As far as physics is concerned, there are two realities: there’s the one we experience every day, where a single object is a single object and nothing teleports through walls or exists in two places at once, and then there’s the one for the very, very small, where those rules don’t hold anymore. These are the worlds of classical and quantum physics, respectively. But everything is made up of quantum particles if you look close enough — and that means that many more of your everyday experiences come down to quantum physics than you realize.
Do You See What I See?
Light is one popular example of the weirdness of the quantum world. It exists simultaneously as a continuous wave and as a discrete particle known as a photon. The most famous demonstration of this bizarre phenomenon is the double-slit experiment, where individual photons that pass through a wall with two slits produce patterns on a screen as if they’re all waves interacting — until you set up a photon detector to measure which slits each photon passes through, at which point they produce two bright lines as if they’re individual photons. The idea is that quantum particles exist as both particles and waves, also known as being in superposition, until they’re measured or observed, at which point they collapse into a single state.
Your eyes create an image of the world thanks to light hitting your retinas. Is it a stretch to call them photon detectors? According to recent research, no: Studies suggest that humans may be able to detect a single photon at a rate better than chance. That opens up the intriguing possibility that our eyes could also detect quantum phenomena. Scientists have already tried to see whether human volunteers can detect quantum entanglement (to mixed results), and there are plans in place to see if they can tell whether a photon is in superposition. In all likelihood, nothing will come of the efforts — most experts think that the eye is too imperfect a measuring device to really detect these phenomena. But you never really know until you try.
Touching the Void
Your sense of touch, too, is based in the quantum world. Even the densest object you’ve ever held is made up of mostly empty space. That’s because matter is made of atoms, and atoms are made up of a very tiny nucleus surrounded by even tinier electrons orbiting a relatively huge distance away. If you blew the nucleus up to the size of a marble, you’d have to traverse the length of a football field to reach the farthest electrons. That’s a whole lot of nothing to contend with.
Things only feel solid because of quantum physics — the Pauli exclusion principle, to be precise. At its most basic, this principle says that there’s a limit to how many electrons can hang out in a specific orbit around an atom. For an electron from an atom in your hand to elbow its way into an atom in your coffee cup would require more energy than your hand is willing to exert. So instead, those electrons repel each other, which feels to you like you’re touching a solid object.
Wake Up and Smell the Tunneling
Your strangest quantum sense of all might be smell. When you catch a whiff of coffee brewing, it’s because airborne coffee molecules have drifted through the air and into your nose, through a thin layer of mucus, and into smell receptors in nerve cells that connect directly to your brain’s olfactory bulb. (Fun fact: these smell-centric nerve cells, or olfactory neurons, are the only part of your central nervous system that’s always exposed to the outside world.) How those receptors translate molecules into scents, though, is anyone’s guess. The most popular theory likens molecules and olfactory neurons to a lock and key: molecules of a certain shape fit into receptors of a certain shape, and your brain goes, “Oh, I know that shape. It’s coffee.”
There’s a problem with this model, though: Molecules of very different shapes can all smell the same, and molecules of similar shapes can smell very differently. There must be some other difference that the receptors are picking up on. That difference may lie in quantum physics.
Molecules all have a different vibrational frequency, depending on their mass, their bonds, and their structure. It’s possible that our noses detect the differences in vibration between molecules, rather than just their shape. There’s some evidence for this: A scientist in the 1990s compared the scents of two molecules with different shapes but identical molecular frequencies and found that they smelled exactly the same.
If our noses really are detecting vibrations, it’s thanks to quantum tunneling. That’s a phenomenon that arises thanks to the way particles can be in many states — and many positions — at once, which sometimes teleports them through solid barriers. Molecular vibrations might provide the right oomph to make an orbiting electron tunnel from one part of an odor receptor to another. When a molecule’s vibrational frequency matches the energy of a certain receptor, that makes it more likely that an electron will tunnel. That particular tunneling rate might trigger particular nerve impulses, which create perceptions of particular scents in your brain.
Sight, touch, and smell aren’t necessarily the only senses that invoke quantum physics; the hair cells in your inner ear are sensitive to movements on the subatomic scale, for instance. It’s just that so far, scientists haven’t delved into the fundamental particles at work in your senses of hearing and taste. Who knows — we may find out that every classical physics phenomenon we experience is really the work of quantum physics. Who says there’s a dividing line between the two, anyway?