Why no alien radio signals?

Many of have heard of the Fermi paradox: on plausible assumptions, the universe should be teeming with intelligent life, so where are they all? The folks behind SETI have responded by going out and looking for it by scanning radio signals, but so far the search has turned up nothing, leading Peter Schenkel to argue in Skeptical Inquirer that SETI needs a skeptical reassessment.

Robin Hanson has taken this conclusion and reasoned that this implies the existence of a “Great Filter” that’s been leading to far fewer alien species than we’d otherwise expect… or perhaps, he suggests, the other species have merely been short lived. If so, we should worry that whatever the Great Filter is, it could one day spell the end of human civilization too.

But for another perspective, I just got done reading a very interesting article by Randall Munroe of xkcd fame, arguing that more efficient communications technologies have made human civilization less visible over time:

You can work through the physics of interstellar radio attenuation, but the problem is captured pretty well by considering the economics of the situation: If your TV signals are getting to another star, you’re losing money. Powering a transmitter is expensive, and creatures on other stars aren’t buying the products in the TV commercials that pay your electricity bill.

The full picture is more complicated, but the bottom line is that as our technology has advanced, less of our radio traffic has been leaking out into space. We’re closing down the giant transmitting antennas and switching to cable and fiber and tightly-focused cell-tower networks.

While our TV signals may have been detectable—with great effort—for a while, that window is closing. In the late 20th century, when we were using TV and radio to scream into the void at the top of our lungs, the signal probably faded to undetectability after a few light-years. The potentially habitable exoplanets we’ve spotted so far are dozens of light-years away, so the odds are they aren’t currently repeating our catchphrases.

But TV and radio transmissions still weren’t Earth’s most powerful radio signal. They were outshone by the beams from early-warning radar.

Early-warning radar, a product of the Cold War, consisted of a bunch of ground and airborne stations scattered around the Arctic. These stations swept the atmosphere with powerful radar beams 24/7, often bouncing them off the ionosphere, and people obsessively monitored the echos for any hints of enemy movement. (I wasn’t alive during most of this period, but from what I hear, the mood was a little tense.)

These radar transmissions leaked into space, and could probably be picked up by nearby exoplanets if they happened to be listening when the beam swept over their part of the sky. But the same march of technological progress that made the TV broadcast towers obsolete has had the same effect on early-warning radar. Today’s systems—where they exist at all—are much quieter, and may eventually be replaced completely by new technology.

The implications for why we have yet to detect any alien civilizations are obvious: maybe they’re out there, but they’ve long since adopted technologies that make themselves invisible to our current means of detecting them.

I have no idea if that’s really right, though. I’m entirely a casual observer of this debate, and what I know of it is mostly what I’ve acquired while trying to learn about other things. So, your thoughts?

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