Continuing our theme from last time, let's take a look at how a free-energy limited civilization might be detected.
Back in the 50's and 60's, when radio was young, the seemingly-magical ability to communicate with anyone in the world wirelessly suddenly made contact with aliens seem very plausible. After all, powerful radios were "advanced" technology, and if we had them so would any spacefaring aliens. Could we listen to their signals? Might they be broadcasting secrets of the Universe? (Folks who have seen or read Carl Sagan's Contact will know it captures this spirit well).
But a major problem was power: how loud would their radio transmitters be? In a groudbreaking paper in 1963, Nikolai Kardashev classified advanced civilizations into three broad categories based on the power generation available for radio transmissions, and hypothesized as to the spectrum of their combined communications:
A type I civilization would be one like ours, with only weak transmitters.
A type II civilization would have a total power supply comparable to the power of its host star, making them detectable from a great distance.
A type III civilization would have a total power supply comparable to its host galaxy
He made the reasonable point that background radio emission from the host galaxy would provide a source of noise, so that the bulk of the radio transmissions to be expected would be where the noise was lowest. By inverting the radio spectrum of the Milky Way, he constructed a plausible radio signature of a galaxy-spanning civilization, and identified two radio sources of unknown nature that possessed this signature, CTA-21 and CTA-102.
The idea that these objects (whatever they were) hosted advanced civilizations was captivating. Even the Byrds got in on it:
The lyrics are:
Year over year receiving you
Signals tell us that you're there
We can hear them loud and clear
We just want to let you know
That we're ready for to go
Out into the universe
We don't care who's been there first
On our radio telescopes
Science tells us that there's hope
Life on other planets might exist.
In a psychedelic self-reference, the song ends with the aliens of CTA-102 listening to the song on their own radio receivers and discussing it in their alien-language. (Since I know the astronomers reading this that don't know the story are dying to know, Kardashev's objects are actually quasars: look up CTA-102 and CTA-21 on NED.)
One major shortcoming of the Kardashev approach is that the power consumption of a civilization and its radio transmissions could easily be ten orders of magnitudes apart (typical very strong radars on Earth are intermittent and have peak power for fractions of a second measured in GW (as in Doc Brown's "1.21 gigawatts!"), but Humanity's average power consumption is over 10,000 times larger than that). In fact, as we switch to co-axial cable and fiber optics, we are getting quieter, not louder!
Of course, you gotta look, and my hat's off to the pioneers who have doggedly maintained SETI's programs to search for such alien signals. But there is another major difficulty to this approach: it is designed to search for signals that are deliberately transmitted, detectably strong, recognizable as alien transmissions, and at frequencies we can guess. Not all of these conditions need be met for SETI success (we could detect radio "leakage", but it would be weaker; or we might not recognize the signal immediately as alien, but we might understand it to be non-astrophysical) but missing any of them makes the task much harder. Alien civilizations might communicate using technologies we cannot guess for purposes we cannot guess at powers we can barely guess.
In 1960 (before Kardashev's paper) Freeman Dyson published a short paper describing a different approach. He noted that the two primary constraints on the growth of a species are energy and material, and that given how much material we use compared to how much energy we use the Solar System contains plenty of material, but a quite finite amount of power. Any civilization that became free-energy limited would have to capture the output of its parent star. Dyson was careful to point out that he was not predicting the future of Humanity (though it is a possibility) but arguing that such an outcome could have easily arisen elsewhere. He argued that we should then be looking not just for bright stars with radio transmission, but for "dark" stars (since the energy was all being trapped) with low temperatures: the waste heat emitted by the elements of the alien biosphere.
Why would a civilization use starlight, instead of some other form of energy generation? Because it is so fantastically ubiquitous compared to the alternatives. Over the course of its 10 Gyr life the Sun produces 1051 ergs of energy. All of the chemical energy in the planets combined (if we burned everything there is to burn) is only about 1042 ergs; all of the kinetic and orbital energy in the Solar System is about the same (interesting coincidence that). Even if we liberate mass-energy with fusion, the total mass-energy of the planets is only 1051 ergs, and unless you're invoking the Penrose process to get high mass-to-energy efficiencies (Anyone got a black hole lying around? No? Then never mind.) you're looking at no better than 0.1% efficiency, so 1048 ergs. Basically, the Sun is the biggest source of mass, and it's doing a great job of efficiently turning a lot of that mass into energy for us.
So any civilization that's going to be around for a long time (and, so, detectable) will have to use the energy of its star, and it will have to re-radiate that energy in the thermal infrared, where we might detect it. (I gave my own version of all of these arguments in Part I, but Dyson's more elegant paper is wonderfully succinct and insightful and worth a read).
So, Kardashev's class I civilizations might not be easily detectable, but a class II civilization, regardless of its actual technology or methods of starlight collection, will have to be detectable by its waste heat.
Dyson's "sphere" would probably not be a solid object (as he pointed out in response to letters about his article) because it would not gravitationally stable; a "swarm" of collectors would be more appropriate, no matter what Star Trek would have you believe:
Apparently they don't have mid-infrared detectors on the Enterprise-D. And apparently the sphere has a binary companion illuminating it. And long-range sensors don't work near solar-mass objects; but they can still scan planets?
Anyway, next entry, I'll take a look at some modifications to Kardashev's scale, and some practical numbers that explain why searching for Dyson spheres is so hard.
We are hiring a postdoc for this project!We have received a grant from the New Frontiers program funded by the John Templeton Foundation.Some press coverage of our award:An article in The Atlantic online"Astronomers assume aliens are more open to solar... Read More