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Radio Telescope
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.
This process 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.