So welcome to the last lecture. Today we're going to talk about the search for extraterrestrial intelligence, the SETI, and what's called the Fermi Paradox. Asking the question, if aliens are out there, why haven't we heard from them yet? Why don't they ever call? So let me begin by going over the outline. We'll first talk about is there life elsewhere? Talk about how difficult space colonization is, and whether we expect it to happen. And then finally, enter into the Fermi Paradox. Let's begin by talking a bit about SETI, or the search for extra-terrestrial intelligence. Which I think really ought to be called the search for extraterrestrial technology, because there could be life out there; life that's very advanced, yet, doesn't desire to communicate with us, or doesn't develop the technology needed, to communicate with us and when we search for life elsewhere in the universe, a search for planets that are habitable, what we are really doing is searching for detectable life. We can begin by thinking about our own planet. We have only just begun in some ways the exploration of the range of environments in which life thrives in our planet. It has only been in recent years that we've discovered the huge biosphere that exists deep down, the deep biosphere. We've talked early with T.C. Onstott about life forms and environments deep underground that we are only now exploring. So if we're only now finding life on our own planet we're just, you know, can just begin to think about what kind of life might be out there, and what kind of life might be detectable. While we look out and try to study life on planets around other stars, we need to be mindful of what we're learning from our own solar system. Europa, with its ocean deep under ice, might host life, might even host intelligent life. There might be, sort of the Europan whales in its deep ocean. But we would not be able to detect with our current technology those Europan whales, nor might they be interested in communicating beyond their deep ocean. So when we talk about searching for either extraterrestrial life or extraterrestrial intelligence, what we're always talking about is searching for detectable extraterrestrial life, or detectable extraterrestrial intelligence. We talk about SETI, what's sort of the fundamental equation of SETI, it's what's called the Drake Equation. The Drake Equation is in many ways a summary of this entire course. So you look at the various terms that entering into this equation, you'll see each term in many ways involved one or several lectures through the course. The number of civilizations in our galaxy, will depend on a bunch several different terms. The rate at which stars form, that's known reasonably well. Our galaxy forms of water a star every year. The fraction of stars that are Sun-like, now we're interested in life, so that's why we're talking about Sun-like stars. But one of the things we discussed already is, there's possibly a wide range of stars that could host life. It could just be g-stars like our sun, or there may be life on stars ranging down in mass to the M-stars. Those M-stars, remember, are colder, longer-lived, if you're orbiting around an M-star, the habitable planet is likely locked, tightly locked so that the same side faces the star all the time. But it's quite possible that those M-stars could host life. The most massive stars are unlikely to host life, simply because their lives, those stars live for such a short time. The most massive stars live for only 10 million years or so, and that probably doesn't provide enough time for life to evolve. So this number here is probably pretty high, certainly tens of percent. Fp, the fraction of stars that host planets was simply not known at the time that Drake wrote down this equation. But one of the areas in which we've had tremendous progress in the past decade, is the search for extra solar planets. And as we've discussed in this class, we're now finding that planets are common. That many stars host planets, at least 10%, probably more like 20%. And many of these stars host planets that are likely habitable. With the Kepler telescope, we have explored orbits out to a bit below the Earth's orbit. With the current analysis of Kepler data, people are trying to extrapolate into the Earth's distance from the sun. Stars that with planets whose periods are comparable to the Earth's period of one year. But based on our current data, our best estimate of these numbers are probably of order, 10 to 20% of all stars host planets that are Earth mass or Earth size at distances comparable to the Earth. Now, what fraction of those are habitable is something we don't know yet. Our own solar system suggests a variety of possible outcomes for planets. A number of times in this course we've compared Venus, Earth and Mars. Planets that have comparable masses, Venus and Earth almost the same. Comparable rates of energy coming in from the sun, yet have very different properties. As we've discussed, Venus is far too hot, Mars is quite cold and the Earth seems to be just right. Though, you know, there are times, like, early January 2014 where quite habitable places like Winnipeg were actually colder than the surface of Mars at the position of the Curiosity rover. So we now know these numbers pretty well. Now when we get to the next four terms, our uncertainties get much larger. Given a planet that could host life say has a liquid water around a star that lives billions of years. What fraction of those planets evolve life? How often does life appear? This is a factor that is very poorly understood. We don't know enough yet, as we've discussed in early lectures, to understand all the steps it takes to go from the simple organic compounds that we do observe through our galaxy. We know that these planets will have carbon and oxygen, hydrogen, the simple building blocks of life. And we know that it is relatively easy to assemble these into amino acids and the other basic building blocks of life. The challenge is how do we go from those basic building blocks to life that is self reproducing and capable of sustaining itself. And this number here is quite uncertain. Equally uncertain are the next couple terms. Once you have simple life, how long will it take to get to intelligence? When, and what fraction of the time does it happen in the age of the star and the planet? On Earth, life evolved relatively quickly. Yet, it took a very long time, nearly four billion years, from when the first simple cells arose to when intelligent life arose. And, you can argue once we got mammals. We have a number of species that distribute significant intelligence from dolphins to various primates including ourselves. And there's a fairly high level of intelligence in animals such as dogs. And we see complex social structures, across a wide range of insect species, and while we might not think of an individual ant as intelligent an ant colony is quite capable of building what you might say are great structures. So, it took a while to get there, but we now actually have very complex life in the last few hundred million years on our planet. But even given complex, intelligent life, it's only been in the last 10,000 years that any species has really developed significant amounts of technology. And only, of course, in the last hundred, technology capable of communicating across long distances. Even when you think about the human species, it's only been in the last 1% of the life of the human species, the last 10,000 years that we have developed agriculture, had technology really take off and of course it's only been in recent times that we've been able to leave the surface of the Earth's planet, travel to our moon. And only now where we're beginning to develop the technologies needed to get to Mars, and start moving beyond Earth orbit. So I would say, the astronomers have done their job. We have been making a lot of progress on figuring out these terms in the equations. While there's been tremendous progress, in biology, these terms are quite poorly known. And, I think our colleagues in anthropology are only beginning. I don't think they've really even thought about the question of, given a species like humans, how long will it take before they start developing technology? I don't think we know how to answer that question. And in many ways, the most uncertain number of them all is a number that, perhaps, comes from political science. Given the existence of a civilization, how long will it last? How long will it be before it destroys itself? Perhaps through things like nuclear war. And at the time when Drake wrote this down, there was a lot of discussion of that. Or destroys itself through developing technologies that are self-destructive. From the point of view of exploring space, and communicating outwards some of it may be its decision for a civilization to stop exploration, to stop going outwards. And we should keep in mind that that has happened on Earth. I think if you look back, you know, several hundred years. Say look back to the 1400s. In 1400, I think many historians would agree that China had the most advanced civilization, technologically, on the planet. And the Chinese, an excellent example is Admiral Zheng treasure fleet, were exploring, much of the Earth and these treasure fleets, which were much larger than the ships that Columbus took, to travel from Europe to the Americas, were exploring all of Asia. And this map just shows where treasure fleets traveled from 1405 to 1430. So well before Columbus, these much larger ships were exploring the Indian Ocean. However the Chinese bureaucrats decided that this was not a worthwhile investment, that these ships were too expensive. And they turned the ships around, and, let them rot. So, you know, sometimes civilizations make the choice to stop exploration, perhaps to stop communication. So we don't know any of these numbers terribly well. 50 years ago, astronomers began to think very seriously about detecting communications from extra terrestrial intelligence. What many of them had in mind were the kind of communications, kind of signals that we were sending out. We were broadcasting I Love Lucy on through our radio and TV channels. Sending that out to space. Sending out our early newsreels. And our early radio programs were traveling out to space. And what we imagined was eavesdropping on other civilizations' radio communications. My own feeling, people disagree on this, is that's now a naive way to think about what we would do, to simply eavesdrop. And that's because as our communication capabilities have evolved, we now want to send signals, when we're talking among ourselves, in a way that's as efficient as possible. We usually compress the signal, what compression does, and this signal that you're getting as you're watching, this program is compressed. The way information is sent, whether you're looking at a movie or watching something on cable, is the information, in the signal is reduced in a way that you remove excess information. When you compress a signal, it's statistical properties look just like noise. You need to know for a compressed signal, what the code is for doing the decompression. So if you were listening in not to a TV signal sent out in the fifties. But, we're monitoring on cable a TV signal today. You find that you couldn't decode that signal, there wouldn't seem to be any information there unless you had the decompression algorithm. So I think we're unlikely to pick up accidental signals. I imagine that alien advanced civilization will want to communicate among themselves in a way that's as energy efficient as possible. So if we are detecting signals, we're likely detecting signals that an alien civilization sends to us because they want us to hear it. We're likely only going to hear from civilizations that call. We're not going to get that much information, I suspect though I may be wrong, by eavesdropping. So what are some of the challenges in detecting extraterrestrial signals. Well one of the first challenges is there's a lot of things out there producing signals, and we need to first distinguish artificial signals from natural signals. One of the most dramatic natural signals that was first interpreted by some as extraterrestrials were pulsars. Pulsars are rapidly rotating neutron stars that have electrons trapped in its magnetic fields, and as that pulsar swings, neutron star swings around, ten, sometimes hundreds of times per second, every time the pulsars beam, and that beam is defined by the neutron, the magnetic field. Of that neutron star, points towards us, we hear a pulse. And when radio astronomers first detected these complex, repeated pulses, one theory was, it was little green men. We now have many observations that show that pulsars are neutron stars. So, we've checked them off the list, but anything, something we need to be mindful of when we have an intriguing detection of a repeated signal, is nature can sometimes produce complex, repeating signals, so we need to be convinced that the signal is truly artificial. We also need to be convinced that the signal's extraterrestrial. We certainly produce lots of radio transmission, lots of radio noise. And if you point your radio antenna in an arbitrary direction, you'll certainly pick up signs of intelligence. It's just intelligence coming from our own planet. And, some of you may argue whether detecting Rush Limbaugh is truly a sign of intelligence. But I think we should be generous in our definition. So if we're going to detect intelligence or detect a signal from space, one of the things we might want to look for is a signal that's intentionally beamed at us. One thing we can exploit to do that is what's called the Doppler effect. This is something we've talked about earlier in the course. If someone's transmitting a radio signal towards me, and I'm moving towards the signal, then that radio signal is shifted to higher frequencies. If I'm moving away from the signal, that signal is shifted towards lower frequencies. If there's an alien civilization intentionally beaming a signal towards Earth, one way they could help us detect that signal is transmit it in a way that corrects for our motion. The Earth's rotation, possibly, but certainly the earth's motion around the sun because that's something they could detect. And if we were trying to transmit our signals towards a planet that we think is habitable, one way we could send the signal, in a way that we would hope would be easier to detect was to correct for the Doppler motion. So send the signal in a way in which the frequency is changing with time to correct for the Earth's motion. Another possibility is even if it's not intentionally sent towards us, we can look for a signal being transmitted by a planet, to detect another planet around the other star and look for its variation. Doing these searches in frequency space is very computationally demanding. And for those of you interested in SETI, I encourage you to go to the SETI at home site. This is a site that helps you, it makes, it involves you directly in this search. What it does is make use of your computer whenever you're not doing something with it to run in background, a program that's going through the enormous amount of data that had been taken by radio telescopes and performing the computationally demanding tasks of searching in that multidimensional space, for the signal. Looking towards the future what probably is the best site that we can think of for SETI searchers is the back side of the moon. That's because the back side of the moon, the side that always faces away from the Earth, is extremely radio quiet. It does not get radio transmissions from Earth, and as a result it's a place that we can tape. Look out in space and signals that we see. We can much more easily detect things that are extraterrestrial. Should note that in addition to searching in the radial, people are searching in the optical and one approach is to search for beamed optical radiation coming at us. When we're doing searches for signals that we don't know what they look like, we don't know where they're coming from and we don't know what frequency at. It really is the search, a searching for a needle in a haystack. And, we don't know the direction or distance. We don't know when the signal's coming. We don't know whether we should be looking radio wave lengths or optical wave lengths or perhaps even at x-ray waves. We don't know the signal strength, we're looking for it. We don't know whether we need to build enormous antennas, or whether they're sending such a strong signal that it can easily be detected. And we don't know the form the signal's coming in. The signal could be compressed, in which case we need a decompression algorithm. And even if it's not, just thinking about the way we transmit radio signals. We transmit signals where the information is carried both, in one case through modulating the amplitude of the signal, that's called Amplitude Modulation or AM radio, another is through modulating the frequency of the signal, that's Frequency Modulation or FM radio. And you need to know what kind of signal you're looking for in order to decode this. But the people who are carrying out SETI are brave. They recognize that their task is difficult. It's not one that's likely to succeed, but if it succeeds, it would have a profound impact on our understanding of the universe and our understanding of ourselves. We think about SETI, I think there's a number of important questions to contemplate. Questions that I don't have any answer to, I don't think anyone does. What should we do if the SETI search succeeds? What do we do if we hear something? How should we respond? What should we do if we don't succeed? What if over the next hundred years, we continue to push our technology, continue to search for extraterrestrial intelligence. I strongly believe over the next 100 years, we'll detect many habitable planets and perhaps, detect signs of life on the planets. But what should we conclude if we don't hear from anyone? We don't detect the signs of intelligence. And finally, should we make noise? Should we be quiet and simply observe the universe. Some of my colleagues argue that we don't know what's out there. What's out there could be very dangerous. And when you find yourself in a forest, don't know where you are, you should perhaps be very quiet, observe the environment around you rather than attract a lot of attention. While others viewing alien life that's won't live does likely not overtly aggressive or likely to come and whine to wipe us out have advocated for trying to communicate as best we can with any planet that looks habitable and might show the signs of life. So let me leave you at this part of the lecture, thinking about whether we should be quiet or transmitting, but then have you go on and go back to the Drake equation. Plug in your best estimates for the numbers in the Drake equation and make your best estimate based on things that you learned in the course, other things you know of the number of communicating civilizations that are likely in our galaxy. Plan to go off and think through that, and then we'll come back and talk some more.