What Is Light?

Light is the connection between us and the universe. Through light, we could experience distant stars and look back at the beginning of existence itself. But, what is light? In a nutshell, light is the smallest quantity of energy that can be transported: a photon, an elementary particle without a real size that can't be split, only created or destroyed.

Light also has a wave-particle duality, being kind of a particle and a wave at the same time (although this is a lie). Also, when we say light, we actually mean visible light, which is a tiny part of the electromagnetic spectrum: energy in the form of electromagnetic radiation. Electromagnetic radiation consists of an enormous range of wavelengths and frequencies.

Gamma rays have the smallest wavelengths because they are the highest energy photons. But most gamma rays are just under ten picometers, which is still way smaller than a hydrogen atom. For reference, a hydrogen atom compared to a cent is about as big as a cent compared to the Moon. Visible light is in the middle of the spectrum in a range of about 400 nanometers to 700 nanometers: about the size of a bacteria. On the other end of the spectrum, radio waves can be up to 100 kilometers in diameter.

The biggest wavelengths we know exist can span from 10,000 kilometers to a baffling 100,000 kilometers, way larger than Earth. From a physics standpoint, all these different waves are the same. They all have the wave-particle duality and travel at 'c', the speed of light, just at different frequencies. So what makes visible light special, then? Well... absolutely nothing.

We just happen to have evolved eyes that are good at registering exactly this part of the electromagnetic spectrum. This is not a complete coincidence, though. Visible light is the only set of electromagnetic radiation that propagates when in water, which happens to be where most eyes first evolved, millions of years ago. That was a smart move because light not only interacts with matter, it’s also altered by it and can be used to gather information about the world around us, with almost no delay. Which is arguably really helpful for survival.

Okay, where does light come from? A vast range of electromagnetic waves are created when atoms or molecules drop from a higher state of energy to a lower one. They lose energy and emit it in the form of radiation. At the microscopic level, visible light is created when an electron within an atom in an excited state drops to a lower energy state and loses this excess energy.

The same way, incoming light can elevate an electron to a higher state of energy by being absorbed by it. Macroscopically, the moving charge of the electron creates an oscillating magnetic field, which creates an oscillating electric field perpendicular to it. These two fields move themselves through space, transferring energy from one place to another, carrying information about its place of creation with it.

Why, of all the things in the universe, is light the fastest thing there is? Let's change the question: what is the fastest way to travel through space in the universe? It's c, exactly two hundred ninety-nine million, seven hundred ninety-two thousand, four hundred and fifty-eight metres per second in a vacuum, one billion kilometres per hour. Electromagnetic radiation just happens to move this fast.

Any particle that has no mass travels at c, without acceleration or any in between. The light that has released from a candle does not speed up until it reaches light speed. At the very instant of its creation, its speed is c. So why is c, the speed of light, finite then? Well, nobody knows. Our universe is just built this way. We don't have the smart answer here.

So light is part of a spectrum, an elementary particle that also behaves like a wave, propelled by two perpendicular fields, travelling at the speed limit of the universe. Okay, that's nice and all, but what about the crazy stuff about travelling at the speed of light, and time, the twin paradox, quantum stuff, things like that? We would have to save them for another video.

For now, let's be happy that we've evolved eyes that pick up waves of information permeating the universe, making us see things and put our existence into perspective.