 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about the search for extraterrestrial intelligence, and not just looking for life in the universe, but for intelligent life with which we are able to communicate. So what do we know here? Well, first of all, how do we travel between the stars? Well, the distances are tremendous. Even if we could travel at the speed of light, it would take four years to reach the nearest star. Our current craft, such as Voyager and New Horizons, would take 80,000 years just to reach the next star. So it's not something that is very doable with current technology. And again, even if we could go accelerate and get up to the speed of light, it still takes light thousands of years to get to many stars. So while there are some in our local neighborhood, actually getting out and traveling to very distant stars would be still a problem. There's also an expense of faster travel. It takes a lot of energy. If we made one trip at 70% of the speed of light, which is way beyond our technology right now, it would take 12 years to reach the nearest star. However, that would cost the equivalent of an energy of hundreds of thousands of years of electric consumption for the United States. So that's how much energy it would take. So we need other ways to try to communicate, because the travel just is not feasible. How about sending messages? Well, we have sent messages on spacecraft, such as the Pioneer, which had a plaque as is shown here, which contains information about our solar system here. So this is our solar system and which planet we're from. So we identify the spacecraft as coming from the third planet there. The sizes of humans here relative to the spacecraft in the background, what the size of a typical human is. And this is the position of the sun relative to 14 pulsars and the center of our galaxy. So that identifies the location of the solar system in space. And then what looks like a pair of little eyes up here is the hydrogen transition. This is the spin flip transition of hydrogen, which is responsible for the 21 centimeter line of hydrogen, which is used to explore gas, hydrogen gas within the galaxy. So this information has been sent out. This is what was sent with Pioneer. On the Voyager, the Voyager sent the golden record with the sounds of earth, including greetings in many languages, different music samples from different cultures on earth, natural and manmade sounds from earth, and encoded diagrams and photographs. However, how likely is it that these will ever get to anyone? Even the Voyager craft now at the greatest distance from earth are still just at the very at the edge of our solar system and are tens of thousands of years from getting anywhere near another star. So that civilization finding them would have to be almost on top of us right now. And who knows what it will mean if maybe millions or tens of millions from years now they are detected. That would be what would have happened here on earth. What would earth be like in the meantime? So since neither of these work, how can we communicate? Well, one advantage is that radio waves travel at the speed of light. So this is a quicker way to make contact, but it is still not a fast way. It would take over eight and a half years to receive a response from the nearest star. That's four point three years to get there. And if they immediately responded, it would take four point three years to get it get back now. We can broadcast signals and have done so with things like the Aeroscebo telescope shown shown here before it was destroyed. And will the signals be strong enough to detect? Where are we going to send these signals? So we have to choose carefully where we want to send them. And then we have to listen for signals. So we have to listen for signals that are coming back. So we really need to be able to do all of this. We'd also need to be able to listen for signals. Because if other people are broadcasting somewhere in the universe, then maybe we'll be able to detect that. How do we determine the difference in a signal? Is it natural or is it artificial? And if you recall, when we discovered the pulsars, the pulsars were thought perhaps at very first could have been a artificial signal, although we later found out that they were a natural signal. Now one signal that has been detected, and this is the only time we've had one, is what is called the wow signal. And that is one spot where we had a significant detection of a signal from space. Now we know it came from space, but we still don't know for sure if it was a signal from another civilization, or some kind of interest different natural signal. But it was much, much stronger than the general background of the signals that we normally see. So interesting to see, but since it's never been repeated in the decades since it was first detected in 1977, it still leaves us questioning exactly what happened there. So how do we detect signals, even for using the radio spectrum? It is large. Where do we listen? Well, we tend to think of listening at what is called the water hole. The water hole is this part of the electromagnetic spectrum between emission of hydrogen on one side and hydroxyl on the other. And hydrogen, H, and OH together make water. It makes sense that maybe a civilization dependent on water might look in this region, much like us. And it's also one of the lowest noise regions in the radio spectrum. If you go too far to either side, whether to high frequencies or to low frequencies, there's a lot of background noise. So this is one of the regions where the noise is at a minimum. What kind of signals might we detect? Leakage, just general signals that are being broadcast around that planet that would also go out into space, those would be very difficult to detect. Things like radio or TV shows that have been broadcast. They would be very difficult to detect elsewhere just because they weren't broadcast there. Beacons, actually messages that are sent, would be much easier to detect. They're a much stronger signal and they are specifically sent in one direction, not just generally out in space. So where are we going to send these signals? Well, sun-like stars, we tend to think that those are the most likely ones to have life like we have on Earth. Stars with known habitable planets, certainly that's a good starting point because at least we know there's the possibility of a habitable planet there. And the question again is will we receive a response? How long will it take? What is the technology level of the civilization? If we had received a signal 200 years ago, it never would have been detected, let alone responded to. So maybe civilization sent us signals for hundreds of years and gave up on us as no life in this area. Are they interested in communication? Are all civilizations interested in communication and in what else is going on? We did send a message in 1974. The Arecibo sent a message toward a globular cluster known as M13, which would have 100,000 stars. So sending it to all these directions of all these stars at once, but it is 25,000 light years away. So while we send it, it would take 25,000 years for it to get there. It still barely started its journey. And if someone did respond, it would take another 25,000 years for that response to get here. So how can we try to understand how many civilizations there are in the galaxy? Well, what we use is the Drake equation. So the Drake equation is a way to estimate a specific thing. It's the number of technological communicative civilizations in the galaxy right now. How many are there now that we could communicate with? It's based on a number of estimates and probabilities. So we can estimate some of the numbers, which are astronomical. We have to get some probabilities for some of the others. And some of them, like this last one is going to be a very difficult to find. But if we can determine these seven terms, all we have to do is multiply them all together to get the number of civilizations. Now, some of these are known better than others. And let's take a look at what they are. So the first one is the rate of star formation in the galaxy. That's an astronomical number. And we have a pretty good idea of what that rate is. Fraction of stars with planets. We're now finding out that lots of stars have planets. So there are a good number of those. So this is the fraction of stars with planets, F with a subscript P. And that one we can probably get a pretty good idea on, as well as getting a good estimate of the number of Earth-like planets in each system. Is it one? Is it two? How many are close enough and could be within that habitable zone? So we could probably get somewhat reasonable estimates here. The question marks are on these ones, especially. Fraction of those planets that develop life. We only know of one planet in the universe that has developed life. Is there other life out there? Possibly. But we don't have any good idea to estimate whether this is one in two of these planets that develop life, or one in a million. And that makes a big difference when you're doing this calculation. If they do form life, does it become intelligent? And that's a good question. Are there lots of planets that have life, but are they just simple animal or plant life and not intelligent life? In other words, these are ones we would not be able to communicate with them. And then the fraction that develop communications. How many of those develop some kind of communications? So in that case it would be probably radio telescopes. Do all intelligent civilizations naturally develop radio communications? Or is it a rarity? And that's another good question. And the biggest question we have here is this one. What is the length of time that a civilization is actively communicating? We've developed radio around a hundred years ago, so technically we've been doing maybe a hundred years. A very small fraction of our civilization. Before that we would not have been able to detect or send messages. So how long does this last? How long does a civilization last? Do they last millions of years? In which case there could be a lot of them there? Do they last hundreds of years? In which case there could be very few. So what is the answer to this equation? Well, there is none. So there is no answer to this equation. Exact. The values have a wide range. We can have a pretty good estimate on the first three, but depending on what you put in for these and for this last one, we could be the only civilization in the galaxy. We could have an N which is less than one. In which case it would just be us. Or there could be thousands or millions of other civilizations. It just depends on what your assumptions are, again especially for the development of life and how long a civilization survives. It's really not an equation meant to give us an answer, more as a discussion point. Let's discuss what the probabilities of life are. And it's unlikely that this is really ever going to have any kind of definite answer. So is there life out there? Well, it's an open question. It's not something we will ever be able to say absolutely not. Because we're never going to be able to study every single possibility of planet that could have life. Could we ever say yes? Well, if we ever do detect it, then it could be answered yes. So it is a question that could be answered yes at some point, but can never really be completely answered no. What we do know is that the building blocks of life are very common in the universe. We know that there are many planets with conditions similar to Earth. What we don't know is how easily does life form and especially how long does an advanced civilization last. How long do we have to communicate with them? Do many civilizations destroy themselves before we would have a chance to communicate and receive responses? So let's go ahead and finish up with our summary. And what we looked at again, the possibility of life in the universe is an open question. We have sent messages by a number of different methods to other civilizations. And we talked about the Drake equation, which allowed us to consider the variables needed to determine if and how many other civilizations are out there. So that concludes this lecture on the search for extraterrestrial intelligence. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.