Human Universe

Professor Brian Cox, Andrew Cohen

• Editore: William Collins
• Anno: 2015
• ISBN:
  9780008129798
• Argomento: Naturwissenschaften
• Categoria: Kosmologie

ARE WE ALONE?

Sometimes I think we are alone in the universe

and sometimes I think we’re not.

In either case the idea is quite staggering.

Arthur C. Clarke

SCIENCE FACT OR FICTION?

There are questions to which knowing the answers would have a profound cultural effect. The question of our solitude is one. Are we alone in the universe – yes or no? One of these is true. The question as posed isn’t a good one, however, because it is impossible to answer in the affirmative. We have no chance, even in principle, of exploring the entire universe, which extends way beyond the visible horizon 46 billion light years away. The answer can therefore never be yes with certainty. Indeed, if the universe is infinite in extent, we have our answer! No, we are not alone. The laws of nature self-evidently allow life to exist, and no matter how improbable, life must have arisen an infinite number of times. In itself, this is quite a challenging statement, and we will explore it in more detail later on. But this isn’t really what most of us want to know.

I’ve always been interested in aliens – the ones that fly spaceships around – and I want to talk to one. On a winter afternoon in 1977 I stood in a queue that went around three sides of the Odeon cinema in Oldham with my dad, shuffling through half-frozen puddles to see Star Wars, and spent the next decade building Millennium Falcons out of Lego. At some point in 1979 I picked up a magazine about Alien, and moved on to Nostromo, which required more bricks. To my delight I saw Alien when I was 11 years old at Friday Evening Film Society at school, and it didn’t put me off. I just realised I really liked the spaceships, and didn’t care much about the organic stuff. Everyone should see Alien at 11. To hell with the ratings; terror, technology and Sigourney Weaver are good for the soul.

Science fiction was a natural home for my imagination. I’d been interested in astronomy for a while, I’m not sure why, but the study of the stars seemed clean and precise and romantic; something done on cold nights before Christmas with mittens and imagination. Star Wars, Star Trek, Alien, Arthur C. Clarke and Isaac Asimov were merged seamlessly with Patrick Moore, Carl Sagan and James Burke, and they remain so; fact and fiction are inseparable in dreams. The superficially orthogonal desires to do science and to imagine distant worlds are closely related: shadows cast by different lights.

So the question ‘Are we alone in the universe?’ might make good science fiction, but it is not well posed in a scientific sense because the universe is too big for us to explore in its entirety. If we restrict the domain of the question, however, we can address it scientifically. ‘Are we alone in the solar system?’ is a question we are actively seeking to answer with Mars rovers and future missions to the moons of Jupiter and Saturn, where the conditions necessary for life may be present on multiple worlds. But even here, the use of the word ‘alone’ in the question is problematic. Would we be alone if the universe were full of microbes? Would you feel alone stranded in a deep cave with no means of escape and a billion bacteria for company? If not being alone means having intelligent beings to communicate with – sophisticated creatures that build civilisations, have feelings, do science and respond emotionally to the universe, then we have our answer in the solar system. Yes – Earth is the only world that is home to a civilisation, and we are alone.

How far might we reasonably expect to extend the domain of our question beyond the solar system? I find it impossible to believe that we’ll ever explore the universe beyond our own galaxy. The distance between the Milky Way and our nearest neighbour, Andromeda, is over 2 million light years, and that seems to me to be an unbridgeable distance, at least given the known laws of physics. But that still leaves an island of several hundred billion stars, 100,000 light years across. We will therefore rephrase our question so that we have a chance of interrogating it in a scientific way, and ask ‘Are we the only intelligent civilisation in the Milky Way galaxy?’ If the answer is yes, then we are in the cosmic equivalent of an inescapable cave and that would have made my 11-year-old self, gazing up at a dark sky of infinite possibilities, extremely sad. There may be others out there amongst the distant galaxies, but we’ll never know. If the answer is ‘No’, on the other hand, this would have profound consequences. Aliens would exist in a truly science-fiction sense; beings with spacecraft, culture, religion, art, beliefs, hopes and dreams, out there amongst the stars, waiting for us to speak with them. What are the chances of that? We don’t know, but at least we have posed a question that can be explored scientifically. How many intelligent civilisations are there likely to be in the Milky Way, given the available evidence today?

THE FIRST ALIENS

On 24 June 1947, Ken Arnold, an amateur pilot from Scobey, Montana, was flying over Mount Rainier, one of the most dangerous volcanoes in the world. Arnold was an experienced pilot with thousands of flying hours, and this implied he was a trustworthy observer. On returning to the airfield, he claimed to have seen nine objects flying in the mountain skies, describing them as ‘flat like a pie pan’ and ‘like a big flat disc’. He estimated the discs were flying in formation at speeds of up to 1920 kilometres per hour. The press jumped on the story – coining the term ‘flying saucer’ – and within weeks hundreds of similar sightings were reported from all over the world. On 4 July a United Airlines crew reported seeing another formation of nine discs over the skies of Idaho, and four days later, the mother of all UFO stories exploded at Roswell, New Mexico, with the confirmation and then rapid retraction by the United States Air Force of a recovered ‘flying disc’ – an alien craft crash-landed on Earth.

I’ll put my cards on the table here: I believe in UFOs. That is to say, I believe that there have been sightings of flying things in the sky that the observers were unable to identify, some of which were objects. But I do not believe for a moment that these were spacecraft flown by aliens. Occam’s razor is an important tool in science. It shouldn’t be oversold; nature can be complex and bizarre. But as a rule of thumb, it is most sensible to adopt the simplest explanation for an observation until the evidence overwhelms it.

My favourite response to the criticism that dismissing the possibility of alien visitations to Earth is unscientific was provided by physicist and Nobel Laureate Richard Feynman in his Messenger Lectures at Cornell University in 1964: ‘Some years ago I had a conversation with a layman about flying saucers – because I am scientific I know all about flying saucers! I said “I don’t think there are flying saucers”. So my antagonist said, “Is it impossible that there are flying saucers? Can you prove that it’s impossible?” “No”, I said, “I can’t prove it’s impossible. It’s just very unlikely.” At that he said, “You are very unscientific. If you can’t prove it impossible then how can you say that it’s unlikely?” But that is the way that is scientific. It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible. To define what I mean, I might have said to him, “Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extraterrestrial intelligence.” It is just more likely. That is all.’

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...