

The Particle That Broke a Cosmic Speed Limit - digital55
https://www.quantamagazine.org/20150514-the-particle-that-broke-a-cosmic-speed-limit/

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batbomb
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.

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ridgeguy
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/...](http://galprop.stanford.edu/elibrary/icrc/2011/papers/HE1.4/icrc1252.pdf)

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batbomb
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.

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simulate
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/)

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georgecmu
_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.

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cshimmin
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_prot...](http://apcauger.in2p3.fr/Public/Presentation/Images/GZK_proton.jpg)

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

