The double-slit experiment is one of the more well-known experiments in physics history. First proposed by early 19th century physicist Thomas Young, this experiment is delightfully simple in its set-up, yet deceptively complex in what it tells us about the world.
While light can behave like a particle, the double-slit experiment was originally used by Young to show that light can also behave like a wave. If you need a refresher on the experiment, you can see a version here:
Other versions of the double-slit experiment that used electrons, or even larger chemical molecules, showed that even these less-ephemeral objects can create wave-like interference patterns.
It wasn't until the early 20th century that physicists working on the then-new field of quantum mechanics arrived on the explanation that still holds to this day: wave-particle duality. The theory holds that—in some senses—light, electrons and other tiny things can behave like both a wave and like a particle. For nearly a hundred years the tenets of quantum physics laid down by some of the biggest names in physics—Einstein, Bohr, Planck, and others—have been used to explain the bizarre results of Young's and other similar experiments. Yet persisting in the background has been another explanation of how the world works, and according to Quanta Magazine, recent laboratory research is causing some physicists to take a second look at the foundations of quantum physics.
According to modern notions of quantum physics, at the very smallest scales—in the realm of electrons and photons and quarks—the world is not obvious, direct and deterministic. Rather, the world is one of probabilities. Electrons seem to exist in a cloud of possibilities, inhabiting an area but no particular space. It isn't until you look that this aura of probability collapses and the electron inhabits a particular place.
For some people, such a probabilistic interpretation of the world is simply unnerving. For others, though, the probabilistic interpretation seems unnecessary from a scientific perspective. There might be another way to explain the weird behavior seen in the double-slit experiment that doesn't devolve into quantum mechanics' usual probabilistic weirdness, says Quanta Magazine.
Known as “pilot wave theory” this line of thinking goes that, rather than electrons and other things being both quasi-particles and quasi-waves, the electron is a discrete particle that is being carried along by a separate wave. What this wave is made of no one knows. But recent experimental research shows that, in the lab, particles being carried around by waves will exhibit many of the same weird behaviors that were thought to be exclusive to the domain of quantum mechanics (as seen in the video above).
Not being able to explain what the wave is is a problem, but so is the inherent randomness of modern quantum physics.
The benefit of pilot wave theory is that, if it pans out, it would allow physicists to explain things that happen at even the smallest sizes with the same rules that apply to larger objects. This isn't the case for quantum mechanics, where different sets of rules seem to apply for tiny objects and for larger ones.
Pilot wave theory was first pitched back at the beginning of the 20th century when the ideas of quantum physics were still being laid down, but it never caught on. For a long time the idea faded from vogue, but the new experiments, says Quanta Magazine, means that pilot wave theory is—at least in some circles—coming back.