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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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