speed becomes. The electron seems committed to not being pinned down. When an electron takes the witness stand, it never agrees to tell the whole truth (both its position and its speed). But does its refusal to tell the whole truth hint at a deeper truth? Are quantum measurements like breezes through a curtain, giving us shifting glimpses of a reality that is never fully revealed?
Some scientists argue that quantum physics predicts outcomes of measurements and nothing more; we shouldn’t even ask the question “What does it all mean?” At least, we shouldn’t claim to know what particles are doing when we’re not measuring them. This is a form of Bohr’s “Copenhagen interpretation,” though the Copenhagen interpretation itself has been interpreted different ways by different people.
Are quantum measurements like breezes through a curtain, giving us shifting glimpses of a reality that is never fully revealed?
People like Einstein were fed up with vagueness, uncertainty, and contradictions. If 1984 had already been written when these physicists were grappling with these qualities of quantum mechanics, Einstein would have accused his opponents of doublethink: “Doublethink means the power of holding two contradictory beliefs in one’s mind simultaneously, and accepting both of them.”1 Surely nature itself is not guilty of doublethink. Surely quantum physics can be massaged and refined, retaining its accuracy while eliminating the fuzziness and absurdity.