“The doctors realized in retrospect that even though most of these dead had also suffered from burns and blast effects, they had absorbed enough radiation to kill them. The rays simply destroyed body cells – caused their nuclei to degenerate and broke their walls.” -John Hersey
Everyone (well, almost everyone) recognizes that radiation is bad for you. And the higher the energy of the radiation, the worse it is for you. The reason is relatively straightforward.
When high energy particles (or photons) come into contact with normal matter, they knock the electrons off of atoms, ionizing them. This action breaks apart bonds, disrupting the structure and function of cells on a molecular level. And, as you might expect, the higher the energy, the more extensive is the damage that the ionizing radiation can do.
Targeted radiation — at cancer cells, for instance — is useful for this exact reason: it destroys the cancer cells. Sure, some of your cells are in the way, too, but radiation therapy is designed to kill the cancer faster (and more effectively) than it kills you.
But too much ionizing radiation will cause too much damage to your body, and spells doom for any human.
Here on Earth, the most intense sources of energetic particles are those that come from the world’s most powerful particle accelerators: at present, that’s the Large Hadron Collider.
But the thing is, you don’t know whether a particle accelerator is on just by looking at it. There are few enough high-energy particles even in the most powerful accelerators that the particles themselves are — and hence the entire beam is — invisible to the naked eye.
You can’t even feel is, much like getting X-rays at the dentist. But, as you may have guessed, there is a trick. An awful, terrible, do-not-try-this-at-home trick. You see, you already know that nothing can move faster than the speed-of-light in a vacuum.
But the speed of light decreases, often quite dramatically, if you’re not in a vacuum.
This is actually the reason why light bends when it passes through a prism, or a straw/pencil appears bent when you immerse it in a glass of water.
The relationship between how much an object appears to bend and the speed-of-light in that medium is actually very simple, and tells you that the speed-of-light in water is only about 75% of what it is in a vacuum.
And in many real-world cases, such as from particle accelerators, nuclear reactors, and radioactive decays, we make particles that — while not faster than light-in-a-vacuum — can travel faster than the speed of light in a medium!
And when that happens, when a particle moves faster than the speed-of-light in a medium, light is produced! That’s what’s going on inside this nuclear reactor and causing this blue glow: the radioactive particles (electrons, in this case) are moving faster than the speed-of-light in water, and hence the particles are emitting Äerenkov Radiation!
What’s Äerenkov Radiation?
The charged particles, passing through this medium at such great speeds, electrically polarize the medium, which then transitions back down rapidly to the ground state. The polarizing of the medium costs the fast-moving particle some energy, slowing it down, while the transition causes the particles in the medium to emit radiation, and that’s where your light — the Äerenkov Radiation — comes from!
So how do you tell if the beam is on?
Horrifically, you stick your closed eye in there!!!
With your eye closed, you should see blackness under normal circumstances. But with the beam on, the high-energy particles entering your eye will see that nice, aqueous fluid that fills your eyeball. And since they’re passing through at — you guessed it — greater than the speed-of-light in your vitreous eye-fluid, they’re going to emit Äerenkov Radiation.
So if the beam is on, you’ll see that light — that Äerenkov light — on the back of your eye. And if it’s off, you won’t.
If that makes you squirm, it should. Physicists used to die from cancer from lack of safety when it came to radiation at alarming rates, and we are no longer (thankfully) allowed to test whether the beam is on or not via methods like this. But this is an interesting bit of history of particle physics that I couldn’t not share with you.
And now, in a life-or-death situation, you know how to tell whether the beam is on or not, consequences be damned!