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Re: This Week's Finds in Mathematical Physics (Week 227)
Posted:
Mar 15, 2006 2:58 PM
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In article <44157629$38$fuzhry+tra$mr2ice@news.patriot.net>,
Shmuel (Seymour J.) Metz <spamtrap@library.lspace.org.invalid> wrote:
>In <dv29o9$oct$1@glue.ucr.edu>, on 03/12/2006
> at 11:10 PM, baez@math.removethis.ucr.andthis.edu (John Baez) said:
>>Since the 1980s, most of the new discoveries in fundamental physics
>>have come from unexpected observations in astronomy. These
>>observations were mostly *not* predicted by theorists.
>Do you count anomalous cosmic ray showers as Astronomy?
Sure!
>That's one
>that will almost certainly require new Physics to explain.
I'm not sure about that, so I didn't include high-energy
cosmic rays in my list - nor the Pioneer effect.
But, it's certainly *possible* that one or both of these will
require new physics for their explanation.
For those not in the know, here's some stuff about high-energy cosmic rays.
I'll use it to update my page on open questions in physics:
http://math.ucr.edu/home/baez/open.questions.html#cosmology
I also need to update the section on gamma-ray bursters!
.......................................................................
What is the origin and nature of ultra-high-energy cosmic rays?
Cosmic rays are high-energy particles, mainly protons and alpha particles,
which come from outer space and hit the Earth's atmosphere producing a
shower of other particles. Most of these are believed to have picked up
their energy by interacting with shock waves in the interstellar medium.
But, the highest-energy ones remain mysterious - nobody knows how they
could have acquired such high energies.
The record is a 1994 event detected by the Fly's Eye in Utah, which
recorded a shower of particles produced by a cosmic ray of about 300 EeV.
A similar event has been detected by the Japanese scintillation array AGASA.
An EeV is an "exa-electron-volt", which is the energy an electron picks up
going through a potential of 10^18 volts. 300 Eev is about 50 joules -
the energy of a one-kilogram mass moving at 10 meters/second, presumably
all packed into one particle! Nobody knows how such high energies are
attained - perhaps as a side-effect of the shock made by a supernova or
gamma-ray burster? The puzzle is especially acute because because particles
with energies over 50 EeV are expected to interact with the cosmic microwave
background radiation and lose energy after travelling only moderate
extragalactic distances, say 30 megaparsecs. This effect is called
the Greisen-Zatsepin-Kuz'min (or GZK) cutoff. So, either our understanding
of the GZK cutoff is mistaken, or ultra-high-energy cosmic rays come from
fairly nearby sources - in cosmological terms, that is.
Right now the data is confusing, because two major experiments on
ultra-high-energy cosmic rays have yielded conflicting results.
The Fly's Eye seems to see a sharp dropoff in the number of cosmic
rays above 100 Eev, while the AGASA detector in Japan does not.
People hope that the Pierre Auger cosmic ray observatory, being built
in western Argentina, will settle the question.
For more, try these:
* HiRes - the High Resolution Fly's Eye experiment.
http://www.cosmic-ray.org/
* AGASA - the Akeno Giant Air Shower Array.
http://www-akeno.icrr.u-tokyo.ac.jp/AGASA/
* Pierre Auger Observatory,
http://www.auger.org/
* D. J. Bird et al, Detection of a cosmic ray with measured energy well
beyond the expected spectral cutoff due to cosmic microwave radiation.
http://arxiv.org/abs/astro-ph/9410067
* A. A. Watson, Observations of ultra-high energy cosmic rays.
http://arxiv.org/abs/astro-ph/0511800
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