If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



估算银河系中可能存在的文明数量. Sal Khan 创建




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 an 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're emitting electromagnetic waves into space that other civilizations like ours can detect them say hey there's someone else out there watching television or using radio or whatever else they might be doing and so what I want to do in this video is not answer that question it'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 of 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're going to do is independently derive our own version of the Drake Equation it's going to be slightly different but it's the same thought process and in a future video I'm going to maybe reconcile what we come up with with the Drake Equation and just so you know the 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 and that's why the formula or the equation has his name but the equation it's not an equation that you can apply on a daily basis and give results that you can use to build things or but what it is is it structures our thinking around around this question of how many detectable civilizations are there in our galaxy now to answer this question I'm going to start a little bit differently than Frank Drake did he starts with the number of new stars that are born each year and we'll see that our definitions are actually pretty close to each other what I want to do what I want to do is start with the total number of stars so we're trying to come up with is I'll call it n and this is the number of detectable civilizations number of detectable detectable civilizations civilizations in the Milky Way in our in our galaxy and once again there could be civilizations look looking back at this star field right over here this star right over here maybe it has a planet that's in the right place it has liquid water and maybe there's intelligent life on that planet but they might not be detectable detectable because they aren't technologically advanced enough that they're using electromagnetic radiation or maybe they just figured out some other way to communicate or maybe they're beyond using electromagnetic radiation you know radio waves and all the rest to to communicate and so we'll never be able to detect them we're talking about civilizations like ours that are to some degree using technology not too different than our own so that's what we mean by detectable so let's think about that a little bit so I like to start with just the total number of star stars in our solar system so let's just start with I'll call it n star and Astrix and this is the number of stars number of stars in our not in our solar system number of stars in our galaxy number of stars in the galaxy and our best guess I said is this is going to be 100 billion to four hundred billion stars we don't even know how many there are some of them are undetectable in the center of our galaxy it's just a big blur to us and what we don't even know what's on the other side of that and we know we can't even that we we can't even see all the stars that are packed into the center so this is our best guess 100 billion to four hundred billion stars now obviously it's going to be a subset of those stars that even have planets so let's multiply it times that subset so let's multiply it times the frequency of having a planet if your star this is the percent chance or the the frequency or the fraction of these plant of these stars that have planets so I'll write it this way fraction that have planets that have planets that have planets so if this is a hundred billion and that let's say I'm making a guess here and we're learning more about this every day there are all these discoveries of exoplanets planets outside of our solar system maybe this is 1/4 then we could say well that means that they're 2100 billion times 1/4 means that they're 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 because the planet 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 a 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 that you need for life maybe we're just not being creative enough that's what we know is life as being so let's multiply this let's multiply this times the average number of life sustaining or planets that could sustain life on them so we don't necessarily that they're gonna have life but they seem like they're 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 exactly what this means but this means average average number so given that there's a solar system with planets what's the average number of planets that are capable of sustaining life average number of planets capable of sustaining life sustaining life and once again we don't know this answer maybe it's it's point one it's kind of you know it's probably it's it's it's it's probably less than one therefore any given solar system that has planets the average numbers capable of sustaining life maybe its point one maybe it's more than one I don't know we don't know the exact answer here but I'll throw an answer here me I'll throw out a guess maybe it is zero point one and here the fraction that have planets I don't know I'll I'll throw that out is and once again I'm just making up these numbers we really don't know the right answer 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 plants are capable of sustaining life now and this would give us a total number because it is average per solar system that has planets this is the total number of solar systems with planets you multiply it out total number of planets in our galaxy see capable of sustaining life now just because you have liquid water and the right temperature and all of the rest ingredients doesn't necessarily mean that you will actually have life happening on your planet so let's multiply that times the fraction that actually generate that actually generate life so this is the fraction that actually have life and this is actually a very we don't know this answer so this is this is a fraction that get that have life on them have life and this is a really big open question maybe if you have the ingredients maybe almost every planet has life maybe it's 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'll throw out a number just do just just for the sake of just to have a number there maybe it's 1 out of every 10 planets that have all of the right ingredients for life actually do generate life my personal guess it's probably higher than that given that life seems such a robust and flexible thing that we've been seeing in all sorts of weird circumstances actually let me make it even a higher number than that so let me make it 1/2 assuming that we have all of the ingredients so this should tell us essentially how many planets if we were to multiply all of these how many planets in our in our in our in our galaxy have had life on them at some point in those planets lives that the life might have come and gone it maybe destroyed itself through nuclear war whatever but this would tell us the number of kind of life planets in our galaxy that have had life on them at at least one point in their in their history now we care about civilizations so let's multiply this times if you make all of these you even get to the point that you have life we care about well do you get intelligent life so maybe if the asteroid never hit earth the dinosaurs would have stayed on earth and they would have never evolved to the point of generating radios and TVs and telephones and all the rest and and and so it's kind of a frico circumstance that we were because they were destroyed these gaps in the ecosystem developed so that we could emerge and be intelligent and do all of these crazy things like make youtube videos and all rest so let's multiply this times the fraction if you get all of this the fraction that actually end up having intelligent the fraction that actually end up having intelligent life and maybe this fraction is so intelligent life intelligent life over here so intelligent life this is maybe I'll throw out a number 110 and probably in the next video I'll calculate at all and this is very important to realize because once again you could have life these are all examples of life right over here these are act this is actually life on our earth on our planet even though this looks quite alien this is a weevil it kind of looked at very close up but there's all sorts of forms of life many of which we probably can't even begin to imagine but what we care is that intelligent life 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 civil sophistication where they will be using radio waves and electromagnetic radiation to communicate with each other you know maybe we might have stagnated at this stage if nothing if the right things didn't happen so what we needed to do now is multiply this so right here we have 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 what we want to do is is Whittle it down even more to the percentage that get to the point that they can that they can develop technology that allows us to detect them so they're kind of that that so let me put it so let me multiply it times the fraction that our output a see here for and maybe they're using communication see for communications that allow us to detect them so this is detectable the fraction that are detectable detectable now you might think that we're done this would give you the total number of civilizations or life-forms in our Galaxy or the planets that have life forms that developed detectable technologies at some point in their history now it would be nice if if civilizations did not kind of be born and then 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 that are in existence now and I'll quote I'll clarify what now means in the next video because it's really if we're detecting something from a star that's ten thousand light years away our now means we're just receiving their signals which means that they release the signals ten thousand years ago but what I want to do is what is the fraction of these whose signals are achieving are reaching us right now and here I'm going to say well what's the average lifespan of a civilization I'll put that l who knows what that is maybe ten thousand years so civilization lifespan lifespan and it's going to be that over the life of the star so that is over I'll put a T here T for the star so the average lifespan for the star and I could say the average lifespan for the planet or whatever but we're assuming that if you know what's our star supernovas there you're not going to have any chance for Earth to develop life on it anymore so maybe this thing up here is ten thousand years and this down here is maybe ten billion years and if you were to multiply all of this out you should get the number of detectable civilizations in our galaxy right now I'll leave you there for this video in the next video we'll discuss it a little bit more and reconcile it with Drake's with Drake's the more famous version of Drake's equation and I'll also try to talk about this piece a little bit because I think this might be a little bit confusing and I'll try to diagram that out a little bit more