Some of the basic assumptions we make about extraterrestrial communication can be woefully naïve. Consider the situation in its gory detail. You decide (perhaps as a species, or perhaps as some resource-rich subset) that you want to ping the cosmos to find out if something else is listening, thinking, and as technological as you are. So you fire up your radio transmitter, or your big laser and start shooting off ‘Hello’ messages.
If our circumstances represent a useful template it means that the earliest possible response might come within about 8 years (Earth years of course). That's assuming that there is a responder in the nearest exoplanetary system, listening and receiving your first message at the right time, ready to fire back a response right away, willing to fire back a response, and capable of firing back something recognizable as a response. So, you start listening carefully 8 years later. But nothing comes in. So, you keep listening, telling yourself that it may take time for anyone to put a response together. And you keep listening.
Meanwhile, you’ve been busy. In the last 8 years you’ve been pinging the next furthest stellar systems. But for these the roundtrip light travel times go up to 10 years, 20 years, 40 years. Within the sphere of space for a 40-year messaging roundtrip are roughly 150 stars.
Time goes by, you decide that star #1 was a dud. But you have to wait longer and longer to find out what happens with the next star and the star after that. And if nothing comes in from the earliest possible responders your assessment of the odds of any randomly chosen star yielding a result has to be revised downwards. As it declines, so too does your resolve.
If you do stick it out, waiting for 40 years (and possibly more, to allow for an unknown delay period as extraterrestrial species get their act together to respond), but still hear nothing, what do you do next? Your options are to carry on pinging the same stars, or push on to more distant ones, or to stop. It’s certainly true that the farther your reach the more stars you access – as the volume of space grows with distance cubed. But at the point where the experiment’s timeline exceeds any individual’s lifespan, you’re going to need an extraordinary amount of patience and determination.
That, I think, is hugely problematic. Not just for us, but for any hypothetical species wanting to discover if there are cosmic neighbors by actively calling out. Unless you are very lucky, or the number of worlds with fully spun-up technological species is immense, you will face a profound barrier of time and willpower. The absence of anyone to talk to among the 150 systems of a 20-light year radius bubble could be true even with a billion talkative species in our galaxy (assuming 200 billion stars in the Milky Way). It would just be a bit of poor luck that there wasn’t one of those billion among the nearest 150 stars – nothing terribly out of the ordinary.
This situation is exacerbated if we allow for other factors. Perhaps a tech-capable species just isn’t looking and listening at the right moment or in the right direction, perhaps the window of technological evolution where a species is ‘hot’ in terms of communication ability is narrow (for reasons of energy-conservation and efficiency, or perhaps interest). And perhaps they simply don’t want to talk back, being happy to just listen in to other chatterboxes.
The conundrum of extraterrestrial messaging (METI) reminds me of the ‘tyranny’ of the rocket equation in space exploration. The faster (and further) you want to go, the more of your rocket has to be devoted to carrying fuel, adding even more mass to the mass that you want to shoot off into the void. The result is a kind of diminishing return (a diminishing return in natural logarithms).
Pinging the cosmos in the hopes of getting a response seems to have its own tyranny; you want to increase the odds of success, but that means an unavoidable increase in how long you have to wait in the hopes of an answer.
While I don’t see an obvious way around this, it does mean one thing. The classic idea of SETI – to listen for signals from elsewhere – does have one overwhelming merit; the signals could be about to reach us from essentially anywhere in the observable universe, with no wait time. Assuming that at least some species out there are very noisy, or have decided they don’t care about the tyranny of METI.
Since we ourselves may belong to both camps I think we can keep our fingers cautiously crossed.