(This is Part 2 of a series on Cherenkov radiation — the “light boom.” Read Part 1 first.)
Before we get to Brad Bradington sprinting down the red carpet, we need to talk about the crowd itself. Because the crowd is where all the magic happens, and the crowd has some very specific properties that make this whole story possible.
Specifically: the crowd slows down light.
In 1865, James Clerk Maxwell published four equations that unified electricity, magnetism, and light into a single framework. It’s one of the towering achievements of 19th century physics — the kind of result that makes you feel like the universe was trying to tell us something, and Maxwell was just the person paying close enough attention to hear it.
One of the things those equations tell you, if you work through the mathematics, is the speed of light. It falls out of two constants — properties of empty space itself — and gives you exactly 299,792,458 meters per second. Not approximately. Exactly. The universe just decided that’s what light does in a vacuum, and Maxwell’s equations are how we know.
But here’s the asterisk: those constants describe the vacuum. Empty space. Put a material in the way, and those effective constants change. The material has its own electric and magnetic properties — its own way of responding to oscillating fields — and those properties act as a drag on the wave. The speed that falls out of the math is now lower.
How much lower depends entirely on the material. Physicists capture this with a single number called the index of refraction — the ratio of the vacuum speed of light to the actual speed in the medium. In air, the index is about 1.0003 — so close to vacuum you’d never notice the difference. In water it’s 1.33, meaning light moves at about 75% of its maximum speed. In glass it’s around 1.5. In diamond it’s 2.4, meaning light is cut to less than half its vacuum speed passing through the stone. HALF. We’ve even engineered special laboratory materials that slow light to walking pace — literally the speed of a person strolling down a corridor, achieved inside ultracold atomic clouds.
This is, if you stop to think about it, genuinely strange. Light doesn’t have mass. It can’t be grabbed or pushed. And yet the mere presence of atoms and molecules — the way they respond to oscillating electric fields, creating their own little ripples that interfere with the original wave — is enough to drag it down from the cosmic speed limit to something a fast cyclist could beat.
Now, WHY this happens in detail is a whole episode on its own. You can picture it a few different ways. You can imagine the light waves interacting with the electrons in each atom or molecule, which then generate their own little electromagnetic waves, which then interfere with the original — slowing the whole thing down like trying to run through a room full of people who all want to stop and chat. You can picture it as individual photons bouncing around in an elaborate quantum pinball machine. Or you can invoke something called phonons, which is my favorite picture because it’s both the most accurate and the most fun to say out loud.
But the HOW doesn’t matter for our story. What matters is the FACT: light inside a material moves slower than c. Sometimes much, much slower.
Here’s where it gets interesting.
In empty space, nothing can outrun light. Einstein’s special relativity closes that door completely, with no exceptions and no loopholes. There is no shortcut, no workaround, no fine print. The cosmic speed limit is absolute.
But what if you filled the stadium with molasses?
Usain Bolt runs at about 10 meters per second. In open air, light outruns him by a factor of thirty million. The gap is not closeable by any conventional means.
But if you fill the stadium with molasses, Bolt slows down. More importantly, light slows down too — and inside certain materials, light slows down dramatically more than a charged particle does. A fast-moving electron barely notices there’s a medium there — it plows through atoms like they’re not its problem. But light gets caught up in all those interactions, all those tiny delays, all that electromagnetic interference.
The result: inside certain materials, a sufficiently energetic charged particle can move faster than light is moving in that same material. Not faster than c — the vacuum speed of light, the true cosmic limit. That’s still inviolable. But faster than the local, in-this-material, slowed-down speed of light. Which is a very different, and entirely permissible, thing.
You’re not breaking any laws of physics. You’re not violating relativity. You’ve just found a material where light has to slog through molasses, and you happen to be a particle that barely notices the molasses is there.
And when that happens — when a charged particle exceeds the local speed of light in its medium?
Brad Bradington has entered the building.
In Part 3, we watch Brad Bradington sprint — and find out exactly what a light boom looks like.