# 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

 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:

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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