Jonathan Falk writes:
A quick search seems to imply that you haven’t discussed the Fermi equation for a while.
This looks to me to be in the realm of Miller and Sanjurjo: a simple probabilistic explanation sitting right under everyone’s nose. Comment?
“This” is a article, Dissolving the Fermi Paradox, by Anders Sandberg, Eric Drexler and Toby Ord, which begins:
The Fermi paradox is the conflict between an expectation of a high ex ante probability of intelligent life elsewhere in the universe and the apparently lifeless universe we in fact observe. The expectation that the universe should be teeming with intelligent life is linked to models like the Drake equation, which suggest that even if the probability of intelligent life developing at a given site is small, the sheer multitude of possible sites should nonetheless yield a large number of potentially observable civilizations. We show that this conflict arises from the use of Drake-like equations, which implicitly assume certainty regarding highly uncertain parameters. . . . When the model is recast to represent realistic distributions of uncertainty, we find a substantial ex ante probability of there being no other intelligent life in our observable universe . . . This result dissolves the Fermi paradox, and in doing so removes any need to invoke speculative mechanisms by which civilizations would inevitably fail to have observable effects upon the universe.
I solicited thoughts from astronomer David Hogg, who wrote:
I have only skimmed it, but it seems reasonable. Life certainly could be rare, and technological life could be exceedingly rare. Some of the terms do have many-order-of-magnitude uncertainties.
That said, we now know that a large fraction of stars host planets and many host planets similar to the Earth, so the uncertainties on planet-occurrence terms in any Drake-like equation are now much lower than order-of-magnitude.
And Hogg forwarded the question to another astronomer, Jason Wright, who wrote:
The original questioner’s question (Thomas Basbøll’s submission from December) is addressed explicitly here.
In short, only the duration of transmission matters in steady-state, which is the final L term in Drake’s famous equation. Start time does not matter.
Regarding Andrew’s predicate “given that we haven’t hard any such signals so far” in the OP: despite the high profile of SETI, almost no actual searching has occurred because the field is essentially unfunded (until Yuri Milner’s recent support). Jill Tarter analogizes the idea that we need to update our priors based on the searching to date as being equivalent to saying that there must not be very many fish in the ocean based on inspecting the contents of a single drinking glass dipped in it (that’s a rough OOM, but it’s pretty close). And that’s just searches for narrowband radio searches; other kinds of searches are far, far less complete.
And Andrew is not wrong that the amount of popular discussion of SETI has gone way down since the ’90’s. A good account of the rise and fall of government funding for SETI is Garber (1999).
I have what I think is a complete list of NASA and NSF funding since the (final) cancellation of NASA’s SETI work in 1993, and it sums to just over $2.5M (not per year—total). True, Barnie Oliver and Paul Allen contributed many millions more, but most of this went to develop hardware and pay engineers to build the (still incomplete and barely operating) Allen Telescope Array; it did not train students or fund much in the way of actual searches.
So you haven’t heard much about SETI because there’s not much to say. Instead, most of the literature is people in their space time endlessly rearranging, recalculating, reinventing, modifying, and critiquing the Drake Equation, or offering yet another “solution” to the Fermi Paradox in the absence of data.
The central problem is that for all of the astrobiological terms in the Drake Equation we have a sample size on 1 (Earth), and since that one is us we run into “anthropic principle” issues whenever we try to use it to estimate those terms.
The recent paper by Sandberg calculates reasonable posterior distributions on N in the Drake Equation, and indeed shows that they are so wide that N=0 is not excluded, but the latter point has been well appreciated since the equation was written down, so this “dissolution” to the Fermi Paradox (“maybe spacefaring life is just really rare”) is hardly novel. It was the thesis of the influential book Rare Earth and the argument used by Congress as a justification for blocking essentially all funding to the field for the past 25 years.
Actually, I would say that an equally valid takeaway from the Sandberg paper is that very large values of N are possible, so we should definitely be looking for them!
So make of that what you will.
P.S. I posted this in July 2018. The search for extraterrestrial intelligence is one topic where I don’t think much is lost in our 6-month blog delay.
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