Don't cross the LHC stream!

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Sep 21, 2010
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There are a lot of questions in science that seem simple, but in fact lead to profound concepts. Why is the sky dark at night? Why does gravity pull me down? Why is the Sun hot?

And some questions seem silly and frivolous, but it turns out are really hard to answer, and in fact scientists might disagree on the answer. Case in point: what happens if you put your hand in the beam of the Large Hadron Collider?

So the folks at Sixty Symbols asked this of several scientists, and the first four minutes of this video are the result:


Fantastic! I love how different scientists think of different angles on this, and come up with different answers. Clearly, they hadn't really thought about this before, so as they realize various aspects of this the answer changes.

It's complicated! You have to think about the energy of the beam, of course, but also the energy of a given proton as it moves at 99.9999% the speed of light. But that number doesn't mean anything if the proton doesn't actually interact with the matter in your hand, so you have to consider the "cross-section" of the atoms in your hand. Think of it this way: if you shoot a gun at a target, you make a hole. But if you shoot a gun at a fishing net, it might pass right through. Most of the area of a fishing net is holes! The nuclei of atoms are very small compared to the atoms themselves, so in a sense most of you is empty space.

And there's more. The beam of the LHC is in a small tube that's a high-grade vacuum -- getting your hand in there would be tough. And there's a magnetic field of ferocious strength there, so I hope you don't have any metal bits in you. And there are actually two beams going in opposite directions.

So taken in total, I'm not sure what would happen. If the total energy of the beams were dumped into your hand all at once, it would act like dropping a bomb on you. One of the scientists in the video says the total energy in the beams is 300 megajoules, which is equivalent to 75 kilograms (165 pounds) of TNT going off. That's about 300 sticks of dynamite, if you're curious.

Bang! Ow.

But depositing all that energy all at once may not be possible; protons are so small they may not all hit you and suddenly stop. That's why I find the scientists' answers so amusing. One says it would explode, but that's because he assumes all the energy gets dumped into your hand. Another thinks nothing would happen, because the energy that actually interacts with you is small. Another thinks about the synchrotron energy produced, which is radiation emitted by the particles themselves as they circle the tube.

The point? The actual answer is hard to state with certainty until you look at everything involved! There are a lot of factors involved, making this seemingly easy question actually a bit of a puzzler.

So I propose that in a few decades, when funding is running low and the LHC's primary objectives are met, we test this idea out. We don't have to use an actual hand -- logistics might be difficult -- but I think a nice slab of rib eye might do the trick. But they better make sure a few interns are on hand to clean the LHC up afterward, just in case the whole 300 sticks of dynamite thing turns out to be the correct outcome.

Tip o' the lead shielding to AstroPixie.

Related posts:

- My excellent CERN adventure
- Breaking: The LHC still will not destroy the Earth
- Brian Cox calls 'em like he sees 'em
- Cox on TED