We might be killed if a bullet penetrates our brain. How about an elementary particle moving with high energy penetrates our brain?

Assume that we can have exactly a single elementary particle for this imaginative experiment.

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    $\begingroup$ It happens all the time thanks to cosmic rays. And by "all the time" I mean several times a second. See also physics.stackexchange.com/q/6435 (which concerns itself with extremely rare "ultra-high energy" cosmic rays---the several a second type average about 1 GeV above the atmosphere). $\endgroup$ – dmckee Jul 10 '12 at 19:15
  • $\begingroup$ The cosmic rays can penetrate or they get lodged? $\endgroup$ – kiss my armpit Jul 10 '12 at 19:19
  • $\begingroup$ Some of each. Most go through. A few decay in flight in there and a few more stop and decay at rest. $\endgroup$ – dmckee Jul 10 '12 at 19:21
  • $\begingroup$ I thought neutrino does that all the time. $\endgroup$ – 吖奇说 Oct 10 '13 at 0:54

2012-07-11 Addendum

Based on excellent inputs, in particular from @annav, my answer is now "no, even a direct worst-case hit by the Oh-My-God particle would not kill you, even by radiation, because there is insufficient distance and angle to generate a fatal radiation cone. Thanks all, and be sure to look at the earlier answer that @dmckee pointed out.

** Original answer**

(My answer seems to differ from the earlier ones that @dmckee aptly pointed out, so I'll go ahead and risk posting it. My main difference is that I suspect that a head on collision with a large nucleus could produce a wide enough horizontal-splatter radiation cone to produce a fatal event.)

Since the 1991 Oh-My-God particle was most likely a proton and had the kinetic energy of a fast baseball, I'm going out on a limb and saying yes, you could be killed by a single particle. This Harvard physics site suggests an approximate energy transfer of about 0.2% in transfers with heavy nuclei, which as I discuss below may be enough to do you in with that kind of particle. But it would be the ensuing radiation event and cone that would do you in, not the kinetic energy of the particle.

The main issue is that your head doesn't have anything in it remotely solid enough to stop or even slow down a particle with that much momentum. So, like a locomotive passing through a cloud of fog, it's going to zip through pretty much as if your head isn't there.

The question, then, is to ask not what the fog will do to the locomotive, but what the locomotive will do to the fog -- that fog being your head.

Even a single solid, exactly head-on collision with a nice fat iron nucleus just as an ultra cosmic ray proton enters your head would probably not be a pretty event in terms of the resulting secondary radiation shower. I'm guessing (nothing more, I haven't tried to calculate anything) that outward splattering of a nice little quark plasma, one created as the iron nucleus vaporizes during the transition event, could produce a sufficiently wide cone of particle-zoo ejecta to irradiate a fatal percentage of your brain. Rapid heating of your brain would not be a problem, however, since 1/500 of the approximately 50 Joules of kinetic energy would work out to be only about 0.1 J of heat energy tops. By comparison a standard firecracker releases about 500 J of energy.

And what are the real odds on such a dead-center strike on a large nucleus near the surface of your brain, assuming you were an astronaut unprotected by out atmosphere? Low almost beyond belief. Look at the date on the Oh-My-God particle: 1991. We haven't seen one quite that feisty since. As @dmckee aptly notes, ordinary cosmic rays or their secondary outputs hit us all the time, and astronauts watch direct collision buzz through their retinas without much harm.

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    $\begingroup$ These particles shower in the atmosphere, they don't make it to the ground. But it's an issue in space-travel. I am not sure your conclusion is valid--- even if there is an energetic collision, the products are flying off too fast to deposit energy in the head, they will just leave the skull, leaving an ionization trail, which might kill a few cells along the path, but generally you shouldn't notice. $\endgroup$ – Ron Maimon Jul 11 '12 at 3:05
  • $\begingroup$ I immediately agree in general about the cone, but that's also why I mention the idea of a quark plasma forming out of the iron nucleus during a direct transition right through its center. The strong force binding of a large nucleus may (or may not) be sufficient for the "passing through a fog" interaction approximation to fail -- parts of the target may shatter with significant orthogonal velocity is another way to put it. But I certainly haven't tried to calculate anything; I'm just putting forward the possibility. $\endgroup$ – Terry Bollinger Jul 11 '12 at 3:21
  • $\begingroup$ I agree you could have an unlucky collision spraying 10,000 particles through your head, but you won't get 2 such collisions, so you just have to worry if those 10,000 particles will deposit all their energy in your head, or leave, with each particle depositing some energy in an ionization trail. You would kill a few million brain cells, but I don't think you would notice. $\endgroup$ – Ron Maimon Jul 11 '12 at 3:33
  • $\begingroup$ You may well be correct! Without any calculations on my part, all the magnitudes are just my wild guesses. There may not be enough room, literally, to create kill-level radiation even if a cone forms. Please feel free to answer! $\endgroup$ – Terry Bollinger Jul 11 '12 at 3:37
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    $\begingroup$ Terry you are talking of a shower starting in the head. A quark gluon plasma, even if it happens as you envisage it, will distribute according to the incoming momentum into maybe 2000 particles again with high momentum. These will leave the head with only some small ionisation damage at the location of the iron nucleus, one cell, ( 10^-36 cm^3, the volume much smaller than a cell, plasma is strong interactions). The probability of a cascade in the head from high energy products is very tiny.Most of the energy will leave with the debris. $\endgroup$ – anna v Jul 11 '12 at 3:54

protected by Qmechanic Oct 9 '13 at 20:23

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