eli5: Why is it significant that light is both a wave and a particle?
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Because it is counterintuitive, and was a debate for a long time, back in the day when people thought something was either described as a wave or a “solid” particle, before being accepted as true. A “pure” particle (theoretical, because they don’t exist) couldn’t self-interfere in a double slit experiment, while “pure” waves didn’t do things like point interactions (like being absorbed by another particle).
We’ve since found out that nothing is a pure particle, and pretty much all waves can have “quasi-particles” (as in, they don’t have all properties a “real” particle have, just some) assigned. And while that might seem weird, the “particles of sound” phonons actually have real quantum effects in supercondutors. It’s a topic waaaay beyond ELI5, but I’m mentioning it to show it’s not just a fun theoretical idea, it has real effects.
Light isn’t both a wave and a particle.
The confusion was because it has some properties that are wave-like and some that are particle-like.
For example, diffraction. Pass light through a narrow slit and it spreads out, like a wave. This wouldn’t happen with individual particles (at least not with larger classical particles) but makes perfect sense if light is a wave. By the end of the 19th century it was widely accepted that light is an electromagnetic wave.
But then there was the photoelectric effect. Basically you have a metal plate with electrons on it and set up some sort of experiment where you can detect how many electrons have been knocked off the plate after you shine a light on it. What was weird is that, with certain wavelengths of light, no electrons would be knocked off the plate, no matter how intense the light or how long you shine it on the plate. This doesn’t make sense if you think of light as a wave, the energy should eventually build up enough to displace the electrons at any wavelength. It does make sense if you think of light as a stream of particles–if the individual particles of light don’t have enough energy, they won’t displace any electrons, no matter how many light particles there are. If each particle does have enough then each one can displace a single electron.
There’s more to it than that and I’m simplifying a lot, but the point is, some experiments only made sense if you think of light as a wave, some experiments only make sense if you think of light as being made of particles. This seems like a contradiction because in classical physics, you can’t be both. Heavy particles don’t seem to act like waves at all, and other types of wave don’t act like particles.
The actual answer is that light is neither. Quantum objects are a different type of thing that act in some ways like particles and in some ways like waves without really being either. But sometimes we model them as particles and sometimes we model them as waves when that makes things easier to understand. Like how we know Newtonian physics isn’t quite right but we still use it anyway because it’s accurate enough a lot of the time, we know that all matter has wave-like properties but those effects are often small enough that we ignore them.
For a long, long time, it was believed that we had more or less figured out the basic principles of science and further details would just support what we already believed: as one scientist said in the late 1800s: “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”
In physics before this point, everything’s behavior could be described either by the rules for particles, or the rules for waves. It seemed like the world fit in these nice boxes.
But light–to paraphrase Einstein–sometimes acts as one thing, sometimes the other, sometimes neither, sometimes both.
The duality of light was one of the earliest indications that physics was not, at all, close to completion.
Because everything is both a wave and a particle. The subatomic particles making up our atoms have similar property of both being a wave and a particle.
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