(Intro)

You've probably heard that nothing can travel faster than the speed of light. And that's true as long as that light is in a vacuum, like space. But in something like water, light travels a little bit slower. This means that other particles can potentially travel faster than light, creating a cool blue glow in the process called Cherenkov radiation.

Light travels at different speeds in different mediums, because of how it interacts with atoms and molecules. And in water, light only travels at around 75% of its speed in a vacuum. Now, this means that certain fast-moving particles can actually travel faster than light in water.

This can happen in nuclear reactors, because of nuclear fission, which is when the nucleus of an atom splits apart and charged particles – like electrons – are flung out at really, really high speeds. As each electron travels through the water surrounding the nuclear reactor, it interacts with some of the the water molecules along its path, which respond by releasing photons of light. When the electrons are travelling faster than light in the water – together, all these photons form a blueish glow called Cherenkov radiation.

And we can see this glow because it's basically a sonic boom, but with light instead of sound. See, when something travelling slower than the speed of sound passes you, like an ambulance blaring its siren, the pitch of the sound it's making seems to change. This is because the sound waves are getting compressed or stretched between you and the ambulance thanks to the Doppler effect.

But when something's travelling faster than the speed of sound, like certain planes, all the sound waves start to bunch up behind it. This creates a cone shape of overlapping sound waves, which start at the plane, and keep expanding. This cone will keep growing as long as the plane's flying at supersonic speeds.

And if you're standing on the ground nearby, the edge of this cone where all the compressed sound waves are overlapping – called the shock wave – will eventually reach you. You'll experience a quick, sudden change in pressure, which will sound like a super loud boom. A sonic boom! And a similar thing is happening in the nuclear reactor, which causes the Cherenkov radiation.

The electron is travelling faster than light in water – like our plane travelling faster than sound in air. So the emitted photons from the water molecules start bunching up behind the electron – just like the plane's shock wave – and form a cone of blue light that can be seen with the naked eye. And in places where lots of these cones form, like a nuclear reactor, you'll just see a general blue glow.

But you can see Cherenkov radiation in other places besides nuclear reactors! For example, scientists are able to see these blue cones at the Sudbury Neutrino Observatory in Ontario, Canada. It happens when neutrinos – which are chargeless particles with a very tiny mass – undergo reactions that release really fast-moving electrons. These electrons move faster than light in the observatory's tank filled with a different form of water, and also produce Cherenkov radiation.

The blue glow helps scientists detect the neutrinos, which would be much harder to do otherwise. And studying these elusive particles are important, because they help scientists better understand events like the Big Bang and nuclear fusion in the Sun.

So, next times someone tries to tell you "nothing can travel faster than light" just remember that that's only true in a vacuum. In something like water, particles like electrons can beat light in a race – and cause a blue glow to prove it.

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