We’re told from a young age that nothing moves faster than light. It’s a rule etched into the fabric of physics, a constant of nature that even Einstein couldn’t override. But the truth is more nuanced, and more interesting.
Because while nothing can outrun light in a vacuum, there are conditions where particles can and do move faster than light. Specifically, faster than light in a medium like water or glass.
Let’s break that down.
Light Has a Speed Limit, But Also a Speed
When physicists talk about light’s speed limit, they’re referring to the speed of light in a vacuum:
299,792,458 meters per second, or about 186,000 miles per second.
That’s the ultimate speed at which information or energy can travel. Nothing with mass can accelerate past it, and no signal can outrun it. But that’s only true in a vacuum, a perfect, empty space.
In real-world materials like water, air, or glass, light slows down. Sometimes by a lot. In water, for example, it moves at roughly 75% of its vacuum speed.
Why?
Why Light Slows Down in Water
Here’s where it gets subtle, and fascinating.
When light enters a material, it interacts with the electrons in the atoms of that substance. The photons aren’t simply gliding straight through. Instead, what happens is a kind of stop-and-go traffic:
- A photon zips through space at full speed (the vacuum speed).
- It gets absorbed by an electron.
- A moment later, the electron re-emits a new photon.
- That photon travels forward… until it’s absorbed again.
This absorb-and-reemit dance creates a kind of delay. Light still moves at full speed between interactions, but the cumulative effect is a slower overall progression through the medium.
That’s why it takes longer for light to pass through water or glass, and why those materials have a refractive index greater than 1.
Now for the Trick: Faster-Than-Light Electrons
So what happens if a particle—say, an electron—travels through water faster than the light in that same water?
It’s still going slower than light in a vacuum (so Einstein’s laws are safe). But because light is sluggish in water, the electron can overtake it.
When it does, something remarkable happens.
It emits Cherenkov radiation, a kind of optical sonic boom.
Cherenkov Radiation: The Glow of Speed
You may have seen photos of glowing blue nuclear reactors. That eerie halo isn’t from the radioactive material itself, it’s Cherenkov radiation, produced when high-energy electrons move through water faster than light can in that medium.
It’s directly analogous to a sonic boom:
- When a plane exceeds the speed of sound in air, pressure waves pile up into a shockwave—boom.
- When a particle exceeds the speed of light in a medium, electromagnetic waves pile up into a brilliant flash of blue light.
So yes, particles can move faster than light, as long as you’re talking about light in a medium, not light in a vacuum.
Light Isn’t Always the Fastest Thing Around
This doesn’t violate relativity. It actually confirms it. The vacuum speed of light remains the unbreakable speed limit. But in materials where light slows down, electrons can steal the spotlight—literally.
And they leave behind a glowing trace of their momentary triumph.
Jayson L. Adams is a technology entrepreneur, artist, and the award-winning and best-selling author of two science fiction thrillers, Ares and Infernum, and his forthcoming novel The Quantum Mirror.
Jayson writes sci-fi thrillers that explore what extreme situations reveal about who we really are. His novels combine high-stakes science fiction with deeper questions about identity, courage, and human nature. You can see more at www.jaysonadams.com.