What is the probability of life in Universe?

Dmitry67

In infinite Universe the probability is irrelevant. I suspect that probability of life is about something 10**-200. But even in such case there are infinitely many planets with life - beyond the cosmological horizon

brother time

I suspect that life is fairly likely (75%) to exist. But I am wondering what is the definition that classifies as life? And isn't life not requiring oxygen and can form based on any element? I did a report on diatoms and found that some in the Antarctic only require sulfur and iron to live.
Cheers, BT

Gannet

In the section Prospects for Life in the {Milky Way} Galaxy
http://www.tufts.edu/as/wright_cente..._1_future.html

The following chart is shown (see attachment)

Attached Images

Figure 8-24.bmp (296.8 KB, 16 views)

phyzguy

The problem with this figure is that the size of the boxes are complete guesses. We have a pretty good idea of the number of stars with planetary systems, but most of the other probabilities, such as:
(1) the probability that a planet is habitable, or
(2) the probability that if it is habitable that it has life, or
(3) the probability that if it has life that it has intelligent life

are complete guesses. Today we have no idea if these probabilities are 1 in 100 or 1 in 10^100.

DaveC426913

Well not quite. We know that the worst the odds can possibly be at the highest level are 1 in 10^11 (one example in 500 billion stars).

phyzguy

How do we know that? Are you basing it on the fact that there is intelligent life on Earth? We cannot draw conclusions of the probability of an event based on one example.

DaveC426913

Your wording was a bit ambiguous.

You were suggesting that the chances of any given planet in our galaxy being habitable could be as low as 1 in 10^100. I am simply saying that the odds for life on a a planet in the galaxy cannot be worse than 10^11, because in the sample 500 billion so far, we've found one example.

i.e. There are 10,000 clover plants in a field. You claim the odds that one of them is a four-leafed variety could be 1 in 10,000,000,000. I'm saying "Nope, I already have one from this field. That means the odds at worst are one in 10,000."

I think what you are trying to say is "the odds of finding another four-leafed variety could easily be 1 in 100,000,000." Which is true, it's just an odd way of calculating probabilities. Kind of like every time you get a positive hit, you toss it away and start counting again.

Dmitry67

?????
This is true only if most of galaxies have life at least on one planet!
You are assuming that the choice of our galaxy is random, which is not of course (anthropic principle) - the sample is not fair - even worse, for obvious reasons this sample is guaranteed to be unfair until you find planets except our own

I claim that that probablility is 10**-200
This number is not only consstent with the data we have, but it is even supported by the observational data - http://en.wikipedia.org/wiki/Fermi_paradox - so called 'Great Silence'

DaveC426913

It hasn't been an assumption. We have been talking only about our galaxy.

phyzguy

You simply can't make probability arguments like this based on one data point. What you are saying is equivalent to the following reasoning:

(1) I deal out two poker hands. One is a royal flush.
(2) Therefore the odds of dealing a royal flush are at least 50%.

DaveC426913

This analogy is flawed. A deck of cards is shuffled and then redealt. Our galaxy is not.

To keep your deck of cards analogy, we would say: this instance of the cards has dealt a royal flush, thus the probability of this instance dealing out a royal flush cannot be worse than 5 in 52 (the odds are, however, better than that, since there's 47 undealt cards that could still make a RF).


Is it possible that you are talking about galaxies, whereas I (and I thought everyone else) is talking about galaxy?

Dmitry67

Our galaxy is SPECIAL because WE are there.
Imagine that probability of life is 10^-200 per planet
So only one of 10^-189 galaxies has life
Observer in that galaxy (based on your logic) would conclude that life is very likely event - because in his sample (one galaxy) there is life.

phyzguy

There are three problems with your argument. First, we have not examined 500 billion star systems and determined that only one has life. We've only examined one star system. Second, by your argument, why stop with only the galaxy? Why not include the entire observable universe, with ~ 10^23 stars? We haven't found any life anywhere. Third, even if we had examined 500 billion star systems, the fact that we only have one example means that the uncertainty in the estimate (which is proportional to the square root of the number of observations) is equal to the estimate itself. Back to my royal flush analogy. If I have two poker hands, and one is a royal flush, I can say that the odds of a royal flush are 50% +/-50% - in other words I know nothing about the odds of a royal flush. That is the situation we are in.

DaveC426913

But we have only been talking about our galaxy.

I must call a point of order here. While this thread is titled "life in the universe", the discussion has evolved since post 54 to "life in our galaxy".

In retrospect, this is not necessarily the shift everyone else has made (though I have been inserting it in every one of my posts), so I state retroactively that my discussions about odds are most definitely only applicable to our galaxy.

That is not a flaw.

If I turn over the top card of a deck and it's an ace, I can say with certainty that the odds of this deck containing aces are no worse than 1 in 52 (even if the chances are much higher than that).

Also not a flaw.

That is the premise of the argument we are in. My odds are based on this galaxy and this galaxy alone.

If you wish to set a different premise, I have no problem with that. The odds will change completely.


Hang on. You've twisted the analogy. You're not comparing apples to apples.

We're not talking about a hand; we're talking about the deck. The odds of a royal flush in the current configuration of cards cannot be 0; it is actually 1.

Deck = our galaxy
Hand = one planet

You are correct that the next hand that is dealt (i.e. the next planet we check) is not affected by the fact that the first hand dealt (the first planet we checked) turned up positive.

I was not saying otherwise.

Dmitry67

*OUR* galaxy is special because we are here.
Even we know that probability of life in OUR galaxy is 1 (at least 1 inhabited planet exists) it does not give us anything.

Returning to your statement:

If we randomly pick any planet in OUR galaxy, the probability that there is life on is >10**-11 - TRUE

If we randomly pick any planet, the probability that there is life on is >10**-11 - FALSE

In general, this number (10^11) is absolutely useless, because it does not give us any estimation about how rare is the life in the Unvierse.

phyzguy

I agree completely. Stated another way, as you said, it is true that the odds of some planet in our galaxy having life is >10^-11, since there is life on Earth. However, this tells us nothing about the odds of there being life on some OTHER planet in our galaxy, which is of course what everyone wants to know.

Returning to your deck of cards analogy, it is correct, as DaveC said that if the first card is an ace, that then we know there is at least one ace in the deck. However, this tells us NOTHING about how many OTHER aces are in the deck.

DaveC426913

Agreed. This whole side-thread came about because someone said the chances of life could be 1 in 10^100. There was some ambiguity about the scope that was applying to.