Mixed metaphors

When people first encounter an idea, they often frame it as a combination of previously understood ideas, like the sea-horse on the left. The reality (right) may be a different thing altogether.

This is the third in a four-part series on quantum mechanics. Previously, I discussed the discrete yet multivalued nature of quantum properties and the fact that quantum cause-and-effect need not be forward in time. Another oddity of quantum particles is that they are sometimes waves, not particles, though this is an altogether different kind of paradox.

While physicists were discovering quantum mechanics, they had already been debating whether matter and light are made of hard, indivisible particles or fluid waves. Early guesses by Democritus and Isaac Newton favored particles, but by the turn of the 20th century, there was strong evidence that light is a wave. These conceptual categories, “particle” and “wave,” are based on experiences with everyday things like pebbles and water, generalized by mathematical abstraction. It is amazing how broadly applicable these simple ideas are to a wide range of natural phenomena, but both fail to describe what matter does at its smallest scales. Quarks, electrons and the rest are neither waves nor particles. They are another thing entirely.

The particle-like aspect of quantum objects is that they may entirely interact when they collide or they may entirely miss each other. This is what little rocks do when they’re thrown at each other: They hit or they miss. It would be very strange for a wave to work this way. Imagine making a splash in Virginia Beach, watching the wave spread out across the Atlantic Ocean, and then seeing the full splash hit someone in Cape Town, Lisbon, Plymouth, Reykjavik or some other single place along the African or European coast. And yet light, which has clear wave-like properties, does entirely hit or entirely miss when individual photons collide.

The wave-like aspect of quantum objects is that they tend to spread out. Moreover, they can overlap each other and sometimes cancel each other out, like carefully timed waves. It’s difficult enough imagining two pebbles occupying the same space at the same time, stranger still to imagine them becoming zero pebbles in the place where they overlap. Electrons engage in this kind of behavior.

Though often framed as a paradox, these properties are not inconsistent with one another, just never seen together at macroscopic scales. Our human notions of particle and wave are both inadequate for the microscopic world. The best metaphor that I think captures the behavior of quantum objects is the splash that propagates across the Atlantic and pops up with full force in some single (random) place. Unlike the time paradoxes, this aspect of quantum mechanics does not push us to the limits of what is conceivable — we just have to approach it with the willingness to make new metaphors.

Next week, I’ll present uncertainty in quantum mechanics and the experiments that tell us that the universe we live in is a quantum universe.

Jim Pivarski