What is the logic of assuming the existence (or nonexistence) of alien life

[primummobile]

a) Colin: Why is this a key issue? I would've thought the key issue would have been whether life is special¹?
b) Initial conditions: There is theoretical evidence that the observable universe started out chaotically. Randomness and order would seem at home in the aftermath .. and it is !
c) Primummobile: 'Mathematically unlikely: Could you please demonstrate for us, the mathematics behind such a statement ? I keep seeing 'mathematics' being used to make a point, and yet there is nothing from mathematics I know of, which would suggest that such a statement is anything more than pure arm-waving and a hijack of mathematics.


Footnotes:
1. I dislike using the term 'special' .. something unique, doesn't have to be 'special'. That being said, I only used the term as a quote of what was said.

I said IF THE EARTH IS NOT SPECIAL, and you may substitute unique for that, the mathematical odds would seem to indicate thrlere may be other life. Latest estimates put the number of stars in the observable universe to be anywhere from 10 sextillion to 1 septillion. If other stars have planets we would consider habitable, (ie Earth is not unique, part of the original premise ) then the chances of finding exo life increases every time you find another earth. I'm not going to do any hand waving and pull numbers out of the air like Drake or Sagan always did. But I can say this: if the likelihood of life developing was equal to rolling a "six" on a six-sided die then the universe should be teeming with life. If, on the other hand, the likelihood of life devoloping is equal to a deck of cards being randomly shuffled and come up in one particular order, then you could say that it is unlikely for there to be other life in the universe. The real likelihood is somewhere in the middle of those two extremes. There is no equation to make because we would have to guess at the numbers. But it still remains that if A sometimes causes B, the more iterations you have of A the more likely you are to have outcome B. There is nothing difficult about that.

You are attacking a straw man here. I didn't say that I assumed anything. Again, I am saying is it better to assume life does not exist because of the lack of evidence, or ia it better to NOT assume life does NOT exist because we just don't have the information to assume ANYTHING. I don't know how many ways I can say this. I'm guessing we may have pretty close to the same opinion on this matter. Not assuming = assuming nothing.

[primummobile]

You said It was just my opinion that the views being taken by some was too minimalist. i.e. "extraterrestrial life does not exist because we have seen no evidence of it."

I said I read the thread in question - I must have missed the bit where somebody said that. Can you provide the quotation?

Obviously, I meant the quotation that supported your claim that some were being too minimalist.

You then gave me a load of quotations, none of which support your claim that some were being too minimalist, none of which stated or implied "extraterrestrial life does not exist because we have seen no evidence of it." So perhaps you can withdraw the claim now.

Ok, one more time. I was saying that someone else was assuming NOTHING. (All he is saying is that you should NOT assume life does not exist. ) The reply was "why not assume that"? In other words, assuming life does not exist. I provided the other quotes to give you context to the conversation. Whether or not how I am reading it is what they meant is the feason, I started this thread. As I have not received an answer to that question yet, it would be a little difficult to retract my assertation.

[primummobile]

Selfsim, I was typing on my phone, and I now see that I left out something in my last post. I had meant to say that if habitable planets are nothing special that the real chances were probably somewhere between the two extremes I happened to mention. I don't want you to think that I am claiming to know any kind of number because the whole point is that I don't and neither does anyone else.

[mutleyeng]

Selfsim, I was typing on my phone, and I now see that I left out something in my last post. I had meant to say that if habitable planets are nothing special that the real chances were probably somewhere between the two extremes I happened to mention. I don't want you to think that I am claiming to know any kind of number because the whole point is that I don't and neither does anyone else.

you cant make a case with numbers and then expect it not to be challenged just because you finish by saying "i dont know and neither does anyone else". Whether you intended to or not, you made a case as to why it is more likely that other life exists than not. That is not a neutral position. That is what i suspect Selfsim meant about being careful how you phrase yourself.
You end by saying you dont know, but in the text you slip in that the mathematical odds suggest there is other life - Thats what is getting the reaction. You said you wernt going to do any hand waving of numbers, and then you went right ahead and did it. ETA ok, you used a math argument without the numbers- no better

[Colin Robinson]

But that is sorta my point - either:
1. There is/was one very precise, extremely unlikely set of conditions and chemicals from which life can appear - which is the one I'm betting on...
or
2. There is a range of conditions and chemicals, or if they are precise, those conditions were (even briefly) widespread - in which case we *would* very likely see distinct descendants (in a regional sense) / differing biochemistry / fossil records

Do we? AFAIK, we don't, and I think we probably should. I'll happily admit I'm not widely read on this aspect of abiogenesis, but I've never seen any references to multiple 'beginnings'. Perhaps the books that were cited give some clues, but it would be nice if this aspect got a bit of coverage here..

I'm not sure I agree. If we had no idea of what we are after, then yes, but we DO know what we are after! We have examples of the primitive starting points of life all around us - they are chemically analysable, we understand the basic processes, the chemical structures... It's a pretty good example of reverse engineering where you not only have the blueprints, you also have as many samples as you need to dissect / analyse..

Of course we can't quantify it meaningfully in mathematical terms, but I think it's fair to say it is really, really difficult to get life started. Maybe even a few more reallys...

So I stand by my point - saying that life is 'abundant' on Earth is misleading, in this context.

There are two questions here:

1. What conclusions can be drawn from the fact that abiogenesis has not be duplicated in laboratories?
2. What about the fact that abiogenesis appears to have occurred only once on this Earth? Why hasn't it been repeated?

1. I think it's a little arrogant of us humans to suppose that something difficult for us must be difficult for nature. For instance, we have yet to make a practical fusion reactor. Whereas nature has made billions, we call them "stars".

2. Maybe because the first organisms cornered the market in easily available energy? That is, they began by taking advantage of the most easily processed chemical energy sources (like a mixture of substances containing both H2 and CO2). Afterwards some of their descendants evolved to use more difficult energy sources like photosynthesis (where you transform light energy into chemical energy, before doing something with the chemical energy), and reactions involving O2 (a challenging stuff to use, because of its tendency to break down life's building blocks, the organic compounds)...

I have another, more general issue with your argument...

Either the Earth is a typical example of what happens on a habitable planet, or it isn't. If Earth is a typical example, then life gets started exactly once on other habitable planets too. On the other hand, if Earth is not typical, why should we suppose that on other habitable planets life typically gets started less times than here? Why not more times?

[Selfsim]

Selfsim, I was typing on my phone, and I now see that I left out something in my last post. I had meant to say that if habitable planets are nothing special that the real chances were probably somewhere between the two extremes I happened to mention. I don't want you to think that I am claiming to know any kind of number because the whole point is that I don't and neither does anyone else.

Well, I think we're roughly on the same page … however, the point needs to be made that the 'numbers game' is a meaningless argument, because the existence of life on Earth, does not imply life exists elsewhere … and this remains no matter how many 'habitable' planets/moons exist out there.

One can portray conditional 'ifs' and possibles' as much as one likes, (usually done to lead others to a desired outcome), but the inescapable fact is, as I've stated above. The mathematical language which follows, (ie: the chances, odds, etc, etc), is not particularly relevant when one sets the initial 'going-in' conditions in concrete, beforehand.

Regards

[Colin Robinson]

Well, I think we're roughly on the same page … however, the point needs to be made that the 'numbers game' is a meaningless argument, because the existence of life on Earth, does not imply life exists elsewhere … and this remains no matter how many 'habitable' planets/moons exist out there.

Is it logically possible for billions of planets to be habitable, yet uninhabited by any sort of life? Yes, it is possible... but the existence of even one habitable-yet-lifeless planet is at present a supposition, a conjecture, and not an established empirical fact...

One can portray conditional 'ifs' and possibles' as much as one likes, (usually done to lead others to a desired outcome),

Do we want discussion about life in the universe to avoid words such as "if" and "possible"? Would that be rigor? Or rigor mortis?

[Selfsim]

Is it logically possible for billions of planets to be habitable, yet uninhabited by any sort of life? Yes, it is possible... but the existence of even one habitable-yet-lifeless planet is at present a supposition, a conjecture, and not an established empirical fact…

Well, there are already a couple of planets which have 'habitable zones', but alas, there are no human beings sending us probes and radio signals from them.

I know ! There must be exceptions !

Under such rationale, will it ever be possible to confirm 'no life' on 'billions of habitable planets' (/moons) ?
Will it ever be practically possible to search every HZ on a such a set of planets, in order to make such a confirmation ? If not, the hypothesis is unverifiable from a practical perspective, and is not a workable hypothesis. (It is falsifiable in practice however .. which demonstrates the bias, yet again).

The statement: "the existence of life on Earth, does not imply life exists elsewhere", on the other hand, is demonstrably present-day fact, and is falsifiable and verifiable in practice, even when projected into an uncertain future.

Do we want discussion about life in the universe to avoid words such as "if" and "possible"? Would that be rigor? Or rigor mortis?

Whatever ..

[Colin Robinson]

Is it logically possible for billions of planets to be habitable, yet uninhabited by any sort of life? Yes, it is possible... but the existence of even one habitable-yet-lifeless planet is at present a supposition, a conjecture, and not an established empirical fact...

Well, there are already a couple of planets which have 'habitable zones', but alas, there are no human beings sending us probes and radio signals from them.

Do you really think "No human beings sending us probes and radio signals" is the same thing as "uninhabited by any sort of life"? Doesn't the category "life" include things like aardvarks, and baobab trees, and yaks, and zooxanthellae?

I know ! There must be exceptions !

If life has appeared only on Earth, that would make Earth an extreme exception.

Under such rationale, will it ever be possible to confirm 'no life' on 'billions of habitable planets' (/moons) ?
Will it ever be practically possible to search every HZ on a such a set of planets, in order to make such a confirmation ? If not, the hypothesis is unverifiable from a practical perspective, and is not a workable hypothesis. (It is falsifiable in practice however .. which demonstrates the bias, yet again).

The statement: "the existence of life on Earth, does not imply life exists elsewhere", on the other hand, is demonstrably present-day fact, and is falsifiable and verifiable in practice, even when projected into an uncertain future.

"Falsifiability" was one of the favorite concepts of Karl Popper, the great philosopher of science. Were you aware of that, Selfsim? I find it funny that you use the term so much, even though you think scientific discussion should avoid philosophy...

[Van Rijn]

Which is why I prefaced those comments with the 'as far as I am aware' bit..

I was referring to the statement, "In all the time before or since, no other life has sprung into existence." That isn't something that can be concluded based on existing evidence. Evidence points to life starting at least once, but it could have started multiple times.

But that is sorta my point - either:
1. There is/was one very precise, extremely unlikely set of conditions and chemicals from which life can appear - which is the one I'm betting on...
or
2. There is a range of conditions and chemicals, or if they are precise, those conditions were (even briefly) widespread - in which case we *would* very likely see distinct descendants (in a regional sense) / differing biochemistry / fossil records

I don't agree those are the only options, and I definitely don't agree with the argument in item 2. Most species don't fossilize well (certainly most bacteria do not) and it's hard to say much about biochemistry unless there are currently living examples. So many things could be missed. Also, if there had been examples of distinctly alternate life, they might have disappeared for many reasons. They might never have had a large ecological niche. They could have been out competed, or faded out when conditions changed, or just could have been unlucky.

Do we? AFAIK, we don't, and I think we probably should. I'll happily admit I'm not widely read on this aspect of abiogenesis, but I've never seen any references to multiple 'beginnings'.

It is sometimes discussed as a possibility. Remember the Mono lake "arsenic life"? That turned out to be wrong, but folks were hoping that would be an example of an alternate path for life.

I'm not sure I agree. If we had no idea of what we are after, then yes, but we DO know what we are after! We have examples of the primitive starting points of life all around us

Sorry, but we don't. In any scientific abiogenesis argument, it's agreed there would have been simpler life forms than any that currently exist. See for instance:

http://exploringorigins.org/protocells.html

Of course we can't quantify it meaningfully in mathematical terms, but I think it's fair to say it is really, really difficult to get life started. Maybe even a few more reallys...

Again, we don't know that. There's a lot about abiogenesis we've yet to learn.

It could be, after we work out some more details, that we'll be able to repeat abiogenesis in the lab when anyone wants to. One day it might become a common school experiment. Or maybe we'll never get there.

We might find life on Mars. Or we might not.

It could turn out that abogenesis is virtually certain in conditions similar to that of early Earth. Or it may be that it would be uncommon, rare, extremely rare, or unique. All of those options would fit existing evidence.

[Van Rijn]

I cannot speak of all discussions on exo-life (some are carefully worded to denote the speculative bases of what is being discussed), but I was referring generally to forum discussions involving speculations which presume the existence of exo-life, as a given.

Around here, people usually get an argument if they make that assumption.

Sure, but don't forget the testability part of a proper hypothesis. In isolation, I don't challenge the 'reasonableness' of the general idea of looking for Earth-life, if one chooses to pursue the search for exo-life ... after all, there is no other choice once one embarks on such a quest. Carbon based biological life tests have to be conducted locally (or via return sample ... oh, SETI signals or ET poking faces at a robotic camera might suffice as well). This limits the practicality of the testing. Such tests cannot be applied over light year distances. This means that the hypothesis is severely constrained by practical testing limitations which ultimately limits the ability to gather the data, and form conclusions. No amount of 'reasonableness' (or logical reasoning) will overcome this practicality imposed limitation. Also, we simply don't have any data to make the firm connection between what we can measure these scales, and the presence/absence of life (whist excluding other unknown non-biological processes).

But our abilities to study exoplanets will continue to improve. If scientists find many Earthlike (and I mean really Earthlike exoplanets - not worlds with five times the mass, or with Venus-like insolation) having a combination of properties associated with life that would be very hard to explain without life, then the most reasonable explanation is that there is life on those planets.

[Selfsim]

But our abilities to study exoplanets will continue to improve. If scientists find many Earthlike (and I mean really Earthlike exoplanets - not worlds with five times the mass, or with Venus-like insolation) having a combination of properties associated with life that would be very hard to explain without life, then the most reasonable explanation is that there is life on those planets.

Oh .. I'm under no misapprehension about that being very likely to happen. That is the stage which has been set for us. But we're expected to accept that life exists on another planet light years away on the basis of an inferred model ?

That might work in classical scientific astronomy circles, (which is dominated by classical Physical Laws), but somehow I can't see it being acceptable when it comes to explaining the existence of complex exo-life. (Y'know ... something about 'extraordinary claims requiring extraordinary evidence' ..??..). I mean for instance, how can one state that science has a complete enough knowledge to rule out other, as yet unknown, natural processes capable of generating so called 'bio-signs' remotely detectable from such an environment ?

Frankly to assume that it will be accepted, seems naive in the extreme !
And what happens when it isn't accepted ? What then ? Nothing is capable of making a trip of light-year distances to gather 'first-hand' data. How will society cope with that ? Is it scientifically responsible to make such an extraordinary claim on the basis of nothing more than an incomplete inferred model, which is based purely on Earth-like conditions ?
You mention the arsenic-based claim ... that would pale into miniscule obscurity in comparison with an exo-life claim based purely on inference !

Has anyone really thought all this through, or are we simply expected to blindly follow the lead set for us by hyped up mass media ?

[iquestor]

Oh .. I'm under no misapprehension about that being very likely to happen. That is the stage which has been set for us. But we're expected to accept that life exists on another planet light years away on the basis of an inferred model ?

That might work in classical scientific astronomy circles, (which is dominated by classical Physical Laws), but somehow I can't see it being acceptable when it comes to explaining the existence of complex exo-life. (Y'know ... something about 'extraordinary claims requiring extraordinary evidence' ..??..). I mean for instance, how can one state that science has a complete enough knowledge to rule out other, as yet unknown, natural processes capable of generating so called 'bio-signs' remotely detectable from such an environment ?

Actually, it should be possible to directly infer or at least provide strong arguments for life processes at work through spectroscopy in 10-15 years. Kepler is finding many cadidates for Earth analogs, showing that reasonably Earthlike Planets (small, rocky planets around similar stars within the liquid water region) are not uncommon. Spectroscopic analysis of light diffracted through the atmosphere (where they exist) of these planets can show the presence of CHON, evidence of photosynthetic processes, and other indicators of life. Detection of artifical elements and compounds in the atmosphere would provide almost irrefutable evidence of industrial intelligent life. See Dr. Garik Israelian's TED Talk See Dr. Garik Israelian's TED Talk

If you see, in the spectrum of a planet host star, strange chemical elements, it can be a signal from a civilization which is there.” (Garik Israelian)

If there is abundant life on one of these planets it is not out of the question that we could provide strong evidence for it using remote technology such as Spectroscopy.

[kzb]

If we are talking about life only, then your only logical position at present is an open mind. The observations to collect any supporting or negating evidence have not been done yet. Hopefully this will be forthcoming in our lifetimes, but probably not absolute proof in either direction.

There will be some interesting evidence from our own solar system, maybe life will be found on Mars or Europa, and then the discussion will centre on whether it has a joint origin with Earth life or not. If it definitely has independent origin, the logical conclusion must be that life is commonplace in the universe.

If we are talking about intelligent, technological life, after our model, then there is plenty of logic that says this is incredibly rare. The central argument being, it would be obvious if we were living in a galaxy-wide civilisation. Lots of people are unconvinced by this argument, citing the extreme difficulty of interstellar travel and communication.

But I personally think we've got the MW at least all to ourselves. It's really comes down to just opinion at this stage of the argument, and we've been round the houses on this several times on here.

[primummobile]

you cant make a case with numbers and then expect it not to be challenged just because you finish by saying "i dont know and neither does anyone else". Whether you intended to or not, you made a case as to why it is more likely that other life exists than not. That is not a neutral position. That is what i suspect Selfsim meant about being careful how you phrase yourself.
You end by saying you dont know, but in the text you slip in that the mathematical odds suggest there is other life - Thats what is getting the reaction. You said you wernt going to do any hand waving of numbers, and then you went right ahead and did it. ETA ok, you used a math argument without the numbers- no better

I'm not trying to make a case. I am trying to ask a question. Is it better to assume nothing, because the only thing going for that is the numbers involved? Is it better to assume life does not exist, because we have not seen any evidence. Which is more persuasive? The ambiguity of the numbers involved is EXACTLY the reason I am asking the question.

This is simple probability theory. They start teaching it in junior high. In any random system, most things have a probability of happening. We assign it a value of between 0 and 1, with 0 meaning it cannot ever happen. 1 means it is a certainty. Personally, I don't know of any events that would be assigned a 0 or 1. Everything I know of is between the two.

If quantum mechanics is true, there is a non-zero probability that if you walk into a wall you will pass right through it. It's awfully close to zero, but it's still not. I don't know what the probability is, but i do know that you would have to wait much longer than the lifetime of the universe before you would actually see it happen. The point I was making is that it doesn't matter what the initial probability is. If it's one in a googol, and you walk into the wall twice, you have doubled the chances you will walk through the wall. Whatever the probability of it happening, your chances are still doubled. I'm asking if the mathematical probability argument is enough to let us assume anything. I'm not saying the numbers increase the probability to closer to 1, because I don't know the answer to that.

As for life in space, we don't know most of the parameters of what we would expect to possibly cause life. The only thing we know is that, unless the universe was designed, the probablility of life arising is not zero. If after we take all the parameters that we do not know into account, and we find that the probability of life arising is equal to the probability of a deck of cards being shuffled and coming out arranged in one particular order, which is 1 in 52!, we would expect to not find any other life. In fact, it would make it surprising that life is even here, since 52! is 43 orders of magnitude larger than the largest estimate I have seen for the number of stars in the universe. But even if those were the odds, two planetary systems with all parameters being equal would still give you twice the chance that one would give you. A million planetary systems of the type needed would give you a million times the chance. It would still be a miniscule chance, (1 in 8.06x10^61 instead of 1 in 8.06x10^68) but your chances are increasing with every instance of the right planetary system.

I'm getting frustrated with this because no one can see that I am not presenting this as evidence of anything. It should be obvious to anyone that the more "stuff" the universe has in it, the more chances you have for life developing. It may be a good chance, or it may be a not-so-good chance. That is why I opened the thread with the questions. They involve a whole lot of different things, and I wanted to have a discussion about it without it getting sidetracked by people asking me for numbers that I wasn't claiming to know. It would involve things like:

1) Is it logical to assume nothing about exo life?
a) Is the probability of other life, while increasing with each new planetary system, so low that we shouldn't expect to see a second instance of it yet?
b) Is the earth unique or special?
c) How many ways could life develop?
All of these deal with the probability of life developing in a random system. I just wanted to have a discussion about it without people asking for equations that no one on the planet can produce since we don't know any of the parameters.

or

2) Based on the lack of evidence, can it be assumed that we are alone?

That's it. No demands for math to prove that probability increases with frequency. No stuff about retracting statements when no one has told me explicitly what they meant. No interpreting my gut feeling to mean that I am advocating a position, because I'm not. This may have started by something I misunderstood, but that doesn't make the question any less valid.

[primummobile]

Selfsim, after reading through what you wrote since I was last on here, I am clear about what you meant. So I withdraw the assertation that you were claiming no other life exists.

[mutleyeng]

This is simple probability theory. They start teaching it in junior high.

yes i know the argument...really i do.
I just dont think its a valid argument, or a needed one as I didnt think people here denied the possibility of other life anyway.

[primummobile]

yes i know the argument...really i do.
I just dont think its a valid argument, or a needed one as I didnt think people here denied the possibility of other life anyway.

I'm not implying that you or anyone else doesn't understand probability. I'm pointing out that I'm talking about only simple probability like they teach to kids, not making any claims about what the probability actually is, or when in multiple instances of favorable planetary systems you would expect to see the first occurrence.

Think of it this way. We know that there is a non-zero chance of life arising in a random system with our physical laws because we know it has happened at least once. I'm asking if the chance is closer to 1 or closer to 0 in similar planetary systems with similar stars. Or would a completely different biochemistry be more likely to develop?

Now, you answered the question, that you don't think it is a valid argument. So do you think the probability is close to 0?

[mutleyeng]

Now, you answered the question, that you don't think it is a valid argument. So do you think the probability is close to 0?

I have absolutely no idea - double the number of supposed environments and i would still have no idea. Use any multitude you wish and i would still have no idea.

[Paul Wally]

Thinking about logical possibilities is fine ... but no progress will be made in the real world, (about the existence or otherwise of life elsewhere), unless such 'thinking' returns useful real-world data.

How so? Theory makes predictions which can be tested empirically in the real world.

I see the discussion of exolife, as not falling into any category of productive reasoning (deductive, inductive or abductive). The problem is there is no data that allows a conclusion to be formed, or allows the construction of a premise.

There already exists a premise to work from; established scientific theory. The question is: What follows from established scientific theory?

(Similarly, the lack of data cannot in itself, be considered as a valid basis for formulation of the premise that exo-life doesn't exist either, as this leads to the logical fallacy of 'argumentum ad ignorantium').

You don't seem to understand the nature of the problem. Why must exo-life existence be formulated as a premise?? Exo-life existence/ non-existence is a possible conclusion of some premise. The premise could then be something like the laws of nature as formulated in our currently established scientific theories. The problem is then, roughly, something like: Do these laws of nature make the emergence of life a likely/unlikely occurrence? Possible conclusions are:
1) Life is an unlikely occurrence.
2) Life is a likely occurence.
3) The question of the emergence of life is undecidable on the premise.

Note that undecidability is something that will have to be proven mathematically, perhaps a Godel-type undecidability, but this hasn't been proven to my knowledge. You keep mentioning that some assume "classical physics", but say we take classical physics as our premise; has any of the above-mentioned conclusions been proven to follow from classical physics? If not, then we cannot say that the assumption of classical physics leads to the conclusion (2): "Life is a likely occurrence". Classical physics does however predict the existence of chaos, so your view that there is some fundamental incompatibility between classical physics and chaos is pure myth, but it's a myth that you strongly believe in because it holds your arguments together.

[Van Rijn]

But we're expected to accept that life exists on another planet light years away on the basis of an inferred model ?

Why not, if the evidence is strong and there aren't any good non-biological models that would fit the evidence? We accept the existence of exoplanets based on evidence and models, why would this be different?

That might work in classical scientific astronomy circles, (which is dominated by classical Physical Laws), but somehow I can't see it being acceptable when it comes to explaining the existence of complex exo-life. (Y'know ... something about 'extraordinary claims requiring extraordinary evidence' ..??..).

What would be so extraordinary about a claim of life on an Earthlike world as compared to a claim of exoplanets around other stars? Reasonable standards for evidence should be expected, naturally, but I'm not aware of any good scientific argument for why there shouldn't be life on Earthlike worlds. It would be no more extraordinary than many other claims that have been accepted based on supporting evidence.

I mean for instance, how can one state that science has a complete enough knowledge to rule out other, as yet unknown, natural processes capable of generating so called 'bio-signs' remotely detectable from such an environment ?

How can we rule out other explanations for the evidence we believe points to exoplanets? Maybe it's all an illusion, and there are no exoplanets.

It is important to be careful, be sure you have solid evidence, and verify there are no reasonable alternative explanations for the evidence. But if you reject solid evidence when there are no reasonable alternative explanations, you're no longer looking at this scientifically.

Has anyone really thought all this through, or are we simply expected to blindly follow the lead set for us by hyped up mass media ?

Yes, many of us have thought this through.

[Selfsim]

Think of it this way. We know that there is a non-zero chance of life arising in a random system with our physical laws because we know it has happened at least once. I'm asking if the chance is closer to 1 or closer to 0 in similar planetary systems with similar stars. Or would a completely different biochemistry be more likely to develop?

Now, you answered the question, that you don't think it is a valid argument. So do you think the probability is close to 0?

No, its not a valid argument - far from it, in fact.

Firstly what is the 'random system' and how does this apply to life in our observable universe ? The physics of our obs. universe at astronomical scales, is certainly dominated by the known fundamental forces, described by a set of physical laws and constants, which have overwhelming empirical supporting evidence … and yet uniqueness and diversity are abundant. There are radically different effects at different scales (eg: quantum effects at small scales .. classical effects at planetary and upwards … relativistic at near light speeds … chaotic at everyday scales … (the list goes on)) .. All this renders physical predictability far from a 'given'.

Secondly, if you are assuming the parent population of the distribution of life in the obs. universe to be a normal one, there is no evidence of that. As a matter of fact, there is measured evidence that the distribution of galaxy clusters is fractal … so at the galaxy cluster level, the distribution of hypothesised life, (under current also invalid assumptions of being galaxy cluster-bound with a one-to-one correspondence), would be fractal with dimension around 2 .. which vastly differs from a normal distribution .... so, once again your assumptions about the parent distribution, renders the subsequential analysis inappropriate and moot (at one's most forgiving). Scale dependency exists !

Thirdly, we know of only one occurrence of life on Earth … there is no 'least' in 'one'. 'One' is fact. 'Least' is an unsupported speculative premise .. whereas it is actually stated as 'fact' in your post above .. !...

Statistics do not reveal 'truths'.
The answer to your question of Mutleyeng, based on the above real-world evidence-based rationale, is a definitive and emphatic: "Unknown" !

[Colin Robinson]

First, the Nessie or Sasquatch question. I was using those as examples of an absurdity. Namely, that no one here is advocating any of the absurd claims that you see from cryptozoologists, but I don't see a real distinction between how some react to cryptozoology claims and how they react to claims that extraterrestrial life exists... We almost universally reject the notion that sasquatch exists because we have explored almost every square foot of the land surface of the earth and never found any evidence to confirm his existence, yet we have found evidence of all other types of life spanning billions of years. Therefore, it is absurd to assume Sasquatch exists because the overwhelming evidence says he does not.

In the case of Sasquatch, we have clear and overwhelming evidence of absence. In the case of alien life, what we have is absence of evidence.

Perhaps another key difference between aliens and sasquatch is this...

As you've mentioned, the proposition that extraterrestrial life does not exist, logically implies that the planet Earth is special -- radically exceptional, in fact. By contrast, the proposition that sasquatch does not exist, does not logically imply anything about a particular place being exceptional.

[primummobile]

Perhaps another key difference between aliens and sasquatch is this...

As you've mentioned, the proposition that extraterrestrial life does not exist, logically implies that the planet Earth is special -- radically exceptional, in fact. By contrast, the proposition that sasquatch does not exist, does not logically imply anything about a particular place being exceptional.

That's a good point.

[primummobile]

No, its not a valid argument - far from it, in fact.

Firstly what is the 'random system' and how does this apply to life in our observable universe ? The physics of our obs. universe at astronomical scales, is certainly dominated by the known fundamental forces, described by a set of physical laws and constants, which have overwhelming empirical supporting evidence … and yet uniqueness and diversity are abundant. There are radically different effects at different scales (eg: quantum effects at small scales .. classical effects at planetary and upwards … relativistic at near light speeds … chaotic at everyday scales … (the list goes on)) .. All this renders physical predictability far from a 'given'.

Secondly, if you are assuming the parent population of the distribution of life in the obs. universe to be a normal one, there is no evidence of that. As a matter of fact, there is measured evidence that the distribution of galaxy clusters is fractal … so at the galaxy cluster level, the distribution of hypothesised life, (under current also invalid assumptions of being galaxy cluster-bound with a one-to-one correspondence), would be fractal with dimension around 2 .. which vastly differs from a normal distribution .... so, once again your assumptions about the parent distribution, renders the subsequential analysis inappropriate and moot (at one's most forgiving). Scale dependency exists !

Thirdly, we know of only one occurrence of life on Earth … there is no 'least' in 'one'. 'One' is fact. 'Least' is an unsupported speculative premise .. whereas it is actually stated as 'fact' in your post above .. !...

Statistics do not reveal 'truths'.
The answer to your question of Mutleyeng, based on the above real-world evidence-based rationale, is a definitive and emphatic: "Unknown" !

Sorry, but nothing can convince me that our existence is due to anything but random chance. Our physical lawz do govern much of how the universe unfolds, but we can't just rule out all the randomness. The quantum realm is rife with randomness and that eventually leads to randomness in ever larger systems. Uniqueness does not imply something isn't random. It implies that there are many different ways for the universe to unfold.

I also don't see any distinction between "one" and "at least one". If I say that I have at least one dollar, it means that I have one dollar and I may have more. If I say I have one dollar, it means I have one. But until I find that second dollar I will never have any more than just one dollar.

What you say does intrigue me, though. I will give it some thought.

[Selfsim]

Is it logically possible for billions of planets to be habitable, yet uninhabited by any sort of life? Yes, it is possible... but the existence of even one habitable-yet-lifeless planet is at present a supposition, a conjecture, and not an established empirical fact…

Under such rationale, will it ever be possible to confirm 'no life' on 'billions of habitable planets' (/moons) ?
Will it ever be practically possible to search every HZ on a such a set of planets, in order to make such a confirmation ? If not, the hypothesis is unverifiable from a practical perspective, and is not a workable hypothesis. (It is falsifiable in practice however .. which demonstrates the bias, yet again).

The statement: "the existence of life on Earth, does not imply life exists elsewhere", on the other hand, is demonstrably present-day fact, and is falsifiable and verifiable in practice, even when projected into an uncertain future.

"Falsifiability" was one of the favorite concepts of Karl Popper, the great philosopher of science. Were you aware of that, Selfsim? I find it funny that you use the term so much, even though you think scientific discussion should avoid philosophy...

So, now it seems, there is wry amusement through ignoring (and undermining) the distinctly different, evidence based, applied philosophical tenets of scientific methodology, in order to defend a fundamentally flawed attempt at a scientific hypothesis, presented embedded in an inference based wrapper ?

A classic reaction to exposure through scientific rigour … and precisely why:

I see the discussion of exolife, as not falling into any category of productive reasoning (deductive, inductive or abductive). The problem is there is no data that allows a conclusion to be formed, or allows the construction of a premise.

[Selfsim]

I also don't see any distinction between "one" and "at least one". If I say that I have at least one dollar, it means that I have one dollar and I may have more. If I say I have one dollar, it means I have one. But until I find that second dollar I will never have any more than just one dollar.

What you say does intrigue me, though. I will give it some thought.

Have a read through this thread (from my post #89 onwards). Caveman1917 explains the basis of validity (from a mathematical principals perspective).

[Colin Robinson]

So, now it seems, there is wry amusement through ignoring (and undermining) the distinctly different, evidence based, applied philosophical tenets of scientific methodology, in order to defend a fundamentally flawed attempt at a scientific hypothesis, presented embedded in an inference based wrapper ?

If your intention is to apply philosophical tenets of scientific methodology, then I respectfully suggest that you take the philosophy of science a little more seriously. I'd specifically suggest that you have a look what Karl Popper says about the relation between falsifiability and conjecture (or "supposition" as Paul Wally calls it), for instance in Popper's book Conjectures and Refutations.

[Selfsim]

But our abilities to study exoplanets will continue to improve. If scientists find many Earthlike (and I mean really Earthlike exoplanets - not worlds with five times the mass, or with Venus-like insolation) having a combination of properties associated with life that would be very hard to explain without life, then the most reasonable explanation is that there is life on those planets.

But we're expected to accept that life exists on another planet light years away on the basis of an inferred model ?

Why not, if the evidence is strong and there aren't any good non-biological models that would fit the evidence? We accept the existence of exoplanets based on evidence and models, why would this be different?

Well, you've qualified the hypothetical scenario under the assumption of 'strong evidence'. Strong evidence would be prior biological test results, sourced from some local planet other than Earth, (after all, we are talking about biology as the subject, aren't we ?), or perhaps evidence sourced from some kind of lab synthesised, second abiogenesis, or the reception of an intelligible SETI signal(s), ET(s) poking faces into a remotely monitored robotic camera, etc, etc.

I am dubious about the non-existence of 'any good non-biological models that would fit the evidence', but, for the sake of the hypothetical, I'll go a long with it.

The existence of life on Earth, does not imply life exists elsewhere, is a present-day fact. It needs to be countered with falsifying new evidence of equal 'weight'. If this is presented, then I see no problems overturning the statement.

That might work in classical scientific astronomy circles, (which is dominated by classical Physical Laws), but somehow I can't see it being acceptable when it comes to explaining the existence of complex exo-life. (Y'know ... something about 'extraordinary claims requiring extraordinary evidence' ..??..).

What would be so extraordinary about a claim of life on an Earthlike world as compared to a claim of exoplanets around other stars? Reasonable standards for evidence should be expected, naturally, but I'm not aware of any good scientific argument for why there shouldn't be life on Earthlike worlds. It would be no more extraordinary than many other claims that have been accepted based on supporting evidence.

In the case of a firm conclusion of exo-life, I think there's a clear need for way more justification, over and above what is needed to support the claim of the discovery of 'Earth-like worlds'. The weight of hundreds of years of observational evidence and physical theory, leading onto planetary development theory, and repeatable local tests on the measurement techniques, tightly constrains interpretation of the measured data, which serves to eliminate uncertainties. Similar constraints don't exist for exo-life, particularly when it comes to the details of the 'precise' environmental conditions when life emerged. (Admittedly, there are constraints for steady-state existence however, in the form of critical sensitivities to environmental habitability).

Anyway, theoretically, there still exists the case of an Earth-like planet which has no life. This scenario still requires evidence in order for it to be eliminated, thus allowing for the firm exo-life conclusion. (I guess this comes back to the no 'good non-biological models to fit the evidence' ... ). In this same category, there is also a class of dynamic system where the tiniest difference in initial conditions (on any scales), could make all the difference in the outcome of a subsequent 'trial'. If life is hypothesised to have emerged from such a system, (and I'm not saying it did), then it would also require elimination from consideration, in order to conclude life (at least in theory). How would one go about doing this ..?.. (same answer as above, I guess).

How can we rule out other explanations for the evidence we believe points to exoplanets? Maybe it's all an illusion, and there are no exoplanets.

.... The weight of hundreds of years of observational evidence and physical theory, leading onto planetary development theory, and repeatable local tests on the measurement techniques, tightly constrains interpretation of the measured data. Similar constraints don't exist for exo-life.
(If you were fishing for Astrophysical/Astronomy ATM from me, you won't find it .. unless I've made an unintentional error somewhere … )

It is important to be careful, be sure you have solid evidence, and verify there are no reasonable alternative explanations for the evidence. But if you reject solid evidence when there are no reasonable alternative explanations, you're no longer looking at this scientifically.

Agreed .. who was rejecting any evidence ? In this case, there isn't any to reject. The explanation: 'Unknown' with accompanying solid evidence, is still a perfectly valid scientific state for something to be in.

Yes, many of us have thought this through.

I look forward to seeing the evidence which might lead me to this conclusion.

Regards

[Selfsim]

Thinking about logical possibilities is fine ... but no progress will be made in the real world, (about the existence or otherwise of life elsewhere), unless such 'thinking' returns useful real-world data.

How so? Theory makes predictions which can be tested empirically in the real world.

When I last checked, 'logical possibilities' (meaning pure speculations) are vastly different from proper theories the latter of which, I agree, should ultimately lead to useful real-world data.

There already exists a premise to work from; established scientific theory. The question is: What follows from established scientific theory?

Which established scientific theory leads to the emergence of universal life ?

You don't seem to understand the nature of the problem. Why must exo-life existence be formulated as a premise??

It doesn't ! Did I say it had to ?

Exo-life existence/ non-existence is a possible conclusion of some premise. The premise could then be something like the laws of nature as formulated in our currently established scientific theories. The problem is then, roughly, something like: Do these laws of nature make the emergence of life a likely/unlikely occurrence?

Specifically, which laws of nature, and which currently established scientific theories are you formulating your premise on ... and why ?

Possible conclusions are:
1) Life is an unlikely occurrence.
2) Life is a likely occurence.
3) The question of the emergence of life is undecidable on the premise.

Note that undecidability is something that will have to be proven mathematically, perhaps a Godel-type undecidability, but this hasn't been proven to my knowledge. You keep mentioning that some assume "classical physics", but say we take classical physics as our premise; has any of the above-mentioned conclusions been proven to follow from classical physics? If not, then we cannot say that the assumption of classical physics leads to the conclusion (2): "Life is a likely occurrence".

But that's just my point ! The usage of purely classical physics theory, to justify either (1) or (2), is inappropriate.

Classical physics does however predict the existence of chaos,

I would've said that mathematics exposed the existence of chaotic behaviours in theory. Measured data then confirmed its existence in nature. The expected determinism inherent in theoretical mathematics however, doesn't nessarily map across into the real world .. and that is one of the big lessons from Chaos Theory.

so your view that there is some fundamental incompatibility between classical physics and chaos is pure myth, but it's a myth that you strongly believe in because it holds your arguments together.

Huh ?? Please explain further the concepts you are trying to express on my behalf !??!
I do not recognise them from your paraphrasing !