It's estimated that our galaxy, the Milky Way, has 100 to 400 billion stars, and when you hear a number like that the obvious question is: Are there civilizations on planets that are orbiting any of these stars and maybe even more interesting question is can we detect any of those civilizations? Have they gotten to the level of technological progress like us, that they are emitting electromagnetic waves into space and other civilization like ours can detect and say, "Hey, there is someone else out there watching television or using radio or whatever else they might be doing." So, what I want to do with this video is not answer that question. That's a big open question, we don't know the answer. We don't have anywhere near enough information to definitively answer that question, but I want to do is come up with a framework for at least thinking about that question, a way of actually estimating how many detectable civilizations there are in just our galaxy, and there is a formula, that you may or may not have heard of, called the Drake Equation. and what we are going to do is independently derive our own version of the Drake Equation. It's going to be slightly different, but it is the same thought process and in the future video, I'm going to maybe reconcile what we come up with, with the Drake Equation, and just so you know Drake equation is named for Frank Drake, who is a Professor at University of California Santa Cruz. He first, kind of put some structure around this problem, that's why the formula or the equation has his name, but the equation is not an equation you can apply on a daily basis and get results that you can use to build things, but what it's, it's the structures of our thinking around this question of How many detectable civilizations are there in our galaxy? And to answer this question, I am going to start little bit differently than Frank Drake did He starts with the number of new stars that're born each year, we will see that our definitions are actually pretty close to each other what I want to do, is start with the total number of stars, So, we are trying to come up with is, I will call it 'N' and this is number of detectable civilizations, number of detectable civilizations in the Milky Way, in our galaxy. and once again, there could be civilizations, looking back in the star field right over here this star right over here, maybe it has a planet that is in the right place that has liquid water and maybe there is intelligent life on that planet, but they might not be detectable, because, they aren't technologically advanced enough that they are using electromagnetic radiation or maybe they just figured out some other way to communicate or maybe they are beyond using electromagnetic radiation, you know, radio waves and all the rest to communicate so we will never be able to detect them. We are talking about civilizations like ours that are, to some degree, using technology not too different than our own. That's what we mean by 'detectable', so let's think about that little bit. I like to start with just the total number of stars in our solar system. So let's just start with, I will call it, N*, so this is the number of stars in our galaxy, and our best guess, I said is this's going to be 100-400 billion stars. We don't even know how many there are, some of them are undetectable, and the center of our galaxy is just a big blur to us we don't even know what's on the other side of that, e can't even see all the stars that are packed into the center, so this is our best guess, 100-400 billion stars. Now obviously, there is going to be subset of those stars that even have planets. So, let's multiply the times that subset; so lets multiply times the frequency of having a planet. If you are a star, this is the percent chance or the frequency or of the fraction of these stars that have planets (f sub p) so, I'll write this way "fractions that have planets", So, if this is a hundred billion, let's say I am making a guess here and we are learning more about this everyday, there are all these discoveries of 'exoplanets'- planets outside our solar systems, maybe this is one fourth. Then you could say, well that means there are 100 billion times one fourth, that means there are 25 billion stars that have planets around them. But that's still not enough to go to civilizations. We also need to think about planets, there could be a planet like Jupiter, and we don't know how life as we know it can survive on a planet like Jupiter or Neptune or Mercury. It has to have planets that are good for sustaining life. Preferably have a rocky core, liquid water on the outside, that's what we think are the ingredients we need for life, maybe we are just not being creative enough, that's what we know as life is being. So, let's multiply this times the average number of life sustaining or planets that could sustain life on them. So we don't necessarily know that they are going to have life, but they seem like they are just the right distance from the star, not too hot, not too cold. They have the right amount of gravity, water, all the other stuff, and we still don't know what this means, but this means average number, so given a number of solar systems with planets, what's the average number of planets capable of sustaining life, and once again, you don't know this answer. maybe it is 0.1, it's probably less than 1. Therefore any given solar system that has planets, the average number capable of sustaining life maybe its 0.1 maybe it's more than 1, I don't know. We don't know the exact answer here, but I will throw an answer in, I 'll throw out a guess, maybe it is 0.1. And here the fraction that has planets (f sub p), i don't know, i will throw that out as, and once again I am making up these numbers, we really don't know the right answers, this is one-fourth. But if we were to multiply this out we would have the average number of planets in our solar system that are capable of sustaining life that are around stars that have planets and these planets are capable of sustaining life. Now, n sub p will give us the total number of planets in our galaxy capable of sustaining life. Now, just because you have liquid water and right temperature and all the other rest of the ingredients doesn't mean that you will actually have life happening on your planet. So let's multiply that times the fraction that actually generate life (f sub l) So f sub p is the fraction that actually have life and this is actually a very , we dont know this answer, so this is the fraction that have life on them And this is a really big open question maybe if you have the ingredients, maybe every planet has life maybe it is a frequent thing that's happening in our galaxy and frankly our universe or maybe it's a very infrequent thing maybe it's just the right kind of freak set of circumstances that just have to happen i will throw out a number, just for the sake of having a number maybe it is one out of every ten planets that have all the right ingredients for life actually do generate life. My personal guess is probably higher than that given that life seems life seems such a robust and flexible thing we have seen in all sort of weird circumstances. Actually let me make it higher number than that so let me make it one half (0.5) assuming that we have all of the ingredients. So this should tell us essentially how many planets in our galaxy have had life on them at some point in those planets' lives. The life might have come and gone maybe destroyed itself through nuclear war. But this would tell us the number of planets in our galaxy that had life at at least one point in their history. Now we care about civilizations, about intelligent life.. So maybe if the asteroid never hit earth... the dinosaurs would have never evolved to the point of generating radios and tvs and telephones and all the rest.. and so it is kind of a freak circumstance that because they were destroyed these gaps in the ecosystem developed so we could emerge and and be intelligent and do all these crazy things like make YouTube videos and all the rest. So, let's multiply this times the fraction if you get all of this, the fraction that actually end up having intelligent life and maybe this fraction is, to a number of 1/10, then probably in the next video I'll calculate it all. Now this is very important to realise because once again, you could have life these are all examples of life right over here This is actually life on Earth, on our planet even though this looks quite alien this is a weeble, kind of looked at from very close-up, but there is all sorts of forms of life many of which we probably can't even begin to imagine but what we care is that if intelligent life starts to emerge on the planet because only intelligent life has a chance, we believe of being able to eventually communicate in ways that are detectable by us. Now I said intelligent life, but maybe not all intelligent life will eventually get to the technological sophistication, where they will be using radio waves, electro-magnetic radiation to communicate with each other maybe we might have stagnated at this stage if nothing of the right things didn't happen.. So what we need to do now, is multiply this so right here, we would have the number of planets in our galaxy that have had intelligent life on them at some point in their history maybe not at a time that coincides with ours, but what we wanna do is bring it down even more to the percentage that get to the point that they can develop technology that allows us to detect them. So, let me multiply times the fraction that are, I'll put a 'C' here, 'C' for, maybe they're using communication; 'C' for communications that allow us to detect them. So this is detectable, the fraction that are detectable. Now you might think that we're done, this will give you the Total Number of Civilizations, or life-forms in our Galaxies or the planets that have life forms that developed detectable technologies at some point in their history.. Now it would be nice if civilizations be not born and die but the reality is, they do die. They might destroy themselves or whatever, and they might exist for only a small period of time for the history of that planet or the history of that Solar System. So in order to make it the number of civilizations that are in existance now and I'll clarify what "now" means in the next video because really if we are detecting something from a star that's 10,000 lightyears away our "now" means we are just receiving their signals so, they released the signals 10,000 years ago. But what I want to do is get to know what is the fraction of these whose signals are reaching us right now. and here, I'll say what's the average lifespan of a civilization I'll put that "L", who knows what is, maybe 10,000 years so civilization lifespan as going to be that over the life of the star So, I'll put a 'T' here, for the average lifespan for the star, and I could say the average lifespan for the planet, or whatever, but we're assuming once our star supernovas, you're not going to have a chance for life on Earth to develop anymore. So, maybe this thing appears ten thousand years and this down here is maybe 10 billion years. and if you were to multilply this out, you should get the number of detectable civilizations in our galaxy right now. I'll leave you there, and next we'll discuss it a little bit more, and reconcile it with the more famous version of Drake's equation, and I'll also try and talk about this little piece of bit, because I think it might be a little confusing, and I'll try to diagram that out a little bit more..