The Drake equation and the search for ET

Instinctively I’m inclined to think there’s other life out there in the universe. They aren’t sure how many stars are in our galaxy but they think it’s between 100 billion and 1,000 billion, and they aren’t sure how many galaxies there are in the observable universe but they think it’s between 100 billion and 2,000 billion.

So that all means there’s between 10,000 billion and 2,000,000 billion stars in the observable universe or, said another way, between 10 trillion and 2 quadrillion stars. And that’s just the observable universe — there are also the bits we can’t see to take into account and nobody’s quite sure how big the unobservable universe is.

From a pure logic point of view, there’s no reason to think we’re living on anything but a fairly typical planet. We may not be of course, but it’s a statistically safe assumption from the outset. Couple that with the sheer number of stars available and life should be fairly prolific in the universe. Furthermore, if our statistical assumption is correct intelligent life should also be fairly prolific.

When I say ‘intelligent’ life, I mean life that’s complex enough to ask why we’re here and has enough technology to look out into the universe to try and answer that question. I’m not considering our social intelligence and I hope passing aliens don’t either otherwise they’d probably give Earth a miss.

In 1961, Frank Drake wrote down an equation that tries to quantify the odds of there being intelligent civilisations in our galaxy. It runs as follows:

$$ N = R * p * e * l * i * c * L $$

The parts of that equation are as follows:

  • $R$: the average rate of star formation in our galaxy.
  • $p$: the fraction of formed stars that have planets.
  • $e$: for stars that have planets, the average number of planets that can potentially support life.
  • $l$: the faction of those planets that actually develop life.
  • $i$: the fraction of those planets that actually develop intelligent life.
  • $c$: the fraction of those planets with intelligent life that actually develop communications that can stretch out into space.
  • $L$: the length of time over which such civilisations transmit said communication. In other words, the length of time such intelligent civilisation exist before something wipes them out.

So you basically take the rate of star formation and then reduce it by a fraction based on the likelihood of all the other parts of the equation.

My guesses on the numbers to plug into the Drake Equation are:

  • $R$: $2.25$. The average of NASA’s estimate that 1.5 - 3 stars are created every year in our galaxy.
  • $p$: $1$. Recent data suggests stars have planets as a rule rather than an exception.
  • $e$: $0.4$. Sky surveys estimate that 40% of systems with planets will have one that could potentially support life.
  • $l$: $0.25$. There is a lot of argument about the fraction of planets that actually do develop life. Some argue it’s very rare and others argue it’s almost certain. So I have personally just taken a moderately cautious view that 25% of planets that can develop life will actually develop life.
  • $i$: $0.25$. Again there’s much argument about whether life, once established, evolves into intelligent life. Some say it will always do so as a product of evolution and others say it’s very rare. I’ve just gone for 25% again.
  • $c$: $1$. This is just my guess but I’d argue that all intelligent civilisations will ask if there’s someone else out there and develop communications to try and find out.
  • $L$: $420$. All we have go by for how long civilisations last is Earth and a chap called Michael Shermer looked at 60 historical Earth civilisations and estimated that their average life span was 420 years.

So plugging those numbers into the equation gives us:

$$ 2.25 * 1 * 0.4 * 0.25 * 0.25 * 1 * 420 = 23.625 $$

So, by my very rough and ready estimate, there are approximately 23 civilisations in our galaxy at the moment we could potentially communicate with.

But this is nothing more than a fun estimate. Respected scientists have plugged their own numbers into the Drake equation and arrived at estimates as low as 0.000000000091 civilisations in our galaxy (i.e. there aren’t any) and as high as 15,600,000 civilisations in our galaxy today.

I should add that there are many criticisms of the Drake Equation itself too, so this all has to be taken with an extremely large pinch of sodium chloride.

However, even if you take the low estimate above and multiply it by the lowest estimate for the number of stars in our observable universe (10,000 billion), we get 910 intelligent civilisations presently active in our universe. If you take my figure or the scientists’ higher estimate there are billions of civilisations in our universe at present.

Furthermore, that’s just life presently active; if we look at whether there’s ever been intelligent life — even taking the most pessimistic figures for the Drake Equation — then we come up with a number in the millions or billions.

I think the probabilities very much suggest there either is or has been intelligent life elsewhere in the universe, and there have possibly been billions of instances of it.

If there is life elsewhere, the next problem is communicating with it. Even if we were astoundingly lucky and intelligent life lived on a planet around the closest star to us (Proxima Centauri), it would take four years for a message to transfer between the planets.

Don’t forget that we’d also have to establish communications protocols during the early messages too. Once the Centauris and us establish there’s actually someone else out there, we need to find some sort of common language to communicate in. It’s unlikely they’ll have a working knowledge of American English like all the aliens in low-grade sci-fi films seem to have.

Imagine sending a message and then waiting eight years for a reply.

I don’t think the problem is the very existence of life elsewhere; our difficulties in having any meaningful contact with it are due to the sheer distances in space and the limiting factor incurred by the speed of light. Wormholes and other shortcuts through space are theoretically possibly but, in my opinion, unlikely to even occur in reality let alone be safe and controllable in such a way as to be safely traversable.

I suppose we should never say never. Putting a man on the moon would have seemed impossible to Ancient Egyptians. Who knows how things will look in 100 or 1,000 years’ time?

It’s possible that life elsewhere is very different to ours. Maybe alien life lives for thousands or even millions of years (or maybe forever) and a few thousand years of travel to them feels like a few days to us.

Maybe they’ve already been here. If so, I wouldn’t be surprised if they’ve dismissed us a too primitive to communicate with at the moment. We’ve been at war with one another from the off, we’re destroying our own planet, we tolerate a society where half the world has more money than they can spend and the other half is starving to death, and we spend an inordinate amount of time watching a box that blurts out nonsense like ‘Keeping Up with the Kardashians’ into our living rooms.

If I were a passing alien I’d leave us well alone too.