- #36

rbj

- 2,227

- 10

rbj said:ask yourself what is the probability, knowing that lifecanemerge somewhere (because it has, at least at one place) in the galaxy, what is the probability that it has never emerged anywhere else in the galaxy. 100 billion stars out there. leaving our sun out of it (because we know what the answer is for that star), if the probability of life emerging on some planet around any particular star is very small,but not zerosince we exist, then the probability of lifenotemerging on a planet around any particular star is not quite equal to 1. so it's 0.9999999999999999999999999999999999 or something like that. now multiply that probability times itself 100 billion times and see how close to 1 it remains and if it is still so likely thatno where(else) life has ever emerged.

it doesn't take a very large probability coming out of the Drake equation for it to be more likely than not that life has emergedsomewhereelse atsome timein the past or the future. not terribly likely to be within 10 or 100 lightyears from us. and not terribly likely that they would become intelligent enough to send out radio signals that would be detected by us in the sliver of time our species would be listening. andveryunlikely that we'll ever detect evidence of their presence since we will not measure any radio signal from them if they live halfway across the Milky Way (they would have to be within 100 lightyears, unless they send Morse code by detonating very large H-bombs in space - they got to compete with the radiant output of stars).

RoshanBBQ said:How on Earth are you coming up with these probabilities

Whovian said:I think that was just a guess, for the sake of example

RoshanBBQ said:Well, if it's just an arbitrary guess, what good is it?

it's good for framing questions. or re-framing questions. would you like me to do that?

so let's say that if you pick

**any**star you see in the Milky Way at random (but leave out

*our*sun, since we know what the outcome is for that particular star).

so the Drake equation (copied from Wikipedia) is:

[tex]N = R^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L[/tex]

where:

*N*= the number of civilizations in our galaxy with which communication might be possible;

and

*R*

^{*}= the average rate of star formation per year in the Milky Way

*f*= the fraction of those stars that have planets

_{p}*n*= the average number of planets that can potentially support life per star that has planets

_{e}*f*

_{ℓ}= the fraction of the above that actually go on to develop life at some point

*f*= the fraction of the above that actually go on to develop intelligent life

_{i}*f*= the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

_{c}*L*= the length of time for which such civilizations release detectable signals into spaceso, just to keep the discussion simpler, i want to fold a bunch of probabilities together into a single probability:

[tex] p = f_p \cdot n_e \cdot f_{\ell} \cdot f_i [/tex]

that appears to me to be about the right expression for the probability that if you pick any old star out there, there will be intelligent life

*at some time*around it. let's all agree that this probability is very, very small. like, for the sake of illustration, it's 0.0000000000000000000000000000000001. i just pulled the number out of my butt.

this means that the probability that, if you pick some star outa the sky at random, the probability that

**no**life has ever existed on any planet around that star is 1-

*p*= 0.9999999999999999999999999999999999 which is less than 1.

now, how many times can you multiply (1-

*p*) times itself to get below 1/2? that is when the likelihood that there are no ETs out there is less than the likelihood that there are. since there are 100 billion stars in the Milky Way, even if

*p*is that small, the likelihood that there are ETs

*somewhere*in the Milky Way still comes out pretty close to 1.

so then invert the question and ask what must

*p*be in order to get the total probability to 1/2?

[tex] (1-p)^{100000000000} = \frac{1}{2} [/tex]

*p*doesn't have to be very large for it to be more likely there are ETs than not.