The act of measuring reality alters what you’ll observe

When we divide up matter into the smallest possible chunks that it’s made of — into the stuff that can be divided or split no further — those indivisible things we arrive at are known as fundamental particles: the elementary quanta that compose our Universe. But unlike the macroscopic, familiar objects that make up our world, we can’t simply assign definitive properties to them like “position” and “momentum” in an unambiguous fashion. Instead, the story gets complicated each time we ask the question: how does each individual quantum behave?

Do they behave like particles? Or do they behave like waves? The most puzzling fact about quantum mechanics is that the answer you get depends on how, or even whether, you look at the individual quanta that are part of the experiment.

If you make certain classes of measurements and observations, they behave and interact like particles; if you make other choices, they behave and propagate like waves. Whether and how you observe your own experiment really does change the outcome, and the double-slit experiment is the perfect way to illustrate how, even more than 200 years after it was first performed. This diagram, dating back to Thomas Young’s work in the early 1800s, is one of the oldest pictures that demonstrates both constructive and destructive interference as arising from wave sources originating at two points: A and B.

This is a physically identical setup to a double-slit experiment, even though it applies just as well to water waves propagated through a tank. The locations marked C, D, E, and F correspond to 100% destructive interference. Back in the late 1790s and early 1800s, the first double-slit experiments were performed by Thomas Young, who was seeking to investigate whether light behaved as a wave or as a particle.

Newton had famously claimed that light must be a particle, or corpuscle, and was able to explain a number of phenomena with this idea. Reflection, transmission, refraction, and any ray-based optical phenomena were perfectly consistent with Newton’s particle-like view for how light should behave. But other phenomena seemed to require a wave-like interpretation in order to explain them: interference and diffraction in particular.

When you passed light through a double-slit, it behaved in a similar fashion to how water waves behave, producing a familiar set of interference patterns in both cases. Instead of the crests and troughs that appear in water waves, light passed through a double-slit would instead exhibit alternative light-and-dark bands, which appeared on the screen behind the double-slit mask. These bands corresponded to constructive-and-destructive interference, indicating that — at least under the right circumstances — light indeed behaves just as a wave does.