It was Einstein who made the real trouble. He announced in 1905 that there was no
such thing as absolute rest. After that there never was.
-- Steven Leacock
Imagine a cloud
of hydrogen gas drifting through the wide empty expanse of space. These clouds are
quite common because 90% of the atomic nuclei in the universe are hydrogen nuclei,
actually single protons. Taking a closer look at each hydrogen atom (a bound electron-proton
pair) we can see that the potential energy of the magnetic moments of the proton
and electron are lower when they are opposite in direction, much as two bar magnets
will attract when aligned south to north, but repell when like poles are adjacent.
However, due to collisions, the cold hydrogen atom can actually have the electron
and proton magnetic moments become aligned and thus be in a higher energy state.
The transition back down to the ground state, when the magnetic moments become anti-parallel
once again, generates a photon with a frequency of 1420.406 MHz and wavelength of
21.11 cm. This particular transition is called a “forbidden” transition
in the terminology of quantum mechanics. That doesn't mean that it cannot happen,
only that it is a transition that is very rare. However, the vast quantities of
hydrogen in interstellar space makes this rare transition one that is relatively
easy to observe.
takes place in the cooler regions of space, not too close to stars, where optical
and ultraviolet radiation would warm the hydrogen out of the ground state.
These clouds of hydrogen gas are dense enough to absorb and then reemit the energy,
which is incident upon them. However, collisions with these atoms are as stated
before, rather rare. The clouds may only be at a density of 1 atom/cm3
and a temp of 100 K. So by observing this phenomenon through a radio telescope we
are able to find these regions and obtain a more complete view of the galaxy.