I was reading up on the Wow! signal today and there was a decent amount made of how the signal was very close to the hydrogen line.
I read on another article how the hydrogen line can be used to map the galaxy. How does that work? What all significance is there to astronomers using 1420Mhz?
I’m very proud of myself for writing this post and not confusing the fields of astronomy and astrology.
If you throw a bunch of photons at some hydrogen, some of the hydrogen atoms will absorb some of the photons. This absorption increases the energy in the hydrogen atom, pushing its sole electron into a higher energy state. This state is unstable, though, and soon the electron falls back down to a lower state and a photon is emitted. These new photons are always emitted with the same energy, creating electromagnetic radiation at a frequency of roughly 1420MHz.
The idea is, if you were an intelligent extraterrestrial, you might take advantage of this by sending information encoded in a 1420MHz signal. All the hydrogen in the universe would resonate and retransmit this signal, acting as a kind of galactic repeater, potentially allowing your message to be broadcast much further than an equivalent signal of the same power on a different frequency.
Actually, 1420 is a terrible choice for interstellar communications, for precisely that reason: It’ll get absorbed and re-emitted. This won’t act like a repeater or resonator, but will rather scatter and attenuate your signal. Nor is it difficult to come up with some natural process that would emit at that frequency. I’m not sure why so much SETI effort is devoted to it.
I read that the 1420 frequency is good for mapping the galaxy and universe. How does that work?
The SETI people really like 1420 MHz, also known as the 21 cm line. There are several reasons for this. This frequency (or wavelength) is the so-called hyperfine transition of a hydrogen atom. Hydrogen is everywhere in the universe and the radio telescopes that were used to look for ETs were mostly built to look for hydrogen, not ETs. This particular line falls in the so-called “water hole,” a spot in the radio spectrum in which our atmosphere is relatively transparent. At higher or lower frequencies, water in the atmosphere absorbs the radio waves. Part of the reasoning was that aliens who could talk to us would naturally be doing astronomy, looking at hydrogen, like we were. If their life was like ours and dependent on water, they would also benefit from observing at 1420 MHz and would have built their antennas, receivers, and transmitters to use that frequency. There are huge advantages to knowing precisely the frequency you are looking for, so the aliens trying to communicate would want to pick a frequency that would be obvious to other aliens, like the Earthlings.
If you look at the plaque that the Sagans designed for Pioneer 10, it actually shows a cartoon of the parallel and anti-parallel spins that are the end points of the hyperfine transition. This is used to establish the unit (21 cm) which shows the aliens how tall humans are and the distances to nearby stars, allowing them to conclude that the spacecraft came from Earth.
Oh, sure, it’s a valuable reference point to establish further communications on. And a frequency in that general range is convenient for long-range communications… just not that exact frequency itself. Personally, I prefer Sagan’s suggestion of “pi times hydrogen” (or maybe e or 2pi, or maybe divided by that instead of multiplied): That’d put it in the same general vicinity, and would communicate the fact that the aliens considered those quantities significant, while being much, much harder to generate naturally.
I’m in the 2pi camp. pi shoulda been 2pi all along!
Radio waves with that particular frequency are weakly absorbed by hydrogen atoms. The exact transition is between the electron and proton spins being antiparallel and parallel. When they’re paralle the energy of the H atom is higher by about 6 ueV (microelectronvolts). Note that the first electronic absorption of H, what you study in freshman physics or chemistry, involves the absorption of about a million times more energy.
I say “weakly” because this is such a low energy transition that at even at the very low temperatures of interstellar space, there isn’t much population difference between the upper and lower energy levels, and the amount of absorption is proportional to the population difference. (There are other quantum reasons for the transition being unlikely and slow, too.) So it takes light-years’ thickness of fairly dense (by interstellar standards) hydrogen to really absorb it, or to generate a noticeable signal. This is why it has been used to successfully map regions of hydrogen throughout our galaxy and even in some other galaxies.
The thinking is that aliens would, first of all, have also used 21cm radiation to do their own radio astronomy. So then they might have thought, hey, if we’re going to use radio to announce we’re here, why not pick that frequency, since we’ve already got big directional antennas built just for it, and everyone else will already be looking at that frequency with big directional antennas for the purposes of astronomy? All we need to do is not look like a hydrogen cloud, which is pretty easy. (They also have to hope we have antennas pointed at them, of course, and there’s some risk there because they’re likely to live in regions of the galaxy that are pretty boring, from the point of view of astrophysics.)
One of the reasons focussing on one particular frequency is a good idea is because the narrower your transmission bandwidth, the higher the power density and the more your transmission stands out above the noise. In fact, this is such an obvious fact that we can be pretty sure by the absence of any signal in or around 21cm received that there are no talkative aliens capable of radio astronomy within the nearest 100 light years or so. Of course, we’re not very talkative ourselves.