First in an occasional series on the nature of reality from the perspective of quantum physics.

The world that we live in, what we comfortably call “our reality,” is an illusion. No need to panic, though. The illusion is not evil in any way. It’s only deceptive, as illusions tend to be.

We think we live in a world of continuity: ocean waves crashing on the beach, the air we breathe, the skin that covers our bodies, the metal pots in the kitchen cabinet. Everything looks smooth and continuous. Even grainy stuff, like sand, still has a reasonable size to it. Or, going smaller, even things that we magnify under a microscope appear whole.

Not true, though. At the smallest levels of molecules and atoms, below the scales of bacteria and viruses, the apparent smoothness vanishes into a puff of graininess. And there, in the realm of the very small, a different kind of reality emerges, one with rules so alien to our everyday perception of things that we would—if we could, like Ant Man, shrink ourselves to those tiny atomic scales—believe we lost our minds.

In the world of the very small, everything vibrates. There is no rest.

No one knew this for sure until the early 1900s, although even in Ancient Greece, pre-Socratic philosophers (known as Atomists) conjectured that matter was not what it seemed to the eye; break it down to its smallest constituents, and you would reach an end point, the smallest bit of stuff, the atoms. This notion of small bits of stuff was revived, on and off, during the past four hundred years of what we call “modern science.” Newton, for one, was an avid defender of the atomistic hypothesis, arguing that light was made of bits of stuff that bounce about at incredible speeds.

The debate whether light is a particle or a wave raged on until it appeared settled in the early 19th century, with the spectacular light interference experiments by English natural philosopher Thomas Young: if light interfered with itself as water waves did, light had to be a wave. But in 1905, Einstein proposed a stunning hypothesis—one he considered his most daring idea—that light was, after all, a particle too. He used light bullets to explain the photoelectric effect, or why metal plates, when lit by certain types of light, like violet, but not others, like yellow or red, acquired an electric charge.

How could something be a particle and a wave at the same time? To physicists of the early 1900s, such possibility caused tremendous distress. There was true drama and much confusion. After all, waves and particles are opposites: one is tiny, the other spreads out. But so it was, and they had to deal with it. Often, Nature will shove something down our throats against our will. We may not like what we measure, but we must accept it, even if it contradicts our most cherished notions. Worldviews do crumble. Science does that. It works as a powerful antidote to the illusions we may fall prey to lest we blindly trust our senses or naïve intuitions.

Heisenberg’s Uncertainty Principle

In the mid-1920s, the German physicist Werner Heisenberg came up with a way of describing—but not explaining—the mystery. He called it the Uncertainty Principle. The smaller the object, the more pronounced its dual wave-particle nature. Imagine holding a little ball in your hand; you know its location with high precision. But that certainty is lost as the size of the ball shrinks to atomic scales. The smaller the object, the more restless it is and the higher the uncertainty in its position. In the world of the very small, nothing stands still. Heisenberg’s principle gives life to this bizarre behavior, making it quantitative.

A baseball moving at 70 kilometers per hour (43.5 miles per hour) has an associated wavelength of about 22 billionths of a trillionth of a trillionth of a centimeter (or 2.2 x 10-32 cm). Clearly, not much is waving there, and we are justified in picturing the baseball as a solid object. This is what we call the “classical” worldview, the one we are used to.

In contrast, an electron moving at one-tenth of the speed of light has a wavelength about half the size of a hydrogen atom. (More precisely, half the size of the most probable distance between an electron at the ground state and the atomic nucleus.) While the wave nature of a moving baseball is irrelevant to understand its behavior, the wave nature of the electron is essential to understand its behavior in atoms.

In the end, we shouldn’t be attributing wave-like or particle-like properties to quantum “entities” in any real sense. Particles and waves are objects we understand concretely from our everyday life, things for which we have an associated mental image and words to describe. However, they are crude extrapolations to what goes on in the quantum realm, and shouldn’t be confused with the real thing. Strictly speaking, we can’t really call them objects in the ordinary sense of the word, as something palpable, that can be seen and touched. Strictly speaking, they are not “things,” like a soccer ball or a car is a thing, but mental idealizations invented to characterize the results of experiments.

We don’t see an electron directly, but its existence is undeniable. What happens, then, is that we identify the results of experiments, which we can see and take note of, analyze and quantify, with idealized objects that we can relate to—the electrons, atoms, and molecules of the quantum world.

There is a very subtle conversation going on here between what experiments tell us and the models we construct to translate the experimental results into material entities. But it’s a long one, and necessitates a whole new day to address. Let’s do it soon.


Gleiser is a professor of natural philosophy, physics and astronomy at Dartmouth College.