Only then I figured, why not go all-modern and crowd-source this question! So I put it to the folks at my Facebook Fan site, spurred by the offer of a prize -- a hardcover first printing of EXISTENCE going to the top vote-getter. We got a fair number of submissions and the top responses are presented here, ranging from the serious and thoughtful to the humorous and ironic...
...starting with our Grand Prize winner, Mr. Tony Farley, a physics teacher from California.
#1 We don't have the capabilities to detect anything but a tightly beamed signal. And like detecting the sound of a jet in the sky, where you can see it, is not where you can detect signals from it. You have to point your microphone behind it. With tightly beamed signals over galactic distances, you have to know the proper motion of the planet and its sun and they have to know our proper motion to beam it to us. If they are ten light years away, they have to beam it to where we were ten years ago and we have to point our detectors to where they were ten years ago. All the SETI searches ignore this and hope a civilization is sending out a ridiculously powerful beam in all directions. –Tony Farley
Tony Farley has also published a physics text, The Electric Force, for the iPad. (I don't suppose he is related to Patrick Farley, the brilliant web artist who created the vivid preview trailer for Existence?)
In fact, Tony, you are partly on-target with this one. But first, where you are wrong. SETI searches engaged in by the top group near Berkeley do compensate for motions and Doppler shifts and orbital variations to a degree that would amaze you. They can detect a signal that is spectrum-varying with time and compensate for that as the source spins and rotates and revolves around a noisy star. These are clever folks.
Still, you are right that they still make untenable assumptions. They search the sky with narrow listening beams... looking for aliens who might be BROADcasting hello signals in all directions. But there's no reason that even a beneficent race would do that, around the clock, for eons. Horribly expensive. They would, as you say, "ping" likely targets like our solar system, maybe once a century. To detect such pings, instead of one expensive SETI program in one place, we should have a thousand backyard receivers, networked, scanning the whole sky at once. Look up Project Argus of the SETI League!
And congratulations on your prize! A hardcover of Existence is winging its way to you.
#2 The universe is big in space AND time. It would be a major accomplishment for a technological society to remain intact for a million years, yet that is just a blip on the scale of the universe. How many galactic empires came and went before the Earth was even capable of supporting life? –Thomas Nackid
A good question. And yes, we might simply not overlap with the others! But note, Thomas, your assumption is that the numbers of tech races must be very small (and that may be the case) in order for the statistical non-overlap idea to work. But if there are numerous long-lived species, then we get the Fermi Paradox. And if they travel? A lot? Colonization changes all the numbers!
Even if they just explore and don't colonize, then the Earth would likely have been visited. But even one toilet flush during the Archaean would have changed life on Earth in ways we'd detect in the rocks.
#3 Life, even intelligent life, is common in the universe, but advanced civilizations are rare, and hard to find in the small window of time that we have been looking, and not all advanced civilizations are nice. Getting between stars and communicating between stars is hard, and having someone close enough to communicate with at the same time you're communicating is rare, and sometimes perilous. We have not found anyone yet because we can only shout at our nearest neighbors, and our local neighborhood is currently empty, probably by chance and possibly by malice. –Ilithi Dragon
I am one of the SETI experts who has been arguing that the Great Silence may be telling us something. "If all the races more advanced than us are being quiet... maybe they know something we don't know?"
Several major voices in the field, Like former NASA SETI chief John Billingham, have joined me in resigning from major committees in protest over the SETI Institute's role in helping clear a path for METI or "MESSAGE to ETI." See our complaint: Shouting at the Cosmos -- or How SETI has taken a Worrisome Turn into Dangerous Territory.
#4 They won't unscramble the signal until we put a deposit down. –Lone Hanks
hrm... you REALLY want to read my novel EXISTENCE! There will come a couple of moments when you just break down with guffaws!
Along those same lines: We haven't yet chosen a intergalactic long distance carrier. --Christopher R. Vesely
#5 The "Do Not Feed the Humans" sign just past Pluto deters all but delinquents making crop circles. –Kevin King
Ditto my answer to #4!
#6 Civilized people do not just drop in uninvited. –Eli Roth
We've been inviting!
Along those same lines: There may be a "Prime Directive" ethos that they stick to. --Glenn Brockett
That's the "Zoo Hypothesis" that comes in dozens of variations... all of which assume either that the ETIS are few and share the same value system, or else have one heckuva police force...
I'll toss in one last one:
As society gets rich enough and technologically sophisticated enough, eventually everyone is able to live in their own personal Matrix, customized to provide them with their ideal life. Soon after the civilization stops bothering to expand any further, as the perfect existence can already be found on their home planet and nothing more could be wanted. Humans have a rare neurological structure that prevents them from being satisfied with this sort of simulation. –Eneasz Brodski
See also a discussion of The Great Filter: Does a Galaxy Filled with Habitable Planets Mean Humanity is Doomed? on io9 -- Robin Hanson’s concept that there may be some obstacle that consistently prevents species from reaching the technological stage where they can traverse interstellar distances. (That's the core topic in my new novel.)
Hey, we've run out of space (get it?) So, Congratulations Mr. Farley... and the rest of you for having lively minds!
