100 - What is Light? A Journey from Ancient Philosophy to Quantum Physics

Dan Hooper (0:01)

Today, we know that light is made of photons or pieces of electromagnetic waves. These photons are particles, but these particles also act very much like waves, which travel through space at an incredible 300 million meters every second.

But long before we knew any of these things about light, human beings have wondered about what light really is and how it really works. Today in the show, we're going to talk about some of the theories of light that people proposed going back to ancient times, and explore how people made sense of light when we were just starting to learn about it, and how we got from where we were then to where we are today. Stick around.

You're listening to Why This Universe, a podcast where we break down the biggest ideas in physics. My name is Dan Hooper.

I'm a professor of physics at the University of Chicago, and I'm going to talk to you about some of the oldest writings in the 5th century BCE, which we had to go out and discover in the world before we even knew they existed. For as long as there have been human beings, I imagine they've had some sort of conception of light. The oldest writings that I could find on the nature of light come from the 5th century BCE, and by writings here, I really mean what people later on said about what these people wrote back in the 5th century BCE. None of the original writings have survived to the present day. So let's start with the pre-Socratic philosopher Empedocles. So Empedocles wrote or said or written that he believed everything in the world was made up of four elements in different combinations. These were fire, earth, air, and water. And he said that Aphrodite, the goddess Aphrodite, made the human eye out of those four elements and lit some sort of inner fire in the eye that would shine out from the eye, and it was this inner fire that made sight possible.

Now I don't know about you, but when I first started to think about this theory, it didn't take me long before I began to notice some problems with it. For example, if it's our eye that's sending out the rays that allow us to see, it would seem to imply that we should be able to see just as easily during the night as we can during the day. But Empedocles, of course, was a smart guy, and he realized this too. So to solve this problem, he postulated there is some kind of interaction that must take place between the rays that were coming from the eye and then rays that were coming from the things we're looking at, what he called effluences. Both of these things had to be present, the rays from our eye and the rays from the light source. So if both of those weren't involved, we wouldn't be able to see something. At least that was his idea. Plato later on would take up some of these ideas, endorsing at least parts of them. And he also argued that vision occurred through some kind of fire emitted by the eyes, a lot like in Petticles, interacting with light from various objects.

Let's step forward a while to about 300 BCE with the Greek philosopher Euclid. So Euclid is most famous for his book Elements, which was really about geometry. He laid out the geometrical system that persisted in place until the Middle Ages. Huge contributions to mathematics and geometry. But in addition to this, he also wrote a book about light that he called Optica. In a lot of ways, Euclid thought about light and vision in the same way that Empedocles and Plato had. But he applied a lot more math to the problem, especially the mathematics and geometry. He did things like argue that light travels in straight lines, which is more or less true. And he also described the phenomena of reflection and wrote down mathematical laws that described how light behaves when it's reflected off the surface of an object.

Euclid argued against Empedocles' theory of vision, pointing out that we see stars instantly when we look up at the night sky. This might be okay if the rays from our eyes traveled infinitely fast or so fast that we didn't notice any time delay. But if you think sight works by sending signals from our eyes to the thing we're looking at, it might be hard to understand why it doesn't take more time for us to see things that are very far away, like stars.

Already, one of the most fundamental and long-standing questions about light was starting to be debated among these ancient Greek philosophers. That question is whether or not light is made up of particles or something else, something non-particle-like. On the one hand, there were figures like Aristotle, who thought that light had to travel through a medium like air or water. This wasn't like a modern theory of wave-like light yet. Aristotle wasn't talking about things like oscillations or interference or any other wave-like features. But it was at least a small step in the direction of something that could begin to resemble a wave-like theory of light. On the other side of this debate were the Greek atomists. The atomists argued that everything, including light, is made up of indivisible particles, or what they called atoms. These aren't the kind of atoms that we talk about in physics and chemistry in the modern era, but it's where we get the name. They basically just mean pieces that can't be broken up into smaller pieces, indivisible particles.