Thursday, October 06, 2016

Detecting Particles By Seeing Them Move Faster Than Light

No, this is not a topic on superluminal particles. Rather, it is an article on how we detect particles by using faster-than-light particles in a medium, i.e. by observing the Cherenkov radiation.

But photons only move at that perfect speed-of-light (c) if they’re in a vacuum, or the complete emptiness of space. Put one in a medium — like water, glass, or acrylic — and they’ll move at the speed of light in that medium, which is less than 299,792,458 m/s by quite a bit. Even air, which is pretty close to a vacuum, slows down light by 0.03% from its maximum possible speed. This isn’t that much, but it does mean something remarkable: these high-energy particles that come into the atmosphere are now moving faster than light in that medium, which means they emit a special type of radiation known as Cherenkov radiation.

The article listed several detectors that make use of this effect, but it is missing A LOT more. Practically all neutrino detectors use this principle (i.e. SuperKamiokande). Auger Observatory also looks out for these Cherenkov radiation.

But the part that I think should fascinate the layperson is when the speed of various things are listed, up to the most accurate decimal places:

It’s true that Einstein had it right all the way back in 1905: there is a maximum speed to anything in the Universe, and that speed is the speed of light in a vacuum (c), 299,792,458 m/s. Cosmic ray particles can go faster than anything on Earth, even at the LHC. Here’s a fun list of how fast various particles can go at a variety of accelerators, and from space:
  • 980 GeV: fastest Fermilab proton, 0.99999954c, 299,792,320 m/s.
  • 6.5 TeV: fastest LHC proton, 0.9999999896c, 299,792,455 m/s.
  • 104.5 GeV: fastest LEP electron (fastest accelerator particle ever), 0.999999999988c, 299,792,457.9964 m/s.
  • 5 x 10^19 GeV: highest energy cosmic rays ever (assumed to be protons), 0.99999999999999999999973c, 299,792,457.999999999999918 m/s.
 Just notice how much energy we had to put in to, say, the proton in going from 0.99999954c to 0.9999999896c. And then, notice how high of an energy cosmic rays have when compared to the LHC. If these types of collisional energy can create "catastrophic blackholes", we would be gone by now, thankyouverymuch!


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