Umm… is this thing on?

“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.

Image credit: Environmental Protection Agency.

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.

Image credit: Christina Macpherson.

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.

Image credit: CERN / LHC, retrieved from

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.

Image credit: KEK e+/e- LINAC.

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.

Image credit: Grimsmann and Hansen.

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.

Image credit: © 2011–2014 Florida Center for Instructional Technology / Dr. Roy Winkleman.

Image credit: © 2011–2014 Florida Center for Instructional Technology / Dr. Roy Winkleman.

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!

Image credit: Matt Howard, Idaho National Laboratory / Argonne.

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 Čerenkov Radiation!

What’s Čerenkov Radiation?

Image credit: Cherenkov Telescope Array in Argentina.

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 Čerenkov Radiation — comes from!

So how do you tell if the beam is on?

Image credit: flickr user ohrfeus.

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 Čerenkov Radiation.

Image credit: The Gale Group, retrieved from

So if the beam is on, you’ll see that light — that Čerenkov 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!


  1. #1 Mark M
    May 15, 2012

    Wait – so this isn’t hypothetical? They actually did this?

  2. #2 Anthony
    May 15, 2012

    The speed of light in air is only 0.997c, so at more than 13x rest energy you’ll get cerenkov radiation in air. That’s not applicable to nuclear reactors, but modern particle accelerators should produce air glow. I understand that astronauts also see flashes of cerenkov light due to cosmic rays.

  3. #3 Ethan (the other one)
    May 15, 2012

    I can’t speak from firsthand experience, but when my father went to work at the University of Rochester in the early 60′s there were physicists who had helped build the cyclotron there and who described using exactly this method to make sure the beam was where it was supposed to be.

    The funny thing was that they knew it was a crazy thing to do. They did it anyway. Real men don’t mind radiation damage in the cause of science.

  4. #4 D. C. Sessions
    May 15, 2012

    Once upon a time the University of Arizona had a research reactor in the Mines and Metallurgy building on what was then near the north edge of campus. You could actually look in through a ground-level back window and see the faint blue glow at the bottom of the tank.

    I’d go there with dates to show them the reactor; some thought it was cool, others unimpressed. On the other hand, it was a pretty fair necking spot in the days when men weren’t allowed in women’s dorms so …

    Yeah, nothing but window glass between a street and a reactor. It was a very different world.

  5. #5 Andrew Foland
    May 15, 2012

    “You can’t even feel is, much like getting X-rays at the dentist.”

    I am pretty sure this is not true. (Luckily, there’s a calculation one could to do find out.) The particles will undergo energy loss as they pass through the matter of your head, I think it works out to be of order 60 MeV per proton. This is pure kinetic energy loss so it will appear as a momentum transfer to the energy loss medium (aka your head). If you carry out the calculation with the beam parameters at LHC, I think you’ll find that you will definitely feel it. In fact I wouldn’t be at all surprised if it turns out that the purely mechanical impact of the beam is lot more deadly, a lot quicker, than the radiation.

    I’ve known older physicists who have aligned beams the way described here, but those beams were very, very low intensity.

  6. #6 Artor
    May 15, 2012

    Gahh! Sticking my head in the beam wouldn’t be my first thought, nor my second or 50th thought for that matter. If you wanted to do that test, why not hold out a jar of water to see if it glows? Did particle physicists learn nothing from Marie Curie?

  7. #7 justawriter
    May 16, 2012
  8. #8 six
    May 16, 2012

    i remember a picture of someone who was in front of a particle accelerator when it was on, and i always wondered why he would have done that. this probably explains that.

  9. #9 Christna Macpherson
    May 16, 2012

    I am very happy to see people using my diagrams. Just would like it if they were correctly attributed to me. “The development of cancer” – my diagram, originally shown in, – no doubt Dr Kamuran Kus found it there.

  10. #10 Phil Shaffer
    May 16, 2012

    there is a hypothesis, supported with some weak data, that low level radiation is actually protective against cancer. The thought is that the minor DNA damage inflicted stimulates DNA repair mechanisms which repair more than just the radiation damage. The term for this is radiation hormesis, if you want to google.
    It wasn’t just physicists who did dumb stuff – radiologists did as well .There is a book – “Martyrs to the Roentgen Ray” that describes the deaths of multiple radiologists. Portions were published in the American Journal of Roentgenology (AJR) some years back. Some of this may be available online.
    I remember one specific episode, it occurred in my city. This did involve a physicist, actually. At the time, this physicist at Ohio State had the only x-ray machine in town. The doctors would send patients from their downtown offices for x-rays. He would test the machinery before the patients arrival by doing an x-ray of himself. At the time, the radiation dose for a standard x-ray was HUGE. We have learned many ways to reduce the dose. But, this fellow got a massive dose of x-rays. Much of it was to his abdominal wall, which he basically burned off. (yes.) He did not do well.
    The typical radiologist injury was to the hands, with radiation dermatitis (redness and flaky skin eventually non-healing ulcers and cancers) due to constantly sticking their fingers into the fluoroscopy beam. I have never seen this in one of my colleagues.

  11. #11 McVier
    May 16, 2012

    This is what happens if you stick your head into a running particle accelerator..:

  12. #12 willys36
    May 16, 2012

    And they call these guys educated? How do they tell of their Harbor Freight branch shredders are on?

  13. #13 rpenner
    May 16, 2012

    Prof. Siegel

    Could you please tell us the source for the John Hersey quote in the epigraph above?

  14. #14 Mu
    May 16, 2012

    Things were different in the early part of the last century, the original publication on phosphorylesters (which later gave rise to compounds like Parathion, Sarin and Tabun) described the flowery smell of the compounds…

  15. #15 Ethan Siegel
    May 16, 2012

    Christina @9, so sorry that I misattributed your figure to someone else who claimed the credit for it. My apologies, and I have corrected it, above.

  16. #16 rpenner
    May 16, 2012

    It appears to be the 1946 New Yorker article “Hiroshima” and later book Hiroshima.

    John Hersey, A Reporter at Large, “Hiroshima,” The New Yorker, August 31, 1946, pp. 15-68

    At page 56 the exact text appears.

  17. #17 dave w.
    May 16, 2012

    FTA: “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! ”

    Yet you continue to tell me faster than light speed travel (Warp Drive, Scotty!) is impossible. I call BS!!! (Another great article, Ethan!)

  18. #18 Bill M.
    May 16, 2012

    Why not just hold a glass of water in the beam and turn out the lights? I’ve heard that the human eye can detect even only a few photons.

  19. #19 Anthony
    May 16, 2012

    According to CERN, the total beam energy of the LHC is 362 megajoules, which somewhat exceeds the total energy requirement to boil a human. Much of the beam would pass right through a human, of course, but a decent fraction would collide with a nucleus on the way through, producing a spray of particles. Even most of that will escape, but you should still get enough energy absorbed to blow a human in half.

  20. #20 Lamy Chopin
    May 16, 2012

    I’m reminded of stories of the old chemists tasting their arsenic compounds as part of their analytical routines.

  21. #21 Alex Besogonov
    May 16, 2012

    >Here on Earth, the most intense sources of energetic particles

    Not really. Cosmic rays can be MUCH more energetic (read about the famous oh-my-god particle – )

    Also, checking if beam is on with your eyes is not that dangerous because the luminosity of the beam is fairly small.

  22. #22 Marcus Ranum
    May 17, 2012

    When I was in Chernobyl last fall, I was talking to some of the survivors of the clean-up and evacuation and apparently the radiation was so intense that some of the crews reported seeing weird dot-flashes appearing inside their eyes. Eep!

  23. #23 Christina Macpherson
    May 17, 2012

    Thank you, Ethan Siegal, for correcting – about the attribution of the diagram. I originally called it “ionising radiation causing cancer”.
    On another point – radiation “hormesis” – mentioned above by Phil Shaffer:
    Even a rank amateur like myself can see the holes in this theory about low level radiation being OK, perhaps even good, for health. For just one example:
    The research studies about this completely ignore “internal emitters”. It’s one thing to have a low level radioactive wave pass through you – as in X rays – an “external emitter”. It’s another thing to have a teensy particle lodged in your lung, or your gut – which continually emits radiation to the nearby tissues. This “internal emission’ is an aspect of low level radiation that is ignored, in these mouse experiments.

  24. #24 Chuckinmontreal
    May 17, 2012

    I once read that thousands of people around the globe must have “seen” the 1987 supernova in the Large Magellanic Cloud hours before its first visible light hit Earth.
    They didn’t need to be looking at the sky, have their eyes open, or even be awake. A flash would have occurred within one of their eyeballs as one of the billions of neutrinos released by the core collapse collided with a nucleus. I never heard of any of them realizing and reporting what had happened to them, but the math supposedly guarantees it did.

  25. #25 Wow
    May 17, 2012

    Chuck, the chance of a collision by a neutrino in the volume of your head (never mind your eyeball) are so tiny as to be indistinguishable from zero, even multiplied by the number of people on earth.

    Neutrino detectors are hundreds of meters across.

    Your eyeball? Not so large.

  26. #26 Ian Kemmish
    May 17, 2012

    I seem to recall the Gemini astronauts reporting similar visual artefacts during early spacewalks?

  27. #27 OKthen
    May 17, 2012

    @24 Chuckinmontreal
    Wow is correct.

    Here’s a correct explantion of interaction with the neutrinos of the 1987 supernova.
    “Almost all of the energy of the [1987] supernova came out in lightweight weakly-interacting neutrinos. About 10^58 neutrinos were produced in the core collapse. On Feb 24, 1987, about 10^13 neutrinos passed through your body from the supernova! About a million people on the Earth had an “interaction” with a neutrino, of course with no noticeable effect.”
    This link also talks about some of the misinterpretation of science by the popular press.

  28. #28 Lycanthrope
    May 17, 2012

    In December and January I was receiving radiation therapy to my brain. The beam went through my temples/forehead area, and must have included my eyes, because when I closed them I saw a weak, ambient blue light instead of darkness. Now I know why.

    (I’m in remission now. It was extranodal non-Hodgkin’s lymphoma, not a brain tumour.)

  29. #29 Filip
    May 17, 2012

    Great article. Just, I remember this story of what would happen if you put your arm in the LHC beam ( So, I suppose the energy of a beam is different, but isn’t it similar to doing what Ethan talks about?

  30. #30 tom campbell-ricketts
    May 18, 2012

    Sounds like a good scene for a nerdy action movie. I think I want to see that movie. I think I want royalties for the idea. (OK, Ethan you can have 10% of whatever I get.)

  31. #31 Jon H
    May 18, 2012

    If particle beams are safe enough to stick your head in without immediate ill effect, why does the LHC have special “beam dump” caverns?

  32. #32 Chuckinmontreal
    May 19, 2012

    @25, 27: Combined volume of two human eyeballs: roughly 1 cubic inch. Total volume of average human: roughly 5,000 cubic inches. Even if (as the article suggests, without elaboration) only a million humans experienced neutrino collisions within their bodies in 1987, about 200 would have occurred inside their eyeballs. We should probably factor in the optical pathways in the brain that could also have been affected. I did say “thousands” saw the supernova before any visible-light telescope detected it. I might have been more correct to have said “hundreds.” Does that diminish my point? Not much.

  33. #33 sean t
    May 21, 2012


    As a chemist and former HS chemistry teacher, I can tell you that there are MANY instances of old-time chemists doing things that would be considered crazy now. One of my favorites is the compound lead acetate, which of course is toxic. It’s pre-systematic name was sugar of lead! That name derived from its sweet taste. Obviously, I can only tell you it’s sweet from what I’ve read in texts, not from personal experience.

  34. #34 David Syzdek
    United States
    June 14, 2012

    Apollo astronauts reported flashes when they had their eyes shut. They performed some experiments with their heads and bodies in different orientations to determine the location of the source of the particles. I believe the source was the sun and these flashes occurred when they were outside the Van Allen Belts.