# Life on Earth

saltydog
Homework Helper
Thanks. I'll spend some time with it.

pattylou said:
I really don't buy the initial premise in the thread that life was too improbable to have occured "by chance."
"What marvelous adaptive tendencies of diversity in chance-fluctuating environments. Such is a beautiful expression of Quantum Mechanical favor"

vanesch
Staff Emeritus
Gold Member
pattylou said:
And since the planet had 500million years or so to work on it, I really don't buy the initial premise in the thread that life was too improbable to have occured "by chance."
The difficulty I have with the statement that life "is quite probable" is: where are the little green men then ?

Let us call L the probability per unit hypercube of spacetime (in units of (10 billion light years)^3 x 10 billion years) to devellop life, including all factors (types of stars, planets, density of stars in the universe, presence of clay...). I take it that if life devellops, then also intelligent life devellops with not too small a probability.
Given the unit I chose, L is also about the probability to find a planet with life on it in the visible universe.
Now, L can take a priori all values: it can be 10^1220 or it can be 10^(-55230). But if we know enough cosmology, planetology, exo geology, chemistry and biology, we can calculate L from a few physical constants (the fundamental constants like the speed of light, planck's constant and so on and a few cosmological parameters such as the matter density in the universe, its density fluctuations etc...), so L is just a physical quantity that is in principle calculable (although it is extremely difficult to do so in practice). The important point is just that L is a quantity that is determined by the laws of nature and a few cosmological parameters (that give us the expected number of stars, their kinds, the distribution of planets, their geology etc...).

If L is a big value, then the universe is FULL of life, so the question is: WHERE ARE THE LITTLE GREEN MEN ?
If L is a very small value, then the odds of life having develloped in the visible universe is rather small, at odds with what we observe, namely that we are here. It is in this range that the anthropological principle has a meaning: amongst zillions of "possible" universes, we picked (of course) one in which we exist."

However, the arguments here seem to imply that L is of the order of unity: the number of times for life to devellop in the visible universe over about 10 billion years independently is a small positive integer: we are not invaded by little green men all the time, and nevertheless we are here. So of all possible values of L, it turns out that L can only take on values in a rather narrow range. So it seems that our universe has by coincidence those right laws of nature (which determine the constant L) for it to fall in this narrow range. Such a coincidence is usually called: parameter fine tuning, and considered a bad thing (there's a remarquable coincidence that one doesn't understand when such a thing happens).

I believe we are not invaded by little green men, due to the vastness of space.

I also believe there is a problem with the notion that life will certainly give rise to intelligent life. I have not seen good speculation on this one way or the other, but consider that for life to be intelligent, it must first be multicellular. For life to be multicellular, it must be able to perform some serious catabolism in order to harvest necessary energy for organising itself ---- in practical terms, I know of no strictly anaerobic multicellular organisms, for example. Aerobic respiration allows more energy harvest per glucose, and requires an oxygen atmosphere.

If (and this is a big if) an oxygen atmosphere is required for the emergence of multicellular life, then certain conditions muct be present on the planet - Oxygen is tough to make and retain (it's very reactive), and plants (and algae, etc) can do it because of being able to harvest energy from the sun.

To sum up the above, although it may be possible that the emergence of simple life is likely to lead to intelligent life, I don't know that this is a foregone conclusion.

Now let's also consider time scales. Life got going here on Earth pretty darn quick. Our technology, which allows radio waves, such as SETI has looked for, is quite recent indeed. There is no reason, as far as I am aware, to assume that aliens will have had sufficiently more time than us, to develop space travel. Any intelligent life "out there" may be roughly where we are, in terms of development. Consider that the nearest star, Vega?, is 25 years away. (I don't know the distance of the closest Sol type star. Anyone?) We earthlings started emitting "intelligent" radio waves in the last 100 years, so think about this - there are precious few stars that would even be able to detect *our* intelligent life yet.

Finally, given the fabulous formaldehyde signatures from Mars reported by the ISA last year, as well as other indications, I believe it is likely we will find simple life on Mars.

-patty

vanesch
Staff Emeritus
Gold Member
pattylou said:
I believe we are not invaded by little green men, due to the vastness of space.

I also believe there is a problem with the notion that life will certainly give rise to intelligent life.
In fact, this doesn't change the argument. Call L then simply the probability density of intelligent life to devellop in the visible universe in about 10 billion years, with all the necessary conditions of getting up to radiobeaming up to the earth.
It is very odd that that probability is a number of order unity ; my same arguments go: if it is much bigger, we have little green men on the TV, and if it is much smaller, we shouldn't be here.

Of course, what has changed is that indeed, the universe might be full of non-intelligent life (intelligent in the sense of technologically able to send TV pictures across the universe), say with density K, which is much bigger than 1 (per hypercube 10billion (light) years ^4), and a tiny fraction (epsilon) of that only is part of the conditions cited above. But again, the funny thing is then, that the probability of going from this (very abundant) non-intelligent life to "radiobeaming life" is JUST FINETUNED so as to yield an L of order unity, because L = epsilon x K

That's the kind of coincidental finetuning which is suspect.