How many intelligent civilizations are out here from a scientific perspective? [Archive]

HumanityBlues

23rd December 2009, 12:17 AM

Here's my thinking (from a non scientist): Size of space+frequency of life sustaining conditions=99.99repating to infinity chance that there are intelligent civilizations throughout the universe, and there are many of them. Could be wrong, but for god's sake, we found water on the freakin moon, mars had water....

So..... someone smarter than me...what are the odds?

I believe Stephen Hawking said that intelligent civilizations probably existed, but that they would never come in contact with civilizations light years away because they would destroy themselves through nuclear war before ever becoming advanced enough to do so.

EDIT: I made a typo in the title for the thread. Should say "out there" not "out here". Damn it.

athon

23rd December 2009, 12:47 AM

Honestly, we can't say. Not even guess.

Firstly, we have absolutely no idea on the possibility of life. We can make some guesses on possible chances of the right temperature, water and complex chemistry, but beyond that any number we come up with is not even a guess, making it useless for anything.

The next consideration would be the possibility of life capable of developing something akin to a nervous system. Again, we can't even guess on what the possibility is. Any number would be a wild throw of the dice.

As for intelligence, I can only assume you mean human-like intelligence. After all, cephalopods independently evolved pretty superior intelligence, but it's nothing like ours because not all forms of intelligence is the same. Ours is more social, for instance and approaches problems in a different way.

So, we can get as far as deducing how common planets and moons with water, the right temperature and right chemistry are...and then we can't even come up with anything that is a useful figure. The range of possibilities would vary by several factors. For all of the information we have at hand, it's as possible that we are the only ones as it is that there are currently several hundreds of billions of sociably intelligent life forms that have or are 'currently' (a rather useless term in itself, given the nature of physics) in existence.

I don't mean to piss on your parade. :) Seriously. Guessing passes the time. But science relies on empirical data on which to base predictions. There simply isn't enough of it to make any answer more reasonable than another.

Athon

wollery

23rd December 2009, 12:56 AM

It's almost impossible to calculate the odds, for two reasons.

First, supposing that life has to be like it is on Earth, i.e. needs a roughly Solar type star, roughly Earth sized planet, liquid water, oxygen atmosphere, etc. Now you have to calculate how many Solar type stars there are, how many of those have an Earth-like planet, how many of those have liquid water, and so on. That gives you a number with the potential for life. Unfortunately, apart from a rough estimate of the number of Solar type stars, we have absolutely no idea about the numbers for the rest of the problem.

Further complicating things is that probably not all of these will harbour life. We don't even know if any will. On top of that, assuming that life does get started, you then have to work out the probability of intelligent life evolving on such a planet.

For a more detailed explanation read up about the Drake Equation (http://en.wikipedia.org/wiki/Drake_equation).

Secondly, this all assumes that life requires the particular set of circumstances that we have here on Earth, but that may not be the case. Until we get out there and find life on other planets, in other solar system, we won't have any idea. The Universe is big, and contains a very diverse range of stars and planets.

So the answer, from a research Astrophysicist (me) is, "we have no idea".

HumanityBlues

23rd December 2009, 12:58 AM

HumanityBlues

23rd December 2009, 01:00 AM

So the answer, from a research Astrophysicist (me) is, "we have no idea".

Ok, well as a research Astrophysicist, what do you think personally?

wollery

23rd December 2009, 02:26 AM

Well, you've certainly pissed on my parade, but honorably and intelligently.

You make fair points. Here's my thinking. We at least know that life has developed on a planet once--and that planet is obviously Earth. We know at least a certain condition a planet can be in where life is sustainable based on Earth. So perhaps the first starting point to creating such a probability would be to estimate how many Earth like planets there might be in the Universe. The answer is probably uncountable.

There must be enough empirical data to come up with something. Do you really think the probability that we are the only intelligent life is the same as the probability that there is not?There is empirical data on the number of Solar type stars. We know how many there are as a proportion of all stars, so we can roughly count the number of Solar type stars in our Galaxy. And that's about it so far.

We don't know what proportion of those have Earth-like planets, since our searches for these are in the very early stages. There have been a small number of "super-Earths" found, but only one (see the thread on "Large watery planet found" (http://forums.randi.org/showthread.php?t=162407)) that's in the habitable zone, so obviously we don't know the proportion of Earth-like planets that are in the habitable zone.

We also only have one planet on which we know life exists, so we can't do any statistical analysis with that either.

So really, when I say we don't know, I mean that we haven't a clue. Not the faintest notion. We don't even have enough data to take a wild guess.

All we know for certain is that it's possible.

Ok, well as a research Astrophysicist, what do you think personally?Personally I think that there is almost certainly life on other planets, some of it is probably intelligent, possibly more technologically more advanced than us. In an infinite Universe it seems ridiculous to me that only one planet harbours intelligent life.

athon

23rd December 2009, 03:19 AM

Of course, size of the universe also needs to be put into perspective.

People have a habit of thinking of the universe as just a big place with lots of stuff in it. But that size is nothing to be flippantly dismissed - the fact that information takes time to move across it makes the notion of life in distant galaxies about as important as life in a parallel universe. Both make for interesting sci-fi novels, but just as it's unlikely given current technology to ever have anything to do with 'parallel' aliens, aliens that might have existed several billion years ago (and hence several billion light years away) will probably never make their presence known, let alone be encountered.

Again, let me be clear - there's nothing wrong with a good science fiction story. I love them, personally, and have no problem speculating. But for much of it, the possibility of ever encountering intelligent aliens is essentially close to zero thanks to simply the sheer scale of the universe.

Athon

alexi_drago

23rd December 2009, 03:32 AM

Mojo

23rd December 2009, 03:39 AM

EDIT: I made a typo in the title for the thread. Should say "out there" not "out here". Damn it.

[Insert obligatory Monty Python reference here]

wollery

23rd December 2009, 04:05 AM

From looking up wiki, it says that the time of the appearance of the first self replicating molecules on earth to our current form today has taken almost 1/3 of the age of the universe. This would suggest to me that the chances of intelligent civilisations coming and going throughout the universe is not nearly as likely as sci fi suggests.

A question I have is about our solar system, does life as we know it depend on our sun being a heavy element rich star? If yes, how long would it take for such stars to appear after the big bang?Current theories of planet formation say that, yes, the star does have do be heavy element rich, or high metallicity (in astronomy, anything other than Hydrogen is considered a metal :rolleyes:).

This is because you need heavy elements in the accretion disc in order to make an Earth-like planet, and also to make the necessary molecules for life as we know it.

TobiasTheViking

23rd December 2009, 04:24 AM

[Insert obligatory Monty Python reference here]

For once, i really have no clue what the reference is....

alexi_drago

23rd December 2009, 04:34 AM

Current theories of planet formation say that, yes, the star does have do be heavy element rich, or high metallicity (in astronomy, anything other than Hydrogen is considered a metal :rolleyes:).

This is because you need heavy elements in the accretion disc in order to make an Earth-like planet, and also to make the necessary molecules for life as we know it.

So, is there an earliest point in the history of the universe that life as we know it could not have existed with respect to the elements that existed at that time?

Alan

23rd December 2009, 04:35 AM

For once, i really have no clue what the reference is....
The Galaxy Song?

Bikewer

23rd December 2009, 04:56 AM

Gawdzilla

23rd December 2009, 05:03 AM

Dave Rogers

23rd December 2009, 05:18 AM

In case nobody else does:

http://xkcd.com/384/

Dave

wollery

23rd December 2009, 05:45 AM

So, is there an earliest point in the history of the universe that life as we know it could not have existed with respect to the elements that existed at that time?I'm not a stellar evolutionary scientist, so I can't answer that with any authority.

But I will say that you need at least 3 generations of stars.

First you need what are called Population III stars. These are virtually zero metallicity, i.e. almost nothing but hydrogen. Theoretically they only form as very massive stars and are nothing like the stars we know - they don't burn so much as explode almost instantly, which in astronomy terms means less than 1 million years. They end their lives as supernovae, throwing the heavier elements they've produced back into the interstellar medium. No Population III stars have ever been directly observed.

Next come the Population II stars. These are low metallicity stars, formed from the remains of the Population III stars. There are plenty of Population II stars around in the Galaxy, although most are low mass because high mass stars burn their fuel quicker, and end their lives as supernovae and novae, again throwing more metal rich gas back into the interstellar medium.

The remains of the high mass Population II stars then form the high metallicity Population I stars, of which the Sun is a member. And it's these Population I stars that you need to form rocky planets and life.

So, first you need to get through 2 populations of stars, which takes time because they have to form, burn their hydrogen and helium, explode, form new clouds of gas, and go through the whole process a 2nd time, plus forming the 3rd generation of stars and their planets. All of this requires the rather slow process of gravitational collapse of gas clouds.

My rough estimate (and remember, this isn't my specialism) would be at least 5 billion years, and probably as much as 7 or 8 billion years, before you could get a fully formed rocky planet around a stable main sequence Solar type star.

Then you can start to think about forming life.

Of course, the above is a rather simplistic version of events, as within each population there is a range of metallicities as stars form continuously with each addition of extra metals into the interstellar medium.

alexi_drago

23rd December 2009, 06:08 AM

I'm not a stellar evolutionary scientist, so I can't answer that with any authority.

But I will say that you need at least 3 generations of stars.

First you need what are called Population III stars. These are virtually zero metallicity, i.e. almost nothing but hydrogen. Theoretically they only form as very massive stars and are nothing like the stars we know - they don't burn so much as explode almost instantly, which in astronomy terms means less than 1 million years. They end their lives as supernovae, throwing the heavier elements they've produced back into the interstellar medium. No Population III stars have ever been directly observed.

Next come the Population II stars. These are low metallicity stars, formed from the remains of the Population III stars. There are plenty of Population II stars around in the Galaxy, although most are low mass because high mass stars burn their fuel quicker, and end their lives as supernovae and novae, again throwing more metal rich gas back into the interstellar medium.

The remains of the high mass Population II stars then form the high metallicity Population I stars, of which the Sun is a member. And it's these Population I stars that you need to form rocky planets and life.

So, first you need to get through 2 populations of stars, which takes time because they have to form, burn their hydrogen and helium, explode, form new clouds of gas, and go through the whole process a 2nd time, plus forming the 3rd generation of stars and their planets. All of this requires the rather slow process of gravitational collapse of gas clouds.

My rough estimate (and remember, this isn't my specialism) would be at least 5 billion years, and probably as much as 7 or 8 billion years, before you could get a fully formed rocky planet around a stable main sequence Solar type star.

Then you can start to think about forming life.

Of course, the above is a rather simplistic version of events, as within each population there is a range of metallicities as stars form continuously with each addition of extra metals into the interstellar medium.

Thanks, that means that only for the last 8 billion years or so there's been rocky planets capable of supporting life as we know? and that it's taken about half that time for life to develop from the first self replicating molecules to what we are today?

I used to be under the impression that life on earth had been around for a much much smaller fraction than that and that countless intelligent civilisations could have risen and died before, probably due to sci fi stories and ideas, but this now makes me think life on earth is not as insignificant as I thought previously and that life might not be all that abundant elsewhere.

EDIT: I suppose there's the 4 billion years before we kicked off but it doesn't quite give me the same feeling.

Gawdzilla

23rd December 2009, 06:18 AM

Thanks, that means that only for the last 8 billion years or so there's been rocky planets capable of supporting life as we know? and that it's taken about half that time for life to develop from the first self replicating molecules to what we are today?

I used to be under the impression that life on earth had been around for a much much smaller fraction than that and that countless intelligent civilisations could have risen and died before, probably due to sci fi stories and ideas, but this now makes me think life on earth is not as insignificant as I thought previously and that life might not be all that abundant elsewhere.

EDIT: I suppose there's the 4 billion years before we kicked off but it doesn't quite give me the same feeling.
If aliens had visited Earth during the first 3/4 of the time life has been here they would have just found cyanobacteria-like organisms. Single celled scum. It wasn't until two of those decided to get together that things got moving.

Arthur Denton

23rd December 2009, 06:28 AM

alexi_drago

23rd December 2009, 06:38 AM

If aliens had visited Earth during the first 3/4 of the time life has been here they would have just found cyanobacteria-like organisms. Single celled scum. It wasn't until two of those decided to get together that things got moving.

Compared to the development of life on all the life supporting planets that we know of, did that take longer or shorter than average to occur on earth?

EDITED:

Gawdzilla

23rd December 2009, 06:45 AM

Compared to the development of life on all the life supporting planets that we know of, did that take longer or shorter than average to occur on earth?

EDITED:

On the non sequitur scale I'd give that a 9.

wollery

23rd December 2009, 06:55 AM

On the non sequitur scale I'd give that a 9.On a nonsensical question scale of 1 to 10 I'd give it an 11.

Cainkane1

23rd December 2009, 06:57 AM

alexi_drago

23rd December 2009, 06:58 AM

On the non sequitur scale I'd give that a 9.

That was the point, you could say the same of your post. ok, we started off with our first few billiion years as a puddle of slime, is that or isn't that going to be the case for life elsewhere or would it take a longer or shorter time due to conditions, was there a particular event or condition that triggered something to happen to the slime or was it continually evolving so at some point it would be able to have a couple of them get together?

alexi_drago

23rd December 2009, 07:02 AM

Whichever it is, that slime is no less part of the evolution of life on earth than any other stage.

wollery

23rd December 2009, 07:13 AM

That was the point, you could say the same of your post. ok, we started off with our first few billiion years as a puddle of slime, is that or isn't that going to be the case for life elsewhere or would it take a longer or shorter time due to conditions, was there a particular event or condition that triggered something to happen to the slime or was it continually evolving so at some point it would be able to have a couple of them get together?As before, the answer is, probably, but we have no idea, because we have no data.

Sorry.

Gawdzilla

23rd December 2009, 07:23 AM

That was the point, you could say the same of your post. ok, we started off with our first few billiion years as a puddle of slime, is that or isn't that going to be the case for life elsewhere or would it take a longer or shorter time due to conditions, was there a particular event or condition that triggered something to happen to the slime or was it continually evolving so at some point it would be able to have a couple of them get together?

Mine post was an observation about the likelihood of life advancing beyond the single cell stage and the amount of time the one known instance took in relation to the total amount of time for life on the planet. It relates directly to the OP.

Matilda

23rd December 2009, 07:25 AM

Another point to consider... Of all the millions of species that have lived on the Earth, both present and extinct...How many have been "intelligent"?

The odds against what we would consider intelligence evolving is considerably higher than life itself, I'd imagine.

That's a really good point. And on top of that, humans took a very long time to develop civilisation.

Though once the environmental conditions were there, we developed civilization multiple times. So even if a species has the potential to create civilization, it may not have the right circumstances to do so.

orange31

23rd December 2009, 07:26 AM

42

Close. Actually the answer is 24.

http://www.youtube.com/watch?v=RkP_OGDCLY0

alexi_drago

23rd December 2009, 07:34 AM

Mine post was an observation about the likelihood of life advancing beyond the single cell stage and the amount of time the one known instance took in relation to the total amount of time for life on the planet. It relates directly to the OP.

Sorry, you'd replied to a quote of mine about earth life being around for half the time that life could possibly exist in the universe by saying that 3/4 of that was spent as scum. I thought you were dismissing the scum period as insignificant.

Gawdzilla

23rd December 2009, 07:35 AM

Sorry, you'd replied to a quote of mine about earth life being around for half the time that life could possibly exist in the universe by saying that 3/4 of that was spent as scum. I thought you were dismissing the scum period as insignificant.

At 3/4 of the time life has been here, I wouldn't do that. Sorry for any confusion.

Perpetual Student

23rd December 2009, 07:47 AM

I'm not a scientist by any stretch of the imagination but right here in our solar system there are many might have beens when it comes to life. mars almost made it. Europa might have life as might Titan. Moons orbiting planets like Saturn and Jupiter could have developed life and even intelligent life. Water is very common in the Universe and there are many combinations and configurations of planets. Planets with water are being discovered which means to me that there are millions of undiscovered watery planets. There is not one reason that some of these planets can't be harboring intelligent beings. Some are gone but others are still alive. We need to find out.

If we find a nice planet with water one day with no life on it we can fix the problem by settling there ourselves one day.

I think the above observations leads to the conclusion that life is pretty common. We have barely begun to explore planets outside of our solar system. The observable universe contains about 80 billion galaxies, each of which which contains an average 400 billion stars.
I also believe that the argument could be made that if the conditions for life exist (as in the early earth), life is inevitable.
How unique was the evolutionary path that led to a species capable of complex symbolic communication? How likely is intelligent life (capable of complex language)? That's the tough part!

Eos of the Eons

23rd December 2009, 08:37 AM

Gawdzilla

23rd December 2009, 08:43 AM

Oh, I thought this thread would be limited to civilizations on this earth. Then we could define "intelligent civilization" and go from there. :P

"Civilization? Great idea! When do we get one?"

I Ratant

23rd December 2009, 08:57 AM

As before, the answer is, probably, but we have no idea, because we have no data.

Sorry.
.
Other than that, I like to think there is life out there, but the physical restraints of distance would prevent any interaction, or... it has come and gone out there... or, it is still evolving.
There's just too many opportunities in all the galaxies for something living to have been generated.

LarianLeQuella

23rd December 2009, 09:00 AM

One thing that makes the Drake equation difficult to use is the amount of time that a civilization will "broadcast". Marconi sent "S" out just over a hundred years ago, and we're already moving away from broadcasting. It may be less than fifty years until we don't send out any significant radio waves. So this window must be considered when thinking about other civilizations. They could be using something unimaginable to us, like fiber optics would have been to Newton.

Well, there is that nasty inverse square law to deal with... Even with our best radio telescope, our leaked radio transmissions are barely detectable WITHIN our own solar system, let alone at the nearest star. http://forums.randi.org/showpost.php?p=4302936&postcount=226 And if you read that entire thread, you'll see a great number of silly assertions about the probablities against (and maybe a few on the probabilities for, but mostly against).

As others have said, we don't know.

I personally would like to think that yes, there is ETI out there. But actual proof or communication is beyond us. :(

Tim Thompson

23rd December 2009, 09:03 AM

So ..... someone smarter than me ... what are the odds?
As you might have figured out by now, any odds calculation would be arbitrary and meaningless. Nobody knows enough about the problem to calculate odds independently from a raft of opinionated assumptions.

I think this is an educational look at the problem: SETI Debate - Carl Sagan vs Ernst Mayr, hosted by the Planetary Society. Optimist Sagan & pessimist Mayr debate the evolutionary value of "intelligence" (which is not a well defined concept in any case) and the odds of success in the Search for Extraterrestrial Intelligence (SETI).

My own take on the subject is simple enough: The most effective way to avoid finding any intelligent civilizations which may exist is to avoid looking for them.

LarianLeQuella

23rd December 2009, 12:32 PM

In case nobody else does:

http://xkcd.com/384/

Dave

Don't forget http://xkcd.com/638/

skeptsci

23rd December 2009, 02:15 PM

. Until we get out there and find life on other planets, in other solar system, we won't have any idea. The Universe is big, and contains a very diverse range of stars and planets.

So the answer, from a research Astrophysicist (me) is, "we have no idea".
And from a biologist point of view I'll add to all the above the problem of "life" definition: If you do manage to get there and find "something" on what criteria are you going to judge it to be "alive", it's not as simple as it might seem in a first thought. Also, Drake type of calculations are mice try. However, it is possible that given all the "right" conditions for life emergent, inteligent development and all, probabilities for these events are so low that they have happened only once in the history of this universe. Go figure.

Hindmost

23rd December 2009, 06:37 PM

so far, the evidence points to zero (0).

glenn

kuroyume0161

23rd December 2009, 07:47 PM

Drake's Equation already gives an 'answer' to this: between 1 and 1,000,000. :D

The problem is that there are many variables, some of which we have no definitive information (yet). The reason for the wide range is that we must guess on those variables.

kuroyume0161

23rd December 2009, 07:49 PM

so far, the evidence points to zero (0).

glenn

QFT

Soapy Sam

23rd December 2009, 07:58 PM

If by out here (there) is meant "the western spiral arm of the galaxy" , my guess is 1.
If what's meant is the entire multiverse , suburbs and parking lots,... millions.
But really, who cares? If we can't talk to them, it makes no difference if they are there or not.
To talk to them, we require a coincidence of evolution and technological development in a very small volume of spacetime. ie either two races must evolve within radio distance of each other and do so at exactly the same time, or one race must be very long lived and like to preserve ancient communication technology. It's just so very improbable.

Unlikely though it is, I find it far more probable that intelligence has occurred on this planet in the distant past ( 30-200MYA say) and been snuffed out by some natural process. I doubt it could have been technological, as I think there would be the odd artifact surviving- but I see nothing inherently impossible in (for example) some Eocene or earlier lifeform getting as far as agriculture before failing. It just seems very unlikely to me that evolution took this long to develop intelligence once. There seems nothing about producing complicated nervous systems that is more difficult than producing wings, which have evolved several times independently.

I still think it possible we will one day find an alien obelisk on the moon. But I bet the critters who put it there, came from here.

portlandatheist

23rd December 2009, 08:59 PM

My unscientific guess is that based on Drake's equation, and the numerous unknowns that I would guess that there is life elsewhere in the universe. One of the supporting arguments of this is that life on Earth appeared virtually immediately after it was possible. It could be possible that life happened and was completely destroyed by meteor bombardment many times before things became more stable here on Earth in a life friendly planet.
My other wild guess and speculation is that even though life is probably scattered all over the universe, intelligent social life is probably very rare and the chances that intelligent social life on separate systems ever come in to contact with each other probably will never happen. FWIW, I imagine a universe with intelligent social life existing independently all over the universe but I also imagine that they never interact with each other. We are remote islands. This is both a reassuring and depressing thought at the same time.

Gawdzilla

24th December 2009, 05:00 AM

I personally would like to think that yes, there is ETI out there. But actual proof or communication is beyond us. :(
I hope there's intelligent somewhere in the Universe. ;)

I think the best thing we can hope for is some as yet unimagined system that hooks us into a "universal telephone exchange". Pure speculation that such is even possible at this point, but it's fun to imagine such might be available.

Molinaro

24th December 2009, 06:19 AM

There have been a small number of "super-Earths" found, but only one (see the thread on "Large watery planet found" (http://forums.randi.org/showthread.php?t=162407)) that's in the habitable zone, so obviously we don't know the proportion of Earth-like planets that are in the habitable zone.

That planet is not in the habitable zone. It's too close in to it's star. It can have liquid water only because of it's very dense atmosphere allows for water to remain in a liquid state at it's surface.

Gawdzilla

24th December 2009, 06:23 AM

That planet is not in the habitable zone. It's too close in to it's star. It can have liquid water only because of it's very dense atmosphere allows for water to remain in a liquid state at it's surface.

So it has been confirmed that there is no life on it?

skeptsci

24th December 2009, 10:23 AM

so far, the evidence points to zero (0).

glenn
Please elaborate. Your short comment hints on creationism. Is this your message to us?

Gawdzilla

24th December 2009, 10:29 AM

Please elaborate. Your short comment hints on creationism. Is this your message to us?

I think he was saying that there are NO intelligent being in the Universe at this time, present company included. As a veteran I tend to agree with him.

skeptsci

24th December 2009, 10:37 AM

QFT
Quantum field theory? Could you explain?

skeptsci

24th December 2009, 10:39 AM

I think he was saying that there are NO intelligent being in the Universe at this time, present company included. As a veteran I tend to agree with him.
L O L Gooood point!

Pixel42

24th December 2009, 10:45 AM

Quantum field theory? Could you explain?

http://en.wikipedia.org/wiki/QFT

"Quoted For Truth" - an acronym commonly used on internet forums.

Gawdzilla

24th December 2009, 10:47 AM

http://en.wikipedia.org/wiki/QFT

Also used as "Quite ------- True". (Apparently "ing" is a bad word.

Dorian Gray

24th December 2009, 11:07 AM

Let's develop a general rule, since it's unpredictable. In about 5 minutes, I came up with this one: We discovered the moon, let's say, 2 million years ago when some earlier hominid became aware enough of it, unless you want to use 3 or 4 million years ago. We landed on the moon 40 years ago. We looked for water and found some, but not nearly as much as we thought. The moon is something like 180,000 miles away.

Someone can take all this and make a formula. I can't, because 1) I just can't, and, well, I don't really need a 2. The formula would use the distance between the Earth and Moon, the time between us 'discovering' the Moon and landing on it, the time between landing on it and discovering water on it, and the paltry amount of water we found, to represent the likelihood that we will find life elsewhere in the universe, how much time it will take, and how scarce life out there actually is.

Just as good as anything, right?

Perpetual Student

24th December 2009, 11:16 AM

Regarding the Drake equation: The factors to the left of fi appear to be reasonably subject to approximation; fi and factors to the right are nothing more than meaningless guesses.

Gawdzilla

24th December 2009, 11:22 AM

Regarding the Drake equation. The factors to the left of fi appear to be reasonably subject to approximation; fi and factors to the right are nothing more than meaningless guesses.

Drake himself will say that it's unsolvable at this time. The point was to bring up the factors that should be considered in any consideration of intelligent life elsewhere. Nobody seriously expects to see a hard number.

quadraginta

24th December 2009, 11:33 AM

JoeTheJuggler

24th December 2009, 11:35 AM

so far, the evidence points to zero (0).

glenn

Not really. We've been having a similar discussion in the SETI thread under podcasts.

In this case, evidence of absence is not absence of evidence, so we simply don't know.

The analogy I've been making goes like this: I've got a dog sitting next to me here in my house. I'm wondering if any extra-my-house dogs might exist. I glance out the window for about 1 second and see no evidence of such dogs. I can't see any dogs or any doggie doo-doo at this distance, and my window is closed and I can't hear any dogs barking.

The fact that I wouldn't necessarily expect to see evidence of a dog from this 1 second glance and listen (and I'd only hear or see a dog if it were very very near my house) doesn't reasonably lead me to conclude that there are no dogs other than the one in my house.

So far, the evidence points to, "We don't know."

JoeTheJuggler

24th December 2009, 11:40 AM

Drake himself will say that it's unsolvable at this time. The point was to bring up the factors that should be considered in any consideration of intelligent life elsewhere. Nobody seriously expects to see a hard number.

Yup. I've always seen the value of the Drake Equation as a way of helping us sort out what we ought look for. I rather think NASA's Kepler mission takes a cue from the direction given in the Equation. (The number of relatively Earth-like planets is probably the first unknown factor reading from the left, now that we know planets are pretty common.)

Gawdzilla

24th December 2009, 11:42 AM

Yup. I've always seen the value of the Drake Equation as a way of helping us sort out what we ought look for. I rather think NASA's Kepler mission takes a cue from the direction given in the Equation. (The number of relatively Earth-like planets is probably the first unknown factor reading from the left, now that we know planets are pretty common.)

It just a starting point, a proposal. I think it was very cool for being impromptu, but then he's smarter than I am.

Perpetual Student

24th December 2009, 12:17 PM

Drake himself will say that it's unsolvable at this time. The point was to bring up the factors that should be considered in any consideration of intelligent life elsewhere. Nobody seriously expects to see a hard number.

From Wkipedia:

"Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were:

R* = 10/year (10 stars formed per year, on the average over the life of the galaxy)
fp = 0.5 (half of all stars formed will have planets)
ne = 2 (stars with planets will have 2 planets capable of supporting life)
fl = 1 (100% of these planets will develop life)
fi = 0.01 (1% of which will be intelligent life)
fc = 0.01 (1% of which will be able to communicate)
L = 10,000 years (which will last 10,000 years).
Drake's values give N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10."

Astronomical observations with advanced technology has given us a basis for beginning to estimate factors to the left of fi. The point is that those factors including and to the right of fi seem to be beyond our ability to estimate regardless of improved technology.

JoeTheJuggler

24th December 2009, 12:27 PM

Astronomical observations with advanced technology has given us a basis for beginning to estimate factors to the left of fi. The point is that those factors including and to the right of fi seem to be beyond our ability to estimate regardless of improved technology.

I disagree that we have any meaningful estimates for anything to the right of fp. Even fp estimates are only based on relatively recent discoveries of extra solar planets (and perhaps the principle that so far whenever we have a technique that can detect planets with certain characteristics--including things like happening to fall in a line of sight between its primary and the Earth--we have found them in abundance).

So far we still don't have any reasonable guess on ne.

Hindmost

24th December 2009, 01:08 PM

Please elaborate. Your short comment hints on creationism. Is this your message to us?

You are going to have to elaborate on how you infer creationism from my comment.

glenn

Hindmost

24th December 2009, 01:13 PM

Not really. We've been having a similar discussion in the SETI thread under podcasts.

In this case, evidence of absence is not absence of evidence, so we simply don't know.

The analogy I've been making goes like this: I've got a dog sitting next to me here in my house. I'm wondering if any extra-my-house dogs might exist. I glance out the window for about 1 second and see no evidence of such dogs. I can't see any dogs or any doggie doo-doo at this distance, and my window is closed and I can't hear any dogs barking.

The fact that I wouldn't necessarily expect to see evidence of a dog from this 1 second glance and listen (and I'd only hear or see a dog if it were very very near my house) doesn't reasonably lead me to conclude that there are no dogs other than the one in my house.

So far, the evidence points to, "We don't know."

The evidence points to zero...the "we don't know" part is because we can't see the entire universe so it leaves the door open to not knowing.

glenn

JoeTheJuggler

24th December 2009, 01:23 PM

The evidence points to zero...the "we don't know" part is because we can't see the entire universe so it leaves the door open to not knowing.

Sorry, you're wrong here. ETIs similar to our own civilization would be indetectable to us at interstellar distances but for the most extremely low probability occurrences (like thrusting your hand into a humongously gigantic haystack and finding a needle on the very first try).

The evidence points to "We don't know" and not to zero. What evidence is there that points to zero?

Is there any evidence that is inconsistent with the existence of ET intelligent civilizations in the universe?

Hindmost

24th December 2009, 01:41 PM

Sorry, you're wrong here. ETIs similar to our own civilization would be indetectable to us at interstellar distances but for the most extremely low probability occurrences (like thrusting your hand into a humongously gigantic haystack and finding a needle on the very first try).

The evidence points to "We don't know" and not to zero. What evidence is there that points to zero?

Is there any evidence that is inconsistent with the existence of ET intelligent civilizations in the universe?

I guess I disagree here..the evidence points to zero, the probability points to "we don't know."

glenn

Cainkane1

24th December 2009, 01:42 PM

I think the above observations leads to the conclusion that life is pretty common. We have barely begun to explore planets outside of our solar system. The observable universe contains about 80 billion galaxies, each of which which contains an average 400 billion stars.
I also believe that the argument could be made that if the conditions for life exist (as in the early earth), life is inevitable.
How unique was the evolutionary path that led to a species capable of complex symbolic communication? How likely is intelligent life (capable of complex language)? That's the tough part!
Think of the combinations. I bet every star in the Universe has planets. A dwarf star could have a watery planey orbiting around it and have life. A hugh star could have a planet way away from it in its orbit. Like I said in my post large planets like Jupiter and saturn could have moons with life. Some of the life on these planets should be intelligent.

JoeTheJuggler

24th December 2009, 01:49 PM

I guess I disagree here..the evidence points to zero,

So what evidence points to zero?

Perpetual Student

24th December 2009, 02:40 PM

I disagree that we have any meaningful estimates for anything to the right of fp. Even fp estimates are only based on relatively recent discoveries of extra solar planets (and perhaps the principle that so far whenever we have a technique that can detect planets with certain characteristics--including things like happening to fall in a line of sight between its primary and the Earth--we have found them in abundance).

So far we still don't have any reasonable guess on ne.

I said, "advanced technology has given us a basis for beginning to estimate factors to the left of fi," not that we have "meaningful estimates."

Hindmost

24th December 2009, 02:49 PM

So what evidence points to zero?

The fact that there is no tangble evidence that indicates there is intellient life...that's all.

The Drake equation is based on mainly on probability with some reasonable estimates for certain terms. Scientifically, there is zero evidence, but reasonable probabilty of life evolving.

glenn

JoeTheJuggler

24th December 2009, 04:59 PM

The fact that there is no tangble evidence that indicates there is intellient life...that's all.

What tangible evidence would you expect?

We can't even determine conclusively that there's no microbial life even in our own solar system yet. What would you expect to see as tangible evidence for intelligent life elsewhere in the universe?

Remember my analogy of glancing out the window for 1 second and failing to see evidence of the existence of dogs other than the one inside my house.

Hindmost

24th December 2009, 06:15 PM

What tangible evidence would you expect?

We can't even determine conclusively that there's no microbial life even in our own solar system yet. What would you expect to see as tangible evidence for intelligent life elsewhere in the universe?

Remember my analogy of glancing out the window for 1 second and failing to see evidence of the existence of dogs other than the one inside my house.

We seem to disagree on the concept of "no evidence" vs "probability" of life in the universe. Whatever reasonable indication of what you consider as evidence of extraterrestrial intelligence...I will accept that...and then show me any of that has been detected.

glenn

fuelair

24th December 2009, 06:29 PM

fuelair

24th December 2009, 06:35 PM

So, is there an earliest point in the history of the universe that life as we know it could not have existed with respect to the elements that existed at that time?

Yes - I don't know what it is, but it would be after the first stars + had died.

JoeTheJuggler

24th December 2009, 06:43 PM

We seem to disagree on the concept of "no evidence" vs "probability" of life in the universe.
No. You claimed the evidence points to the conclusion that no ETIs exist. I said the lack of evidence only points to the conclusion, "We don't know."

Whatever reasonable indication of what you consider as evidence of extraterrestrial intelligence...I will accept that...and then show me any of that has been detected.

No. You're the one claiming the existence of evidence that points to a conclusion. (I assume that you mean the absence of some evidence points to that conclusion, which is why I'm asking what expected evidence is missing.)

Again, consider my dog analogy. Does the absence of evidence point to the conclusion that no other dogs exist?

ETA: For the record, we know it's possible for intelligent civilizations to exist because we have an example of one. There is no reason to think that it's only possible here and nowhere else in the universe.

kuroyume0161

24th December 2009, 06:47 PM

Yes - I don't know what it is, but it would be after the first stars + had died.

The theory goes that the universe started as subatomic particles and aggregated into clumped matter, almost exclusively Hydrogen. So, yes, it would have taken at least a couple star life-cycles for enough complexity and variety in elements to be available for life to come about (depending upon how we define 'life'). How long that might have been is probably around a billion years (a complete guess - any more authoritative information is welcomed).

Hindmost

24th December 2009, 07:16 PM

No. You claimed the evidence points to the conclusion that no ETIs exist. I said the lack of evidence only points to the conclusion, "We don't know."

No. You're the one claiming the existence of evidence that points to a conclusion. (I assume that you mean the absence of some evidence points to that conclusion, which is why I'm asking what expected evidence is missing.)

Again, consider my dog analogy. Does the absence of evidence point to the conclusion that no other dogs exist?

ETA: For the record, we know it's possible for intelligent civilizations to exist because we have an example of one. There is no reason to think that it's only possible here and nowhere else in the universe.

This is my last post on this...I did not conclude anything. I did not say "ETI does not exist." Possible is not evidence, it is related to probability. I said, and this is still valid, so far the evidence points to zero.

glenn

Malerin

24th December 2009, 07:44 PM

It's also been argued the absence of any Von Neumann probes is some evidence for the absence of technologically advanced life.

Suppose a civilization wanted to broadcast a welcome message that would be detectable to other civilizations with radio telescopes within, say, a hundred light year radius. Targetting the beam to specific stars, how powerful would it have to be to be detectable by our best radio telescopes?

Malerin

24th December 2009, 08:08 PM

quadraginta

24th December 2009, 08:21 PM

kuroyume0161

24th December 2009, 09:00 PM

Also, a civilzation with a growth rate like ours (1-2%), and the ability to produce generation ships, would fill up all the habitable planets in the galaxy pretty quickly. A seed colony of 1000, with a population growth rate of 2%, would need to start looking for a new habitable planet before 1000 years were up.

If they seeded 10 planets to start with, they would expand to 20 in the next 1000 years, then 40, 80...within a ridiculously short period of time, they will have run out of planets.

After reading and reading and considering and considering, I think that these estimates are way, way extremely overly optimistic.

1. Any sentient lifeform will more than likely need to overcome the harshness of space, taking or having the ability to accumulate/recreate the necessary requirements for sustenance. There are no warp drives and cryogenic suspension is a hypothetical science fiction.

2. Star to star, at near light speed, can take many years (12 to our nearest). 'At near light speed', unless Einstein was a baffoon, requires monumental amounts of energy to achieve (at light speed = infinite energy). Galaxy to galaxy, need I comment. I think that I computed the time, at light speed, to go from one galaxy to another takes about a million years. That'll slow down that expansionistic dreaming very quickly.

3. Extending 1., the best way to colonize/expand into the universe would require massive (as in MASSIVE) consumption of resources. And that requires the ability to gather, process, store, distribute these resources. We're talking about quintillions of megatons of matter - entire solar systems need to be consumed to move radially outwards. I can't fathom how this would be possible.

It may be possible but is it practically improbable - as in would it even be achievable. Our galaxy doesn't seem to be infested with any particular form of life and that is a good indicator of the actual probability - virtually none.

Malerin

24th December 2009, 09:35 PM

After reading and reading and considering and considering, I think that these estimates are way, way extremely overly optimistic.

Possibly.

1. Any sentient lifeform will more than likely need to overcome the harshness of space, taking or having the ability to accumulate/recreate the necessary requirements for sustenance. There are no warp drives and cryogenic suspension is a hypothetical science fiction.

Actually, if you can just maintain a constant 1G acceleration, you wouldn't even need a generation ship. 20 years ship time at 1g acceleration/decceleration would give the ship a 137 light year radius. We can't do it with technology we have now, but look where we were 100 years ago.

2. Star to star, at near light speed, can take many years (12 to our nearest). 'At near light speed', unless Einstein was a baffoon, requires monumental amounts of energy to achieve (at light speed = infinite energy). Galaxy to galaxy, need I comment. I think that I computed the time, at light speed, to go from one galaxy to another takes about a million years. That'll slow down that expansionistic dreaming very quickly.

Well then, just tack on an extra 200 years for travel to nearby habitable planets. So every 1,200 years, they seed another 10 planets, then 20, 40, etc. You still run out of habitable planets in the Milky Way very quickly.

3. Extending 1., the best way to colonize/expand into the universe would require massive (as in MASSIVE) consumption of resources. And that requires the ability to gather, process, store, distribute these resources. We're talking about quintillions of megatons of matter - entire solar systems need to be consumed to move radially outwards. I can't fathom how this would be possible.

But we're not talking moving billions of people around in space. You start a small colony and let exponential population growth kick in. Then that colony branches off, etc. Making a ship that could hold a couple hundred colonists and maintain 1g acceleration wouldn't require "quintillions of megatons". We have drawing board ideas of how it could be done already.

It may be possible but is it practically improbable - as in would it even be achievable. Our galaxy doesn't seem to be infested with any particular form of life and that is a good indicator of the actual probability - virtually none.

Right, we haven't encountered any colonists, so there are several possibitilies:
1. They don't exist
2. They don't have any population growth (or desire to colonize extensively)
3. They're too low tech
4. Habitable planets are too rare for sub-light speed travel to be practical
5. They haven't reached us yet

Roboramma

24th December 2009, 10:40 PM

This is my last post on this...I did not conclude anything. I did not say "ETI does not exist." Possible is not evidence, it is related to probability. I said, and this is still valid, so far the evidence points to zero.

glenn

By similar logic I have no evidence that you have money in the bank. Yet, I wouldn't say that the evidence points to you not having money in the bank.

For one thing, there is direct evidence and indirect evidence. The indirect evidence for ETI is things like:

- the fact that once the earth cooled down, life appeared very quickly
- intelligence has evolved convergently in many different lineages, though we are the most intelligent species yet. For instance the ancestor of both us and the crow was less intelligent that either of us, and this is true of many species. This suggests that there are many situations in which increased intelligence is selected for.
- The number of stars in our galaxy, and the number of galaxies in the universe, which mean that even if it is very rare, it's still likely to exist.
- Our current findings that many stars do, in fact, have planets: though we should be careful here as we still don't know how common "life-bearing" planets are, or even what constitutes a planet with the right sort of environment for life to evolve.
- etc.

The is also similar indirect evidence against the proposition that ETIs exist:
- The fermi paradox.
- The fact that complex multicellular life took a long time to evolve on earth: suggesting that it may be uncommon (though this doesn't tell us whether or not its something that, once you're got life, takes an average of 100 million years, 3 billion years, or happens only once on every 10,000 such life bearing planets).
- The fact that we are the first known technological civilization on earth.
- etc.

Personally, I find the first class slightly more convincing than the second, but really, neither form of indirect evidence is really at all persuasive, and because we have no direct evidence, we are left not with the conclusion that there are zero ETIs, but that the evidence simply does not point to a particular conclusion.

It doesn't seem that you really disagree with that (let me know if you do), and if not I apologize for engaging in a semantic argument, but simply want to make this point clear.

Roboramma

24th December 2009, 10:44 PM

Right, we haven't encountered any colonists, so there are several possibitilies:
1. They don't exist
2. They don't have any population growth (or desire to colonize extensively)
3. They're too low tech
4. Habitable planets are too rare for sub-light speed travel to be practical
5. They haven't reached us yet
All of which are quite reasonable and possible. Here's another:

6. They find the rewards of colonization not worth the expense. (this differs from 2, because they may have a desire, just not one which is strong enough to merit undertaking the endeavor).

Which, given the expense of interstellar travel, shouldn't surprise anyone.

Hindmost

25th December 2009, 07:49 AM

....snip....

The Carl Sagan quote I provided several times best expresses my position. Here it is again:

Originally Posted by Sagan
I'm often asked the question, "Do you think there is extraterrestrial intelligence?" I give the standard arguments--there are a lot of places out there, and use the word billions, and so on. And then I say it would be astonishing to me if there weren't extraterrestrial intelligence, but of course as yet there is no compelling evidence for it. And then I'm asked, "Yeah, but what do you really think?" I say, "I just told you what I really think." "Yeah, but what's your gut feeling?" But I try not to think with my gut. Really, it's okay to reserve judgement until the evidence is in.

I got this quote from Sagan's introduction to The Outer Edge: Classic Investigations of the Paranormal edited by Joe Nickell.

By similar logic I have no evidence that you have money in the bank. Yet, I wouldn't say that the evidence points to you not having money in the bank.

For one thing, there is direct evidence and indirect evidence. The indirect evidence for ETI is things like:

- the fact that once the earth cooled down, life appeared very quickly
- intelligence has evolved convergently in many different lineages, though we are the most intelligent species yet. For instance the ancestor of both us and the crow was less intelligent that either of us, and this is true of many species. This suggests that there are many situations in which increased intelligence is selected for.
- The number of stars in our galaxy, and the number of galaxies in the universe, which mean that even if it is very rare, it's still likely to exist.
- Our current findings that many stars do, in fact, have planets: though we should be careful here as we still don't know how common "life-bearing" planets are, or even what constitutes a planet with the right sort of environment for life to evolve.
- etc.

The is also similar indirect evidence against the proposition that ETIs exist:
- The fermi paradox.
- The fact that complex multicellular life took a long time to evolve on earth: suggesting that it may be uncommon (though this doesn't tell us whether or not its something that, once you're got life, takes an average of 100 million years, 3 billion years, or happens only once on every 10,000 such life bearing planets).
- The fact that we are the first known technological civilization on earth.
- etc.

Personally, I find the first class slightly more convincing than the second, but really, neither form of indirect evidence is really at all persuasive, and because we have no direct evidence, we are left not with the conclusion that there are zero ETIs, but that the evidence simply does not point to a particular conclusion.

It doesn't seem that you really disagree with that (let me know if you do), and if not I apologize for engaging in a semantic argument, but simply want to make this point clear.

In a bit of Irony, from the other thread, JoeTheJugger has provided my position quoting Carl Sagan.

I see your arguement as reasonable, but it still relates to the probability of life evolving and not evidence. Based on your reasoning, someone that has never left australia could conclude there are kangaroos and Koalas all over the world. It would be the wrong conclusion as that person had no evidence, just scientific probability.

glenn

shadron

25th December 2009, 09:24 AM

I used to be under the impression that life on earth had been around for a much much smaller fraction than that and that countless intelligent civilisations could have risen and died before, probably due to sci fi stories and ideas, but this now makes me think life on earth is not as insignificant as I thought previously and that life might not be all that abundant elsewhere.

Current thought is that abiogenesis occurred on Earth very soon after the late bombardment took place about 4.3 bya. Single cell life more-or-less wandered along before going multicellular about 700 mya. It had it's ups and downs, and may have been pushed back into the muck a couple of times during snowball-Earth episodes, but the last of that series finally resulted in a 20% oxygen atmosphere that many think was required to build collagen and energize multicellular creatures.

Soapy Sam

25th December 2009, 08:44 PM

Actually, if you can just maintain a constant 1G acceleration, you wouldn't even need a generation ship. 20 years ship time at 1g acceleration/decceleration would give the ship a 137 light year radius. We can't do it with technology we have now, but look where we were 100 years ago.

I see a teensy problem here. A 1g acceleration (.0098km/s/s) will take you past lightspeed in about 354 days by my calculator.
Would you mind explaining for this numerical illiterate where the 137 LY radius comes in?
(Hint- 19 1/2 or thereabouts light years would seem like the maximum possible radius to me)

RecoveringYuppy

25th December 2009, 09:03 PM

You never pass light speed. You need relativistic calculations. And note he said ship time. We'd be talking about 250 years of Earth time.

Soapy Sam

26th December 2009, 10:27 PM

Ship time. I missed that. Duh.

JoeTheJuggler

26th December 2009, 10:32 PM

In a bit of Irony, from the other thread, JoeTheJugger has provided my position quoting Carl Sagan.

I don't follow. I quoted Carl Sagan as the best expression of my own position--we don't know. You said the evidence points to the conclusion that there are zero ETIs. Those are contradictory positions.

Sagan most definitely does not say that the evidence points to the conclusion that no ETIs exist.

JoeTheJuggler

26th December 2009, 10:39 PM

I see your arguement as reasonable, but it still relates to the probability of life evolving and not evidence. Based on your reasoning, someone that has never left australia could conclude there are kangaroos and Koalas all over the world.
No no no no! You're simply flat out wrong.

By my reasoning, someone who has never left Australia (and had no access to other evidence about the fauna of other continents) would say that the evidence only points to the conclusion, "We don't know."

You keep claiming I'm talking about probabilities and not evidence, but I am in fact talking about evidence (or lack thereof).

Your reasoning in the same analogy, would be that having no evidence regarding the fauna of other continents points to the conclusion that not only are there no koalas or kangaroos elsewhere (even though lacking any evidence, you really have no evidence to support that conclusion), but indeed that there is no life of any kind on other continents.

I prefer my analogy: there is a dog here inside my house, but when I glance out the window at my backyard for a second, I see no evidence whatsoever of other dogs. What can I conclude? Simply, that I don't know whether or not dogs outside my house exist. The only "looking" we've done for ETIs elsewhere in our galaxy (and forget about life in other galaxies!) has been analogous to that quick glance at our own backyard.

Malerin

26th December 2009, 11:02 PM

No no no no! You're simply flat out wrong.

By my reasoning, someone who has never left Australia (and had no access to other evidence about the fauna of other continents) would say that the evidence only points to the conclusion, "We don't know."

You keep claiming I'm talking about probabilities and not evidence, but I am in fact talking about evidence (or lack thereof).

Your reasoning in the same analogy, would be that having no evidence regarding the fauna of other continents points to the conclusion that not only are there no koalas or kangaroos elsewhere (even though lacking any evidence, you really have no evidence to support that conclusion), but indeed that there is no life of any kind on other continents.

I prefer my analogy: there is a dog here inside my house, but when I glance out the window at my backyard for a second, I see no evidence whatsoever of other dogs. What can I conclude? Simply, that I don't know whether or not dogs outside my house exist. The only "looking" we've done for ETIs elsewhere in our galaxy (and forget about life in other galaxies!) has been analogous to that quick glance at our own backyard.

But it's not really a glance, is it? We know (or are reasonably sure) that Earth was never colonized by any aliens. There's no evidence any Von Neuman devices have been at work in the solar system (as far as we can tell).

JoeTheJuggler

27th December 2009, 09:56 AM

But it's not really a glance, is it? We know (or are reasonably sure) that Earth was never colonized by any aliens. There's no evidence any Von Neuman devices have been at work in the solar system (as far as we can tell).

Yes--that's what I would call our current level of space exploration, from a cosmic scale, a glance for about 1 second into our own backyard.

I've addressed this line of thinking many times now.

Fermi's Paradox may be explained a number of ways other than, "They don't exist."

All we can say doesn't exist is a civilization whose presence is ubiquitous in the galaxy.

1. It could be that no civilization lasts that long.

2. It could be that the technology isn't possible.

3. It could be that a civilization lasts that long, and the technology exists, but they never discover it.

4. It could be that they discover it, but are unmotivated to use it.

5. It could be that they're motivated to use it, but it's not feasible to do so (costs outweigh the benefits).

6. It could be that they've almost filled the galaxy with evidence of their existence but just barely missed us by a mere 1 million or 1 thousand years, or that they're about to get around to us in the next hundred or thousand or million years.

7. It could be that they're intentionally hiding evidence from us.

Or it could be that they don't exist.

Based on the current evidence, we don't know.

For us to conclude that they don't exist, we'd have to make a number of very grand assumptions. To conclude that they don't exist, you'd need to refute each one of the numbered points.

You have to assume intelligent civilizations can live for millions of years, even though we don't know that. You'd have to assume technology not available to us is possible. You'd have to assume discovering that technology is inevitable for any intelligent civilization. You'd have to assume that every intelligent civilization is motivated to fill the galaxy with evidence of their existence. And you'd have to assume they have done so before now and that they have no way or no reason to hide that evidence from us.

Hindmost

27th December 2009, 04:30 PM

I don't follow. I quoted Carl Sagan as the best expression of my own position--we don't know. You said the evidence points to the conclusion that there are zero ETIs. Those are contradictory positions.

Sagan most definitely does not say that the evidence points to the conclusion that no ETIs exist.

Since you are misquoting me again, I have to respond. I never stated any conclusion that ETIs do not exist. I stated that the evidence points to zero.

Sagan stated that there is no compelling evidence. I see his statement as similar to what I stated-the only difference is the word "compelling."

Again. I did not conclude anything.

Glenn

skeptsci

28th December 2009, 02:38 AM

http://en.wikipedia.org/wiki/QFT
Thanks

Roboramma

28th December 2009, 05:42 AM

In a bit of Irony, from the other thread, JoeTheJugger has provided my position quoting Carl Sagan.

I see your arguement as reasonable, but it still relates to the probability of life evolving and not evidence. Based on your reasoning, someone that has never left australia could conclude there are kangaroos and Koalas all over the world. It would be the wrong conclusion as that person had no evidence, just scientific probability.

glenn

I think you and I pretty much agree and are just using words a little differently. For instance, I would say that we use evidence to determine "scientific probability". Its clear from the way that you're using the words that you aren't using them the way I am, and I'm pretty sure any disagreement stems from that.

:)

skeptsci

28th December 2009, 06:12 AM

And from a biologist point of view I'll add to all the above the problem of "life" definition: If you do manage to get there and find "something" on what criteria are you going to judge it to be "alive", it's not as simple as it might seem in a first thought. Also, Drake type of calculations are mice try. However, it is possible that given all the "right" conditions for life emergent, intelligent development and all, probabilities for these events are so low that they have happened only once in the history of this universe. Go figure.

so far, the evidence points to zero (0).

glenn

You are going to have to elaborate on how you infer creationism from my comment.

glenn

Sorry for the late response, must have missed your post. I quoted here the relevant posts exchange. I understood your post as a response to mine in which I wondered how small could the probability of intelligent life spontaneous emergence be (see quote). You responded "(0)". In other words, no such spontaneous emergence is possible due to improbability (hence, intelligent life must have been created) . This is the exact claim used by what is called "scientific creationism".

Hindmost

28th December 2009, 07:06 AM

Sorry for the late response, must have missed your post. I quoted here the relevant posts exchange. I understood your post as a response to mine in which I wondered how small could the probability of intelligent life spontaneous emergence be (see quote). You responded "(0)". In other words, no such spontaneous emergence is possible due to improbability (hence, intelligent life must have been created) . This is the exact claim used by what is called "scientific creationism".

I think the confusion is that I was not responding to your post, I was responding to the OP. I have elaborated that in several posts since relating to the probability of intelligent life etc...

glenn

skeptsci

28th December 2009, 11:23 AM

By similar logic I have no evidence that you have money in the bank. Yet, I wouldn't say that the evidence points to you not having money in the bank.

For one thing, there is direct evidence and indirect evidence. The indirect evidence for ETI is things like:

- the fact that once the earth cooled down, life appeared very quickly
- intelligence has evolved convergently in many different lineages, though we are the most intelligent species yet. For instance the ancestor of both us and the crow was less intelligent that either of us, and this is true of many species. This suggests that there are many situations in which increased intelligence is selected for.
- The number of stars in our galaxy, and the number of galaxies in the universe, which mean that even if it is very rare, it's still likely to exist.
- Our current findings that many stars do, in fact, have planets: though we should be careful here as we still don't know how common "life-bearing" planets are, or even what constitutes a planet with the right sort of environment for life to evolve.
- etc.

It doesn't seem that you really disagree with that (let me know if you do), and if not I apologize for engaging in a semantic argument, but simply want to make this point clear.
I will disagree with you.
It seems there is a logical fallacy involved here. I'll describe it by giving as example. If you go to the statistical history of the lottery in any country (civilized) and query about people that won a substantial prize upon their first ticket, I know for sure you will find several. Now, if the winner is to commit the same fallacy, coming to assess the probability of winning in qualitative terms they will argue:" I won on my first try, hence, probability must be very high – right?" – Dead wrong!!! There is no such lottery. The mistake originate from the fact that you used a sample size of one to assess the probability distribution. This is the mistake you did by claiming that probability for life, given the conditions on earth circa 3 B years ago, was high because it happened quickly. No way you can know it by this deduction.
I have to disagree also on the conclusion drawn about the high probability of intelligence evolution (your evolution convergence claim) but am not going to elaborate on it now. Just think that there might have been one (and only one), very low probability breakthrough in the brain/mind evolution that enabled future higher probability (given the breakthrough) events.

skeptsci

28th December 2009, 11:31 AM

I think the confusion is that I was not responding to your post, I was responding to the OP. I have elaborated that in several posts since relating to the probability of intelligent life etc...

glenn
Got you. This is the reason I use quotes almost always because if your post is right after another one which deals with sort of the same topic... you know...

JoeTheJuggler

28th December 2009, 11:50 AM

Since you are misquoting me again, I have to respond. I never stated any conclusion that ETIs do not exist. I stated that the evidence points to zero.

Sagan stated that there is no compelling evidence. I see his statement as similar to what I stated-the only difference is the word "compelling."

Again. I did not conclude anything.

Awright--how exactly does the statement "so far, the evidence points to zero (0)" in the context of the question asked by the OP differ from my saying that you said the evidence points to the conclusion that no ETIs exist?

Can evidence point to a proposition, but that proposition not be a conclusion?

My first reply to your statement was to say that the evidence only points to, "We don't know". You then defended the position that no ETIs exist, but kept saying I wasn't responding to evidence but to probability. I pointed out that you were wrong, and you wrongly applied my reasoning to your Australia analogy.

If you meant to agree with me, that we don't know, you could have said so.

ETA: And again, the Sagan quote most certainly did not say that the evidence points to there being zero ETIs. He said that the only conclusion we can draw from the evidence is that we don't know. Those are not the same conclusions. In fact, they're contradictory propositions.

Hindmost

28th December 2009, 12:23 PM

Awright--how exactly does the statement "so far, the evidence points to zero (0)" in the context of the question asked by the OP differ from my saying that you said the evidence points to the conclusion that no ETIs exist?

Can evidence point to a proposition, but that proposition not be a conclusion?

My first reply to your statement was to say that the evidence only points to, "We don't know". You then defended the position that no ETIs exist, but kept saying I wasn't responding to evidence but to probability. I pointed out that you were wrong, and you wrongly applied my reasoning to your Australia analogy.

If you meant to agree with me, that we don't know, you could have said so.

ETA: And again, the Sagan quote most certainly did not say that the evidence points to there being zero ETIs. He said that the only conclusion we can draw from the evidence is that we don't know. Those are not the same conclusions. In fact, they're contradictory propositions.

The short answer is yes, if one includes the word "so far" indicating that nothing compelling has been received. In Sagan's sound bite, the query of "what does your gut say..etc." was responded to with "I have already answered it and preferred to wait for the evidence" So, he concluded nothing either. I agree with that. A single premise does not necessarily imply a conclusion if you ask me.

And again, I never stated or implied that ETIs don't exist...you have inferred it several times now. I have defended the position that we have no evidence.

You seem to be thinking "we don't know" and "so far, evidence points to zero" are mutually exclusive. I don't think so. I was being Spock-ish...he never concluded stuff until he had sufficient data to make his estimate. Using them both in a sentence..."So far, the evidence points to zero, but with all the glaxies, stars and planets, there is a reasonable possibility of ETIs, but we just don't know.

Arthur C Clarke said something to the effect:

Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case, the idea is quite staggering.

glenn

JoeTheJuggler

28th December 2009, 12:41 PM

It's also been argued the absence of any Von Neumann probes is some evidence for the absence of technologically advanced life.
But not very well. See again the argument I made above with the numbered points. I purposely generalized it so that it covers any argument based on the lack of evidence of ETIs (whether probes, colonies, or whatever).

That ETIs don't exist is only one possible explanation of Fermi's Paradox. Since there are many others, you can't use the absence of evidence in this situation as evidence of absence.

That's the difference between "We don't know" and "Zero" as the answer to the question asked. (And strictly speaking, the way the question is phrased in the title, the answer is "At least one".)

Suppose a civilization wanted to broadcast a welcome message that would be detectable to other civilizations with radio telescopes within, say, a hundred light year radius. Targetting the beam to specific stars, how powerful would it have to be to be detectable by our best radio telescopes?

We've had an authority on that subject--Active SETI or METI (Messaging to ETIs) on the other thread (see A. Zaitsev's posts and links on about page 24 (http://forums.randi.org/showthread.php?t=98847&page=24)).

My own answer is that power isn't so much the problem as how long such a signal is sustained. It's still a needle in a gigantic haystack--especially if the "needle" is only visible-- even if you are near enough to see it and looking in its direction-- for a tiny bit of time.

JoeTheJuggler

28th December 2009, 12:55 PM

And again, I never stated or implied that ETIs don't exist...you have inferred it several times now. I have defended the position that we have no evidence.
That's not a true statement.

You said (my bolding):

so far, the evidence points to zero (0).
When I disagreed saying the lack of evidence points to, "We don't know", you said:

The evidence points to zero...the "we don't know" part is because we can't see the entire universe so it leaves the door open to not knowing.

I asked what evidence you were talking about (pretty sure you mean the absence of evidence), and you replied:
The fact that there is no tangble evidence that indicates there is intellient life...that's all.
So at this point it's clear that you're talking about the absence of expected evidence as evidence for the conclusion that ETIs don't exist.

So I asked what tangible evidence you expected or were lacking (the lack of which you're asserting as evidence for the non-existence of ETIs).

That's when you began to claim that I was talking about probability rather than evidence, which is simply not true.

---

You seem to be thinking "we don't know" and "so far, evidence points to zero" are mutually exclusive.

Yes, because they are.

Claiming the evidence points to something that it does not is not the same as saying we lack the evidence to answer the question (or we can only answer the question, "We don't know").

See Robramma's analogy about the lack of tangible evidence of whether or not he has money in the bank (post #84). Your claim is analogous to claiming that the lack of evidence leads me to the conclusion that his balance is zero (0). My position is that we don't know. These are two different positions. One of them follows from the available evidence, and one does not.

HumanityBlues

28th December 2009, 12:55 PM

Interesting stuff all around. I don't understand the half of what everyone is talking about, so suffice to say, the answer is Pie.

Hindmost

28th December 2009, 01:50 PM

That's not a true statement.

You said (my bolding):

When I disagreed saying the lack of evidence points to, "We don't know", you said:

I asked what evidence you were talking about (pretty sure you mean the absence of evidence), and you replied:

So at this point it's clear that you're talking about the absence of expected evidence as evidence for the conclusion that ETIs don't exist.

So I asked what tangible evidence you expected or were lacking (the lack of which you're asserting as evidence for the non-existence of ETIs).

That's when you began to claim that I was talking about probability rather than evidence, which is simply not true.

---

Yes, because they are.

Claiming the evidence points to something that it does not is not the same as saying we lack the evidence to answer the question (or we can only answer the question, "We don't know").

See Robramma's analogy about the lack of tangible evidence of whether or not he has money in the bank (post #84). Your claim is analogous to claiming that the lack of evidence leads me to the conclusion that his balance is zero (0). My position is that we don't know. These are two different positions. One of them follows from the available evidence, and one does not.

You keep indicating that I implied a conclusion...let me state again...I have not implied any conclusion. I use the word "tangible" and Sagan uses "compelling" and somehow you infer that I have concluded there are no ETIs but Sagan hasn't. I am having difficulty with that logic as I can't see any tangible/compelling difference.

Absence of evidence does not mean evidence of absence...Although I generally agree with this statement, I think you are taking it too far--absence of evidence does not imply any conclusion. In talking about psychic abilities, I may have inductively concluded to date that is doesn't exist but will change my mind with new evidence. With ETIs, there is no tangible evidence, but it would be wrong to conclude anything due to the limited nature of that evidence. In both cases there is a definite absence of evidence. I don't believe in psychic abilities as the evidence points to none, but do consider life in the universe possible.

glenn

kuroyume0161

28th December 2009, 02:56 PM

The evidence (physical, real, tangible) is 1.5. 1 is for Earth - definitely life here. 0.5 for Mars since the modicum of evidence we have is still under dispute. Beyond that, no actual evidence of life anywhere else.

That said, the probability for life existing elsewhere is very high. And I think that the proposition that it has existed, does exist, will exist elsewhere throughout the universe is almost a certainty. That does not mean that we can assume that it does in an evidential sense. We can only assume in a probabilistic sense.

As noted, our best stance is 'we don't know'. It would certainly go against our understanding of life's genesis and evolution here if it only happened here because there are too many opportunities, many probably extremely close to the Earth's, for it to happen.

JoeTheJuggler

28th December 2009, 03:06 PM

You keep indicating that I implied a conclusion...let me state again...I have not implied any conclusion. I use the word "tangible" and Sagan uses "compelling" and somehow you infer that I have concluded there are no ETIs but Sagan hasn't. I am having difficulty with that logic as I can't see any tangible/compelling difference.
The difference is that you said the evidence points to zero ETIs, but Sagan said the best answer we have is that we don't know. (See again my first reply to you on this thread.)

Absence of evidence does not mean evidence of absence...Although I generally agree with this statement, I think you are taking it too far--absence of evidence does not imply any conclusion. In talking about psychic abilities, I may have inductively concluded to date that is doesn't exist but will change my mind with new evidence. With ETIs, there is no tangible evidence, but it would be wrong to conclude anything due to the limited nature of that evidence. In both cases there is a definite absence of evidence. I don't believe in psychic abilities as the evidence points to none, but do consider life in the universe possible.

And I have been very careful to avoid claiming that the absence of evidence is never evidence of absence. However, the absence of evidence where we have no reason to expect evidence (even if the thing in question exists) is not evidence of absence.

I agree, we can draw no conclusion based on the current evidence beyond, as Kuroyume has been pointing out, the existence of an intelligent civilization here on Earth.

But there is also no evidence to the support the proposition that the total number of ETIs in the universe is zero. That is the proposition you have indeed asserted several times that I take issue with.

JoeTheJuggler

28th December 2009, 03:13 PM

The evidence (physical, real, tangible) is 1.5. 1 is for Earth - definitely life here. 0.5 for Mars since the modicum of evidence we have is still under dispute. Beyond that, no actual evidence of life anywhere else.

Strictly speaking, though, the question is about intelligent civilization. So we only have evidence for the one. I took Hindmost's "zero" to be in answer to the question of the existence of ETIs.

That said, the probability for life existing elsewhere is very high. And I think that the proposition that it has existed, does exist, will exist elsewhere throughout the universe is almost a certainty. That does not mean that we can assume that it does in an evidential sense. We can only assume in a probabilistic sense.

As noted, our best stance is 'we don't know'. It would certainly go against our understanding of life's genesis and evolution here if it only happened here because there are too many opportunities, many probably extremely close to the Earth's, for it to happen.

Yup, I agree.

We also have the history of assertions that humans are somehow unique or central or important in the universe. (They've failed over and over.) Just applying inductive reasoning on this point, I would conclude that we are probably not unique in the universe.

Also, there's the problem of words like "probable" and "rare" and "common". Since they're not quantified, what exactly do they mean?

As I said often on t he other thread, there's rare and then there's rare.

Is one in 10 million (intelligent civilizations per star) probable or improbable? And when you consider time, is even one in a billion ETIs to the lifetime of a star really rare? (There would still be hundreds in our galaxy alone. And even at that, they'd likely be so spread apart in space and time--assuming they're not exceptionally long-lived--that no two would ever make contact or even encounter evidence of each other.)

But for now, when you consider the evidence (or lack thereof), all we can say is, "We don't know." The evidence (wrt ETIs) doesn't point to 0 or 10 or 100 or 1 million or any number. It's completely inconclusive at this point.

It's analogous to glancing out the window into the backyard for 1 second and not seeing any evidence of the existence of dogs, and being asked whether or not dogs outside the house exist based on that evidence alone.

JoeTheJuggler

28th December 2009, 03:34 PM

Right, we haven't encountered any colonists, so there are several possibitilies:
1. They don't exist
2. They don't have any population growth (or desire to colonize extensively)
3. They're too low tech
4. Habitable planets are too rare for sub-light speed travel to be practical
5. They haven't reached us yet

And also,
6. They don't exist.

They're all possible, and probably other explanations we haven't thought of yet. We just don't know which is/are true.

Jungle Jim

28th December 2009, 04:03 PM

Interesting stuff all around. I don't understand the half of what everyone is talking about, so suffice to say, the answer is Pie.

Would that be apple or cherry?:)

Corsair 115

28th December 2009, 04:34 PM

Also, a civilzation with a growth rate like ours (1-2%), and the ability to produce generation ships, would fill up all the habitable planets in the galaxy pretty quickly. A seed colony of 1000, with a population growth rate of 2%, would need to start looking for a new habitable planet before 1000 years were up.

There is the assumption inherent in the above that only planets are the best habitats for an intelligent species. Surely one with enough technological sophistication could construct artificial purpose-built habitats rather than relying on nature's random chance to provide them with new living space. Even in human terms there have been proposals to build colony habitats in space (see the works of Gerard K. O'Neill for some examples).

We need to hurry up, before all the good spots are taken.

I believe this is often referred to as the 'colony rush' phase of Galactic Civilizations or Master of Orion... :D

Actually, if you can just maintain a constant 1G acceleration, you wouldn't even need a generation ship. 20 years ship time at 1g acceleration/decceleration would give the ship a 137 light year radius. We can't do it with technology we have now, but look where we were 100 years ago.

Seems to me there are two little problems with that: (1) the propulsion technology required, and (2) the fuel which would need to be carried. Both of these are rather consequential matters it seems to me.

Malerin

28th December 2009, 06:17 PM

But not very well. See again the argument I made above with the numbered points. I purposely generalized it so that it covers any argument based on the lack of evidence of ETIs (whether probes, colonies, or whatever).

I agree. There's too many possibilities than can account for the Fermi Paradox.

We've had an authority on that subject--Active SETI or METI (Messaging to ETIs) on the other thread (see A. Zaitsev's posts and links on about page 24 (http://forums.randi.org/showthread.php?t=98847&page=24)).

My own answer is that power isn't so much the problem as how long such a signal is sustained. It's still a needle in a gigantic haystack--especially if the "needle" is only visible-- even if you are near enough to see it and looking in its direction-- for a tiny bit of time.

That was interesting. Thanks for the link!

Malerin

28th December 2009, 06:19 PM

And also,
6. They don't exist.

They're all possible, and probably other explanations we haven't thought of yet. We just don't know which is/are true.

"They Don't Exist" was first on my list.

kuroyume0161

28th December 2009, 07:42 PM

Strictly speaking, though, the question is about intelligent civilization. So we only have evidence for the one. I took Hindmost's "zero" to be in answer to the question of the existence of ETIs.

To be pedantic, Hindmost is correct. We have 'zero' evidence for ETIs. We're not ETIs, we're TIs. :) Except for one 'Wow' moment with SETI, there is no evidence for ETIs.

Probability wise is a different situation of course. I think that the probability for ETI is greater than zero but much smaller than for life in general. There have only been two on Earth over the past 3.5 billion years of life and they were very closely related (homo sapiens sapiens and homo neanderthalensis). That just shows that there was evolutionary variety in the process as one might expect.

The reason that Drake's equation is a lot of guesswork and that we have to use words like 'probable', 'common', 'rare' is because the parameters are vague and detection almost futile. As you said, we can't make any conclusions so we must simply say 'we don't know', yes.

JoeTheJuggler

28th December 2009, 07:51 PM

"They Don't Exist" was first on my list.

D'oh! :blush:

At any rate, I've been making roughly the same points you make here for over a year (on that other thread). And still people think they can say "Fermi's Paradox" to support the conclusion that no ETIs exist.

To be pedantic, Hindmost is correct. We have 'zero' evidence for ETIs. We're not ETIs, we're TIs. :) Except for one 'Wow' moment with SETI, there is no evidence for ETIs.
That's why I was correcting you when you said the number is 1. (And also the 1.5. The 1 is not ET and the "0.5" is not I.) [ETA: Though I understood what you meant, and what Hindmost meant, which is why I wasn't trying to make a big deal of the point. The way the question is worded in the title is asking for how many Is there are, not ETIs, but I understood Hindmost to be addressing the question of ETIs.]

But actually Hindmost said the evidence points to the existence of zero ETIs, which is not true. Again, there is a difference between saying we have no evidence for the existence of ETIs and we have evidence that they do not exist.

The reason that Drake's equation is a lot of guesswork and that we have to use words like 'probable', 'common', 'rare' is because the parameters are vague and detection almost futile. As you said, we can't make any conclusions so we must simply say 'we don't know', yes.

On the other thread, someone said rare meant almost certainly no ETIs in our galaxy and no more than a dozen in the entire universe. I've been arguing against that conclusion (again, because there is no evidence to support it).

Also, the arguments offered for that position have been a mixture of stuff from the Rare Earth book and this rehashing of Fermi's. And the thinking is often backward, the same way the Fine Tuning argument (the latest guise of Creationism) is.

Hindmost

28th December 2009, 09:37 PM

D'oh! :blush:

At any rate, I've been making roughly the same points you make here for over a year (on that other thread). And still people think they can say "Fermi's Paradox" to support the conclusion that no ETIs exist.

That's why I was correcting you when you said the number is 1. (And also the 1.5. The 1 is not ET and the "0.5" is not I.) [ETA: Though I understood what you meant, and what Hindmost meant, which is why I wasn't trying to make a big deal of the point. The way the question is worded in the title is asking for how many Is there are, not ETIs, but I understood Hindmost to be addressing the question of ETIs.]

But actually Hindmost said the evidence points to the existence of zero ETIs, which is not true. Again, there is a difference between saying we have no evidence for the existence of ETIs and we have evidence that they do not exist.

On the other thread, someone said rare meant almost certainly no ETIs in our galaxy and no more than a dozen in the entire universe. I've been arguing against that conclusion (again, because there is no evidence to support it).

Also, the arguments offered for that position have been a mixture of stuff from the Rare Earth book and this rehashing of Fermi's. And the thinking is often backward, the same way the Fine Tuning argument (the latest guise of Creationism) is.

Please do not assert what I have or have not stated. I disagree with what you have stated above and have pointed that out several times. Disagree with what I post, but please do not place your interpretation as what I stated.

In this statement:

But actually Hindmost said the evidence points to the existence of zero ETIs, which is not true. Again, there is a difference between saying we have no evidence for the existence of ETIs and we have evidence that they do not exist.

I have never stated that the evidence points to the existence of zero ETIs. Again I stated: "SO FAR, the evidence points to zero." I was not stating that ETIs don't exist because there is no evidence--that would be stating a conclusion which I indicated numerous times that I was not drawing any conclusion. Your inference or interpretation is completely wrong.

And, I was actually addressing ETIs and TIs.

glenn

kuroyume0161

28th December 2009, 10:30 PM

That's why I was correcting you when you said the number is 1. (And also the 1.5. The 1 is not ET and the "0.5" is not I.) [ETA: Though I understood what you meant, and what Hindmost meant, which is why I wasn't trying to make a big deal of the point. The way the question is worded in the title is asking for how many Is there are, not ETIs, but I understood Hindmost to be addressing the question of ETIs.

I see. Yes, I meant life in general in that response and not specifically intelligent or sentient life.

But actually Hindmost said the evidence points to the existence of zero ETIs, which is not true. Again, there is a difference between saying we have no evidence for the existence of ETIs and we have evidence that they do not exist.

I agree. Actual evidence for ETIs is zero but evidence that ETIs might exist is not. In a sort of backward sense, we are evidence that ETIs have a probability of existence. That is what makes the question interesting. But it doesn't answer the question without more exploration.

On the other thread, someone said rare meant almost certainly no ETIs in our galaxy and no more than a dozen in the entire universe. I've been arguing against that conclusion (again, because there is no evidence to support it).

There is no evidence that precludes existence of other ETIs in our galaxy. We can barely make out planetary systems around stars using very cunning technologies. Have we found every planet in our galaxy and determined that there are no ETIs (or have never been)? I don't think so. In this case, absence of evidence isn't evidence of absence. It is just absence of ways to gather the evidence.

Also, the arguments offered for that position have been a mixture of stuff from the Rare Earth book and this rehashing of Fermi's. And the thinking is often backward, the same way the Fine Tuning argument (the latest guise of Creationism) is.

Fermi's Paradox makes too many assumptions. It assumes that interstellar space travel/colonization is possible/expected/efficacious all too readily. Evidence to the contrary from our own very (very,very) limited space explorations shows that it is, by no means, as simple as strapping a rocket to a bunch of organic beings and "away we go!" :) How far have we traveled? To our own moon. That is a not even dipping a part of a toe in the cosmic ocean. There are many other considerations when one goes from 'three days each way' to months, years, centuries, eons.

JoeTheJuggler

29th December 2009, 09:24 AM

Please do not assert what I have or have not stated.
Sorry, but I choose to ignore this directive.

I have never stated that the evidence points to the existence of zero ETIs. Again I stated: "SO FAR, the evidence points to zero."
Yes, and that's what I said that you said. And it's not true. The evidence (so far, or to date) does not point to zero (in response to the question in the thread title, understanding that you meant ETIs).

I was not stating that ETIs don't exist because there is no evidence
Except that you did.

Here is the question: "How many intelligent civilizations are out here from a scientific perspective?"

Here is your answer, "so far, the evidence points to zero (0)."

You already said you weren't talking about a probability, but what the evidence points to. It does not point to anything. We are not lacking any evidence we would expect if there were ETIs, contrary to the argument based on Fermi's Paradox (that reaches the same conclusion you do--that there are no ETIs or else we would have evidence of them).

Your inference or interpretation is completely wrong.

I'm not inferring or interpreting anything. This is what you said. And when I replied that the only thing the evidence points to is that we don't know, you defended your position (that the evidence points to the existence of zero ETIs).

And then you launched into a discussion of the difference between probability and evidence. (You can--and ought--use evidence to assert probabilities, so I see no major difference anyway. I think the only difference you're referring to is between talking about what the evidence supports and what you feel in your gut. As Sagan said, I prefer not to think with my gut.)

Again, saying there is no evidence for the existence of ETIs is not the same as saying the evidence points to zero (in answer to the question, how many ETIs are there?).

JoeTheJuggler

29th December 2009, 09:27 AM

Fermi's Paradox makes too many assumptions. It assumes that interstellar space travel/colonization is possible/expected/efficacious all too readily.
Yep. And I would add "inevitable" to that list.

Evidence to the contrary from our own very (very,very) limited space explorations shows that it is, by no means, as simple as strapping a rocket to a bunch of organic beings and "away we go!" :) How far have we traveled? To our own moon. That is a not even dipping a part of a toe in the cosmic ocean. There are many other considerations when one goes from 'three days each way' to months, years, centuries, eons.
And apparently the argument also denies that it's possible that ETIs might be at roughly our own level of development. By the standard given, an Earth-like civilization located a thousand LY away from us would conclude that we don't exist since we Earthlings haven't colonized the galaxy (or otherwise made evidence of our existence ubiquitous in the galaxy).

And none of this even addresses ETIs in other galaxies! The universe is really really really big!

Hindmost

29th December 2009, 10:08 AM

Sorry, but I choose to ignore this directive.

Yes, and that's what I said that you said. And it's not true. The evidence (so far, or to date) does not point to zero (in response to the question in the thread title, understanding that you meant ETIs).

Except that you did.

Here is the question: "How many intelligent civilizations are out here from a scientific perspective?"

Here is your answer, "so far, the evidence points to zero (0)."

You already said you weren't talking about a probability, but what the evidence points to. It does not point to anything. We are not lacking any evidence we would expect if there were ETIs, contrary to the argument based on Fermi's Paradox (that reaches the same conclusion you do--that there are no ETIs or else we would have evidence of them).

I'm not inferring or interpreting anything. This is what you said. And when I replied that the only thing the evidence points to is that we don't know, you defended your position (that the evidence points to the existence of zero ETIs).

And then you launched into a discussion of the difference between probability and evidence. (You can--and ought--use evidence to assert probabilities, so I see no major difference anyway. I think the only difference you're referring to is between talking about what the evidence supports and what you feel in your gut. As Sagan said, I prefer not to think with my gut.)

Again, saying there is no evidence for the existence of ETIs is not the same as saying the evidence points to zero (in answer to the question, how many ETIs are there?).

I am finished here with this final caveat. I find part of your arguement intellectually dishonest...you should not state someone else's position when you are misrepresenting it. I keep informing you that I implied no conclusion and then you assign a conclusion to my words that does not reflect my meaning. Your interpretation of what I posted in this thread does not represent my thoughts.

glenn

wollery

29th December 2009, 06:59 PM

I am finished here with this final caveat. I find part of your arguement intellectually dishonest...you should not state someone else's position when you are misrepresenting it. I keep informing you that I implied no conclusion and then you assign a conclusion to my words that does not reflect my meaning. Your interpretation of what I posted in this thread does not represent my thoughts.

glennHindmost, I'm sorry to butt in on this exchange, but I think you must be using an entirely different meaning of the phrase, "the evidence points to zero" than would normally be understood from it.

You keep saying that you aren't drawing a conclusion, but the phrase "the evidence points to" usually implies a conclusion drawn from the evidence, and in the context of how many ETIs there are, the phrase, "the evidence points to zero" would therefore usually imply that the evidence points to the conclusion that there are zero ETIs.

Since you have stated that this is clearly not the case (i.e. that the evidence points to zero ETIs) then I'm afraid that I'm at a loss as to how to understand the phrase that you keep defending.

Maybe you need to explain better what you specifically mean by that phrase, rather than simply repeating that it doesn't mean what it has been interpreted to mean.

JoeTheJuggler

29th December 2009, 08:41 PM

I am finished here with this final caveat. I find part of your arguement intellectually dishonest...you should not state someone else's position when you are misrepresenting it. I keep informing you that I implied no conclusion and then you assign a conclusion to my words that does not reflect my meaning. Your interpretation of what I posted in this thread does not represent my thoughts. [/COLOR]

Yet again, I was not interpreting anything you said. I quoted your very words.

Language works by convention. If you say the evidence leads to a proposition, that proposition is being put forward as a conclusion (even if that conclusion is only provisionally held pending more evidence, it is nonetheless a conclusion).

If you misspoke and meant to say there is no evidence of ETIs, you could have cleared that up after my first reply, but instead you've been defending your statement.

Again here is the question: "How many intelligent civilizations are out here from a scientific perspective?"

And here is your answer, "so far, the evidence points to zero (0)."

I've quoted it verbatim several times, but my paraphrase, that it is the claim that the evidence points to the conclusion that no ETIs exist, is valid.

Roboramma

30th December 2009, 12:02 AM

I will disagree with you.
It seems there is a logical fallacy involved here. I'll describe it by giving as example. If you go to the statistical history of the lottery in any country (civilized) and query about people that won a substantial prize upon their first ticket, I know for sure you will find several. Now, if the winner is to commit the same fallacy, coming to assess the probability of winning in qualitative terms they will argue:" I won on my first try, hence, probability must be very high – right?" – Dead wrong!!! There is no such lottery. The mistake originate from the fact that you used a sample size of one to assess the probability distribution. This is the mistake you did by claiming that probability for life, given the conditions on earth circa 3 B years ago, was high because it happened quickly. No way you can know it by this deduction.
I disagree. :)
First, as I said, this is a weak form of evidence, it's certainly not conclusive, and so as you say I can't really accurately asses the probability distribution.

However, let's say that I have a random number generator that will randomly choose a number between 1 and X. I present you with the outcome, and your job is to tell me, what is X.

I tell you the number is 2.

Of course, this doesn't tell you, "Oh, X is 2, because otherwise the outcome would have been higher." Obviously, if X were 10 an outcome of 2 wouldn't be particularly surprising.

On other hand, if X were 1,000,000,000,000,000,000,000,000,000,000,000,000, an outcome of 2 would surprise you.

You can say, "no, it shouldn't surprise, because by that logic, any number would surprise you."

But let me put it this way, if X is 2, the outcome 2 is much more likely than it is when X is 1*1036. So this outcome is (weak) evidence that X is a low number.

If we repeated this experiment and had another low number, that would be more evidence.

As to your lottery example, yes, obviously sometimes unlikely events happen, but if we take one event at random, it while it may fall outside the expected norm, it likely doesn't.

As I said, it's not strong evidence, because the sample size is very small, but it's still evidence.

I have to disagree also on the conclusion drawn about the high probability of intelligence evolution (your evolution convergence claim) but am not going to elaborate on it now. Just think that there might have been one (and only one), very low probability breakthrough in the brain/mind evolution that enabled future higher probability (given the breakthrough) events.
There may have been some sort of very unlikely to evolve thing that we passed through, but I don't see any evidence to suggest that.

As I said in another thread: the blue whale may be the largest animal that ever lived, but that doesn't suggest that similarly large animals are so unlikely to evolve that most planets with otherwise earth-like biospheres doesn't ever see them evolve.

skeptsci

31st December 2009, 02:26 AM

I disagree. :)
First, as I said, this is a weak form of evidence, it's certainly not conclusive, and so as you say I can't really accurately asses the probability distribution.

However, let's say that I have a random number generator that will randomly choose a number between 1 and X. I present you with the outcome, and your job is to tell me, what is X.

I tell you the number is 2.

Of course, this doesn't tell you, "Oh, X is 2, because otherwise the outcome would have been higher." Obviously, if X were 10 an outcome of 2 wouldn't be particularly surprising.

On other hand, if X were 1,000,000,000,000,000,000,000,000,000,000,000,000, an outcome of 2 would surprise you.

You can say, "no, it shouldn't surprise, because by that logic, any number would surprise you."

But let me put it this way, if X is 2, the outcome 2 is much more likely than it is when X is 1*1036. So this outcome is (weak) evidence that X is a low number.

If we repeated this experiment and had another low number, that would be more evidence.

As to your lottery example, yes, obviously sometimes unlikely events happen, but if we take one event at random, it while it may fall outside the expected norm, it likely doesn't.

As I said, it's not strong evidence, because the sample size is very small, but it's still evidence.

There may have been some sort of very unlikely to evolve thing that we passed through, but I don't see any evidence to suggest that.

As I said in another thread: the blue whale may be the largest animal that ever lived, but that doesn't suggest that similarly large animals are so unlikely to evolve that most planets with otherwise earth-like biospheres doesn't ever see them evolve.
As much as I hate it, I still must not agree. First, I do notice that you consider your argument to be indicative only ("weak" as you put it). But (in my opinion) it cannot be even this. Let's conduct a thought experiment. Somehow we are God's little helpers, so that we know all that is happening in the universe. Now, we know that, given a range of "correct" background conditions, the chances of emergence of intelligent (if this is what we are) life is say 10-30 (pretty low, right?). We observe the universe for 17 B years and then, in a pale blue planet, in a forgotten fringe of a galaxy, one such event occurs. ONE. Thrilled, we go there to watch the statistical wonder. We especially observe the symbol of intelligence – the scientific community and to our great shock we find them working hard on Drake equation: "If life was created here" they claim "then given so many billions of planets harboring the "right conditions" there are surely several millions, well at least tens of thousands intelligent civilizations out there". End of experiment. This mistake might happen because some of us draw conclusions on the whole population based on a sample of the size of 1. It happened here that's for sure. It could have happened 0 more times , 2, 10, 1000, 106, pick. We have no way of knowing at all (not even "weakly"). This is in the hard core logical view. Personally, I also tend to suspect there "should" be others there but this is totally in the realm of belief.
In your example of 1036 if probability of occurrence of any of the numbers is equal than surely I would expect a number that is bigger than say 106 simply because there are more numbers there than there are in the population of numbers smaller than 106. I don't think this has any useful bearing on our issue.

HumanityBlues

10th January 2010, 12:31 AM

jadey

10th January 2010, 05:51 AM

Just remembered this article I read a long time ago. Finally found it: http://www.cnn.com/2009/TECH/space/02/25/galaxy.planets.kepler/index.html

In it, it says that there may be 100 billion earth like planets in the Milky Way Galaxy alone! If we build off that assumption, would that change anyone's opinion?

I've heard estimates of 200 to 400 billion stars in the Milky Way galaxy. That estimate would imply 1/2 to 1/4 of stars have earthlike planets. Seems awfully optimistic to me.

HumanityBlues

10th January 2010, 02:26 PM

I've heard estimates of 200 to 400 billion stars in the Milky Way galaxy. That estimate would imply 1/2 to 1/4 of stars have earthlike planets. Seems awfully optimistic to me.

Well, even if 1/2000 had earthlike planets, wouldn't that still be significant?

Edit: Here is an interesting video (http://www.cnn.com/2009/TECH/space/02/25/galaxy.planets.kepler/index.html#cnnSTCVideo) in conjunction with the article I just posted. Explains things in a little more detail. A little less optimistic than I would've thought, but still very interesting.

jadey

11th January 2010, 03:05 PM

Well, even if 1/2000 had earthlike planets, wouldn't that still be significant?

Yes, it would be extremely significant. That still sounds way optimistic, but who am I to say. I would think 1 in a billion would still be extremely significant.

Corsair 115

11th January 2010, 03:54 PM

Well, even if 1/2000 had earthlike planets, wouldn't that still be significant?

Does Earth-like planet include having a large, natural satellite to stablize the planet's axial tilit? Does Earth-like planet include having plate tectonics? Does Earth-like planet include having a solar system with orbitally stable and orderly gas giant planets?

In other words, simply having a ball of rock roughly the same mass and density of the Earth doesn't really mean that much by itself in terms of potential life. There are a lot of other factors that play a role as well.

wollery

11th January 2010, 05:24 PM

Corsair 115

11th January 2010, 10:55 PM

Actually we have no idea what factors play a role.

Actually, we do, as you yourself noted:

So far we only have one planet known to have life...

Granted, it's only one example, but it's a data point nonetheless. The question is whether it's the rule or the exception.

And some roles are purely the work of physics—a large satellite stablizes axial tilt, which would otherwise wander. A wandering axis means dramatic climate shifts, and that would mean a tougher place for life to maintain a foothold. Similarly, a solar system with an unstable collection of gas giants means any terrestrial worlds will either be pushed into the star or ejected from the system, both of which would spell disaster for any life upon that terrestrial world.

Titanic Explorer

11th January 2010, 11:07 PM

Should we assume there is no life on other planets, and counter that conclusion only when we discovery evidence of extra terrestrial life?

Or should be assume there IS life on other worlds- despite the lack of evidence?

I'm open to the idea of extra terrestrial bacterial, floral and small animal life, but not humanoid aliens with human like technology. All life on earth evolved in accordance to the earth environment- If there is life out there, it should be nothing like on earth- and as civilizations and technology is a human concept- i doubt we'll find other worlds with civilizations or human like technology.

Alien life could be in the form of creatures that swim through gas giants atmosphere the way fish live in water...

Perhaps there is silicon based life...

There is no alien 'Federation of Planets' or 'Galactic Empire' out there- That is the product of the human imagination....

Forget about outer space- what about inner space? There might be micro universes- entire micro galaxies smaller than a molecule....

If we truly master space exploration and off world colonization, I think we will never be able to explore beyond this solar system (with anything other than unmanned probes)...

kuroyume0161

11th January 2010, 11:23 PM

That's the problem. While Drake's Equation is a good summary of required variables for the consideration of the number of possible sentient civilizations, the variables are interdependent and hard to determine. The difference in magnitude between 100 million 'Earth-like' planets and 10000 changes the results very dramatically.

My thoughts are that 'Earth-like' planets are rarer than the more optimistic estimations by magnitudes. First you need a nebulaic cloud star nursery which contains at least the atomic elements needed to seed a planet with rock, water, and other components like Earth in a forming star system. Then you need a star similar to ours (as best we know). Then you need that planet to be within a temperature zone satisfactory to liquid water (as best we know for life) and an accomodating gaseous atmosphere (as best we know). Then you might need a satellite or two to act as 'first responders' to early cataclysmic asteroid bombardments (as best we know).

As my response goes, we are very much working on a single data point with no general principles so everything we guesstimate is 'as best we know'. Not very convincing for any deduction on firm probabilities of the number of sentient lifeforms out there, even in our own galaxy let alone generally.

In the history of the universe, are we the only sentient lifeforms? Doubtfully. Can we get a firm idea on how many might exist (generally)? Maybe in time - a long time. We have to assume, unfortunately, that there might be but that we may never know certainly. There are really only several ways we will know certainly (from current physics):

1. An extra-terrestrial probe or signal (of directed and incomprable clarity) is encountered.

2. We find a way to travel great interstellar distances at FTL speeds.

3. They have found a way to travel great interstellar distances at FTL speeds.

That's about it. Not much else. You know, in a million years, the existence of our 'great civilization' of homo sapiens sapiens will be all but dust. Even a lunar 'time capsule' might be compromised in a few ten million years from radiation or asteroid impacts. Nothing lasts forever.

Corsair 115

11th January 2010, 11:31 PM

In the history of the universe, are we the only sentient lifeforms? Doubtfully. Can we get a firm idea on how many might exist (generally)? Maybe in time - a long time. We have to assume, unfortunately, that there might be but that we may never know certainly.

There's also the fact that the universe could be positively overflowing with intelligent life, and yet we'll never know about. Assume, for the moment, that, on average, there is one intelligent species per galaxy. With some 100 billion galaxies in the universe, that would mean about 100 billion intelligent lifeforms—that's a hell of a lot of intelligent races out there. But with intergalactic distances dwarfing the interstellar distances within a galaxy, all those intelligent civilizations would never get to know about any of the others because they are simply too far apart.

wollery

11th January 2010, 11:32 PM

Actually, we do, as you yourself noted:

Granted, it's only one example, but it's a data point nonetheless. The question is whether it's the rule or the exception.Yay, let's do meaningful statistics with a single data point!

And some roles are purely the work of physics—a large satellite stablizes axial tilt, which would otherwise wander. A wandering axis means dramatic climate shifts, and that would mean a tougher place for life to maintain a foothold. Similarly, a solar system with an unstable collection of gas giants means any terrestrial worlds will either be pushed into the star or ejected from the system, both of which would spell disaster for any life upon that terrestrial world.Neither of which necessarily makes life impossible. Who said that life could only exist on a terrestrial planet?

Oh, and one thing that every extrasolar system we've observed so far has in common - stable orbital solutions.

RecoveringYuppy

12th January 2010, 09:32 AM

[quote=wollery;5504713
Oh, and one thing that every extrasolar system we've observed so far has in common - stable orbital solutions.[/quote]

Are we just presuming they are stable because they exist? Or have we actually ruled out that they are in transient orbits?

Corsair 115

12th January 2010, 03:48 PM

Yay, let's do meaningful statistics with a single data point!

What can I say, I enjoy the occasional nitpick. :D

Neither of which necessarily makes life impossible. Who said that life could only exist on a terrestrial planet?

There's a difference between life and intelligent civilizations, which is what the OP mentions. Simple life can probably exist almost anywhere; complex life tends to favour ever more restricted environments.

Oh, and one thing that every extrasolar system we've observed so far has in common - stable orbital solutions.

"Hot Jupiters" are extremely unlikely to have formed where they have been observed. That means those planets had their orbits migrate inwards from where they were originally formed. Such a process means either the ejection of terrestrial planets from the system or the terrestrial planets are pushed into the star. Highly elliptical orbits of other large gas giant planets found in extrasolar systems would likewise spell doom for the orbits of any terrestrial planets.

So, while those systems have attained orbital stability now, the process by which they attained that final stability would have meant the end of any small, rocky worlds there beforehand.

BigSlowTarget

12th January 2010, 04:33 PM

What's the number of otherwise potentially habitable systems in which we have observed a planet(s) where the location of the observed planet(s) would prevent formation of a rocky planet in a habitable zone? It seems that might create some kind of upper bound for the frequency of such planets - if the percentage is significant.

wollery

12th January 2010, 05:54 PM

What can I say, I enjoy the occasional nitpick. :DAs do I. ;)

There's a difference between life and intelligent civilizations, which is what the OP mentions. Simple life can probably exist almost anywhere; complex life tends to favour ever more restricted environments.Fair point, but we still don't know what those restrictions are.

"Hot Jupiters" are extremely unlikely to have formed where they have been observed. That means those planets had their orbits migrate inwards from where they were originally formed. Such a process means either the ejection of terrestrial planets from the system or the terrestrial planets are pushed into the star. Highly elliptical orbits of other large gas giant planets found in extrasolar systems would likewise spell doom for the orbits of any terrestrial planets.

So, while those systems have attained orbital stability now, the process by which they attained that final stability would have meant the end of any small, rocky worlds there beforehand.Actually current models of planet formation have a very hard time explaining hot Jupiters at all. The reason they migrate is dynamical friction with the debris disk, but there are problems with the timescale calculations.

Also, if you have a smaller planet orbiting inside the massive planet that sweeps a gap in the disk that stops the migration. This suggests that hot Jupiters would only form in systems that don't have inner planets to start with.

And again, we have fewer than 500 extrasolar planets, in even fewer systems, and the reason most of them are hot Jupiters is that that's what the surveys are sensitive to. Give it a few years and we'll start to get more and more rocky planets, and more and more Solar system analogues. Then we can start to do some real statistics, and develop reasonable theories of planet formation.

wollery

12th January 2010, 06:06 PM

Corsair 115

12th January 2010, 11:09 PM

Actually current models of planet formation have a very hard time explaining hot Jupiters at all. The reason they migrate is dynamical friction with the debris disk, but there are problems with the timescale calculations.

That's not my understanding, but if you've got links, I'll happily read them. I've always liked astronomy. :)

wollery

13th January 2010, 12:02 AM

That's not my understanding, but if you've got links, I'll happily read them. I've always liked astronomy. :)Ah, sorry, got that from a long and very interesting conversation with a planet formation theorist at a conference last year. I can't remember the details, but it all boils down to the planets migrating too fast. Unless the theorists can find a mechanism to slow them down they end up getting swallowed by the star.

I just had a quick look, and found this review paper (http://ukads.nottingham.ac.uk/cgi-bin/nph-data_query?bibcode=2009sf2a.conf..333F&db_key=AST&link_type=ARTICLE). It's a pdf download. The relevant part is section 4. I'll look for a more detailed paper when I have time.

Obviously hot Jupiters can form, and apparently do it fairly well, as we keep finding them. As usual in astronomy, we don't know nearly enough to make accurate models.

Titanic Explorer

13th January 2010, 10:14 AM

That's not my understanding, but if you've got links, I'll happily read them. I've always liked astronomy. :)

is it true Jupiter has all the key ingredients that make up a sun? I wonder if Jupiter could ever expand into a real heat emitting star...

kuroyume0161

13th January 2010, 10:21 AM

is it true Jupiter has all the key ingredients that make up a sun? I wonder if Jupiter could ever expand into a real heat emitting star...

Mass. There is a critical mass required for gravity to start crushing the elements to kickstart the thermonuclear fusion/fission reaction. Jupiter has about 60 times too little mass. That should give a good indication of the size of the Sun comparatively.

JoeTheJuggler

13th January 2010, 10:21 AM

Does Earth-like planet include having a large, natural satellite to stablize the planet's axial tilit? Does Earth-like planet include having plate tectonics? Does Earth-like planet include having a solar system with orbitally stable and orderly gas giant planets?

In other words, simply having a ball of rock roughly the same mass and density of the Earth doesn't really mean that much by itself in terms of potential life. There are a lot of other factors that play a role as well.

But you don't know that.

To be clear, the Rare Earth argument says that the role a large moon is to to prevent axial precession and orbital precession from resonating and causing climatic swings.

The problem with this (and many of the arguments in Rare Earth) is that environmental change is what seems to drive evolution rather than "friendliness". You have no evidence that what happened on Earth is the ideal situation. It could be that we're an especially late bloomer because of our nice big moon.

The climatic changes would take place quickly on a geological scale, but slowly to biology--that is, they'd take place over thousands of years.

If you're sold on the arguments made in Ward and Brownlee's Rare Earth, you might want to read Life Everywhere by David Darling. He's got a chapter that refutes the Rare Earth arguments point by point, and he points out that at least some of their arguments are based on the collaboration of Guillermo Gonzales (http://en.wikipedia.org/wiki/Guillermo_Gonzalez_(astronomer)), someone who fully admits that his "science" is motivated by his belief in Creationism/ID.

UGottaB

13th January 2010, 11:20 AM

When does a neural response imply intelligence? Are you supposing that humans are intelligent. We are just as capable of making a non-correct decision as a correct decision and often do which bares the question.

wollery

13th January 2010, 04:04 PM

kuroyume0161

13th January 2010, 09:08 PM

More like 75 times too small (if current theory is correct).

Well, if you want to be pedantic. ;) I did get my information from a Wiki so you get what you get. :)

UGottaB

14th January 2010, 11:11 AM

Mass. There is a critical mass required for gravity to start crushing the elements to kickstart the thermonuclear fusion/fission reaction. Jupiter has about 60 times too little mass. That should give a good indication of the size of the Sun comparatively.

Strange how it is that the center of a large body has a net effect of 0 gravity. As you move further away from the center gravity increases. So where does the crushing take place. It would seem to me that gravitational crushing isn't the ignition switch. Maybe outward acceleration can provide the speed necessary to bang atoms together producing free neutrons for fission or binding nuclei together and the heat necessary to sustain the reaction.

RecoveringYuppy

14th January 2010, 12:54 PM

Strange how it is that the center of a large body has a net effect of 0 gravity. As you move further away from the center gravity increases. So where does the crushing take place.
Not quite. There may be no net gravity at the center but alll those gravitational vectors all over the planet are pointing to the center and the forces they create transmit through the planet.

Corsair 115

14th January 2010, 04:37 PM

To be clear, the Rare Earth argument says that the role a large moon is to to prevent axial precession and orbital precession from resonating and causing climatic swings.

The problem with this (and many of the arguments in Rare Earth) is that environmental change is what seems to drive evolution rather than "friendliness".

Well, the argument in the book is that, based on the history of life on Earth, very long periods of relative climatic stability are needed to go from bacterial life to the more complex forms represented by animals. Earth had bacterial life for a couple of billion of years but nothing more complicated than that arose. Note too that the Rare Earth hypothesis is not that life is rare, but rather complex life like animals is rare. Bacterial life is theorized to be very common and widespread.

In any case, you've mentioned a new book to take a look at, which hopefully I will be able to do. :)