XKCD #962

We should be able to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy according to the Drake equation:

Drake equation

By plugging in realistic values we obtain the result that there should be other civilizations in our galaxy, and this is what fundamentally motivates the search for extraterrestrial intelligence effort.

Now, the fact is that we haven't detected any, leading to Fermi's paradox: where is everybody?

Carl Sagan proposed, in the 70's, that this could only explained by the fact that civilizations almost invariably self-destruct once they have thermonuclear arsenals.

I am skeptical of the following claim: That Fermi's paradox implies civilizations will self destruct inevitably in a short time. I would like to see scholarly articles supporting Sagan's claims or debunking it.

In Sagan's own words: http://www.youtube.com/watch?v=0Ztl8CG3Sys

Same claim by Stephen Hawking.

  • 3
    For all those who downvoted. It is a contentious question and might promote a torrent of opinion. But, it cries out for good skeptical analysis: how much do we actually know about the key assumptions; what range of uncertainty is reasonable? It isn't easy to provide evidence; but surely it is productive to try even if a definitive answer isn't possible. Downvote the bad answers not the question.
    – matt_black
    Commented Oct 10, 2011 at 23:39
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    Relevant: smbc-comics.com/comics/20120107.gif
    – Borror0
    Commented Jan 7, 2012 at 13:57
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    "By plugging in realistic values we obtain the result that there should be other civilizations in our galaxy, and this is what fundamentally motivates the search for extraterrestrial intelligence effort." I think this should either be a quote or at least cited. The fact that we know what realistic numbers for the other variables happen to be is an extraordinary claim.
    – Christian
    Commented Jul 4, 2012 at 8:57
  • @Christian I provided a link to the claim, what else do you need?
    – Sklivvz
    Commented Jul 4, 2012 at 19:20
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    @Sklivvz Just because someone says they plugged realistic values into the equation doesn't mean that the values in fact are realistic. In fact the highest voted answer to this question argues that those values aren't. If you accept them as realistic then Fermi's paradox holds.
    – Christian
    Commented Nov 2, 2012 at 0:27

2 Answers 2


The Drake equation isn't exact science and it was never intended to be. In fact, Drake himself didn't intend it be used as such. It was merely a tool to focus the discussion at the Green Bank conference:

When Drake came up with this formula, he had no notion that it would become a staple of SETI theorists for decades to come. In fact, he thought of it as an organizational tool - a way to order the different issues to be discussed at the Green Bank conference, and bring them to bear on the central question of intelligent life in the universe.

All but one of the probabilities used in the equation are the result of wild speculation. As a general rule, when people go beyond the viability of their model, the results ends being brutally wrong.

In fact, just looking at the process used to determine each value is enough to make anyone skeptical:

The Green Bank meeting weighed in on each of the elements in the equation, and came up with generally optimistic estimates. The rate of star formation (R*) was the only element in the equation about which some reliable information existed, and the conference settled on a conservative estimate of about one star per year. Otto Struve was the resident expert on extrasolar planets (fp), and he suggested that planets orbiting distant stars were, in fact, very common. Su-Shu Huang gave an optimistic assessment about the likelihood of planets having life supporting environments (ne), and Calvin and Sagan suggested that on suitable planets life would ultimately emerge (fl). Lilly gave an optimistic assessment on the likelihood of intelligence emerging on a life-bearing planet (fi), based on his work on dolphins. If at least two intelligent species emerged on Earth, doesn't that suggest that intelligence is common? Lilly's views, however, were highly controversial, and often dismissed by mainstream biologists. Even the sympathetic audience at Green Bank was quick to note that dolphins were not a technological species, and would be unlikely to send radio beams into space.

The final two elements in the equation were in the field of social science: how likely are intelligent beings to communicate with other civilizations (fc), and how long do civilizations last (L)? Significantly, there were no social scientists at Green Bank. But while lamenting their absence, Morrison also pointed out that even specialists were unlikely to have the answers for such grand questions. In their absence, he suggested that based on Earthly experience civilizations were likely to develop advanced technology, and that curiosity and the urge to communicate appear to be universal. Furthermore, he suggested, if civilizations are able to overcome the dangers of nuclear self-destruction, they can probably sustain themselves for very long periods of time.

In summarizing their discussions, the conference members concluded that the number of communicating planets could range from fewer than 1000 to more than a billion. Most of them thought the higher number a more likely estimate. On this basis they called for a vigorous radio search for extraterrestrial intelligence, using a 300-foot dish, very large computers, and patience to search for at least 30 years.

You'll notice that, at best, scientists will refer to the equation as interesting. Take David Darling, an English astronomer, who says:

Despite the enormous uncertainties involved in using the Drake Equation, which can result in a value of N from less than one to more than a billion, it is at least interesting and instructive to consider each of the factors involved.

As for Sagan's assumption, it's just that: an assumption. It might be valid, or it might not be.

We can't derive the likelihood of a civilization to self-destruct from humanity's experience. We are only one data point. We will have to encounter many more habitable planets before pronouncing ourselves on that. As such, I would label Sagan's claim that civilizations almost invariably self-destruct once they have thermonuclear arsenal as nothing more than galactic ethnocentrism.

We really have no way of knowing.


The Drake equation is controversial, as most of the elements are very hard to 'guess'. For instance, see this quotation by T.J. Nelson (via wikipedia)

The Drake equation consists of a large number of probabilities multiplied together. Since each factor is guaranteed to be somewhere between 0 and 1, the result is also guaranteed to be a reasonable-looking number between 0 and 1. Unfortunately, all the probabilities are completely unknown, making the result worse than useless.

You must also not forget that the equation has a specific goal: a conversation piece, not a scientific equation at all.

The problem with the paradox might be simply a matter of "the number of civilizations is smaller then currently guessed".

There is no reason to start from the amount of expected civilisations to a conclusion about self-destruction, especially as specific as thermonuclear arsenals. Any global event, intentionally or not, caused by a civilization or not, could be the cause. Logic would say that though some civilizations would go down because of something, there is no reason to state that all would.


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