Tuesday, December 28, 2010

IceCube Neutrino Observatory: Again

"The World's Biggest, Iciest Particle Detector"
Dave Mosher, Wired Science (December 28, 2010)

"Construction of the world's largest particle detector is now complete after 10 years of drilling deeper than a mile into ultraclear Antarctic ice.

"Called the IceCube, the three-dimensional array of sensors can detect neutrinos expelled by some of the universe's most violent sources, including black holes, supernovas and energetic stars.

"Neutrinos weigh hardly anything, so the particles usually travel through matter — including the sun and Earth — without interacting. But every now and then they slam into the cores of atoms to create nuclear particle showers. The events emit faint blue trails of light which IceCube's 5,160 sensors can track with extreme precision...."

If the IceCube neutrino observatory sounds familiar, maybe you read the Lemmings micro-review on another article, ten days ago.

The Wired Science article is short, but has a favorable content/fluff ratio. And, pictures. Also this extra content on the article's home page:

"...Video: Animations show the drilling of IceCube's 1.5-mile-deep holes, the completed array of light-detecting sensors and a simulation of a neutrino collision event.
"Credit: NSF, IceCube/University of Madison-Wisconsin and Chris Bickel...."

Another page, "Last Sensor," displays photos of the sensors: and a closeup on the last one to be set in place. The crew autographed in.

That page also explains a little more about just how the IceCube array detects neutrinos. The sensors pick up Cherenkov radiation: "Light emitted by particles that move through a medium in which the speed of light is slower than the speed of the particles." ("Appendix A Glossary of Nuclear Terms," The Nuclear Wall Chart, The ABC's of Nuclear Science, Lawrence Berkeley National Lab)

That "through a medium in which the speed of light is slower than the speed of the particles" isn't as exotic as it sounds. Although warp ships (the Lemming will get back to that) would almost certainly generate Cherenkov radiation, neutrinos in ice are a more ordinary sort of thing.

The speed of light is, as far as physicists can tell, as fast as anything can travel through space. That's speed of light in a vacuum. Light slows down, a little, when it travels through matter. Like ice. light doesn't slow down much, but a neutrino screaming in from space is going faster than the speed light makes, going through ice. Which is, again, why IceCube works.

There's a pretty good explanation of Cherenkov radiation at a page titled, reasonably enough, "Cherenkov Radiation," at the University of Pennsylvania, Philadelphia's website. (Department of Physics & Astronomy)

"Warp Ship?!" What, the Lemming's a Trekkie?

The Lemming enjoyed the original Star Trek series, and quite a few of the subsequent series and movies. There are even episodes that I'd be willing see again. But Scottie's warp drive, with it's not-entirely-consistent performance limits, isn't real.

What a physicist named Alcubierre came up with a few years ago is. Or, rather, might be. The power requirements are, literally, astronomical. But it looks like once we're able to generate enough power, and can use the energy to bend space, it's possible to create a semi-independent pocket of space, and move that pocket around through the rest of space at any speed.

The math is beyond what the Lemming's learned: but the last I heard, quite a few other physicists were taking the Alcubierre equations seriously. The last I heard, the 'common sense' physicists are saying that an Alcubierre warp is possible: but isn't stable. (Links in "Humanity in Space: The Next Few Centuries")

Now, Back to IceCube and Frozen Photomultipliers

Since IceCube's senors are lowered down very deep shafts melted in Antarctica's icecap, and left to freeze in place as water flows back in and turns to ice: once they're in place, they're not coming out.

The good news is that it's a comparatively low-maintenance piece of hardware. The bad news is that if something goes wrong with a sensor, there's no way to pull it out for repairs.

More good news, though: The array should be good for about 20 years of observations. And the researchers already have a little data from the parts of the array that are already online. ("Sky Map")

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