Lance wrote:My personal view, as I tried to explain, is not a denial of other intelligent life in the universe. The universe is so enormous that such a view will likely be wrong. However, such limited evidence as we have, backed up by logical extrapolation, would suggest that intelligent, technologically advanced extraterrestrial species are few and far between.

I think that it's equally important to not just conceive of the huge 3 dimensions of space, but also the huge dimension of time.

So humans arose say 200kya. Sometime around 60kya, we either reached a critical mass, or some cultural or mental change occurred that lead to behavioral modernity. From there we expanded around the world and began shaping the ecosystems we encountered. Then, sometime around 10kya, we began engaging in a more controlled version of hunting and gathering, by keeping animals and growing plants. Around 6kya this proliferated and the abundance of surplus food permitted us to maintain denser populations. This in turn lead to specialisation, urbanisation, political stratification and to writing. Writing finally allowed a greater transmission of culturally and experientially acquired knowledge to be transmitted down through the generations - wheels did not need to be reinvented, but could be built on. Even then, it took thousands more years for appropriate economic and social changes to occur that lead to a systematic methodology of knowledge acquisition.

Not all our discoveries were made in the last few hundred years, but it's not a great stretch to say that the most life-changing ones occurred in this handful of generations. And here we are today, just a step down the road and our planet is groaning under our presence; we're actively engaged in a systematic destruction of ecological diversity, we're seriously damaging the tiny little ring of atmosphere that houses us, and we're frittering the finite mineral resources which could act as a spring-board to the stars.

I don't think it's a safe bet yet that we're going to make it out to colonising even our solar system, let alone the galaxy. The universe is fundamentally restricted by thermodynamic exchanges, life is as well. Perhaps the explanation for the apparent lack of technological civilisations roaming around the galaxy is best explained by the nature of becoming such a civilisation in the first place - the over-exploitation of the rock they evolve on. It's one shot, and it depends very much on whether the civilisation recognises this and starts to put their resources towards space expansion in sufficient time before they run out of the materials they need to achieve it. This is, i think, a variation on the doomsday argument, and while it doesn't follow that such an occurrence means the end of life on that particular rock, it could doom future, perhaps more conservative civilisations, to being tied to their planet thanks to their predecessors having frittered the one-time mineral windfall. History shows that even the greatest empires ebb, perhaps civilised species do too.

Lance wrote:The chances of life appearing on another planet would seem, by a form of simple logic (and we know how dangerous that is), to depend on the conditions prevalent on that planet.

For example : we know that the planets of our solar system have almost circular orbits, which allow for a relatively stable climate. Most extrasolar planets so far discovered, have relatively wildly elliptical orbits, which would create substantially varying temperatures and seasons. This factor alone would appear to make the formation of life on other planets somewhat rare and unusual.

To begin with I want to get the obvious argument that we've only found a few hundred exoplanets as we've only been searching for a few years. It now is beginning to look like that suns without planets may be the rare exception. Making an extrapolation of the average exoplanet's orbit on the relatively small amount of data we have at the moment is premature.

But that's not the point I wanted to make in response to your above point. It may well turn out that the "goldilocks zone" is an overrated concept, and with it the need for circular orbits. Take Jupiter's moon Europa as an example. It exists far from our system's goldilocks zone and it's current status would likely be similar if Jupiter existed in a highly elliptical orbit. The warmth that makes it likely that water exists in liquid form under it's ice crust comes from the gravitation tidal forces it's subjected to from Jupiter and its sister moons. And yet most astronomers will tell you that Europa is the most likely place to find life in the solar system outside our biosphere.

There are some hints from our local understanding, that the origins of life may have been tied up with volcanic activity like hot springs. If so, life would not appear on planets without tectonic activity. We do not know how prevalent tectonic activity is on extrasolar planets, but there appears to be little or none on our moon and on Mars. Even Venus is problematic on this count. It is very likely that tectonic activity is unusual, making the appearance of life also unusual.

Even taking the possibility that life is only possible on tectonically active bodies as a definite tectonic activity has been shown to likely exist on our gas giants' moons. The reason tectonics don't exist on the Moon and Mars is purely because they are not large enough. So of the two bodies we know of large enough for tectonics that is not caused through gravitational tidal forces fifty percent have them.

Earth has a very unusual moon, which is massive in proportion to its parent. This provides a stabilising effect. It may be possible that this is an essential factor for life to survive?

How can it be said to be very unusual? There are only three planets of relative size in our solar system and we lack the ability at the moment to detect moons on the exoplanets we've found, and most of them are huge compared to the Earth.

John Gribbin has put together a number of such effects in his book, and suggested that this makes life a great rarity. Of course, this thesis must be seen as speculative, and it will be a long, long time before humanity will be in a position to make a reliable judgement. It makes for fascinating speculation, though.

I agree. I find all this hugely interesting. It is possible though that it will not be a long time before we are much more clued up on the subject. If another tree of life exists within our solar system that is unrelated to ours it is entirely feasible that we will find it within a lifetime. If this is the case, then we will have a better idea on the prevalence of life in the universe. If not, then yes, we'll likely be in the dark for a long, long time.

I would like to have the cloud tops of Venus properly researched - they're a fairly good place to expect life to appear, whatwith temperatures in rather average terms for terrestrial life, a somewhat nice chemistry, and relatively nice pressure too.

Zwaarddijk wrote:I would like to have the cloud tops of Venus properly researched - they're a fairly good place to expect life to appear, whatwith temperatures in rather average terms for terrestrial life, a somewhat nice chemistry, and relatively nice pressure too.

It's an interesting idea - I think you mean the tropopause btw - about 50km from the surface where temperatures are stable and balmy. Just 65km from the surface it's -20c, so it's a thin band. The only question is the sulfuric acid - if something's alive there, it's a super-extremophile! I recall there being some chemical production that's not to be expected, maybe carbonyl sulfide? Can't recall off the top of my head, but there are a few pointers suggestive of microbial action. Anyway I like it as it fits my personal conjecture that life is almost inevitable wherever it's possible in this thermodynamic universe.

Not wanting to resurrect abiogenesis. But the problem with the idea of life in Venus' clouds is that we do not know the conditions under which life can arise. I would be happy to imagine bacteria evolving to thrive in the atmosphere of Venus. After all, there are bacteria living in sulphuric acid here on Earth. eg. that cave in Mexico.

But could life actually arise under those conditions? We do not know, but it seems unlikely to me.

I hold to the view, that if we discover life on other planets within our own solar system, it will be essentially Earth bacteria. We know that the asteroid that killed the dinosaurs had enough energy to throw Earth rocks into space. We also know that some bacterial spores can survive the cold, radiation, and vacuum of space, if they are inside a rock, for very long periods. So a rock drifting through space and intercepting Venus or Mars, and seeding it with Earth bacteria, is a theoretical possibility.

Lance wrote:Not wanting to resurrect abiogenesis. But the problem with the idea of life in Venus' clouds is that we do not know the conditions under which life can arise. I would be happy to imagine bacteria evolving to thrive in the atmosphere of Venus. After all, there are bacteria living in sulphuric acid here on Earth. eg. that cave in Mexico.

But could life actually arise under those conditions? We do not know, but it seems unlikely to me.

That's the problem with going back to anything about abiogenesis... what seems unlikely is terminally irrelevant. Our brains are not evolved to work on these kind of things - something a billion to one against is inevitable if you run a trillion 'throws' and keep doing so every millisecond.

Lance wrote:I hold to the view, that if we discover life on other planets within our own solar system, it will be essentially Earth bacteria.

Unless it is actually Earth bacteria, it won't necessarily be anything like it. While there may be something functionally equivalent to DNA as a unit of inheritance; a replicator... the construction of it would be completely different.

Lance wrote: We know that the asteroid that killed the dinosaurs had enough energy to throw Earth rocks into space. We also know that some bacterial spores can survive the cold, radiation, and vacuum of space, if they are inside a rock, for very long periods. So a rock drifting through space and intercepting Venus or Mars, and seeding it with Earth bacteria, is a theoretical possibility.

Or vice-verse - life on Earth might have originated in spores from Mars when it had an atmosphere.

Spearthrower wrote:Or vice-verse - life on Earth might have originated in spores from Mars when it had an atmosphere.

Indeed, if panspermia is a reality then it's possible that Earth was not the cradle of our tree of life.

One problem with the idea that life arose on Mars is time. It appears that Mars may have had liquid water for 100 to 200 million years. Earth had liquid water for 500 million to a billion years before it had life. And it took another billion years before that life evolved to the point of being able to produce oxygen. What are the odds against life on Mars arising, and evolving to the point where it could seed the Earth in only 100 to 200 million years? Pretty slim, I would say.

Earth has now had life for 3 to 4 billion years. So the odds of Earth seeding Mars are much higher than those for Mars seeding Earth.

There is another problem. Why did Mars have liquid water? A leading hypothesis is that it came from bombardment. Water containing comets crashing into Mars would heat the place up, When the bombardment stopped, without an appreciable atmosphere, Mars cooled rapidly, and the water froze. So life would have to arise and evolve in the middle of a massive bombardment from space. Very unlikely.

My take on anything revolving around this topic is that arguments from probability are really an expression of our incredulity rather than anything necessarily real.

Spearthrower

This whole thread is arguments from probability.

Is it though, in truth?

It's certainly an argument of possibilities. But as been said by numerous people so far until we have more data it's impossible to make any kind of significant calculation on those possibilities.

Yes, it is. The probabilities are, of course, largely unknown, and many may be so small as to be almost certainly wrong. But it is still about probabilities. A possibility after all, is simply a small probability.

Now let's argue semantics!

Lance wrote:Yes, it is. The probabilities are, of course, largely unknown, and many may be so small as to be almost certainly wrong. But it is still about probabilities. A possibility after all, is simply a small probability.

Now let's argue semantics!

Actually, I think your 2nd sentence says why it's really not useful. The probabilities are largely unknown. Someone will say 'that's very improbable' based on any given instant, but really the statement is meaningless with respect to the time scales involved. Even an extraordinarily rare occurrence will happen given enough time and enough potential interactions. And further, shit does just happen.

This post by Cali deals with some of the fallacious reasoning that plagues people's thoughts about probablistic processes.

http://www.rationalskepticism.org/creat ... t1121.html

While it's obviously pointed at the specious arguments set forth by religious proponents wishing to give their criticisms of abiogenesis a scientific veneer, the 2 notions 'one true sequence' and 'serial trials' are well worth considering with respect to any topic like this that covers a lot of space, and a lot of time.

Lance wrote:One problem with the idea that life arose on Mars is time. It appears that Mars may have had liquid water for 100 to 200 million years. Earth had liquid water for 500 million to a billion years before it had life.

I'm not convinced.
Do you have a reference to support this?

rainbow wrote:

Lance wrote:One problem with the idea that life arose on Mars is time. It appears that Mars may have had liquid water for 100 to 200 million years. Earth had liquid water for 500 million to a billion years before it had life.

I'm not convinced.
Do you have a reference to support this?

Which bit do you contend? It would help to specify.