are we alone in the universe it's a question we've been asking ever since we figured out that there may be other worlds besides this one the famous Drake Equation tries to estimate the number of technological civilizations currently existing in our galaxy it gives that number as a combination of Astrophysical biological and sociological factors each of which narrows the range of stars in our galaxy that may have produced a surviving civilization these factors include the rate of star formation in the Milky Way the number of planets per star that could support life the fraction of life-bearing planets that produce intelligent civilizations and of those the fraction that developed technology that we might detect and finally you need to factor in the average lifetime of the typical advanced civilization when Frank Drake formulated his equation in the early 60s most of the factors going into it were hopelessly unconstrained Drake's original calculations gave a range from 20 and 50 million current technological civilizations in the Milky Way although he felt they were probably at least a thousand still there is a huge range at the low end we're unlikely to ever see one assuming they don't come to us and at the high end there should be civilizations within a hundred light years which may have detected our own radio transmissions by now in the last 50 years we've learned a lot more about some of Drake's factors there's some progress with the biological factors as we push the fossil record back closer and closer to the formation of the earth and we see just how quickly life arose here we also have a slightly better understanding of abiogenesis the initial evolution of life and perhaps even the subsequent evolution of intelligence although these are still a long way off giving us hard numbers the sociological factors and in particular the average lifespan of a technological civilization are still the subject of wild guesswork the area where we've made the most progress is in the S physical factors we now have a very good idea of how many planets there are in the Milky Way the good potentially support life the Kepler mission allowed us to estimate that our galaxy both something like 40 billion terrestrial planets in the Goldilocks zone of their parent star so 40 billion rocky world's capable of supporting liquid water which may be an essential ingredient for life we call these habitable planets although we have no idea how many are inhabited around 11 billion of those are earth-like planets around sun-like stars with such a tight constraint on the astrophysics but with the other factors being pretty hopeless some researchers have decided to reframe the Drake Equation astrophysicists Adam Frank and Rudra Sullivan asked a different question instead of figuring out the number of technological civilizations still existing in the Milky Way they asked the following Indian deeply pessimistic assumption that Humanity is the only technological civilization in the entire known universe what would that say about the biological and sociological factors in the Drake Equation in fact they figured out for there to be only a 1% chance for one advanced civilization to have ever arisen in the history of the known universe the chance for each habitable planet to produce such a civilization would need to be less than 2.5 by 10 to the power of minus 24 so less than a 1 in 400 billion trillion chance that's so insanely small that we're forced to conclude that there must have been civilizations out there before they also ran the numbers for the Milky Way they got that in order for humans to be the only advanced civilization to have ever appeared in our galaxy ever then there would need to be only a 1 in 60 billion chance for any suitable planet producing something like us that's still a very small number if the true probability of producing civilizations is higher then there have been and perhaps still our advanced species in our galaxy ok so I have a challenge question for you the best chance we have finding other civilizations is if they're close I want you to follow Frank and Sullivan's method plus whatever other information you care to include to answer this if Humanity is the only technological civilization to have arisen on any habitable planet within 100 light years how low would that probability of technological emergence need to be what does this tell you about how close our nearest neighbors might be to answer this you should take a look at Frank and Sullivan's paper link below and check out some of the other useful info I'll leave in the description let's also do an extra-credit question answer the above but also the following consider tabbies star an otherwise normal looking f-type star 1,500 light-years away which the Kepler mission revealed to be experiencing some weird dimming some people like to think this is evidence of some sort of alien mega-structure eclipsing the star of course it's almost certainly not but to say it was if we now say that those guys are the one advanced Dyson swarm building civilization in the entire Kepler sample how close to us is the nearest such civilization not in that sample what does your answer say about the likelihood of tabi star hosting such a civilization submit your answers with full work to PBS space time at gmail.com within two weeks of release of this video for a chance to win a space time t-shirt make sure you use the subject line galactic civilization challenge to be in the running because we filter by subject line see you next week for a new episode of space time you [Music]