 This video is about the Drake equation in the search for extraterrestrial life. So I will first explain what the Drake equation is, and I will then explain why it is completely and utterly useless. So the Drake equation tries to estimate the number of civilizations we can communicate with by multiplying together a lot of numbers. So the numbers are as follows. First of all is our star, which is something to do with the rate of star formation, how often stars are formed in the galaxy. Then there's a number Fp, and this gives you the chance that a star will have planets, not plants. Next we have a number Np, which is an estimate of how many planets are suitable for life. So that's right, suitable for life. And then we have a number Fl, which is the chance that a planet that is suitable for life, whatever that means, will actually develop life. We have a factor Fi, which is the chance that life will develop intelligent life. And then we have a factor FC, which is the chance that intelligent life will develop enough to be able to send signals. And finally we have a factor L, which is the length of civilization. And if we multiply all these together, we get a number N, which is an estimate for the number of civilizations in the galaxy we might be able to communicate with. So this was introduced by Frank Drake in about 1961. There's nothing terribly wrong with this equation as Frank Drake stated it. The trouble is it's been grossly misapplied rather often. People attempt to derive conclusions from it that aren't supported. So I'll explain what the problems with it are. First of all, there are some minor problems. So let's list the minor problems. So here's some minor problems. So first of all, it doesn't take into account various effects. Civilizations might colonize other planets, which would make it much more likely to detect them. This leads to something called the Fermi Paradox. So Fermi sort of pointed out that if you have a civilization, it should be able to colonize the galaxy within maybe a hundred million years or so. So if there are any such civilizations, we really ought to have seen them and we haven't. This sort of weekly suggested that it might actually be quite difficult for intelligent civilizations to develop. On the other hand, going from one star to another is enormously more difficult than most people seem to imagine. At the speed of our fastest space probes, it might take tens of thousands of years to travel to a nearby star. And colonizing the galaxy is, I mean, it's going to take far more than the length of time that humans have been around for. But anyway, that's actually by the standards of this subject, that's actually only a minor problem. The next problem is we have a short range in which you can detect a civilization. So if there was another Earth-like planet emitting radio signals in the way that we are doing, we probably couldn't detect it if it were more than a few light years away, which means that even if there is another civilization in the galaxy, unless it's in a nearby star, we probably just wouldn't be able to detect it. So Frank Drake kind of forgot about that effect when writing this down. But again, that's a fairly minor problem. The third problem is there might be other life forms that we can't imagine. For example, as far as we know, neutron stars might have life living in the center of them. It'd have to be a really weird form of life and we have no evidence for this, but we know so little about neutron stars that for all we know the entire galaxy is teeming with life inside them. We can't even explain most of the mass of the galaxy. We have no idea what it is. My guess would be it probably doesn't have life in it, but we really have no idea. So there's an amazing amount of uncertainty about what life actually is. Well, these are all just minor problems. There's a really major problem with this equation. The major problem is we have no idea what the numbers are. So let's go through this and point out what the problem is. First of all, with the rate of star formation and the number of planets, these are under reasonable control now. We sort of know pretty well how many stars there are in the galaxy and how many galaxies there are and observations in the last couple of decades have shown that it's actually fairly common for stars to have planets. Most of the remaining ones, we simply have no idea what these numbers are. People have tried to guess and their estimates differ by huge factors. So how many planets are suitable for life? Well, we again have no idea. We know the Earth is suitable for life by observation, but we have no idea if this is quite common or if it was due to some freakishly improbable event that happened in the Earth's formation. Most of the other planets in the solar system, as far as we can tell, have no sign whatsoever of life on them, other than that we don't know. The really big one is, big problem is this one here, that if a planet is suitable for life, we don't have the faintest idea how likely it is that life will actually develop. Estimates for this, given by people, range from between nearly one, people think life is almost certain to develop, to other people who think that life is so improbable that the chance of it developing is one in 10 to the power of some absurdly large number. One problem is, we simply have no idea how life started on Earth. If you look inside a primitive cell, it's got this incredibly complicated mechanism of the genetic code and lots of proteins and so on for copying the DNA from one cell to another. And our guess is this probably evolved somehow, but we have no idea how it is evolved. And some people have suggested that it's so unlike to have evolved that maybe it only happens in one in every 10 to the million times we have a suitable planet. So we cannot estimate this factor here to within a factor of one in 10 to the million. I mean, it's just, we have no idea what it is. And we have a similar problem for how often life becomes intelligent. So how many different species have there been on Earth? We don't really know, a billion, a trillion could be either. And there's some evidence that one of these species has developed intelligent life although there's a strong case for saying that no species on Earth have ever developed intelligent life, but whatever. So again, the chance of intelligent life developing we simply don't know. It might have been some sort of freak event on Earth or it might be common. We can't pin this down to within a factor of a billion. Intelligent life developing signals, we have a better idea of this. It's probably more than about one in a thousand and less than one. So we can actually guess this number to within a factor of a thousand, which in this subject is actually pretty accurate. How long does a civilization last for? Well, a lot of civilizations on Earth live a few hundred years. On the other hand, if you read science fiction books, which is as good a way of getting information about this as anything else, you can find examples of alien species that have lasted for billions of years. You may think science fiction isn't terribly accurate, but more is anything else in this area to be frank. So this number here is between say a hundred and a billion. So what we do is we multiply together these five numbers that we essentially know absolutely nothing about and claim we've got something useful from it. Well, I've gone through the Drake equation putting in reasonable error estimates for what we know and I can give you the following conclusion. It is very likely, say more than 50% like that the number of civilizations in the visible universe is somewhere between zero and 10 to the power of 30. So this is the true conclusion that the Drake equation gives. As you see, it's not really terribly useful. Anybody giving more precise numbers than that is probably just wildly guessing.