Irrespective of the veracity of the stories of mutilated cows, crop circles and violated Midwesterners at the hands of these alien visitors, the cultural impact of these early sightings was very real. America quickly entered into a media-fuelled love affair with alien invaders in shiny discs brandishing anal probes (why didn’t they use MRI scanners, a non-Freudian would surely ask?). Of all the hundreds of thousands of references to flying saucers that began to appear in the media, a cartoon by Alan Dunn published in the New Yorker magazine on 20 May 1950 found its way into the lunchtime conversation of a group of scientists at the Los Alamos National Laboratory in New Mexico.
Enrico Fermi was one of the greatest twentieth-century physicists. Italian by birth, he conducted his most acclaimed work in the United States, having left his native country with his Jewish wife Laura in 1938 as Mussolini’s grip tightened. Fermi worked on the Manhattan Project throughout World War Two, first at Los Alamos, and then at the University of Chicago, where he was responsible for Chicago Pile 1, the world’s first nuclear reactor. In a squash court underneath a disused sports stadium in December 1942, Fermi oversaw the first man-made nuclear chain reaction, paving the way for the Hiroshima and Nagasaki bombs.
After the war Fermi settled as a professor in Chicago, but he often visited Los Alamos. During one of these visits, in the summer of 1950, Fermi settled down for lunch with a group of colleagues including Edward Teller, the architect of the hydrogen bomb, and fellow Manhattan Project alumni Herbert York and Emil Konopinski. At some point, talk turned to the recent reports of UFO sightings and the New Yorker cartoon, stimulating Fermi to ask a simple question that turned a trivial conversation into a serious discussion: ‘Where are they?’
Fermi’s question is a powerful and challenging one that deserves an answer. It has become known as the Fermi Paradox. There are hundreds of billions of star systems in the Milky Way galaxy. Our solar system is around 4.6 billion years old, but the galaxy is almost as old as the universe. If we assume life is relatively common, and on at least some of these planets intelligent civilisations arose, it follows that there should exist civilisations far in advance of our own somewhere in the galaxy. Why? Our civilisation has existed for around 10,000 years, and we’ve had access to modern technology for a few hundred. Our species, Homo sapiens, has existed for a quarter of a million years or so. This is a blink of an eye in comparison to the age of the Milky Way. So if we assume we are not the only civilisation in the galaxy, then at least a few others must have arisen billions of years ahead of us. But where are they? The distances are not so vast that we cannot imagine travelling between star systems in principle. It took us less than a single human lifetime to go from the Wright Brothers to the Moon. What might we imagine doing in the next hundred years? Or thousand years? Or ten thousand years? Or ten million years? Even with rocketry technology as currently imagined, we could colonise the entire galaxy on million-year timescales. The Fermi Paradox simply boils down to the question of why nobody has done this, given so many billions of worlds and so many billions of years. It is a very good question.
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FERMI’S PARADOX
The Fermi Paradox is the apparent contradiction between the high probability of extraterrestrial civilisations’ existence and humanity’s lack of contact with, or evidence for, such civilisations.
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LISTEN VERY CAREFULLY
For three days in 1924, William F. Friedman had a very important job. As chief cryptographer to the US Army, Friedman was used to dealing with National Security responsibilities, but from 21 to 23 August he was asked to search for an unusual message. On these dates Mars and Earth came within 56 million kilometres of each other, the closest the two planets had been since 1845, and they would not be so close again until August 2003. This offered the best opportunity since the invention of radio to listen in on the neighbours.
To make the most of the planetary alignment, scientists at the United States Naval Observatory decided to conduct an ambitious experiment. Coordinated across the United States, they conducted a ‘National Radio Silence Day’, with every radio in the country quietened for five minutes on the hour, every hour, across a 36-hour period. With this unprecedented radio silence and a specially designed radio receiver mounted on an airship, the idea was to make the most of the Martian ‘fly-by’ and listen in for messages, intentional or otherwise, from the red planet.
Conspiracy theories notwithstanding, William F. Friedman didn’t decipher the first message from an alien intelligence, and the American public soon tired of the disruption to their news bulletins, but the principle of the experiment was sound. The idea that we might listen in to aliens had first been proposed 30 years earlier by the physicist and engineer Nikola Tesla. Tesla suggested that a version of his wireless electrical transmission system could be used to contact beings from Mars, and subsequently presented evidence of first contact. He wasn’t right, but in 1896, one year before the publication of War of the Worlds, it was certainly a plausible claim. Tesla wasn’t alone; other luminaries of the time shared his optimism, including the pioneer of long-distance radio transmission, Guglielmo Marconi, who believed that listening to the neighbours would become a routine part of modern communications. By 1921 Marconi was publicly stating that he had intercepted wireless messages from Mars, and if only the codes could be deciphered, conversation would soon begin.
The failure of the National Radio Silence Day brought a temporary halt to the organised search for extraterrestrial signals, and the idea dropped out of scientific fashion until the post-war flying saucer boom. One of the first scientists to make 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 $30 million to fund the construction of the project, consists of 42 radio antennae able to scan large areas of the sky at multiple radio frequencies, including the 21cm hydrogen line. If there are any civilisations making a serious attempt to contact us with technology at least as advanced as our own within a thousand light years, the Allen Array will hear them.
In the early 1960s, the scientific community was sceptical about such endeavours and Frank Drake was perceived as a maverick. It’s important to be sceptical in science, but as Fermi understood, a back-of-the-envelope calculation with some plausible assumptions suggests that the search for ET may not be futile. Indeed, the alternative view that our civilisation is unique or extremely rare in a galaxy of a hundred billion suns appears outrageously solipsistic, and the sceptical finger might as easily be pointed at the cynics. There was, however, a handful of scientists who understood the importance of asking big questions, and together with Peter Pearman, a senior scientist at America’s prestigious National Academy of Sciences, Drake organised the first SETI conference in November 1961. The Green Bank meeting was small, but the list of attendees, who named themselves The Order of the Dolphin, was impressive.
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FIRST SETI CONFERENCE ATTENDEES
PETER PEARMAN
conference organiser
FRANK DRAKE
PHILIP MORRISON
DANA ATCHLEY
businessman and radio amateur
MELVIN CALVIN
chemist
SU-SHU HUANG
astronomer
JOHN C. LILLY
neuroscientist
BARNEY OLIVER
inventor
CARL SAGAN
astronomer
OTTO STRUVE
radio astronomer
GIUSEPPE COCCONI
particle physicist
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Philip Morrison was there, as was his co-author of the seminal 1959 Nature paper, Giuseppe Cocconi. I have a professional connection with Cocconi, who was a noted particle physicist and director of the Proton Synchrotron accelerator at CERN in Geneva. Cocconi was instrumental in discovering early experimental evidence for the pomeron, an object in particle physics known as a Regge trajectory that I have spent most of my career studying. The eminent, highly respected astronomer Otto Struve also attended. Struve publicly stated his belief in the existence of intelligent extraterrestrial life, perhaps because he had recently suggested a method for detecting alien planets outside our solar system (see here). Nobel Laureate Melvin Calvin, most famous for his work on photosynthesis, was present, along with future Hewlett Packard vice president for R&D Barney Oliver, astronomer Su-Shu Huang, communications specialist Dana Atchley and the colourful neuroscientist and dolphin researcher John Lilly. The most junior attendee was a 27-year-old postdoc. called Carl Sagan. I would love to have been there, although I’d have spent the whole time chatting with Cocconi about pomerons.
In preparation for the meeting, Drake drew up an agenda designed to stimulate a structured conversation amongst the group. If the search for intelligent extraterrestrial life was to be taken seriously, it was clear in Drake’s mind that the discussion should be rigorous and provide a framework for future research. The way to do that is to address the problem quantitatively rather than qualitatively; to break it down into a series of probabilities that can be estimated, at least in principle, using observational data.
Drake focused on a well-defined question – the one we discussed above: how many intelligent civilisations exist in the Milky Way galaxy that we could in principle communicate with? Drake’s brilliant insight was to express this in terms of a simple equation containing a series of probabilities. What is the fraction of stars in the galaxy that have planets? What is the average number of planets around a star that could support life? What is the fraction of those planets on which life begins? What is the probability that, given the emergence of simple life, intelligent life evolves? Given intelligence, how likely is it that the intelligent beings build radio telescopes and are therefore capable of communicating with us? Multiply all these probabilities together, and multiply by the number of stars in the Milky Way, and you get a number – the number of intelligent civilisations that have ever existed in the Milky Way.
This isn’t all Drake did, however, because he was interested in the number of civilisations that we might be able to speak to now, and that requires the addition of a rather thought-provoking term – the average lifetime of civilisations from the moment they develop the technology to communicate. If a civilisation arose a billion years ago and vanished shortly afterwards, then we would never be able to talk to them. The question of the lifetime of a civilisation may have been more vivid in the early 1960s than it is today. The Manhattan Project had been the training ground for many of the great physicists, and the Cuban missile crisis was less than a year away, propelling the world, in Soviet Premier Khrushchev’s words to President Kennedy, towards ‘… the abyss of a world nuclear-missile war’. To me, and to the participants at the Green Bank conference, the idea that a civilisation might destroy itself is both ludicrous and likely. We are pathetically inadequate at long-term planning, idiotically primitive in our destructive urges and pathologically incapable of simply getting along. More of this later! Putting the lifetime term into the equation was therefore scientifically valid and a political masterstroke; merely confronting the question should give us pause for thought at the very least.
To complete the equation with the lifetime term included – recall that it should give the number of currently contactable civilisations in the Milky Way – a little thought will convince you that the whole lot must be multiplied by the current rate of star formation in the galaxy. That might not be immediately obvious, but I have confidence you can demonstrate to yourself that it’s the correct thing to do. Homework is good.
The completed equation, which is known as The Drake Equation, is shown here.
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THE DRAKE EQUATION
N = R* × fs × fp × ne × fl × fi × fc × L
where:
N
the number of civilisations in our galaxy with which radio communication might be possible
(i.e. which are on our current past light cone)
R*
the average rate of star formation in our galaxy
fp
the fraction of those stars that have planets
ne
the average number of planets that can potentially support life per star that has planets
fl
the fraction of planets that could support life that actually develop life at some point
fi
the
fraction of planets with life that actually go on to develop intelligent life (civilisations)
fc
the fraction of civilisations that develop a technology that releases detectable signs of their existence into space
L
the length of time for which such civilisations release detectable signals into space
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When Drake wrote down his equation, only R was known with precision. Star formation had been closely studied in parts of our galaxy and the data suggested a value of around one new star per year. The rest of the terms were unknown in the 1960s, and we will spend the majority of this chapter exploring them, given over 50 years of astronomical and biological research. Despite the lack of experimental data, however, the Green Bank participants spent the meeting debating each one of the terms in the Drake Equation. This is the power of Drake’s formulation. It’s not yet possible to make a measurement of the fraction of planets on which life emerges with any sort of precision, but it is possible to look at the experience we have on Earth, and increasingly in the wider solar system, and make an informed guess. The probability of the emergence of intelligence given simple life is also a difficult question, but we do know that it took over 3 billion years on Earth, and that may give us a clue. Drake’s equation is valuable therefore because it provides a framework for discussion and debate, focuses the mind and suggests a direction for future research, just as Drake intended.
Human Universe Page 7
