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The Question
(Submitted November 02, 2005)
What is the upper limit to the maximum energy that a cosmic ray can
have? What is the upper limit on how much energy we can expect to
see from the maximum energy cosmic rays hitting Earth?
The Answer
Theoretically, cosmic rays with energies above 1020 electron volts
should rapidly lose their energies by interacting with the cosmic
microwave background photons. Technically, this is called the GZK
cut-off. However, several events above the GZK cut-off have been
observed. The highest energy event ever recorded has about the
same energy as a major league fastball (if you're not familiar with
baseball, think the fastest tennis ball at Wimbledon).
In addition, we don't know how cosmic rays can attain energies below
the GZK cut-off but above the "knee," see:
http://imagine.gsfc.nasa.gov/docs/features/topics/snr_group/cosmic_rays.html
You can find more information at the website of the Pierre Auger
Observatory, which is a facility dedicated to the study of highest energy
cosmic rays:
http://www.auger.org/rays/big_events.html
Hope this helps,
Koji & Kevin
for "Ask an Astrophysicist"
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