杏吧原创

Thin Cosmic Rain: Particles from outer space by Michael Friedlander, Harvard
University Press, 拢18.50, ISBN 0674002881

COSMIC radiation was discovered by the Austrian physicist Victor Hess in
1912. We now know that it is made up of atomic particles, mainly protons but
with a sprinkling of heavier nuclei and electrons. They range up to the most
energetic particles known鈥攔ather more than 10²⁰ electronvolts (eV).
Today鈥檚 largest particle accelerators achieve a mere 10¹² eV or so鈥�100
million times less energetic.

Michael Friedlander, a physicist of international stature from Washington
University in St Louis, has understandably chosen to concentrate on the area of
cosmic radiation research that he knows best: the nuclear physics of particle
interactions. In an excellent survey of the complex and fascinating history of
the subject, he gives a blow-by-blow account of the discoveries and the details
of what he calls 鈥渁 disproportionately large number of Nobel prizes鈥� in this
field.

His readers, and he deserves to have many, will learn about increasingly
elegant detection techniques, about cosmic rays from the Sun, the energy spectra
of the particles and details of the tiny鈥攂ut important鈥攆lux of gamma
rays. As a source book for someone wanting 鈥渄ata鈥� as distinct from 鈥渋deas鈥�, this
is a fine book. But the tenor of the book is, alas, summed up by its title A
Thin Cosmic Rain鈥攂oth 鈥渢hin鈥� and 鈥渞ain鈥� suggest insubstantial and
rather inconsequential matters. Nothing could be further from the truth, because
there are some incredibly exciting aspects of cosmic radiation that Friedlander
really should have described. Most of these relate to 鈥渢he origin problem鈥�.

There are conflicting ideas as to where the most energetic particles come
from, but it is generally believed that supernova remnants passing through the
interstellar medium are responsible for those particles with energies up to
about 10¹⁵ eV. (Supernovas themselves are not implicated.) Friedlander
documents this aspect well. But we find a 鈥渒nee鈥� in the energy spectrum鈥攁
kink in the line of particle density plotted against energy鈥攊n the region
around 10¹⁵ eV, suggesting a completely different origin. This possibility
arouses passions in the cosmic ray research community, which Friedlander fails
to reflect.

Similar excitement reigns at the very highest energies. What are the sources?
Are exotic particles of dark matter in our Galaxy鈥檚 halo giving rise to the
ultra-energetic particles? Could a completely new physics even be involved
here? Perhaps colliding galaxies play a role鈥攁 case can certainly be made
for their contribution.

Gamma-ray astronomy is a growth area too, but is also barely touched upon by
Friedlander. It was studies of gamma rays and the gas content in three
galaxies鈥攐urs and the Large and Small Magellanic Clouds鈥攖hat led to
the conclusion that most cosmic ray particles are produced in our own Galaxy.
These studies left the tiny flux of ultra-high-energy particles to be accounted
for by extragalactic sources. Studies of cosmic gamma rays also played a role in
the deeper questions of cosmology, showing that there can be no symmetry between
the amount of matter and anti-matter in the Universe on any scale. This should
have been given a mention.

Friedlander covers solar cosmic rays well鈥攅xcept for one dramatic
omission: solar particles and solar plasma, which play a major role in modifying
the Earth鈥檚 atmosphere. This is another research growth area. Less importantly,
he should also have mentioned the effect of solar cosmic ray flares on human
life in the past鈥攁nd presumably the future, if humanity survives its own
environment-destroying activities for long enough.

Friedlander writes well: this book is a good example of that. However, what
we need now is a more up-beat contribution, perhaps entitled 鈥淐osmic Rays: the
truth is way out there鈥�.