As Edward Teller, Emil Konopinski, Enrico Fermi and Herbert York walked to lunch one of them noticed a New Yorker cartoon taped to a door blaming trashcan thefts on aliens. It was 1950 and the country was in something of a UFO craze. They laughed and continued.
Unidentified flying objects have been a thing in the US since the late 1880s. As soon as flying machines captured imaginations UFO sightings were reported. Sometimes explained as foreign war machines, more often they were from other planets in the solar system falling in line with science fiction in the penny press and dime novels. Jules Verne and HG Wells created more sophisticated aliens and UFO sightings reported rockets and flying saucers during the Great Depression. The movie comic book industries picked up on it and fanned the enthusiasm.
After WWII the atom bomb - something right out of science fiction for most people - and conspiracy theories growing out of secret government programs gave a sense to many in the public that these events were real. The four physicists knew everything so far had a down-to-earth explanation - and if there was something, they'd probably be among the first to know.
They sat down to lunch and physics talk when Fermi, who had been uncharacteristically quiet, suddenly exclaimed "where are they?" Everyone at the table knew what he was talking about. He scribbled away with some guesses about life supporting planets, the probability of life and the probability of interstellar travel.
° there are billions of stars similar to the Sun. Many are much older than the Sun
° Many probably have planets and some are probably Earth-like.
° Some must have developed intelligent life. Some of these civilizations are likely to be much more advanced than us.
° even at conservative interstellar travel speed of a half of one percent of the speed of light it would be possible to spread life from one location throughout the out galaxy in under 100 million years. Not a huge amount of time compared to the geological age of the Earth.
We should have been visited .. regularly, in fact. But where are they? Although it's not a paradox it is commonly called the Fermi Paradox. A large number of explanations have been given. Here's an unordered partial list of the top of my head:
° extraterrestrial life is very rare.. we indeed may be alone
° life might be common, but evolution to intelligent life is very rare.
° intelligent life destroys itself before it develops interstellar travel - a popular conjecture during the Cold War and one Carl Sagan warned about.
° civilizations are too far apart in space-time
° it's too expensive to mount such expeditions.
° AIs will replace biological intelligent life and they may be focused elsewhere.
° we don't know how to look
° they're not interested in us or they purposely are avoiding us
° they're already here .. or their sensors are
° they're too alien
° and on and on
Frank Drake modified a technique particle physicists use to create the first serious attempt to quantify how common life is in the Milky Way galaxy.1 The Drake Equation differs from the Fermi Paradox in that physical contact isn't necessary.. it's a counting rather than a visit. It's easy to criticize as many of the terms are little more than educated guesses and some of them depend on poorly defined terms like life and intelligence. It was also conceived before the Big Bang was established and assumed a steady state universe. Lots of issues, nut it fires the imagination and encourages dialog to the point where some progress has been made.
In its original form there are seven terms that are multiplied together .. note the fractions f are between 0 and 1.0.
R* the rate of star formation
fp fraction of stars with planets
ne number of planets that might allow life to evolve per star
fl fraction of planets that could have life that have life
fi fraction of planets with life that evolves to intelligent life
fc fraction of planets with intelligent life that have the capability of interstellar communication
L the length of time such a civilization can broadcast and listen
As you might imagine there's some serious guesswork involved. The last three terms in particular give such a spread that it has been criticized as an exercise in personal bias. But we've learned a lot since 1961. Now we have a good estimate of the probability a star has planets .. it's very common. Furthermore there's been a good deal of work on the signatures of life and physically where to look - so called goldilocks zones. One of the goals of the James Webb Space Telescope - the follow-on to the Hubble Space Telescope - is probing for signs of life. That's for one of the next posts.
My own view, admittedly not worth much as I don't have enough empirical evidence, is that life must be very common, but intelligent life (whatever that means) is fairly rare. Still the Universe is so large that I think there must be many civilizations. But I also like to think we may be very rare and that humanity is special and worth preserving. My belief is that Elon Musk and Stephen Hawking are very wrong when they say we need to leave Earth very soon to preserve humanity. We have a very special and amazing place that is worth saving.
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1 The form has its roots in 19th century system efficiency calculations. If a machine or process has parts each with their own efficiency and everything connected together you simply multiply the efficiencies to get the total system efficiency. You can use the same technique to estimate the probability of finding a potential mate:-)
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