6 ms·

The Particle That Broke a Cosmic Speed Limit

38 points by digital55 11y ago

batbomb 11y ago
I worked in the Cosmic Ray Research group at Utah as an undergrad and postgrad. I started at the tail end of HiRes, which was the sucessor to Fly's Eye, and helped develop, test, deploy, run etc... parts of the Telescope Array experiment. I know a bunch about these experiments if anybody is curious. Also, AFAIK, Telescope Array has the only approved outdoor LINAC^H^H^H^H^H "electron light source" they fire directly into the sky. We joked about it giving birds cancer. It's also a potentially an awesome lightning rod.
- Trombone12 11y ago
  Is there a chance of seeing these showers by eye, or are they too weak even for the extremely high events compared to the stars?
  - batbomb 11y ago
    You can't see them by eye, we detect indirectly through nitrogen fluorescence with Photomultiplier tubes and mirrors, and for Telescope Array, also with scintillation detectors. That being said, there are EAS everywhere.
- Trombone12 11y ago
  How does this really work? I imagine looking at the starry sky for brief flashes of light is not without complications, what are the biggest backgrounds?
  - batbomb 11y ago
    To elaborate, when a proton (or iron) particle hits the atmosphere, it starts interacting with matter it hasn't interacted with before. These interactions produce pions and kaons, which decays further until you end up with a bunch of electrons/positrons, gamma rays, and muons. These particles ionize the atmosphere, which puts the electrons in the N2 orbitals in a frenzy, causing fluorescence and Cerenkov radiation (also light). This light is more energetic (shorter wavelength, more blue, near-UV, etc...) than the human eye can detect. In theory you could maybe see some light (in high humidity environments, but I think there's a bunch of favorable conditions that would be required for that to happen.
- ridgeguy 11y ago
  I was very curious about "the only approved outdoor LINAC...". I presume it's what this article describes? http://galprop.stanford.edu/elibrary/icrc/2011/papers/HE1.4/icrc1252.pdf http://galprop.stanford.edu/elibrary/icrc/2011/papers/HE1.4/...
  - batbomb 11y ago
    Yep, that's it. We used to calibrate with laser shots into the night sky, but direct excitation of the sky with the LINAC is much better, because you are directly measuring fluorescence.
simulate 11y ago
Here's an older article that discusses how that one proton would be able to travel to the Andromeda galaxy in under four minutes and to the edge of the edge of the universe in 19 days (as observed from the proton). https://www.fourmilab.ch/documents/OhMyGodParticle/ https://www.fourmilab.ch/documents/OhMyGodParticle/
cshimmin 11y ago
The title is a bit misleading. The particle doesn't break any "speed limits", in the sense that there is nothing fundamental preventing particles from traveling faster (or more precisely, having more energy). The GZK cutoff simply implies that particles are likely to interact with relic photons above this speed. So, the correct interpretation is that either 1) the observed particle started out with much more energy, and lost some via the GZK mechanism, 2)it didn't travel very far to get to us, or 3) we were just "lucky" to see it (which is already built into the rarity of such high energy particles). See also this figure [1], showing expectation energy vs. propagation distance for various initial proton energies. You can see that above 10^20 eV, particles will on average loose energy pretty quickly until they are at or below this energy, on about a 250 million lightyear distance scale. [1] http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_proton.jpg http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_prot...
- MatthewWilkes 11y ago
  There's nothing fundamental stopping cars breaking a normal speed limit, either. /s
georgecmu 11y ago
The rarest of all, those that are labeled “ultrahigh-energy” and exceed 1 EeV, strike each square kilometer of the planet only once per century. In other words, 14 thousand of these particles strike Earth every day.

38 points by digital55 11y ago

I worked in the Cosmic Ray Research group at Utah as an undergrad and postgrad. I started at the tail end of HiRes, which was the sucessor to Fly's Eye, and helped develop, test, deploy, run etc... parts of the Telescope Array experiment. I know a bunch about these experiments if anybody is curious. Also, AFAIK, Telescope Array has the only approved outdoor LINAC^H^H^H^H^H "electron light source" they fire directly into the sky. We joked about it giving birds cancer. It's also a potentially an awesome lightning rod.

Trombone12 11y ago
Is there a chance of seeing these showers by eye, or are they too weak even for the extremely high events compared to the stars?
- batbomb 11y ago
  You can't see them by eye, we detect indirectly through nitrogen fluorescence with Photomultiplier tubes and mirrors, and for Telescope Array, also with scintillation detectors. That being said, there are EAS everywhere.
Trombone12 11y ago
How does this really work? I imagine looking at the starry sky for brief flashes of light is not without complications, what are the biggest backgrounds?
- batbomb 11y ago
  To elaborate, when a proton (or iron) particle hits the atmosphere, it starts interacting with matter it hasn't interacted with before. These interactions produce pions and kaons, which decays further until you end up with a bunch of electrons/positrons, gamma rays, and muons. These particles ionize the atmosphere, which puts the electrons in the N2 orbitals in a frenzy, causing fluorescence and Cerenkov radiation (also light). This light is more energetic (shorter wavelength, more blue, near-UV, etc...) than the human eye can detect. In theory you could maybe see some light (in high humidity environments, but I think there's a bunch of favorable conditions that would be required for that to happen.
ridgeguy 11y ago
I was very curious about "the only approved outdoor LINAC...". I presume it's what this article describes? http://galprop.stanford.edu/elibrary/icrc/2011/papers/HE1.4/icrc1252.pdf http://galprop.stanford.edu/elibrary/icrc/2011/papers/HE1.4/...
- batbomb 11y ago
  Yep, that's it. We used to calibrate with laser shots into the night sky, but direct excitation of the sky with the LINAC is much better, because you are directly measuring fluorescence.

simulate 11y ago

Here's an older article that discusses how that one proton would be able to travel to the Andromeda galaxy in under four minutes and to the edge of the edge of the universe in 19 days (as observed from the proton). https://www.fourmilab.ch/documents/OhMyGodParticle/ https://www.fourmilab.ch/documents/OhMyGodParticle/

cshimmin 11y ago

The title is a bit misleading. The particle doesn't break any "speed limits", in the sense that there is nothing fundamental preventing particles from traveling faster (or more precisely, having more energy). The GZK cutoff simply implies that particles are likely to interact with relic photons above this speed. So, the correct interpretation is that either 1) the observed particle started out with much more energy, and lost some via the GZK mechanism, 2)it didn't travel very far to get to us, or 3) we were just "lucky" to see it (which is already built into the rarity of such high energy particles). See also this figure [1], showing expectation energy vs. propagation distance for various initial proton energies. You can see that above 10^20 eV, particles will on average loose energy pretty quickly until they are at or below this energy, on about a 250 million lightyear distance scale. [1] http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_proton.jpg http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_prot...

MatthewWilkes 11y ago
There's nothing fundamental stopping cars breaking a normal speed limit, either. /s

georgecmu 11y ago

The rarest of all, those that are labeled “ultrahigh-energy” and exceed 1 EeV, strike each square kilometer of the planet only once per century. In other words, 14 thousand of these particles strike Earth every day.