SETI stands for “Search for Extraterrestrial Intelligence”. Yup, we’re looking for aliens.

And, contrary to what the conspiracies say, we haven’t found any yet. But it’s helpful to understand why. After all, we’ve been looking for 50 years now, surely if they were out there we’d know by now? Wouldn’t we?

Not surprisingly, it isn’t that simple.

Let’s start with the Drake Equation. In 1961, the astronomer Frank Drake asked where everyone was. If there’s other life in the galaxy, where is it? Surely, if other societies existed somewhere we would have seen signs by now. After all, they could have evolved intelligence a million years before us, or ten million, or a hundred. Enough time to have visited every interesting star in the galaxy.

Drakes equation was an attempt to calculate the probability of other intelligent societies in the galaxy based on what we knew. The trouble is, it’s clear that we don’t really know much at all. To see what I mean, here’s the equation:

   N = R * fp * ne * fl * fi * fc * L

Each of those terms represents one thing that has to happen for intelligent life to exist. This is what they define:

  • R  – the average rate of star formation in the galaxy
  • fp – the fraction of those stars that have planets
  • ne – of those stars, the average number of planets that can support life
  • fl – the fraction of those planets where life actually develops at some point
  • fi – the fraction of planets where that life develops intelligence
  • fc – the faction of those civilisations that become detectable
  • L  – length of time that civilisations remain detectable

There are two things in particular to say about this – it leaves a lot out, and we don’t know any of the values, even estimating the last four is enormously difficult. Everyone who use the equation to prove something chooses different values with the best intentions and comes up with a different number. Depending on what you choose to believe, the number of existing civilisations in our galaxy right now can vary between 10 an a million. If there are 10, they’re way too far apart to contact each other, if there are a million, it might take a century, and we’ve only been trying for 50 years.

So, the distance is the first part of the problem. The second is how to contact them, or even learn about them.

Radio is so common these days we barely consider it. But in the 1930s, Karl Jansky built the first directional radio antenna and noticed that it picked up a lot of background noise. At first he thought it was from the sun, but later realised that it came from the centre of the galaxy. Without knowing it, he had effectively invented radio astronomy. Eighty years later, we know quite a lot more about natural sources of radio waves in space, including the frequencies which are particularly quiet.

If aliens want to attract our attention, it seems reasonsble to start listening to frequencies that are naturally quiet, where they are more likely to transmit. That way, anything artificial will have a better chance of being recognised for what it is. But, do you send out a signal in all directions at once, which is easy enough, or do you pick a direction and transmit to a single star, or to a small group of stars? It makes a difference. Transmitting everywhere at once takes a lot more energy.

It works like this – the radio signal is transmitted in a sphere, and as you get further from the transmitter, the sphere gets bigger. Twice the distance means four times the size. Ten times the distance, one hundred times the size. But the strength of the radio signal doesn’t change, so when you’re ten times further away from the transmitter, the signal will be 100 times fainter. And because we’re talking about the distance between stars here, the signal gets ridiculously faint very quickly.

There’s an urban legend that an interested alien 50 light years away could be watching _Here’s Lucy_, a popular, light TV comedy series starring Lucille Ball that started in 1968. The reality is that the TV transmitters in 1968 didn’t have enough power for the show to be recognised after travelling 50 light years, it would be indistinguishable from the general background mush. And that sums up the problem with directionless transmission. The further you want the signal to be recognised, the more powerful the transmitter needs to be, until you’re trying to outshine the galaxy itself, and that’s not going to happen.

So, in practice, you choose a target star and transmit. But how long do you stay focused at one star before moving on to another? If there is a civilisation able to and interested in listening, how long do they listen to a star before they move on to another. The whole thing is a bit like one of those kids games where everyone is blindfolded, no-one is allowed to speak and you have to find the person holding something important.

And this assumes that they use radio and are interested in transmitting. If everyone listens, nothing happens, but if everyone transmits, nothing happens then either! The whole subject is a mixture of fascinating and frustrating. More so when you appreciate how our use of radio has changed over the previous fifty years. During the Cold War, the most powerful radio signals came from the ballistic missile early warning radars. Today the radars are still there, but the frequency has changed and the power is reduced because the receivers are more sensitive. Which means, we are not accidentally broadcasting strong signals into space which may be picked up by civilisations out there. And the same will be true for other races. As experience with radio increases the amount of wasted energy in the broadcasts will reduce, so, if we hear anyone else it’s going to be because they want us to, not because we’re picking up their equivalent of I Love Lucy.

When you think about it a bit more, it’s no surprise at all that we’ve not found anyone out there yet. Of course, if you don’t look, you never will, so we keep trying.

Suggested Reading

Making Contact: Jill Tarter and the Search for Extraterrestrial Intelligence

The Eerie Silence: Are We Alone in the Universe?