We must conclude from the work of those who have studied the origin of life, that given a planet only
approximately like our own, life is almost certain to start. Of all the planets in our own solar system we arc
now pretty certain the Earth is the only one on which life can survive. Mars is too dry and poor in oxygen,
Venus far too hot, and so is Mercury, and the outer planets have temperatures near absolute zero and
hydrogen-dominated atmospheres. But other suns, stars as the astronomers call them, are bound to have planets
like our own, and as the number of stars in the universe is so vast, this possibility becomes virtual certainty.
There are one hundred thousand million stars in our own Milky Way alone, and then there are three thousand
million other Milky Ways, or Galaxies, in the universe. So the number of stars that we know exist is estimated
at about 300 million million million.
Although perhaps only 1 per cent of the life that has started somewhere will develop into highly complex
and intelligent patterns, so vast is the number of planets that intelligent life is bound to be a natural part of the
universe.
If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from
outer space yet ? First of all, they may have come to this planet of ours thousands or millions of years ago, and
found our then prevailing primitive state completely uninteresting to their own advanced knowledge.Professor
Ronald Bracewell, a leading American radio-astronomer, argued in Nature that such a superior civilization, on
a visit to our own solar system, may-have left an automatic messenger behind to await the possible awakening
of an advanced civilization. Such a messenger, receiving our radio and television signals, might well
re-transmit them back to its home-planet, although what impression any other civilization would thus get from
us is best left unsaid.
But here we come up against the most difficult of all obstacles to contact with people on other planets--the
astronomical distances which separate us. As a reasonable guess, they might, on an average, be 100 light years
away. (A light year is the distance which light travels at 186,000 miles per second in one year, namely 6
million million miles.) Radio waves also travel at the speed of light, and assuming such an automatic
messenger picked up our first broadcasts of the 1920's, the message to its home planet is barely halfway there.
Similarly, our own Present primitive chemical rockets, though good enough to orbit men, have no chance of
transporting us to the nearest other star, four light years away, let alone distances of tens or hundreds of light
years.
Fortunately, there is a 'uniquely rational way' for us to communicate with other intelligent beings, as
Walter Sullivan has put it in his excellent recent book, We are not alone. This depends on the precise
radio-frequency of the 21-cm wavelength, or 1420 megacycles per second. It is the natural frequency of
emission of the hydrogen atoms in space and was discovered by us in 1951; it must be known to any kind of
radio-astronomer in the universe.
Once the existence of this wave-length had been discovered, it was not long before its use as the uniquely
recogniz

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