Wednesday, October 1, 2008

Cherenkov blue

I have been digging around in all the "draft" posts, things that I have written for this blog and never actually posted. The following was written in spring. I think I refrained from posting it because I thought it was silly to pick apart one little thing from a story, in this way. Now that I have read it again I think it's kind of OK anyway -- I just take the story as a starting point for discussing some physics which has nothing much to do with the story itself. Take it as it is-

I have read a good story with a not-so-catchy title ("The Sky that Wraps the World Round, Past the Blue and Into the Black") by Jay Lake, in Clarkesworld Magazine.

I really liked this story. I'm not going to write a review, just use it as a starting point for some thoughts (as I often do). But first, I want to quote one line that I just love:

A billion billion years from now, even General Relativity might have been demoted to a mere Captain.

Go read the story. And then, come back here again if you want to talk about details and physics.

You see, I'm going to show my geeky side again. I have nothing against taking some poetical liberties in fiction, but some things just make me itch to tell someone how it really is. Anyone. It's not that I think it has to be straightened out, it's just that I get enthusiastic when I notice that I have some expert knowledge. Therefore: Cherenkov light. But first the paragraph that got me started:

I've been told the specks of light are the excitation trails of neutrinos passing through the aqueous humor of the human eye. They used to bury water tanks in Antarctic caves to see those things, back before orbit got cheap enough to push astronomy and physics into space where those sciences belong. These days, all you have to do is go for a walk outside the planet's magnetosphere and be patient.

Can you see blue flashes from neutrinos in your eyes, if you go outside the magnetosphere? Well, hmm, almost. Neutrinos themselves are not ionizing -- they don't leave "excitation trails" -- but if they happen to interact with matter (a rare event, most neutrinos just pass through without caring the slightest about any matter around it) an electrically charged particle can be created. This particle will kick the electrons in the atoms it passes and disturb them: the atoms might be excited or ionized.

The flash of blue the author is talking about is Cherenkov light. A charged particle is a source of an electric field, and when it's moving through matter it will will be like a ripple in the electric potential the electrons experience. The atomic electrons will wobble a little as the particle passes by, and that electon motion will create a little electromagnetic wave -- light. Every wobbling electron is like a tiny antenna emitting a wave front. If the material is transparent to light, and if the particle happens to move faster than the electromagnetic wavefronts from all of the disturbed electrons, the crests of all the little waves will coincide and build up to a stronger wave. This wave is the visible Cherenkov light, the spooky radiation glow that you might have seen from the water around nuclear reactors or storage of radioactive materials. (As people like to point out Cherenkov radiation is analogous to a sonic boom, but I suspect most people don't think enough about sonic booms for this to be very helpful.)

That thing about water tanks in Antarctic caves is not entirely true either, but almost. Neutrino detectors are located underground to sheild them from cosmic rays. The thing about Antarctica is not the caves, but the ice -- so a neutrino detector in Antarctica is of course using the natural ice as a detector medium. And shielding. Why not use it, when it's three kilometers thick?

The interesting question is now: if all humans close their eyes and watch for blue flashes, could we detect neutrinos? Yes, it's not theoretically impossible, but we would detect far more cosmic rays than anything else so we would never be able to tell the neutrino induced flashes from everything else.

Because we do have cosmic rays (particles moving fast) inside atmosphere, even though the magnetic field is protecting the earth from part of the flux. Among them we have lots and lots of muons, a particle which is like a heavy electron. The muons are very penetrating, and it's mainly because of them we have to hide the neutrino detectors (and dark matter detectors) underground. The muons are actually created in the atmosphere, when cosmic rays in the form of for example protons collide with atoms in the air. They have a short life, which actually demonstrates time dilation in special relativity -- if this effect did not exist, they would decay before reaching the ground.

So, as you can understand, if we could take all human eyes and take note of all the flashes we can see, almost all of them would be caused by muons from cosmic rays.

Another inconvenient aspect of the human eye as a neutrino detector is of course that with 6.7 billion separately operated detectors it would be hard work to interpret the data!

1 comment:

Dan said...

I found this because I heard Alex Filippenko say (in his astronomy lecture series for "The Learning Company") that SN1987A may have been perceived by one in one thousand humans. I started searching and Google sent me here when I searched for "neutrino cerenkov eye".

I think Wikipedia said that the burst put out some 10^58 neutrinos, and they were 99% of the energy output. Neutrino observatories detected some two dozen or so events thought to be from SN1987A.

Just thought it was interesting that something so numerous, ghostly, and directly related to the same type of event that made everything heavier than H, He and Li (ie "us") could be directly perceived by us. A strange loop indeed.