The Search for Extra-terrestrial Intelligence! Is there anybody out there?
Summary: we will investigate (a) the Drake Equation and (b) the factors that determine how many, if any, other civilizations are out there
Our planet, the Earth, is the only place in the Universe where we know for sure that life exists. But our Earth is just an ordinary planet, one of nine, circling the Sun, which we know is just an ordinary star.
and our Sun is just one of 100 billion stars, all circling around in our Galaxy - the Milky Way So surely, amongst those billions of stars, there must be other stars just like our Sun, with planets just like our Earth, with civilizations like ours or more advanced? Where are they and why are they not coming here? Enrico Fermi (famous Italian Physicist 1945)
How many civilizations are out there (Drake Equation)?
Sanity warning! It s great fun to speculate on life forms that may be totally different from us. For example, perhaps they can be based on Silicon instead of Carbon, or take the form of huge gas clouds or slime patches rather than solid bodies like ours. However, because we don t know anything about such strange lifeforms, or even whether they could in principle exist, it s almost impossible to make any sensible statements about, for example, our chances of being able to communicate with them. So, in this and nearly all scientific discussions of extra-terrestrial intelligence, we make some rather conservative assumptions: For example, we assume the other civilizations are carbonbased, live on a planet which has plenty of liquid water (necessary for carbon chemistry), and require billions of years to evolve to intelligence. In other words, we are searching for other beings rather like ourselves, and the following discussion ignores the additional possibility of finding beings totally unlike ourselves. (for these purposes, this guy is rather like ourselves!)
(a) The Drake Equation: Number of communicative civilizations in our Galaxy N = N *. f p. n e. f l. f i. f c. f L N * is the number of stars in our n f is the of life-forms intelligent that life- is f Galaxy p is the of those stars that have planets e f the i c number of planets in each planetary l is the of those life-capable f system L is the fraction of the lifetime of the star around planets evolve forms which them that intelligence on is which choose capable life to of actually try bearing to communicate evolves life for which this communicating life-form with other civilizations lasts Now let us look at each of these factors in detail, and also try to estimate a number for each one.
(b) Determining the Factors N * - the number of stars in our Galaxy This one s easy! We know from studying our Galaxy that it contains about 100 billion stars So: N * = 10 11
f p - the fraction of stars that have planetary systems - I This was completely unknown until 1995, when two Swiss scientists, Mayor and Queloz, accidentally discovered the first true planetary system outside the solar system. Since then over 350 planets have been discovered, d mainly by the team led by Geoff Marcy (in the US) and Paul Butler (in Australia). Marcy and Butler search for planets by looking for a wobble in the star caused by the planet.
f p -the fraction of stars that have planetary systems - II The technique used by Marcy and Butler is sensitive only to the most massive planets, mostly, only Jupiter-like planets have been discovered and one Earth like by other techniques. So, f p =0.1 However, it is believed, based on theoretical modelling, that most of these planetary systems will probably also have smaller planets like our own Earth. At present, it seems that about 10% of stars searched have planetary systems
n e - the number of planets in each planetary system which h is capable of bearing life Assuming (see sanity warning) that the lifeform is somewhat like us, we can estimate what sort of planet is needed. n e itself depends on three separate factors : n e factor 1: time to evolve If life takes (like us) about 4 billion years to evolve, then the star must last at least that time. That rules out planets around very large, hot, luminous stars (O,B stars) which only last a short time before they explode.
n e factor 2: presence of water Assuming the life form needs liquid id water, then the planet must not be too close to the star (like Mercury or Venus) where the water would boil, or too far away (like Mars or Jupiter) where the water would freeze. For any star, we can calculate l the size of the habitable zone which is the range of orbital distances that t will allow a planet to have liquid id water. From this we can estimate the fraction of planets that lie within the habitable zone. Habitable Too hot zone Too cold
n e factor 3: chemistry The planet must have the right chemicals for life. We now know that the chemicals we have on Earth are common throughout the Galaxy, and so will probably be common on any planet that is about the right temperature. n e = 0.1
f l - the fraction of habitable planets on which life actually evolves This is a hard one! The problem is that, at present, we have no idea how life starts off. Pessimistic view: We know all the building blocks like amino acids are easily synthesized by ygeological g processes, and are probably common even in space, but to get from there to the first RNA molecule is very tricky. So some people believe that the formation of life may be a very rare event for life to start, and would guess at f l being close to zero. On the other hand, we know that it has occurred at least once (here on Earth) so we can estimate that f l is greater than about 10-8.
f l - Optimistic view We know that on the Earth, life started about 4 billion years ago, almost as soon as the Earth cooled down enough for the chemicals for life to appear. So, many people believe that, given the right conditions, life will always appear quickly. The fact that we don t understand how life gets from simple amino acids to the first RNA molecule is then a reflection of our ignorance, not a reflection of the difficulty of the process! So, these people would estimate f l as being close to unity. In summary, we know only that f l is somewhere between 10-8 and 1.
f i - the fraction of lifeforms that evolve intelligence - an optimist s i t view On Earth, about one hundred thousand years ago, several intelligent species of hominid appear to have evolved separately from our ape-like ancestors. One, homo neanderthalensis ( Neanderthal Man ) evolved in Europe, and the other, homo sapiens (ourselves) evolved in Africa. Each of these two species survived happily for a long period, until homo sapiens moved from the Africa into Europe and the Neanderthals quickly became extinct, only thirty-five thousand years ago. So, on Earth two separate species have evolved intelligence this shows that intelligence evolves easily and naturally, so any lifeform will naturally evolve intelligence. So: f i = 1
f i - the fraction of lifeforms that evolve intelligence - a pessimist s i view In fact the only thing we are really sure about is that hardly any (if any!!) of the species on Earth have evolved intelligence! So intelligence is a very rare thing,and it probably takes some special accident of evolution to achieve it. Of the million species of creature on Earth, only one has clearly l evolved intelligence (us?), so f i = 10-6
Are we off the chart smart? Humans have comparatively large brains Does that mean our level of intelligence is improbably bl high? h?
f c - the fraction of intelligent species who are communicative By communicative here, we mean able to build equipment to transmit and receive signals to other worlds Humankind entered this phase about 80 years ago, when radio broadcasts first became powerful enough to leave the earth and potentially be received by other civilizations with sensitive receivers.
It is easy to assume that other civilizations will evolve as we have, and develop technology and a desire to explore the universe, but this assumption may not be correct. Other civilizations may, for example: be content to live a pastoral existence, and never develop technology, or develop alternative technologies (still unknown to us) and never need to use radio waves for communication
So what is the value of f c? We cannot possibly even make an intelligent guess at f c - we know of only one intelligent civilization and that did develop technology. All we can say is that f c is greater than 0 and not more than 1. 0 < f c < 1
f L - the fraction of the stars lifetime for which h the civilization ili survives Since we know how long stars last (our Sun will keep shining for a further 5.2 billion years), this question is the same as asking: How long will a civilization survive? We have so far been civilised in the sense of being able to communicate with our interstellar neighbors for 70 years, so we know the age of our civilization is somewhere between 70 and 5.2 billion years!
What other things might cut our civilization short? Global nuclear war Destruction of our atmosphere An asteroid hitting the Earth A new virus against which we have no immunity A supernova close to the Sun Other horrors which we don t yet know about 2012? The first three of these are either preventable or survivable with our present technology. The last are not,but are hopefully rare events.
There is no way, at our present state of knowledge, that we can estimate t the likelihood lih of these various catastrophes. t All we can say with certainty is that the lifetime of our civilization will be between 70 years and 5.2 billion years. Since the Sun has a lifetime of 9.8 billion years, we can work out that 7 0 x10-9 <f L 7.0 x10 < f L < 0.5
The answer Multiplying l i each of these numbers together, th for an optimistic and a pessimistic scenario, gives the following o answers s for N, the number of civilizations in the Galaxy: Pessimistic: st N=0 Optimistic: N = 5x10 8 In the optimistic i view, about 1 in every 200 stars has a communicative civilisation. That means our nearest neighbour is less than 100 light-years away. In the pessimistic view, we are alone in the In the pessimistic view, we are alone in the Galaxy!
So what s the right answer? At our present state of knowledge, it is impossible for us to tell whether the right answer is the optimistic one, the pessimistic one, or somewhere in between. So rather than pontificating any longer on what is the correct value for any one of these factors, the time has come to take our radio-telescopes send messages and go and take a look!
Arecibo message to M13 EXTRA CREDIT Figure out number code
Likely place for ET to Phone Home!
SETI=SEARCH FOR EXTRA-TERRESTRIAL INTELLIGENCE JOIN THE SEARCH your computer can be a valuable tool! SEE THE ASTROPHYSICAL OBSERVATORY SITE via CSI Quick Links
How does SETI work?
SETI experiments look for deliberate signals from E.T.
We ve even sent a few signals ourselves Earth to globular cluster M13: Hoping we ll hear back in about 42,000 years!
Y t h l! SETI @ H ith Your computer can help! SETI @ Home: a screensaver with a purpose.
24.5 Interstellar Travel and Its Implications to Civilization ili Our goals for learning How difficult is interstellar travel? Where are the aliens?
How difficult is interstellar travel?
Current Spacecraft Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars. Pioneer plaque Voyager record
Going there ourselves? Jet plane 1,000,000 yrs to nearest star. This Voyager, the fastest spacecraft will take 25,000 yrs maybe a Huge ship with supplies can get a few whose future generations can make it but will they be human? If we try to get there at half speed of light then We need 40,000 times yearly energy of U.S. and will get there in 10 years!
Greetings From Planet Earth. Follow the map To our home! IS THIS A GOOD IDEA?
This Professor worries about sending our address Did you see Independence day Or perhaps the Aliens are giant lobsters They would not be happy about how we treat Their relatives here on earth..in fact.
Starship design for Interstellar travel: We don t give up so easy! Pusher Plate Project Orion: Nuclear pulse Propulsion repeated H-bombs
Anti-matter would be great but We cannot get enough or control It at present! Sorry Trekies! Project Daedalus: Controlled Nuclear Fusion reactor get push!
Solar sail or beamed energy propulsion: a laser to push on sails to c/2 needs 1,000 X current human power consumption! EH?
Interstellar Ramjet: Giant scoop collects Hydrogen for fusion fuel!
Difficulties of Interstellar Travel Far more efficient engines are needed Energy requirements are enormous Ordinary interstellar particles become like cosmic rays Social complications of time dilation
Where are the aliens? Fermi s Paradox Plausible arguments suggest that civilizations should be common, for example: Even if only 1 in 1 million stars gets a civilization at some time 100,000 civilizations So why we haven t we detected them?
Possible solutions to the paradox 1) We are alone: life/civilizations much rarer than we might have guessed. Our own planet/civilization looks all the more precious
Possible solutions to the paradox Civilizations are common but interstellar travel is not. Perhaps because: Interstellar travel more difficult than we think. Desire to explore is rare. Intelligent Civilizations destroy themselves before achieving interstellar travel And the other pessimistic ends we considered before are possible Alien civilizations have quarantined our planet considering us too violent a species to visit? These are all possibilities, but not very appealing
Possible solutions to the paradox 3) There IS a galactic civilization and some day we ll meet them