# Drake's equation

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 46009.  Mon Jan 16, 2006 12:31 pm What is the probability that there are other thinking beings* in the universe? In the 1960s, Frank Draks, a professor at Cornell, worked out an equation to calculate those chances. You divide the number of stars in a selected part of the universe by the number of stars that are likely to have planetary systems. Divide that by the number of planetary systems that could theoretically support life. Divide that by the number on which such life advances to a state of intelligence** Even with the most conservative input, the number of advanced civilisations in the Milky Way works out in the millions. So why haven't we seen them?*** First of all, space is.... very large. The average distance between any two such civilisations would be at least 200 light years. Thus, any such beings watching us through telescopes would see a world in which Nelson had just died preserving England's superiority at sea and Wellington had yet to decisively defeat Napoleon on land, and we still lit houses with candles and lamps and travelled everywhere by the four-legged version of horsepower. Carl Sagan wrote that space was so empty that if you were randomly inserted into the universe your chances of being on or near a planet would be 10 to the power of 33 - 10 followed by 32 zeroes. * Always assuming that we so classify ourselves. ** See above *** Assuming we haven't

 46170.  Tue Jan 17, 2006 6:08 am The Drake Equation was developed by Frank Drake in 1961 as a way to focus on the factors which determine how many intelligent, communicating civilizations there are in our galaxy. The Drake Equation is: N = N* fp ne fl fi fc fL The equation can really be looked at as a number of questions: N* represents the number of stars in the Milky Way Galaxy Question: How many stars are in the Milky Way Galaxy? Answer: Current estimates are 100 billion. fp is the fraction of stars that have planets around them Question: What percentage of stars have planetary systems? Answer: Current estimates range from 20% to 50%. ne is the number of planets per star that are capable of sustaining life Question: For each star that does have a planetary system, how many planets are capable of sustaining life? Answer: Current estimates range from 1 to 5. fl is the fraction of planets in ne where life evolves Question: On what percentage of the planets that are capable of sustaining life does life actually evolve? Answer: Current estimates range from 100% (where life can evolve it will) down to close to 0%. fi is the fraction of fl where intelligent life evolves Question: On the planets where life does evolve, what percentage evolves intelligent life? Answer: Estimates range from 100% (intelligence is such a survival advantage that it will certainly evolve) down to near 0%. fc is the fraction of fi that communicate Question: What percentage of intelligent races have the means and the desire to communicate? Answer: 10% to 20% fL is fraction of the planet's life during which the communicating civilizations live Question: For each civilization that does communicate, for what fraction of the planet's life does the civilization survive? Answer: This is the toughest of the questions. If we take Earth as an example, the expected lifetime of our Sun and the Earth is roughly 10 billion years. So far we've been communicating with radio waves for less than 100 years. How long will our civilization survive? Will we destroy ourselves in a few years like some predict or will we overcome our problems and survive for millennia? If we were destroyed tomorrow the answer to this question would be 1/100,000,000th. If we survive for 10,000 years the answer will be 1/1,000,000th. When all of these variables are multiplied together when come up with: N, the number of communicating civilizations in the galaxy. :-) Tas Source: http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html

 46200.  Tue Jan 17, 2006 8:10 am As Tas has pointed out, at least 2 of the variables (I could probably argue for more) are completely unknown such that they vary from 0% to 100%. If either of these variables (of the combination thereof) was 0%, then N would be 0. Clearly that would be wrong, as we exist (allegedly). However, if either variable was 1/(number of planets in the universe), then N would be 1, i.e. us! There is currently no reason to suppose that the combination of all these variables doesn't add up to 1/(number of planets in the universe). For this reason, anyone who says "Even with the most conservative input, the number of advanced civilisations in the Milky Way works out in the millions" is talking utter rubbish. Not that I'm having a go at Jenny here. I've heard lots of people trot out that quote over the last years and it gets my goat every time I hear it. It ranks up there with the opening chapter of Chariots of the Gods where von Danniken uses a similar (or possibly the same, it's been a while since I read it) equation to "prove" that alien life exists.

46226.  Tue Jan 17, 2006 8:57 am

 Quote: Clearly that would be wrong, as we exist (allegedly). However, if either variable was 1/(number of planets in the universe), then N would be 1, i.e. us! There is currently no reason to suppose that the combination of all these variables doesn't add up to 1/(number of planets in the universe).

However, if either one (or both) was greater than one, then we are not alone, right?

My personal view is that, given that there are so many stars and planets out there, then life is a certainty (or at least has an infetessimally small chance of not existing). I think the universe is geared toward life, not against, and life seems to be the most tenacious of things, existing where we thought it previously could not, and in ways unimaginable just a few decades ago.
Whether intelligent species will be able to communicate over vat distances, or even if they will want to, however, is another matter entirely. Maybe we are the only ones who are THAT curious. It would be amazing if/when it did happen, I think many of us could agree on that!

:-)

Tas

46322.  Tue Jan 17, 2006 12:22 pm

It seems fitting to also mention the Fermi paradox:
 Quote: The story goes that, one day back on the 1940's, a group of atomic scientists, including the famous Enrico Fermi, were sitting around talking, when the subject turned to extraterrestrial life. Fermi is supposed to have then asked, "So? Where is everybody?" What he meant was: If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth? This has come to be known as The Fermi Paradox. Fermi realized that any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could rapidly colonize the entire Galaxy. Within a few million years, every star system could be brought under the wing of empire. A few million years may sound long, but in fact it's quite short compared with the age of the Galaxy, which is roughly ten thousand million years. Colonization of the Milky Way should be a quick exercise. .... Bracewell-Von Neumann Probes: While interstellar distances are vast, perhaps to vast to be conquered by living creatures with finite lifetimes, it should be possible for an advanced civilization to construct self-reproducing, autonomous robots to colonize the Galaxy. The idea of self-reproducing automaton was proposed by mathematician John von Neumann in the 1950's. The idea is that a device could 1) perform tasks in the real world and 2) make copies of itself (like bacteria). The fastest, and cheapest, way to explore and learn about the Galaxy is to construct Bracewell-von Neumann probes. A Bracewell-von Neumann probe is simply a payload that is a self-reproducing automaton with an intelligent program (AI) and plans to build more of itself. Attached to a basic propulsion system, such as a Bussard RamJet (shown above), such a probe could travel between the stars at a very slow pace. When it reaches a target system, it finds suitable material (like asteroids) and makes copies of itself. Growth of the number of probes would occur exponentially and the Galaxy could be explored in 4 million years. While this time span seems long compared to the age of human civilization, remember the Galaxy is over 10 billion years old and any past extraterrestrial civilization could have explored the Galaxy 250 times over. Thus, the question arises, if it so easy to build Bracewell-Von Neumann probes, and they has been so much time in the past, where are the aliens or at least evidence of their past explorations (old probes). So Fermi Paradox becomes not only where are They, but why can we not hear Them and where are their Bracewell-von Neumann probes? .... Possible solutions to Fermi's Paradox fall in the following categories: They Are Here They Were Here and They Left Evidence UFO's, Ancient Astronauts, Alien Artifacts: all fall under the heading of proposals that aliens are here now (and they call themselves Republicans) or have been here in the recent past. Problem: evidence for aliens is non-existent. They Are Us Humans are the descendents of ancient alien civilizations. Problem: where are the original aliens? Where are all the other alien civilizations Zoo/Interdict Scenario The aliens are here, and they are keeping us in a well designed zoo (cut off from all contact) or there is an interdiction treaty to prevent contact with young races (us). Problem: scenario lacks the ability to be tested. Takes only one ET to break embargo. They Exist But Have Not Yet Communicated They Have Not Had Time To Reach Us Speed of light slows communication levels, relativity makes space travel long. ET's message may not have reached us yet. Problem: Galaxy has been around for billions of years, even if one ET civilization formed a few million years before us, the Galaxy would be filled with Bracewell-von Neumann probes. They Are Signaling, But We Do Not Know How To Listen EM radiation, gravity waves, exotic particles are all examples of methods to signal. Problem: they may use methods we have not learned yet, but if there are many civilizations someone would use EM methods. Berserkers The Galaxy is filled with killer robots looking for signals. ET is keeping low. Problem: where are the berserkers coming after us? They Have No Desire To Communicate ET has no interest in conversing with lesser beings. Problem: with millions of possible civilizations, someone would have some curiosity. They Develop Different Mathematics Mathematics is the universal language. But humankind may have a unique system of mathematics that ET cannot understand. Problem: then where are their incomprehensible signals? Catastrophes Civilizations only have a limited lifetime, They are all dead. Overpopulation Nanobots -> Gray Goo Problem Dangerous Particle Physics They Do Not Exist We are the First, Life is New to the Galaxy Life is new to the Galaxy, evolution takes time, we are the first civilization. Problem: Sun is average star, if other stars formed a million years ahead of us, then They would be a million years ahead of us in technology. Planets With the Right Conditions are Rare Planetary systems are rare Habitable zones, proper distance from star for liquid water, are narrow Galaxy is a dangerous place (gamma-ray bursters, asteroid impacts, etc) Earth/Moon system is unique (large tides needed for molecular evolution) Life Is Rare Life's Genesis is rare Intelligence/Tool-Making is rare Language is unique to humans Technology/Science is not inevitable In general, solutions to Fermi's paradox come down to either 1) life is difficult to start and evolve (either hard for the process or hard to find the right conditions) or 2) advanced civilizations destroy themselves on short timescales. In other words, this is an important problem to solve in the hope that it is 1 and not 2.

http://zebu.uoregon.edu/~js/cosmo/lectures/lec28.html

Basically, given Drake's equation proposing that life should be common in not only the universe but our galaxy, coupled with the vast age and timespans available for life to have spread we should see some evidence of them, somewhere. The fact that we don't is either telling or misleading.

Interesting subject.

46329.  Tue Jan 17, 2006 12:33 pm

 Tas wrote: However, if either one (or both) was greater than one, then we are not alone, right?

Correct. Now all you have to do is prove that :)

 Tas wrote: My personal view is that, given that there are so many stars and planets out there, then life is a certainty (or at least has an infetessimally small chance of not existing).

You are perfectly free to believe in that if you wish. However, at the moment there exists not one shred of scientific evidence to back up that belief.

46396.  Tue Jan 17, 2006 4:27 pm

dr.bob wrote:
 Tas wrote: However, if either one (or both) was greater than one, then we are not alone, right?

Correct. Now all you have to do is prove that :)

 Tas wrote: My personal view is that, given that there are so many stars and planets out there, then life is a certainty (or at least has an infetessimally small chance of not existing).

You are perfectly free to believe in that if you wish. However, at the moment there exists not one shred of scientific evidence to back up that belief.

I think we can be reasonably certain that life exists on at least one planet in the universe.

46408.  Tue Jan 17, 2006 5:01 pm

 Quote: You are perfectly free to believe in that if you wish. However, at the moment there exists not one shred of scientific evidence to back up that belief.

All too true, but it is very unlikely that another planet has not got similar conditions enough for life not to have spawned in the same way that this one has. There is, I concede, a chance, however small that we are unique. I do doubt it very much, though.

:-)

Tas

 46439.  Tue Jan 17, 2006 7:51 pm What would seem very interesting is the question how long it took, after the conditions on earth had been formed in which life (as we know it) had the possibility to emerge (in hindsight), for life to actually emerge. If that period was relatively short that could indicate that the emergence of life could be quite likely to occur. The longer it took life to emerge in those circumstances the less likely it is for life to be 'inevitable'. I guess people would find it easier to accept the chances of similar lifeless conditions to occur on other planets.

46480.  Wed Jan 18, 2006 7:06 am

 Tas wrote: but it is very unlikely that another planet has not got similar conditions enough for life not to have spawned in the same way that this one has

Really? What are you basing that particularly bold claim on?

There are nine planets (arguments about Pluto aside) in our solar system and only one has the correct conditions for life as we know it. Even the planets either side, which are only slightly nearer or further from the Sun, have completely different environments which make life unsupportable. The so-called "Goldilocks" region (where conditions are neither too hot nor too cold but just right) in extremely narrow.

There are also theories which state that life was able to devlop due to the lack of asteroid impacts on the Earth caused by having a very large planet (Jupiter) with a gravity well deep enough to help hoover up all the stray material left over from the formation of the solar system. This theory states that, if Jupiter didn't exist, it's likely that conditions on the Earth would not have supported life.

All of this implies that it's fantastically lucky that the conditions on Earth were just right for life. Even given the size of the Universe, there are no guarantees that the same conditions have arisen anywhere else.

 QI Individual wrote: If that period was relatively short that could indicate that the emergence of life could be quite likely to occur.

I draw the reader's attention to the number of incidences of the word "could" in that sentence :)

Though before you all start yelling at me, perhaps I should point out that I do actually believe that life may well have developed elsewhere in the Universe, though I'm extremely sceptical as to the possibility of intelligent life. However I think it's important to understand that this view is completely based on faith and gut feeling and has no hard scientific evidence to back it up. At least, not yet.

46481.  Wed Jan 18, 2006 7:10 am

 Quote: Really? What are you basing that particularly bold claim on?

 Quote: However I think it's important to understand that this view is completely based on faith and gut feeling and has no hard scientific evidence to back it up. At least, not yet.

errr.....there ya go

:-)

Tas

 46497.  Wed Jan 18, 2006 8:24 am Fair enough. I was just making sure we were on the same page :)

 46504.  Wed Jan 18, 2006 9:19 am The Earth was bombarded by asteroids for a very long time after its initial formation, and then again a billion or so years later (The Late Heavy Bombardment), and the available evidence suggests that life got started very soon after that ceased, possibly as a result of comet-borne hydrocarbons. According to the best current theoryThe Moon was formed by a huge impact with Earth. Might make a good question: What is the Moon made of? A: The Earth. http://archives.cnn.com/2000/TECH/space/12/07/lunar.cataclysm/ http://nai.arc.nasa.gov/students/focus1199/

46509.  Wed Jan 18, 2006 9:42 am

 Quote: A: The Earth.

Theoretically!

:-)

Tas

 46519.  Wed Jan 18, 2006 10:47 am Also to be taken into account is the 'Rare Earth Hypothesis'. The book 'Rare Earth: Why Complex Life Is Uncommon' in the Universe by palaeontologist Peter Ward and astronomer Donald Brownlee uses an extended Drake equation to answer the Fermi paradox. This includes such arguments as the right kind of star being required (not too bright not too dim) with enough metalic compounds to allow a terrestrial planet to form a stable orbit within the habitable (goldilocks) zone. It isn't only the planets orbit which must be just so but the star's own orbit around the galactic centre cannot take it too close in or the entire system will be exposed to massive radiation. Our sun is a 'G' Type and these account for only 5% of the stars in our galaxy, the majority being Red Dwarfs (cooler) which are unlikely suitable as to be within the goldilocks zone of these would expose a life-supporting planet to frequent solar flares and higher radiation. Stars that are too hot may burn out too quickly to allow intelligent life to develop. On top of all of this we have the Giant-Impact theory which deals with the formation of our moon (The real one, not any of these pretenders that lurk hidden). This theory states that a young Earth was impacted by a Mars sized planet early on which chucked up enough debris to form what we now call the Moon. The Moon is thought to not only stabilise our orbit and tilt but also act as an extra defense shield against more frequent impacts. It is thought that the conditions must have been just right to allow the moon to form after this Giant Impact or else both planets would have been destroyed in the collision. Just like this: All of this is one of the reasons why it is so important to discover if life ever existed on Mars and if so, did it develop independantly. This would show that life is somewhat inevatable and all of these theories would be out of the window, somewhat. The whole argument lacks evidence one way or the other.

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