The coolest things about science are often at the frontiers. We love to press the limits of nature: to cool things down as close as possible to absolute zero, to raise energies as high as possible, and to speed them up as fast as possible. But when it comes to speed, there’s an absolute limit: the speed of light in vacuum.
Relativity tells us why this is the absolute limit: the faster you want to go, the more energy it takes. But, as you get close to the speed of light, you effectively increase the mass of your speedy particle, which makes it ever harder to accelerate it. In fact, it would take an infinite amount of energy to get even a tiny, massive particle (like an electron) to reach the speed of light, much less exceed it. The fastest we’ve ever made an electron go on Earth is 299,792457.9968 m/s, just a few millimeters per second slower than the speed of light.
Before the LHC (Large Hadron Collider) was set up to collide protons, it was called LEP, the Large Electron-Positron collider, and it achieved these speeds. By comparison, light in air is only 99.97% the speed of light in vacuum, or hundreds of kilometers per second slower than the speed of light in vacuum. What happens when a bunch of electrons, moving at almost the speed of light in vacuum, suddenly enter the air?
Well, you see blue light, known to physicists as Čerenkov radiation. What causes this? Nothing can move faster than the speed of light in vacuum, but anything can move faster than light in any material other than vacuum! As soon as this speedy particle enters this “slower-than-vacuum” medium, the medium turns on the brakes. All the charged particles (protons and electrons) that make up the medium interact with the delinquent speeder, and cause it to lose energy.
In this case, energy translates into photons — or particles of light — getting spit out. The diagram above shows one photon; in reality, light gets spit out in all directions, and it gets spit out perpendicular to the direction of motion. When you continue traveling through the medium, you wind up with a cone-shaped beam of light:
And you continue to emit it until you find yourself back in a vacuum again or you slow down to a velocity below the speed of light in that material. Why should you care? Because all you need to build a high-speed particle detector is a tank of water! In water, the speed of light is down to about 3/4 of light in vacuum, so even something as simple as radioactive metals will emit tiny amounts of blue light if you put them next to water. This is incredibly useful to scientists looking for fast-moving particles or rapidly moving decay products. Just build a tank of water and put some blue-light detectors around the edge. When something goes too fast, you see this:
And just like that, you’ve tricked this elusive particle into showing you all of its secrets! (And that’s how we find astrophysical neutrinos, for you fans of the little guy.)