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.