I’ve been thinking about the detection of alien lifeforms. As our search for exoplanets continues, it seems like we’re finding that planets are really quite common around stars; this should not be surprising since our star is really rather ordinary when compared to those around us. Probably most of the stars in the galaxy have planets.
So, asks Enrico Fermi- where are all the aliens? Why haven’t we detected them?
I think it’s because we haven’t been looking hard enough, or at the right time.
As our communications technology improves and we switch from simple modulations like AM, FM and QAM to things like DSSS and OFDM, the radio spectrum of our civilization looks more and more like white noise. In a hundred years or so, I would not expect to see anything that looked remotely like a carrier wave between DC and light.
This gets more convincing if you’re looking at the world from a long way away. We are already seeing a shift from highly powerful single transmitters (say, that AM radio transmitter kicking out a megawatt on a few hundred kHz, one per country) to non-synchronized repeaters (say, that FM network at 100 MHz where each transmitter only reaches a few dozen miles). The ranges on the surface are limited because of the curvature of the earth, so the frequencies are reused outside of line of sight. What this looks like from space is noise as all the transmitters interfere with each other- and not in a predictable way, since their master oscillators all drift at different rates.
Then there’s the ongoing shift from broadcast (which necessarily uses a small number of very powerful transmitters) to unicast media like cellphones; there isn’t the slightest chance you could even tell there was a cellphone network on the ground from space, since the frequencies are reused on a radius of less than 25 km; from a lightyear away picking out a single base station would require an unfeasibly large aperture (which would be no good for a sky search unless you had a ridiculously long time to perform it).
And then there’s the modulation issue; while GSM uses GMSK which can be relatively easily found with an untargetted search, UMTS and CDMA use spread spectrum, which is to say, a modulation based on a known pseudorandom number sequence; if you don’t know the PN sequence, you have absolutely no chance of figuring out there’s even a signal there without a damn strong hint (like a known pilot tone).
This gets even worse with OFDM, where the spectrum is intentionally as similar to a block of white noise as possible (after all, the more similar a signal is to white noise, the more effectively you are using the spectrum).
Try computing the power spectrum for an uncompressed text file a couple hundred kbytes long, and then again for a bzip2 compressed version of the same file.
Therefore the absence of alien signals is quite possibly just because they’re a century more advanced than us, or a century less advanced (and thus unable to make strong enough signals to pick up with our primitive VLA or whatever).
Let’s say it took us five billion years to evolve (close enough, right?) And it took a few billion years for the stellar nursery our Sun came from to acquire enough “metals” (astrophysics term) to make a star with a dirty enough composition to make complex chemistry. (Let’s assume chemistry is a prerequisite for intelligent life.) Therefore the window of possibility for the age of neighbouring civilizations is a few billion years wide.
Two hundred years in a few billion years is not a big window of detectability, but that’s all it would take to make them undetectable unless they’re actually *trying* (and spending a respectable fraction of their planet’s resources on terawatt radio transmitters) to be found.
Some of these ideas have been seen elsewhere — primarily that the window of time during which a civilization communicates by radio is very narrow. Other points include the fact that we’re rapidly switching to fiber and other technologies that don’t radiate at all.
Your point that any sufficiently advanced technology is indistinguishable from white noise is one I hadn’t seen before, but makes a lot of sense.
It seems that the only way SETI is going to find other civilizations is if they’re deliberately transmitting a signal that’s intended to be detected. This plotline was used in “The Listeners” by James Gunn, and later in Sagan’s “Contact”.
Then of course, there’s Stanislaw Lem’s “Star Diaries” where the great philosopher Master O comes up with his own method of finding other civilizations: The “A Priori Clue”, in which you look for new stars in the sky, which indicate a civilization that has discovered nuclear power and just blown itself up.
Hmm. I’m not entirely convinced by this. I agree that the transmission from (say) CW spark transmissions to AM international short-wave to FM local radio to cellular/wifi networks has decentralized transmitters and whitened the emitted spectra, but white spectra – noise – are exactly what radio astronomers spend our lives looking for. Even spectral line work looks at noise, it’s just narrow-band noise. What’s more, the move to higher frequencies makes the radio more able to pass out through the ionosphere, and lowers the background (typical Galactic radio background falls off like the square of the frequency). If we have moved from a handful of megawatt transmitters to hundreds of thousands of ten-watt cell phone base station transmitters (or millions of 2-watt GSM phones), that’s potentially an easier signal to pick out in spite of being more broadband. And in fact, there have been proposals to find exoplanets using radio astronomy (though by looking for analogues of the Earth’s auroral kilometric radiation or Jupiter’s radio emission).
So I’ll grant that it might be much harder to recognize the stray signals of an intelligent civilization with today’s technology than that of fifty years ago, but I’m not sure it’s any harder to detect that there’s *something* there. The SETI folks looking for narrow-band birdies might be in trouble, though.