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Dark
Night Sky: Olbers Paradox
It was long before science became
compartmentalized.
Professions could freely intermingle and overlap. Wilhelm Olbers was a
German physician who enjoyed exploring the night sky with his
telescope. His work on comets and asteroids earned him a fairly
respectable niche in astronomy, still he continued to be a physician.
But something about the night sky kept bothering him. Almost towards
the end of this career he brought it in the open. In 1826 he wrote a
paper and asked the question – Why is the sky dark at night?
Until then and even through the nineteenth century
it
was generally believed that the universe was infinite with infinite
number of stars. Newton argued in its favor and explained why it should
be so. Newton’s eminence and stature notwithstanding Olbers
was
not convinced. He reasoned that if there were infinite number of stars,
the sky would be covered with stars. No matter how far they were the
cumulative effect of the star light would ultimately make the sky as
bright as the sun. Yet the night sky is dark.
Olbers was the
first to address the scientific
community
with this problem and this is known as Olbers paradox. It turns out
though that a Swiss astronomer Cheseaux had made the same observation
half a century ago and had mentioned it in his book. Apparently Olbers
never read that book and came upon the idea on his own. He tried a
couple of explanations but found them unsatisfactory. Finally he
assumed that the light from the stars was absorbed by the interstellar
gas and rested his case. Later though it turned out that this
explanation would not fit either. The interstellar gas will be
eventually heated to bright glow and the sky will be again as bright as
the sun. So the paradox remained unresolved.
A century after
Olbers had put his question Edwin
Hubble
discovered the expansion of the universe from the observation of
redshifts. The answer to the question was: the sky is not dazzling
bright because the universe is spending its energy in expanding. The
radiation from the galaxies and stars is mostly dumped into the
redshift. Whatever remains is so thinned out over the vast distance as
to be negligible. The night sky thus remains dark.
Redshift is the
term used for the shift of the
frequency
of light towards the red end of the spectrum due to the source moving
away from the observer. This occurs due to a phenomenon known as
Doppler Effect. The frequency emitted by a source increases if it is
approaching and decreases if it is receding. For example the whistle
from an approaching train becomes higher pitched if it is approaching
and lower pitched if it is moving away. In the case of visible light
the two ends of the spectrum are blue and red. For stars or galaxies
approaching the earth the spectral lines will have a blueshift, for
those receding they will have redshift. All distant galaxies show
redshifts directly depending on their distance, which shows that the
universe is expanding.
Dharmbir Rai
Sharma is a retired professor with
electrical engineering and physics background. He obtained his M.S.
degree in physics in India and Ph.D. in electrical engineering at
Cornell University. He has taught in universities here and also in
Brazil, where he spent sometime. He maintains a website http://www.cosmosebooks.com
devoted mainly to philosophy and science.
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