Answer Happy

In this lab we will continue our calculation of the Drake equation to estimate the number of civilizations that might currently exist in our galaxy, and that might produce signals that we can detect from Earth. In the previous lab you came up with estimates for the first three factors in the equation, which you might call the astrophysical and biological factors - they give the number of habitable planets around suitable stars (astrophysical) and then the number of these on which primitive life begins (biological). In this lab we will estimate the last three factors. We can think of these as evolutionary, technological, and sociological factors. They determine the number of planets with primitive life that go on to develop technological civilizations, also factoring in how long those civilizations last. These factors are much more difficult to estimate that the first three, but it is very interesting to explore how these numbers influence the number of alien civilizations we expect! For this lab we are using this modified version of the Drake equation: N = S*. np.fi.fi.fc. P where: N=the number of civilizations in our galaxy with which communication may be possible; S.=the number of suitable stars in the Milky Way: ne= the number of habitable planets per star; fu=the fraction of the above that actually go on to develop life at some point; fi=the fraction of the above that actually go on to develop intelligent life; fe=the fraction of civilizations that develop a technology that releases detectable signs of their existence into space; P= the chance that a technological civilization will exist at the same time as us You already have your estimates for S*. np and fu,. Today we will come up with estimate for fi fc, and P.

The fi parameter This is the fraction of planets that have already developed primitive life that go on to develop intelligent life at some point. This is the same as the probability of a given life- baring planet producing intelligent life. This one is tough to estimate, because we know of only one planet where this happened, and there is something called the anthropic bias - which just says that we will always observe a part of the universe that is able to produce observers like us. So, even if the chance of making intelligent life is one in a trillion (so there is only one in the whole Milky Way), by definition that is where we will be. However, just as we did when we looked at the probability of forming simple life in our fL parameter, we can look at how long it took for intelligent life to form on Earth. The earth is -4.5 billion years old. The earliest life on earth is 3.5 billion years old. But really, it has only been possible to make intelligent life for about half a billion years - that's how long complex life has been around. How do we use this to estimate fi? We have to guess. Consider pessimistic and optimistic cases and come up with a number for a minimum and maximum fi. maximum fi= minimum fi = The fc parameter This is the fraction of intelligent species that go on to produce become technological civilizations and which also do something (anything!) that might allow us to notice them That could include: sending signals out into the galaxy that we could detect or sending fleets of explorers or colonists visit other star systems or building gigantic structures like Dyson spheres that we might notice from Earth. Let us estimate this parameter in two steps: first, what are the chances that an intelligent species will go on to develop the beginnings of technology that might get them noticed? Humans are definitely the only species on Earth to have done that so far, but there are other species that might have. For example, various cousins of us homo sapiens were certainly intelligent, like Neanderthals and Denisovans. They used tools, and some may have language and culture, but they went extinct before developing radio transmitters or spaceships! We can estimate the probability of an intelligent species developing technology as just the ratio of genus homo went on to develop technology. Find out how many independent branches there were to the genus homo. That is, how many branches split off directly from the first genus homo ancestor 2 million years ago?

Only one of those developed technology (homo sapiens) so the chance of an intelligent species developing tech is just one over the number of homo branches you found. This number will be your optimistic or maximum estimate for fc. We can get a pessimistic estimate by arguing that only one technological species emerged from all intelligent-ish species on Earth. How many intelligent species are there? Instead of just counting genus homo, we could count all the Great Apes, as well as some other species - perhaps whales and dolphins, octopus, certain birds, and others you can think of. So, one over the number of intelligent-ish species gives us our pessimistic, or minimum fc. maximum fc = minimum fc = The P parameter Multiply all the above numbers together and you should get the number of technological civilizations that develop in the Milky Way over the entire lifespan of the galaxy. But even if a civilization appears, we will not notice it unless we overlap - we need to either exist at the same time or exist whenever that civilizations radio signals are passing by the Earth. The Milky Way is 10 billion years old, but what if the average technological civilization only lasts for, say, 1000 years? That means that chance of any two civilizations overlapping is 1,000/10,000,000,000 - or one in ten million! We need to estimate this last factor before we determine the number of civilizations that we might one day hope to contact. To do that, we need to estimate the average lifespan of a technological civilization. This will take some guesswork. How long do you think before humanity will no longer be around? Will we last a billion years as we travel through the Galaxy? Or will we wipe ourselves out in the next century by war, environmental disaster, or other catastrophes? Divide these numbers by the age of the Milky Way to get the maximum and minimum probability that two civilizations will overlap. maximum P = minimum P = We now have everything we need to solve the Drake equation. Combine your values from last week with the numbers you came up with today to make two estimates. First combine all of your optimistic or maximum values, and then all of your pessimistic or minimum values. Here is that equation again: N=S*. np.fl.fi.fc.P Maximum Nlife = Minimum Nlife = Does this range sound reasonable? If not is it more or less than you expected? Based on these numbers, is it surprising that we have seen no strong evidence for the existence of alien civilizations? And finally - what can that lack of evidence tell us about the parameters that we just estimated? At least one of those parameters needs to be pretty small. Which do you think it is?