Now, even a few hundred light-years away means that we must not imagine much in the way of dialogue. It's more monologue. They talk, we listen, because otherwise they would say, let us imagine, "Hello. How are you?" and we would say, "Fine, thank you, and you?" and that exchange would take, say, six hundred years. It's not what you might call a snappy conversation.
On the other hand, it's very clear that one-way transmission of information is something that can be enormously valuable. Aristotle talks to us. We do not, except for spiritualists, talk to Aristotle. And I have grave doubts about the spiritualists. (Although Aristotle is almost never on their list of contacts.)
Now, let's therefore say a few more words about this idea of radio communication. What we imagine is that beings on a planet of another star know that emerging civilizations will stumble upon radio. It's part of the electromagnetic spectrum; it is, as I will show you in a moment, a clear channel through the Galaxy. The technology is relatively simple and inexpensive. Radio waves travel at the speed of light, faster than which nothing can go, so far as we know. The information that can be transmitted is enormous, not just "Hello, how are you?" Put another way, if an identical system were at the center of the Galaxy and we were here using our present detection technology, we could pick up that signal coming from thousands of light-years away. It gives you an idea of the remarkable power of this technology, which has in fact been only lately brought up to its actual capabilities.
There is a question of frequency. What channel would you listen on? There's an enormous number of possible radio frequencies. We have here the radio frequency spectrum in gigahertz, thousands of millions of cycles per second, against the noise background from various sources in degrees absolute. And what we see is that at the low frequencies there is a background from charged particles in magnetic fields in the Galaxy, the galactic background. It's noise. And it gets to be very substantial noise.
This is not where you would want to transmit or receive. At the high-frequency end, there is another source of noise, intrinsic to the quantum nature of radio detectors. And in the middle there is a broad region where the noise is low, and this is the window in which it makes sense to transmit. In this window there are certain spectral lines, for example, of atomic hydrogen, the most abundant atom in the universe, at specific frequencies. So for this reason there is now a very sophisticated search program going on at Harvard, in Massachusetts, a cooperative project with Harvard University and the Planetary Society, a hundred-thousand-member worldwide organization, and it is remarkable that dues and contributions to a private organization are able to maintain by far the most sophisticated search for extraterrestrial intelligence yet attempted. [4]
fig. 33
This illustration might convey a sense of how a success would be noted. The slanting line indicates a very weak signal from an extraterrestrial source. You listen at many frequencies for a while and see if there's anything happening. The Planetary Society system was recently upgraded, so that 8.4 million separate channels are being listened to simultaneously. The antenna points to some part of the sky. And some places have peaks. They may be due to radio interference on the Earth, satellites in Earth orbit, automobile ignitions, diathermy machines. But each of those has a particular kind of signature, and it is possible to imagine signals that don't look like any of those things, which the computer immediately would cull out of the noise, leaving no doubt that this was an artificial signal of extraterrestrial origin, even if we had no opportunity, no ability, to understand what it meant.
Now, as I said, the expectation is that they send and we, newly emerged, the youngest communicative civilization in the Galaxy, we listen. Not the other way around.
Let me stress that this is the one respect in which our civilization is probably unique in the Galaxy. No one even slightly more ignorant can communicate at all. Let me say this in a better way: A civilization only a few decades behind us would not have radio astronomy and therefore could not tumble to this technique. Or maybe they could tumble to it, but they couldn't manifest it. And anyone, therefore, whom we hear from is likely to be ahead of us, because if they're even a little bit behind us, they can't communicate at all.
So the most likely situation is communications from beings vastly more advanced than us. And this therefore raises the ques-
fig. 34
tion, could we possibly understand what they're saying? What we have to remember here is that if this is an intentional message from them to us, then they can make it easy. They can make allowances for civilizations. And if they do not choose to do that, then we will not understand the message.
Maybe you would say advanced civilizations communicate with each other by zeta waves. And I'd say, "What is a zeta wave?" And you reply, "It is something fantastic for communication that I can't give you any details about, because it won't be invented for another five thousand years." Well, that's wonderful, and if those fellows can communicate with zeta waves, that's terrific. But if they wish to communicate with us, they will have to wheel out some ancient, creaking radio telescope from the technology museum and use it, because that is all that young civilizations will be able to understand and detect.
Now, suppose we get a message. What would it be like? Here is a possibility: There would be a powerful beacon or announcement signal, something that makes it very clear that we are unambiguously receiving a message from an advanced civilization. It might, for example, be highly monochromatic; that is, a very narrow radio frequency band pass, and/or it might be a sequence of pulses that could not possibly be of natural origin. For example, a sequence of prime numbers, numbers divisible only by 1 and themselves-1, 2, 3, 5, 7, 11, 13, 17, 19, and so on. There is no natural process that could produce such numbers.
Then, having established unambiguously that the message was from intelligent beings in space, it is perfectly possible to imagine a vast amount of additional information conveyed in ways that we can understand. For example, it is perfectly possible to transmit pictures. In fact, it's done by radio all the time. That's what your television set does. It is possible to send mathematics. It's very easy. I mean, suppose they set out the numbers-beep, that's one; beep beep, that's two; beep beep beep, that's three-and so on. And then they do (I'm just going to make this up) beep glagga beep wonk beep beep. Well, a few more like that and you decide a glagga means "plus" and wonk means "equal." But suppose they now do beep glagga beep beep wonk beep beep? And then there's some symbol after that. That symbol, that new symbol must mean "false." And you can immediately see that abstract concepts like true and false could be communicated very quickly. And between these two modes-the use of mathematics, which we would, of course, share in common, and the transmission of pictures-it is possible that a very rich message could be conveyed. What that message would be, clearly none of us are in a position to say.
Now, I would like you to just think about contrasting this open-minded, experimental approach, which consists of some plausibility arguments that no one takes too seriously, with the more traditional approach to intelligent life in space: the one in which there are no experiments, in which there is no withholding of opinion until the evidence is in, in which we are asked merely to take it on faith. The contrast is, in my opinion, very stark. There is quite a different approach in method. And I remind you about how powerfully we were fooled by the Martian canal situation, where passions and emotions were heavily engaged.
[4] In 2006 the Planetary Society and Harvard University inaugurated the SETI Optical Telescope, the first-ever optical observatory dedicated to the search for intelligent extraterrestrial signals. For the history of the Planetary Society and SETI, see www.planetary.org, and for the thrill of actually participating in the search, go to www.setiathome.ssl.berkeley.edu/.