Human Universe by Professor Brian Cox Page A

the search for ET scientifically acceptable again was Philip Morrison, a contemporary and colleague of Fermi. It is not known whether they discussed the Fermi Paradox directly, but the idea of answering it certainly played on Morrison’s mind throughout the 1950s. At the end of the decade Morrison published a famous and influential paper with another of Fermi’s collaborators, Giuseppe Cocconi, laying out the principles of using radio telescopes to listen for signals. ‘Searching for Interstellar Communications’ was published in the prestigious journal
Nature
, and proposed a systematic search of the nearest star systems on a very specific radio frequency – the so-called 21cm hydrogen line.
    Morrison and Cocconi chose the hydrogen line because it is a frequency that any technological civilisation interested in astronomy will be tuned in to. Hydrogen is the most abundant element in the universe, and hydrogen atoms emit radio waves at precisely this frequency. If we could see these wavelengths with our eyes, the sky would be aglow, and this is why astronomers tune their radio telescopes to the 21cm line to map the distribution of dust and gas in our galaxy and beyond. If a technological civilisation wants to be heard, then under the assumption that anyone with any sense does radio astronomy, the 21cm line would be the most obvious choice for a message.
    Morrison and Cocconi’s paper inspired the birth of one of the most widely debated and controversial astronomical projects of modern times. Within a year of its publication, the 85-foot radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia, was pointing towards two nearby stars – Tau Ceti and Epsilon Eridani – listening in to the 21cm hydrogen line for any signs of unnatural order in the signals from the stars. The project, known as Ozma after a character from L. Frank Baum’s
Land of Oz
, was the brainchild of Frank Drake, a young astronomer from Cornell University. Drake chose Tau Ceti and Epsilon Eridani as the first target star systems because of the stars’ similarity to our own Sun and their proximity, just 10 and 12 light years away from Earth. In 1960 Drake had no idea if these stars harboured planetary systems, because no planets had been detected outside our solar system at that time. We now know that Drake’s guess was a good one. Tau Ceti is thought to have five planets orbiting the star, with one of them in the habitable zone. Epsilon Eridani is also thought to have at least one gas giant planet with an orbital period of around seven years. After 150 hours of observation, Drake heard nothing, but for him this was the beginning of a lifetime dedicated to the search for extraterrestrial intelligence, a search commonly known by its acronym, SETI.

 
     
     
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    21CM LINE
    Hydrogen atoms consist of two particles – a single proton bound to a single electron. Protons and electrons have a property called spin, which for these particular particles (known as spin ½ Fermions, named after Enrico Fermi himself) can take only one of two values, often called spin ‘up’ and spin ‘down’. There are therefore only two possible configurations of the spins in a hydrogen atom: the spins can be parallel to each other – both ‘up’ or both ‘down’, or anti-parallel – one ‘up’ and one ‘down’. It turns out that the parallel case has slightly more energy than the anti-parallel case, and when the spin configuration flips from parallel to anti-parallel, this extra energy is carried away as a photon of light with a wavelength of 21cm.

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    Today SETI is a global scientific effort, analysing data from telescopes used primarily for radio astronomy. The organisation also has a dedicated collection of telescopes designed specifically to detect signals from extraterrestrial civilisations at the Hat Creek Radio Observatory near San Francisco. The Allen Array, named after Microsoft founder Paul Allen who donated over