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. Newtons 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. |
