What is Fermi’s paradox?
The following extract is from The Great Silence by Milan M. Ćirković and introduces Fermi’s Paradox, named after the physicist Enrico Fermi.
Fermi’s paradox presents arguably the least understood of the ‘Grand Questions’ posed in the history of science, as well as the biggest challenge for any practical SETI activity. As is now well known (having been established by the diligent research of Eric M. Jones), the key argument follows a lunchtime remark of the great physicist, Enrico Fermi: Where is everybody? It turns out that, when Fermi was having lunch with Edward Teller, Emil Konopinski, and Herbert York at Los Alamos on a day in the summer of 1950, at one point the conversation touched on how there had recently been a flood of UFO sightings all over the United States. Some trash cans had gone missing in New York City at the time as well, and a New Yorker cartoon (of 20 May, 1950) had charged the interstellar visitors for the misdeed. In the relaxed atmosphere, Fermi remarked that extraterrestrial visitations could indeed have been the single common cause of two independent empirical phenomena — in this case, UFO sightings and missing trash cans — in the best tradition of scientific methodology: searching for a small number of causes for many different phenomena. The talk then veered towards the general issue of existence, or otherwise, of extraterrestrial intelligence. While they did not take the flying saucers stories seriously, Fermi and his companions earnestly discussed topics such as interstellar — and even superluminal — travel. Then, after some delay — and, as one might imagine with such key moments, which are always more the domain of legend and imagination than historical fact, in the midst of some tasty dish — Fermi allegedly asked his famous question.
Where, indeed, is everybody? His friends understood that the great physicist was talking about extraterrestrials.
In a nutshell, Fermi’s reasoning was as follows: both time and space are large in our astronomical environment, but there is an important sense in which the temporal scale is larger. The Galaxy is about 100,000 light years from edge to edge, which means that a star-faring species would need about 10 million years to traverse it if moving at a very modest velocity of just 1 per cent of the speed of light. Since the Galaxy is about a thousand times older than this, any technological civilization will have much more available time for such expansion and colonization of all planetary systems that exist in the Milky Way. If one species fails in this endeavour, another won’t. Consequently, if intelligent species were out there in any appreciable numbers, they would have been here already. And yet, we do not see them on Earth or in the Solar System. For Fermi and many thinkers since, this constituted a paradox.
All this was happening, one should keep in mind, 7 years before Sputnik 1 and a full 11 years before the first human cosmic flight by Yuri Gagarin. Fermi, sadly, did not live to witness either of these two epoch-making events for humanity, having died in 1954, only 4 years after his Los Alamos lunch. However, his powerful intuition applied excellently to this case, since he could not see any reason interstellar travel would be impossible (we do not nowadays either, in spite of the repeated assertions), and even if he underestimated the size of the Galaxy, it was at best a factor of a few orders of magnitude. In any case, he found a way to formulate, clearly and memorably, an obvious — but still not understood enough — fact: that humanity is a new phenomenon on the cosmic scene and that extraterrestrial intelligence, if it exists, is likely to be older than us on the basis of that fact alone. Adding the modern-day values for the age and size of the Galaxy and other astrophysical data just strengthens this conclusion and makes likely that the difference in ages becomes only larger.
Milan M. Ćirković is a research professor at the Astronomical Observatory of Belgrade, and a research associate of the Future of Humanity Institute at Oxford University. His primary research interests are in the fields of astrobiology (Galactic habitable zone, SETI studies, catastrophic episodes in the history of life), risk analysis (global catastrophes, observation selection effects, epistemology of risk), and philosophy of science (anthropic principles, philosophy of physics, future studies). He is the author of The Great Silence: Science and Philosophy of Fermi’s Paradox.
