How Light Gets Made: Feynman on Electromagnetism

What’s Waving When Light Waves?

Look, we can see light. We measure its speed. We split it into colors with a prism and marvel at rainbows. But here’s the question that should bother you: what IS it? What’s actually traveling through empty space from the sun to your eye right now?

It’s not like sound, which needs air molecules to bump into each other. Light crosses a hundred million miles of vacuum without breaking a sweat. For centuries, physicists convinced themselves there must be some invisible substance filling space—they called it “aether”—that light waves through like sound waves through air. They were wrong, beautifully wrong, because the truth is even stranger.

Light isn’t carried by anything. Light IS the wave. More precisely, it’s a self-sustaining dance between electric and magnetic fields, and understanding this mechanism is one of the most elegant pieces of physics you’ll ever encounter. When James Clerk Maxwell wrote down his equations in the 1860s, he discovered something shocking: the math predicted waves that travel at exactly the measured speed of light. Not approximately. Exactly. That’s not a coincidence—that’s nature telling you something profound.

Fields That Make Themselves

Here’s what’s really going on. Start with electric fields. Put an electron somewhere, and it creates an electric field around itself—think of it as invisible fabric permeating all of space, turned “blue” by negative charge. Put a proton down, it creates a “red” field. Particles move away from their own color and toward the opposite. Simple enough.

Now add motion. When charges move—when you get an electric current flowing through a wire—something new appears: a magnetic field. Moving electrons create magnetic fields that loop around their direction of travel. Wind the wire into a coil, let current flow, and you’ve built an electromagnet. Same principle as the permanent magnets on your refrigerator, except you can turn it on and off.

But here’s where it gets interesting. These aren’t separate phenomena. Electric and magnetic fields are two faces of one unified electromagnetic field. What you see as “electric” or “magnetic” depends on your reference frame, on whether you’re moving relative to the charges. Magnetism is literally just electricity viewed from a different perspective—it’s a relativistic effect. When you move past a current-carrying wire, electric forces that perfectly cancelled when you were stationary suddenly don’t cancel anymore. That imbalance IS the magnetic force.

Maxwell’s genius was recognizing the full circle: not only do moving charges create magnetic fields, but CHANGING magnetic fields create electric fields, and CHANGING electric fields create magnetic fields. The fields make each other. Once you start the process, it can sustain itself without the original charge.

How to Make an Electromagnetic Wave

Let’s build a wave from scratch. Take a charged particle—an electron, say—and shake it. Accelerate it back and forth.

When that electron was sitting still, it had a static electric field around it. But when you accelerate it, several things happen at once. First, the motion creates a magnetic field. But because you’re accelerating—changing the motion—you’re creating a CHANGING magnetic field. And here’s the key: that changing magnetic field disturbs the electric field.

Now you’ve got a disturbance in the electric field. That disturbance propagates outward, and as it does, it creates a changing electric field. Which disturbs the magnetic field. Which disturbs the electric field further out. Step by step, the fields influence each other in a cascading mutual disturbance.

The ripple propagates outward from your shaking charge. You’ve created an electromagnetic wave.

This is why all warm objects glow, even if you can’t see it. Temperature is just atoms jiggling. Every atom is a collection of charged particles—positive nuclei and negative electron clouds connected like tiny springs. When they vibrate with thermal energy, those charges accelerate. Accelerating charges radiate electromagnetic waves. Your body at 37 degrees Celsius constantly broadcasts infrared radiation. Thermal cameras don’t see heat—they see the electromagnetic waves your accelerating atoms emit.

The faster you shake the charge, the higher the wave frequency. Wiggle it slowly, you get radio waves. Faster, microwaves. Faster still, infrared, then visible light, then ultraviolet, x-rays, gamma rays. Same phenomenon, different frequencies. The electromagnetic spectrum isn’t multiple types of radiation—it’s one unified thing.

Maxwell’s Speed Limit

Now comes the beautiful part. When Maxwell wrote down his four equations describing how electric and magnetic fields behave, something remarkable fell out of the math.

The equations said: electric charges source electric fields. Magnetic charges don’t exist—magnetic poles always come in pairs. Changing magnetic fields disturb electric fields. Changing electric fields or electric currents disturb magnetic fields.

Those last two create the circular coupling that lets fields sustain each other. But when you work through the mathematics, when you calculate how fast these mutual disturbances propagate, you get a specific number. The speed depends on two fundamental constants: epsilon-zero and mu-zero, which describe how space responds to electric and magnetic fields.

Calculate it: $c = 1/\sqrt{\epsilon_0 \mu_0}$. Plug in the measured values.

You get 299,792,458 meters per second.

That’s the measured speed of light. Exactly.

Maxwell must have had quite a moment when he realized that. The equations describing electricity and magnetism—phenomena physicists had been studying separately for decades—were predicting waves that travel at precisely the speed of the mysterious rays we call light.

Light IS electromagnetic waves. Not carried by electromagnetic waves. Not similar to electromagnetic waves. Light is the thing itself—ripples in the electromagnetic field propagating through space.

This unified three completely separate fields of physics. Electricity, which Franklin studied with his kites. Magnetism, which sailors used for navigation. And optics, the study of light and lenses. All the same thing. All manifestations of one electromagnetic field doing what fields do.

Why We Don’t Need Aether

So what’s waving? The field itself. The electric field creates the magnetic field creates the electric field, over and over, propagating outward at the speed dictated by how space responds to fields.

There’s no medium. No aether. No invisible substance filling space that light travels through. The fields ARE the medium, each sustaining the other through their intimate connection. They’re as fundamental as you can get.

When people ask “what is light made of?”, the honest answer is: it’s made of fields. Quantum mechanics adds another layer—at the smallest scales, these field oscillations come in discrete packets we call photons—but the wave nature remains. Light is still electromagnetic fields doing their self-sustaining dance.

This is what good physics looks like. You start with seemingly unrelated phenomena. Through careful measurement and mathematical description, you discover they’re not separate at all—they’re different aspects of one underlying reality. When your equations predict exactly what nature does, when the speed of light falls out of electricity and magnetism equations, you know you’ve found something true.

The electromagnetic field isn’t some abstract mathematical fiction. It’s as real as matter, maybe more fundamental. Wiggle a charge here, and the influence propagates through the field at light speed, disturbing other charges there. That’s how forces work. That’s how the sun warms your face. That’s how your eyes read these words.

Fields all the way down.