EXTRAORDINARY ENERGY, RARITY: The
highest energy of a detected cosmic ray is 3x1020 eV (electron volts),
equivalent to about 12 calories, more than ten-million times greater than
the maximum energy achievable with the largest planned particle accelerators
on Earth. This recently detected event was concentrated in a single "elementary"
particle, the highest energy such fundamental chunk of matter yet to be
found in nature. Only seven other events with energy above about 1020 eV
have been observed.
LIKELY EXTRAGALACTIC ORIGIN: There
is no known mechanism in our Galaxy which can accelerate particles to the
observed energies. Additionally, the magnetic field of our galaxy is too
weak to contain the highest energy cosmic rays, if they are charged particles.
UNKNOWN SOURCES: If they are extragalactic,
the highest energy cosmic rays will interact with the pervasive three-degree
thermal cosmic microwave background radiation, which degrades their energy.
As a result the observed cosmic rays cannot originate from distances greater
than a few percent of the distance to the horizon of the universe. There
are no plausible sources this close to our galaxy.
NEW PHYSICS? Possibly the highest
energy cosmic rays have an exotic, "top down" source. The annihilation
of topological defects, e.g. "strings", remaining from phase
transitions in the early Universe, would produce charged particles with
energies extending well above the highest energy cosmic rays.
AN EXPERIMENT: High energy cosmic
rays may be observed when they interact with the earth's atmosphere, producing
a cascade of millions of secondary particles which move through the atmosphere
at the speed of light and produce fluorescence. The fluorescent light from
these cascades may be observed from space, and the energy and arrival direction
of the cosmic rays determined.