Where is ET? Fermi’s paradox turns 65



Gravitational lensing revealing multiple copies of the same supernova


Sixty five years ago, in 1950, while having lunch with colleagues Edward Teller and Herbert York, who were chatting about a recent cartoon in the New Yorker depicting aliens abducting trash cans in flying saucers, Nobel physicist Enrico Fermi suddenly blurted out, “Where is everybody?” His question is now known as Fermi’s paradox.

Fermi’s line of reasoning was the following: (a) Most likely there are numerous (maybe millions) of other technological civilizations in the Milky Way galaxy alone; (b) if a society is less advanced than us by even a few decades, they would not be technological, so any other technological civilization is, almost certainly, many thousands or millions of years more advanced; (d) within a million years or so (an eye-blink in cosmic time) after becoming technological, a society could have explored or even colonized most of the Milky Way; (e) so why don’t we see evidence of the existence of even a single extraterrestrial civilization?

Clearly the question of whether other civilizations exist is one of the most important questions of modern science. And a discovery of such life, say by analysis of microwave data, would certainly rank as among the most significant and far-reaching of all scientific developments. For one thing, it would lend credence to the suggestion by some eminent scientists, such as Freeman Dyson, that the universe is primed for intelligent life.

The Drake equation

At a conference in 1960, Frank Drake (1930-) and some other scientists proposed what is now known as Drake’s formula to estimate the number of extraterrestrial civilizations in the Milky Way galaxy:

N = R* fp ne fl fi fc L


N = number of civilizations in our galaxy that can communicate
R* = average rate of star formation per year in galaxy
fp = fraction of those stars that have planets
ne = average number of planets that can support life, per star that has planets
fl = fraction of the above that eventually develop life
fi = fraction of the above that eventually develop intelligent life
fc = fraction of civilizations that develop technology that signals existence into space
L = length of time such civilizations release detectable signals into space.

The values used by Drake in 1960 were R = 10, fp = 0.5, ne = 2, fl = 1, fi = 0.01, fc = 0.01, L = 10,000, so that N = 10 x 0.5 x 2 x 1 x 0.01 x 0.01 x 10,000 = 10. This was a rather conservative estimate — other researchers, then and now, estimate far more.

The SETI Project

In the wake of these analyses, scientists proposed the Search for Extraterrestrial Intelligence (SETI) project, to search the skies for radio transmissions from distant civilizations in a region of the electromagnetic spectrum thought to be best suited (because of low background noise) for interstellar communication. The SETI project and related activities have by now searched the radio spectrum for several decades, at higher and higher frequency resolution, with ever-more-sophisticated equipment and computer processing facilities.

But after 50 years of searching, the bottom line is that nothing has been found. If there are indeed numerous technological civilizations in the Milky Way, why have we not been able to detect any signals or other evidence of their existence? Why are they making it so hard for us to find them? In Fermi’s parlance, “Where are they?”

Proposed solutions to Fermi’s paradox

Numerous scientists have examined Fermi’s paradox and have proposed solutions. Here is a brief listing of some of the proposed solutions, and common rejoinders [Webb2002, pg. 27-231]:

  1. They are under strict orders not to disclose their existence. Rejoinder: Although this explanation (often termed the “zookeeper’s hypothesis”) is preferred by some scientists (including Carl Sagan, for instance), it falls prey to the inescapable fact that it just takes one small group in the extraterrestrial society to dissent and break the pact of silence. Given our experience with human society, it seems utterly impossible to think that a ban of this sort could be imposed, without a single exception over millions of years, on a vast extraterrestrial civilization dispersed over multiple stars and planets.
  2. They visited Earth and planted seeds of life or left messages in DNA. Rejoinder: Although the notion that life began elsewhere (i.e., “directed panspermia”) has been advanced by scientists such as Francis Crick, the co-discoverer of DNA, there is no evidence in DNA of anything artificial, and, what’s more, this does not solve the problem of the origin of life — it just pushes it away to some other star system.
  3. They exist, but are too far away. Rejoinder: Such arguments typically ignore the potential of rapidly advancing technology. For example, once a civilization is sufficiently advanced, it could send “von Neumann probes” to distant stars, which could scout out suitable planets, land, and then construct additional copies of themselves, using the latest software beamed from the home planet. Simulations of this scheme indicate that a single society could explore (via its probes) the entire Milky Way galaxy within at most a few million years, which is a tiny fraction of the galaxy’s lifetime (see below). Communication can similarly be greatly facilitated by futuristic, but entirely feasible, high-tech means (see below).
  4. They exist, but have lost interest in interstellar communication and/or exploration. Rejoinder: Given that Darwinian evolution, which is widely believed to be the mechanism guiding the development of biology everywhere in the universe, strongly favors organisms that explore and expand their dominion, it is hardly credible that each and every individual, in each and every distant civilization forever lacks interest in space exploration, or (as in item #1 above) that a galactic society is 100% effective, over many millions of years, in enforcing a ban against those who wish to communicate or explore.
  5. They are calling, but we do not yet recognize the signal. Rejoinder: While most agree that the SETI project still has much searching to do, this explanation doesn’t apply to signals that are sent with the express purpose of communicating to a newly technological society, in a form that this society could easily recognize. Indeed, the current SETI project program assumes that the remote civilization is making some effort to signal its existence using technology we can detect. And as with item #1, it is hard to see how a galactic society could forever enforce, without any exceptions, a global ban on such targeted communications.
  6. Civilizations like us invariably self-destruct. Rejoinder: This contingency is already figured into the Drake equation in the L term (the average length of a civilization). In any event, from human experience we have survived at least 100 years of technological adolescence, and have not yet destroyed ourselves in a nuclear or biological apocalypse. Global warming presents a major challenge at the present time, and has recently been explicitly suggested as a negative solution to Fermi’s paradox. But we now understand the situation fairly well and are rapidly developing affordable green technologies, leading some, including Al Gore, to change their minds and be cautiously optimistic. Additional, more exotic, technologies are in the works, and at least some of them may bear fruit. In any event, within a decade or two human civilization will spread to the Moon and to Mars, and then its long-term existence will be largely impervious to calamities on Earth.
  7. Earth is a unique planet with characteristics fostering a long-lived biological regime leading to intelligent life. Rejoinder: The latest studies, in particular the detections of extrasolar planets (see below), point in the opposite direction, namely that environments like ours appear to be quite common.
  8. WE ARE ALONE, at least within  our home in the Milky Way galaxy. Rejoinder: This hypothesis flies in the face of the “principle of mediocrity,” namely the presumption, dominant since the time of Copernicus, that there is nothing special about Earth or human society. This may be a philosophically satisfying answer to some, but scientifically speaking it is rather disquieting.

Numerous other proposed solutions and rejoinders are given in books by Stephen Webb (2002) and by John Gribbin (2011). Frankly, none of the proposed solutions is very convincing.

Extrasolar planets

Two key terms in the Drake equation are fp (the fraction of stars that have planets) and ne (the average number of planets that can support life, per star that has planets). Scientists once thought that stable planetary systems in general, and Earth-like planets in particular, were a rarity. But beginning in the 1990s, scientists detected unmistakable evidence of planets orbiting around other stars. Since then, many more Earth-like planets in habitable zones have been identified. A November 2013 report found that roughly 20% of all sun-like stars (which constitute 20% of all stars) have Earth-like planets in the habitable zone, so that there are possibly as many as 40 billion such planets in the Milky Way. A March 2015 analysis concluded that many billions of planets in the Milky Way have 1-3 planets in the habitable zone. And in January 2015, a team of astronomers announced a total of over 4,000 confirmed extrasolar planets, including eight new planets in the “Goldilocks” zone about their respective suns (where liquid water is possible).

In short, among the factors in the Drake equation, two that have proven amenable to experimental study so far have been found to have entirely reasonable values, roughly in keeping with what Drake and his colleagues first estimated in the 1960s. If anything, the terms fp (the fraction of stars that have planets) and ne (the average number of planets that can support life, per star that has planets) appear to be somewhat higher than estimated by Drake.

Exploration of the Milky Way

One frequently proposed solution to Fermi’s paradox is that such extra-terrestrial civilizations may in fact be numerous in the Milky Way, but they are too far away for us to detect them using radio telescopes, and too far away for any of them to have visited us or to have left any trace of their existence. However, researchers point out that a distant society could deploy “von Neumann probes,” namely self-replicating robotic spacecraft that travel to a nearby star system, send data back to the home planet, construct replicas of themselves, and launch these craft to even more distant systems.

Von Neumann probe scenarios have been studied at length. In the latest such analysis, researchers at the University of Edinburgh employed a computer simulation to explore the scenario where each probe travels at fairly modest speed under powered flight (roughly 10 km/sec), but employs a “slingshot” technique to enhance its speed and reduce its need for fuel. These researchers found that with this scenario, 99% of all star systems in the Milky Way could be explored in only about five million years, which, as mentioned above, is an eye-blink in the multi-billion-year age of the Milky Way.

Communication and imaging using gravitational lenses

“Exploring” the Milky Way telescopically is even easier. SETI pioneer Frank Drake observes that we could do this by taking advantage of the fact that the sun can act as a “gravitational lens,” according to the equations of general relativity. With such a facility, which is nearly feasible at the present time, we could obtain rather high-resolution images of distant planets, and even listen in to their microwave or optical communications and respond in kind.

Technologies such as von Neumann probes and gravitational lens-based communication may seem futuristic, but keep in mind, as emphasized above, that other technological civilizations are almost certainly many thousands or millions of years more advanced than our own. So where are they? Why have they made no attempt to contact us (as far as we can tell)? Why have we been unable to detect evidence of their existence?

The great filter

Some writers have suggested that there is a great filter that explains the eerie silence — some major barrier to a society becoming sufficiently advanced to explore the Milky Way. Possibilities here range from the hypothesis that it might be extraordinarily unlikely for life to begin at all, or that the jump from prokaryote to eukaryote cells is similarly unlikely, or that our combination of planetary dynamics and plate tectonics is exceedingly unlikely, or, as suggested above, that civilizations like ours invariably self-destruct, or that some future calamity, such as a huge gamma-ray burst from a nearby star, invariably ends societies like ours before they can explore the cosmos.

One disquieting aspect of this line of thinking is that it then follows that either (a) we are first such technological society, since the great filter is behind us, or else (b) we are in deep trouble, since the great filter, possibly a great catastrophe, is still ahead of us. Along this line, Nick Bostrom, among others, hopes that the search for extraterrestrial life, either on Mars or on an extrasolar planet, comes up empty-handed, because if life were found, either ancient or present-day, this would reduce the number of possible candidates for the great filter being behind us, and it would increase the likelihood that the great filter still lies ahead of us.


With every new research finding of extrasolar planets in the habitable zone, or of potential life-friendly environments within the solar system, the mystery of Fermi’s paradox deepens. Indeed, “Where is everybody?” has emerged as one of the most intriguing scientific questions of our time. There is no easy answer.

Astronomer Paul Davies concludes his latest book (2011) on the topic by stating his own assessment: “my answer is that we are probably the only intelligent beings in the observable universe and I would not be very surprised if the solar system contains the only life in the observable universe.” Nonetheless, Davies reflects, “I can think of no more thrilling a discovery than coming across clear evidence for extraterrestrial intelligence.”

John Gribbin, a prominent British scientist, agrees:

They are not here, because they do not exist. The reasons why we are here form a chain so improbable that the chance of any other technological civilization existing in the Milky Way Galaxy at the present time is vanishingly small. We are alone, and we had better get used to it.

For additional discussion, see our 2011 blog in The Conversation, and also Tim Urban’s interesting 2014 article in the Huffington Post.

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