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Thread: Evidence for ET is mounting daily, but not proven.

  1. #451
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    Quote Originally Posted by A.DIM View Post
    Thanks for that Trakar.
    It corroborates what I said.
    actually it is distinctly different.

    Certainly, and as I said, there are very few, if any?, scientists who'd suggest life couldn't have arisen elsewhere.
    what you said was
    ...I know of no scientist who'd suggest life is not likely to have arisen elswhere.
    again very different to anyone interested in accuracy.

  2. #452
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    The gist is identical.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Jeff Root View Post
    You have insisted over and over again that if ETs were common,
    they would be obvious to us, yet, long after being asked, you still
    have not said what would make them visible. Why?
    I have answered that question about 5 times. Maybe you should just go back and read over my previous posts. But I'll reiterate them here too.

    As Isaac Kuo says, "there are many possibilities!" How about a crashed probe, or the communications of a galactic radio station? How about more blinking of stars as great solar panels orbit them? We have observed the night sky a lot and not seen any sign of interstellar ET.

    But as I have said before the real compelling evidence is that we even evolved here at all, seemingly in vacuo relative to life outside the Earth. It really would appear there has never been any interaction between life on Earth and life outside the Solar System. If there had been, it's quite possible we wouldn't be here at all, as evidenced by the fact that we humans were the first species to evolve intelligent civilization here-- may not have been possible had some other species beat us to it.

  4. #454
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    Quote Originally Posted by IsaacKuo View Post
    We, in contrast, can see that there are many possibilities.
    Isaac, nowhere have I said anything about a lack of possibilities. My arguments are centered around the fact that the lack of evidence shows that ET is not "common" as Jeff Root stated.
    Quote Originally Posted by IsaacKuo View Post
    Note that I'm talking about ET. ET means extra-terrestrial life. If you want to use the term "ET", I will hold you to using it in the correct manner. ET does not mean extra-terrestrial technological civilizations.
    Oh gee, thanks for the lesson in terminology. I wasn't aware of that. I guess rather than address my real arguments, its easier to spam the thread with pedantics?

    Quote Originally Posted by IsaacKuo View Post
    As for your choice of "hundreds of millions of years"...that's a very odd choice. Humans haven't even been around for hundreds of millions of years. If aliens did something observable, and only dinosaurs were around to observe it...so what?
    This shows that you've missed my point again. Their effect is more than just a probe.

    You take our civilization and extrapolate that we will persist for millions of years and expand to many star systems.

    If such civilizations were common, then in hundreds of millions of years, one would've expanded to our system. I'm not talking about just a probe observed by dinosaurs, I'm talking about interaction that completely changes the course of the evolution of life on Earth. Because likely it's not just a probe, maybe at first its a probe, then within a hundred years later its a spore that colonizes Earth.

    So our existence alone, by virtue of evolving here unperturbed is evidence that there are not a lot of other humanlike civilizations who have passed this way over the billion year history of Earth.

    I'm not saying they're impossible, I'm just saying that by any good definition of the word common, something that has never been observed despite reasonable arguments for why it SHOULD have been observed, should not be deemed "common."

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    Quote Originally Posted by CaptainToonces View Post
    I have answered that question about 5 times. Maybe you should just go back and read over my previous posts. But I'll reiterate them here too.
    As Isaac Kuo says, "there are many possibilities!" How about a crashed probe, or the communications of a galactic radio station? How about more blinking of stars as great solar panels orbit them? We have observed the night sky a lot and not seen any sign of interstellar ET.
    None of these possibilities are any more compelling than alternatives Isaac Kuo and others have pointed out.
    I find it difficult to think advaced ET could traverse interstellar distances and then ... crash when they get here. I find it difficult to think advanced ET would utilize "galactic radio stations" transmitting on only one frequency, that of hydrogen, the only one we're listening in on. I find it difficult to agree humans have sufficiently surveyed the heavens to determine no astroengineering has been observed, especially through the haze of the galactic center.
    We really have no idea what we're looking for when it comes to advanced ET and likely wouldn't know it if we saw it.

    But as I have said before the real compelling evidence is that we even evolved here at all, seemingly in vacuo relative to life outside the Earth. It really would appear there has never been any interaction between life on Earth and life outside the Solar System. If there had been, it's quite possible we wouldn't be here at all, as evidenced by the fact that we humans were the first species to evolve intelligent civilization here-- may not have been possible had some other species beat us to it.
    Maybe so, Captain Toonces, but I don't find that any more compelling as evidence. It's just as likely Earthlings are but a primitive jungle society, a la "zoo hypothesis," immersed in a far more advanced civilization than can be conceived.
    Where the telescope ends, the microscope begins. Which of the two has the greater view?

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    To me the most compelling argument that suggests that we would easily see an alien civilisation is one derived from the Fermi Paradox. If alien civilisations were both common and diverse, then one or more of them would become expansive, and in a very short time compared to the current lifetime of the universe, fill all the star systems with colonies. It may be the case that most do not do this, but we are not concerned with the stay-at-homes; expansive civilisations would be visible simply because they are expansive.

    If there were expansive civilisations anywhere in our galaxy they would have been here long ago; they would be us. We would be speaking a local dialect of Old Galactic and learning billions of years of ancient history. We could easily see evidence of ancient advanced civilisations by looking in the mirror.

    There are many suggestions as to why this hasn't happened; but the most consistent and compelling suggestion is that they quite simply aren't there.

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    Quote Originally Posted by eburacum45 View Post
    If there were expansive civilisations anywhere in our galaxy they would have been here long ago; they would be us. We would be speaking a local dialect of Old Galactic and learning billions of years of ancient history. We could easily see evidence of ancient advanced civilisations by looking in the mirror.
    I think this is not that simple. Civilisation as is is too unstable to survive significant (in astronomical scale) time, and bilions of years are even more unprobable. They either wither and die or Get Outta Here. Our universe is fine cradle and all of that, but it have rather annoying constraints.

    Of course, this does not solve Fermie paradox, but things like "bilion year old galactic empires" simply cannot exist, especially if constraint of v < c upholds.

    Why it does not solve, if eveyone either die off or move out of cradle? It takes just one. Even if fragmented due to v<c, even if stagnated (neccessary - progress will inevitably lead to going out, and recess will inevitably kill), we would be seeing in mirror latest batch of unlucky survivors of past rises and falls. I personally do not see very many so long persisting in stagnation civilisations, but even if we would be as we really are - fresh batch - you could not fart wihout hitting some lying artifact from previous eras belonging to former tentants of this rock.

    So... no solution (no surprise here), but still fun speculation.

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    Quote Originally Posted by eburacum45 View Post
    To me the most compelling argument that suggests that we would easily see an alien civilisation is one derived from the Fermi Paradox. If alien civilisations were both common and diverse, then one or more of them would become expansive, and in a very short time compared to the current lifetime of the universe, fill all the star systems with colonies. It may be the case that most do not do this, but we are not concerned with the stay-at-homes; expansive civilisations would be visible simply because they are expansive.

    If there were expansive civilisations anywhere in our galaxy they would have been here long ago; they would be us. We would be speaking a local dialect of Old Galactic and learning billions of years of ancient history. We could easily see evidence of ancient advanced civilisations by looking in the mirror.

    There are many suggestions as to why this hasn't happened; but the most consistent and compelling suggestion is that they quite simply aren't there.
    :Applause:
    Not that I agree in all details, but it is the essence of Fermi's paradox

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    Quote Originally Posted by Colin Robinson View Post
    I agree that is rash to go from the fact that organisms here on earth seem to have a common ancestor, to infer that abiogenesis happened here just once, then to conclude that it must be extremely rare in the universe.

    It may be, as you've suggested, that there were several instances of abiogenesis, out of which one succeeded and the others fizzled out, because they were crowded out by the offspring of the successful species. Or it may be that the first organism that could metabolize and reproduce (even if at first it did both those things very slowly and clumsily) went on to fill the niches available for very simple organisms, before any unrelated competitor even got started.

    While I wouldn't take the Copernican principle so far as to assume there is nothing unusual (or improbable) about our planet, nor do I think we should assume its improbability without strong evidence. Maybe on a planet with the right host conditions, abiogenesis is probable or even inevitable; but once evolution has begun, a 2nd abiogenesis (on the same planet) is then unlikely.
    Thank you for the response, but I am really making 2 points.

    #1 You addressed. The possibility that other (other than the chain that we evolved from) non-surviving "life forms" arose from other abiogenesis events in the early Earth but were not robust enough to survive or were crowded out by the one surviving chain of life from which we all hail.

    #2 For those that contend that abiogenesis happened only once on Earth, do they literally mean that on March 23rd at 3:35pm GMT at long 54.55/lat 34.36 that "the abiogenesis" event took place and all subsequent reproduction, evolution, mutation and ultimately resulted in us, OR, that because all of the raw ingredients were the same and the conditions were the same that multiple abiogenesis events took place at different locations and at different times, but all yielded the identical genetic result (again from which we all hail). Logically, I would think that the latter occurred (basically saying that one could replicate the same lab experiment a week from now as long as the conditions were the same) and not the former. Within this latter scenario I do think that it suggests that abiogenesis is more likely to occur elsewhere because it is a more robust process that can be replicated over time as long as the ingredients/conditions are the same.

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    While I've defended abiogenesis as more likely common than not, what I am not so clear on is if intelligence isn't far more rare. To our knowledge, there were no highly intelligent tool-makers among the dinosaurs, in spite of all the survival pressures, changing ecological niches, millions of years and so on. I am not aware of arguments showing intelligence to be preferentially selected for in the broadest sense; ie, no overarching evolutionary imperative for its emergence (like to hear an argument if someone has one).

    The pace of natural extinction events may also be high enough in most systems to reduce long-term chances for intelligence even further. In my understanding, extinction events bathe the galactic core, making the outer rim the goldilocks area for intelligence, no? I wonder if it is indeed valid to suggest that if there is life elsewhere in our galaxy it must have had time to expand. That assumes an early starting date in galactic evolution, while it could be generally true that stable life-supporting systems are later rather than early developments. Is there a reason that cannot be so? Perhaps you kind astronomers can fill me in.

    We may very well be the first in our galaxy, or even part of the first fledgling batch, I reckon.
    For each man, according to the measure of his intelligence, must speak what he can speak, and do what he can do. - Alfred, King of Wessex
    Calm down, have some dip. -George Carlin

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    Quote Originally Posted by eburacum45 View Post
    To me the most compelling argument that suggests that we would easily see an alien civilisation is one derived from the Fermi Paradox. If alien civilisations were both common and diverse, then one or more of them would become expansive, and in a very short time compared to the current lifetime of the universe, fill all the star systems with colonies... There are many suggestions as to why this hasn't happened; but the most consistent and compelling suggestion is that they quite simply aren't there.
    One or more of them might "become expansive" but in order to fill the galaxy it would need to remain expansive for 50 thousand years at least, assuming travel close to speed to light...

    Which may seem a likely scenario, IF you assume that interstellar travel is not only possible, but becomes easier, cheaper and generally more attractive the more you do of it... Which is comparable to what has happened with cars and planes in the last century...

    But what if interstellar travel is not really comparable?

    At present we have certain knowledge of

    * one instance of abiogenesis (Earth),
    * one instance of a planet with a technological civilization (Earth),
    * zero instances of a civilization which sends colonists to other stars.

    One hypothesis -- subject of an article by Geoffrey Landis of the NASA Lewis Research Center, published in 1998 -- is that going from one star system to another is indeed possible, but intrinsically difficult and costly. Even if a culture does become expansive and succeeds in establishing a colony in a nearby stellar system, the colony takes time to develop, it becomes essentially a new culture, and there is a substantial chance it won't dedicate its resources to further colonization. Landis also suggests that it would be still more difficult -- perhaps impossible -- to colonize another stellar system if a technological civilization is already there.

    The bottom line of Landis' argument... You would then get civilizations spreading through space in a non-uniform way: a process of percolation, where some regions of the galaxy do get colonized, while others do not.

    http://www.geoffreylandis.com/percolation.htp

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    Quote Originally Posted by KABOOM View Post
    #2 For those that contend that abiogenesis happened only once on Earth, do they literally mean that on March 23rd at 3:35pm GMT at long 54.55/lat 34.36 that "the abiogenesis" event took place and all subsequent reproduction, evolution, mutation and ultimately resulted in us, OR, that because all of the raw ingredients were the same and the conditions were the same that multiple abiogenesis events took place at different locations and at different times, but all yielded the identical genetic result (again from which we all hail). Logically, I would think that the latter occurred (basically saying that one could replicate the same lab experiment a week from now as long as the conditions were the same) and not the former. Within this latter scenario I do think that it suggests that abiogenesis is more likely to occur elsewhere because it is a more robust process that can be replicated over time as long as the ingredients/conditions are the same.
    I see the logic in what you are saying. But compared to laboratory experiments, natural processes tend to be more complex and less predictable. Scientists have different conceptions about how abiogenesis happened, but the descriptions I've seen all involve a degree of randomness. The French biochemist Jacques Monod, who didn't think there was anything inevitable about life on Earth, compared it to a "Monte Carlo game". On the other hand the biologist Harold Morowitz and the physicist Eric Smith have developed a theory (involving principles of energy and entropy) where the appearance of life actually is inevitable, like the formation of snow flakes from water vapour at a particular temperature and pressure. But if you look at snow flakes under a microscope, you are unlikely to find two identical ones.

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    Quote Originally Posted by KABOOM View Post
    Thank you for the response, but I am really making 2 points.

    #1 You addressed. The possibility that other (other than the chain that we evolved from) non-surviving "life forms" arose from other abiogenesis events in the early Earth but were not robust enough to survive or were crowded out by the one surviving chain of life from which we all hail.

    #2 For those that contend that abiogenesis happened only once on Earth, do they literally mean that on March 23rd at 3:35pm GMT at long 54.55/lat 34.36 that "the abiogenesis" event took place and all subsequent reproduction, evolution, mutation and ultimately resulted in us, OR, that because all of the raw ingredients were the same and the conditions were the same that multiple abiogenesis events took place at different locations and at different times, but all yielded the identical genetic result (again from which we all hail). Logically, I would think that the latter occurred (basically saying that one could replicate the same lab experiment a week from now as long as the conditions were the same) and not the former. Within this latter scenario I do think that it suggests that abiogenesis is more likely to occur elsewhere because it is a more robust process that can be replicated over time as long as the ingredients/conditions are the same.
    Until we know a lot more about the abiogenesis process, these issues are impossible to properly address. My original consideration went along the lines that there may have been many examples of life on the early proto-Earth, but that the last major impact event destroyed all but a varient which had adapted and evolved to deep hot-rock environments an early extremophile, and thus survived as the rest of the planet was sterilized. This group then gradually recolonized the planet either preventing any further instances of abiogenic competition, or other conditions were substantively different so that abiogenesis was no longer viable. But this is largely speculation without much evidenciary support.

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    Quote Originally Posted by CaptainToonces View Post
    Isaac, nowhere have I said anything about a lack of possibilities.
    You constantly insist that the only possibility is what you find plausible.
    Oh gee, thanks for the lesson in terminology. I wasn't aware of that. I guess rather than address my real arguments, its easier to spam the thread with pedantics?
    I let it go many times, but enough is enough. ET has a particular meaning, from the very title of the thread.

    I have addressed your arguments many times already. You don't agree. Maybe you don't even "get it".
    This shows that you've missed my point again. Their effect is more than just a probe.
    Or maybe the effect is just a probe. That's my point. Your point is that the effect MUST be more than just a probe. I get that. And I don't agree.
    You take our civilization and extrapolate that we will persist for millions of years and expand to many star systems.

    If such civilizations were common, then in hundreds of millions of years, one would've expanded to our system.
    Maybe, maybe not. You think it's the only possibility. I think not. We don't expand to every place on Earth. Similarly, we wouldn't expand to every place in the galaxy.
    I'm not talking about just a probe observed by dinosaurs, I'm talking about interaction that completely changes the course of the evolution of life on Earth. Because likely it's not just a probe, maybe at first its a probe, then within a hundred years later its a spore that colonizes Earth.
    Or maybe it's just a probe. If they're so interested in studying Earth because it's just so incredible and awesome, one possible way they might want to study Earth is to STUDY Earth, not colonize it. There are plenty of star systems out there to colonize.

    Plausibly, other star systems are far more desirable. For example, I like to point out the incredible potential of black hole systems. Fast interstellar travel and expansion between black hole systems could actually be rather cheap, compared to travel and expansion to pathetic star systems like ours. Why colonize low desirability star systems with little power, few resources, and which are expensive to travel to, when it's cheap to travel to high desirability star systems with plentiful power and resources?

    And you also discount the possibility of aliens causing things which we simply don't recognize as alien caused.

    Okay, let's say aliens decided that dinosaurs were ugly. For whatever reasons, they decide to wipe them out. Cheapest easiest way to do this? Deflect a NEO onto a collision course. Voila! No more ugly dinosaurs. Mission accomplished!

    So, even if aliens did something that we have massive evidence of, and even if it had a massive effect on our history, that doesn't mean we would recognize it as alien activity.
    So our existence alone, by virtue of evolving here unperturbed is evidence that there are not a lot of other humanlike civilizations who have passed this way over the billion year history of Earth.
    You know, we have not evolved here "unperturbed". Mother nature has perturbed our planet with incredible changes and mass extinction events. I doubt any of these were caused by aliens. But what if some of them were caused by aliens? It may look all the same to us.

    Anyway, you have a highly restrictive definition of "humanlike" civilization which doesn't seem to include human civilizations. We humans don't expand everywhere. We leave the majority of the Earth uninhabited and practically undisturbed because it's simply not desirable to us.
    I'm not saying they're impossible, I'm just saying that by any good definition of the word common, something that has never been observed despite reasonable arguments for why it SHOULD have been observed, should not be deemed "common."
    You're not giving reasonable arguments for why it SHOULD have been observed. The basic problem is that you insist that alien civilizations SHOULD behave in the way you expect them to. You think falsely that humans expand everywhere, and that therefore aliens must expand everywhere.

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    Quote Originally Posted by eburacum45 View Post
    To me the most compelling argument that suggests that we would easily see an alien civilisation is one derived from the Fermi Paradox. If alien civilisations were both common and diverse, then one or more of them would become expansive, and in a very short time compared to the current lifetime of the universe, fill all the star systems with colonies.
    You mean, fill all of the star systems in the galaxy, or maybe the nearby galaxies. The universe is a terribly big place, compared to how long the universe has been around.
    If there were expansive civilisations anywhere in our galaxy they would have been here long ago; they would be us. We would be speaking a local dialect of Old Galactic and learning billions of years of ancient history. We could easily see evidence of ancient advanced civilisations by looking in the mirror.
    I used to believe this argument. That was before I really thought about it. Expansive civilizations wouldn't be able to just expand everywhere they wanted. They'd run up against each other. If we just look at one galaxy, then plausibly the first civilization to go interstellar will expand throughout the galaxy before anyone else has a chance. Even if a few challengers pop up before total galactic domination, the challengers will be hopelessly outnumbered by the first one.

    Okay...then what? Will this first expansive civilization colonize all star systems? Or just the ones which they find desirable?

    Years ago, I thought our star system was desirable so I thought this factor didn't really matter. But the more I learned about other star systems, the more I realized just how much is out there. Now, I can list many reasons why our star system is not so great. (The one I'm most annoyed by is the lack of a warm jupiter...that would really open up the solar system to us.)
    There are many suggestions as to why this hasn't happened; but the most consistent and compelling suggestion is that they quite simply aren't there.
    I don't find it the most consistent, nor the most compelling, for one big reason--we exist. If we exist, but there is no other life in the universe, then that is to me very strange. Life on Earth didn't pop up after billions and billions of years, which is what you'd expect if it were a stupendously rare chance event. It only took hundreds of millions of years, maybe less.

    Now, I would be extremely excited at the prospect that we're alone in the universe. If so, then my various concepts of how to implement intergalactic transportation and colonization could come to fruition someday. There are so many great things that would be possible, from my point of view. But I would find it extremely odd.

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    Quote Originally Posted by IsaacKuo View Post
    You mean, fill all of the star systems in the galaxy, or maybe the nearby galaxies. The universe is a terribly big place, compared to how long the universe has been around.

    I used to believe this argument. That was before I really thought about it. Expansive civilizations wouldn't be able to just expand everywhere they wanted. They'd run up against each other. If we just look at one galaxy, then plausibly the first civilization to go interstellar will expand throughout the galaxy before anyone else has a chance. Even if a few challengers pop up before total galactic domination, the challengers will be hopelessly outnumbered by the first one.

    Okay...then what? Will this first expansive civilization colonize all star systems? Or just the ones which they find desirable?

    Years ago, I thought our star system was desirable so I thought this factor didn't really matter. But the more I learned about other star systems, the more I realized just how much is out there. Now, I can list many reasons why our star system is not so great. (The one I'm most annoyed by is the lack of a warm jupiter...that would really open up the solar system to us.)

    I don't find it the most consistent, nor the most compelling, for one big reason--we exist. If we exist, but there is no other life in the universe, then that is to me very strange. Life on Earth didn't pop up after billions and billions of years, which is what you'd expect if it were a stupendously rare chance event. It only took hundreds of millions of years, maybe less.

    Now, I would be extremely excited at the prospect that we're alone in the universe. If so, then my various concepts of how to implement intergalactic transportation and colonization could come to fruition someday. There are so many great things that would be possible, from my point of view. But I would find it extremely odd.
    Statistically it would be an extremely rare and unusual event to roll twenty large straights in a row in yahtzee, but the odds are the same regardless of whether the series comes near the beginning of a long test run, or not until near the end of a long test run. In Poker, the odds of me getting a royal flush is around 1 in ~650,000, that doesn't mean I have to play 650,000 hands of poker before I get a royal flush. Yes, it would be odd, but perhaps life is odd.

    This doesn't mean that there are no others, merely that, at this time, in our relatively close neighborhood, we appear to have some elbow room, in the future that may not be the case. If we can survive our own death wish, and all the boogeymen the natural world can toss at us, perhaps we'll live to test our mettle against the children of other stars, but I'm not in any hurry.

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    The big limiting factor to me, is the speed of light. When we look out at the universe, we look out and back. There could be massive EM signatures, stars being shifted into orderly structures and all manner of detectable signs headed our way at the speed of light right this minute, from massive technological civs that are tens-of-thousands/hundreds-of-thousands/millions of years in advance of our own, but advanced technological life is rare and recent enough that the nearest such civs are hundreds-of-thousands/millions/tens-of-millions of light years distant. The goldilocks zone, must also have a temporal component, life as we know it (not that others aren't possible, merely that we have no evidences of such and so really can't speak to it one way or the other) couldn't have come about in the early universe. Our Sun and planetary systems formed between 5-6 Billion years ago (The first few High Straights in a row),...

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    Quote Originally Posted by Trakar View Post
    The big limiting factor to me, is the speed of light. When we look out at the universe, we look out and back. There could be massive EM signatures, stars being shifted into orderly structures and all manner of detectable signs headed our way at the speed of light right this minute, from massive technological civs that are tens-of-thousands/hundreds-of-thousands/millions of years in advance of our own, but advanced technological life is rare and recent enough that the nearest such civs are hundreds-of-thousands/millions/tens-of-millions of light years distant.
    This is such an excellent point I don't think very many consider at all.

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    Quote Originally Posted by Trakar View Post
    The big limiting factor to me, is the speed of light.
    Well, there are billions of stars within a 50,000 light year radius of us, so if our solar system having technological life is more common than 1 in a billion, there would only be a 50,000 year communications delay linking us to this hypothetical sister civilization. That's a long time, but, 1,500 chunks of time that large have passed since the dinosaurs died out.

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    Quote Originally Posted by Trakar View Post
    Statistically it would be an extremely rare and unusual event to roll twenty large straights in a row in yahtzee, but the odds are the same regardless of whether the series comes near the beginning of a long test run, or not until near the end of a long test run. In Poker, the odds of me getting a royal flush is around 1 in ~650,000, that doesn't mean I have to play 650,000 hands of poker before I get a royal flush. Yes, it would be odd, but perhaps life is odd.
    It's true that the odds for a given roll are the same. However, the odds for a series of rolls gets higher as the number of rolls gets higher. If the chances of getting a royal flush in one try is around 1 in 650,000, the chances of getting a royal flush in ten tries is around 10 in 650,000. If you try 650,000 times, the chances are about 63% of getting at least one royal flush.

    It's certainly possible to try and calculate the probability of abiogenesis based on this sort of thing. You yourself posted a link to one attempt:
    Quote Originally Posted by Trakar View Post
    The Implications of the Early Formation of Life on Earth - http://arxiv.org/PS_cache/arxiv/pdf/...807.4969v1.pdf
    This paper refers to other attempts, which reach different conclusions due to different assumptions. (It's a matter of Bertrand's paradox, where you have different choices of probability distribution for a "random" chord. I do not agree with the author of that paper, because he chooses to analyze things in terms of an open ended infinite range--and it's fundamentally impossible to assign a neutral flat distribution on an infinite range. I feel his variable choice of "expected time to first abiogenesis" to be odd, inappropriate, and possibly deliberately contrived to support his conclusion.)

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    Quote Originally Posted by Colin Robinson View Post
    One hypothesis is that going from one star system to another is indeed possible, but intrinsically difficult and costly.
    Good post Colin, thanks.

    It may just be that interstellar travel is extremely difficult and not particularly necessary. By "not necessary" i mean it could be that there's plenty of room for a civilization to expand within its own solar system as technology improves.

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    Quote Originally Posted by Trakar View Post
    The big limiting factor to me, is the speed of light. When we look out at the universe, we look out and back.
    Yes, but not really all that far back. In our own galaxy, we're only looking up to a hundred thousand years back. Even the Andromeda galaxy is only looking a million years back.

    It's far more of a limiting factor in the other direction. We have only really been sending out signs of advanced technology for some decades. That severely limits the amount of the galaxy which could have seen this by now. For example, if there is an alien civilization which keeps tabs on various star systems using black hole gravity lens telescopes, it could be centuries before they see us develop modern technology.

  23. #473
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    Quote Originally Posted by Colin Robinson View Post
    One hypothesis -- subject of an article by Geoffrey Landis of the NASA Lewis Research Center, published in 1998 -- is that going from one star system to another is indeed possible, but intrinsically difficult and costly. Even if a culture does become expansive and succeeds in establishing a colony in a nearby stellar system, the colony takes time to develop, it becomes essentially a new culture, and there is a substantial chance it won't dedicate its resources to further colonization. Landis also suggests that it would be still more difficult -- perhaps impossible -- to colonize another stellar system if a technological civilization is already there.
    The bottom line of Landis' argument... You would then get civilizations spreading through space in a non-uniform way: a process of percolation, where some regions of the galaxy do get colonized, while others do not.
    http://www.geoffreylandis.com/percolation.htp
    We have discussed percolation theory before on this forum, but not recently. It falls down somewhat because of the highly limiting assumptions Landis makes in that article. Some daughter colonies might well be expected to send out more expeditions to other systems, rather than less; some systems could send out two, or five, or a million. The colonies could communicate with each other and discuss just how many unclaimed stars are left. None of these possibilities are explored in detail by Landis, and any one of them might prevent this kind of incomplete percolation.
    Last edited by eburacum45; 2011-Jan-27 at 05:54 PM.

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    Quote Originally Posted by eburacum45 View Post
    We have discussed percolation theory before on this forum, but not recently. It falls down somewhat because of the highly limiting assumptions Landis makes in that article. Some daughter colonies might well be expected to send out more expeditions to other systems, rather than less; some systems could send out two, or five, or a million.
    This is directly addressed by his assumptions. According to his assumptions, there are only five or so systems within range because it is assumed that colonization missions are limited to perhaps 30 light years in range.

    He also directly addresses the issue of stellar drift, by assuming that every star system--including the "colonizing" ones, will evolve into "stable" non-colonizing systems within timescales of a million years. This assumption is, to me, dubious to say the least. But certainly Landis does address it explicitly.

    For example, suppose there is some "short" range for which even "stable" systems will send colonization missions? Whether this "short" range is 1,000AU or 10,000AU makes a huge difference.

    Using orbitsimulator's Close Stellar Encounter calculator, we can see that close encounters at 10,000AU take place at a rate of about one per 100 million years. If we assume that 100% of star systems are desirable, and we assume that all systems will colonize within a range of 1,000AU, then that means a doubling every 100 million years. A billion years means multiplying the number of colonized star systems by 1000x. After 4 billion years, that means colonizing the entire galaxy.

    If the "short" range is 1,000AU, then this would take on the order of 400 billion years--far longer than the age of the universe so far.

    One question is whether or not colonized systems would expand to their oort clouds. If so, then 10,000AU doesn't even require a special mission. They're already there. Civilizations with the ability and desire to expand to oort clouds would have perhaps a radius of 50,000AU for "cheap" colonization. But I find it plausible that colonized systems may deflect oort cloud objects inward rather than expand outward.

    In any case, this is all assuming 100% of star systems are desirable. I don't buy into that. If, say, 10% of the star systems are desirable, then this increases the doubling time by a factor of ten.

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    Quote Originally Posted by CaptainToonces View Post
    Well, there are billions of stars within a 50,000 light year radius of us, so if our solar system having technological life is more common than 1 in a billion, there would only be a 50,000 year communications delay linking us to this hypothetical sister civilization. That's a long time, but, 1,500 chunks of time that large have passed since the dinosaurs died out.
    Can you say that every star has an equal chance of producing a planet within it's habitable zone that is capable of producing an abiogenic event? what if it takes a star and planetary system greatly identical to ours to have a chance of producing an abiogenesis event. What if only 1 out of billion of such greatly identical stars/planets actually produces life,...only 1 out of a billion of those actually evolve complex life,...one out of billion of them produces a technological species?

    Granted, those odds may be high (but they could be low as well). Until we know a lot more about abiogenesis itself it is difficult to lay hard odds on the parameters necessary.

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    Quote Originally Posted by IsaacKuo View Post
    It's true that the odds for a given roll are the same. However, the odds for a series of rolls gets higher as the number of rolls gets higher. If the chances of getting a royal flush in one try is around 1 in 650,000, the chances of getting a royal flush in ten tries is around 10 in 650,000. If you try 650,000 times, the chances are about 63% of getting at least one royal flush.

    It's certainly possible to try and calculate the probability of abiogenesis based on this sort of thing. You yourself posted a link to one attempt:

    This paper refers to other attempts, which reach different conclusions due to different assumptions. (It's a matter of Bertrand's paradox, where you have different choices of probability distribution for a "random" chord. I do not agree with the author of that paper, because he chooses to analyze things in terms of an open ended infinite range--and it's fundamentally impossible to assign a neutral flat distribution on an infinite range. I feel his variable choice of "expected time to first abiogenesis" to be odd, inappropriate, and possibly deliberately contrived to support his conclusion.)
    Well, all such considerations are going to shape and be shaped by the foundations we establish in those considerations. It is normal that one's conclusion follows from one's considerations of the evidences. I don't know that this author used any improper methods or valuations, but perspective is always present in all of our considerations, the subjectivity of being human.

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    Quote Originally Posted by IsaacKuo View Post
    Yes, but not really all that far back. In our own galaxy, we're only looking up to a hundred thousand years back. Even the Andromeda galaxy is only looking a million years back.

    It's far more of a limiting factor in the other direction. We have only really been sending out signs of advanced technology for some decades. That severely limits the amount of the galaxy which could have seen this by now. For example, if there is an alien civilization which keeps tabs on various star systems using black hole gravity lens telescopes, it could be centuries before they see us develop modern technology.
    I don't know that we've produced many/any signals that would be clearly discernible even with a gravity lens, but your point is nonetheless possesses validity. Technologically speaking a few thousand years is an incredible gap, and a few million years, nearly unimaginable.

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    Quote Originally Posted by Trakar View Post
    Well, all such considerations are going to shape and be shaped by the foundations we establish in those considerations. It is normal that one's conclusion follows from one's considerations of the evidences. I don't know that this author used any improper methods or valuations, but perspective is always present in all of our considerations, the subjectivity of being human.
    I don't know if I'm just weird, but I have always been highly skeptical of Bayesian analysis in general. To me, it's nothing but Bertrand's paradox over and over again, with arbitrary choice layered over arbitrary choice, to the point of meaninglessness! Maybe I just don't like Bayesian analysis because it doesn't make hard unambiguous scientifically testable predictions like traditional probability and statistical analysis.

    Anyway, my impression of that paper was..."But aha! What if we bias the probability distribution in favor of extremely low abiogenesis probabilities? Wow! The result is that extremely low abiogensis probabilities look more likely!"

    Of course, that would be a little too baldly obvious. So instead, the author transforms the problem to use "expected time to first abiogenesis" rather than "abiogenesis probability". And since there's inherenly no neutral flat choice of probability distribution for an open-ended infinite range, it's less suspicious to make an arbitrary choice.

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    Quote Originally Posted by IsaacKuo View Post
    ...One question is whether or not colonized systems would expand to their oort clouds. If so, then 10,000AU doesn't even require a special mission. They're already there. Civilizations with the ability and desire to expand to oort clouds would have perhaps a radius of 50,000AU for "cheap" colonization. But I find it plausible that colonized systems may deflect oort cloud objects inward rather than expand outward.

    In any case, this is all assuming 100% of star systems are desirable. I don't buy into that. If, say, 10% of the star systems are desirable, then this increases the doubling time by a factor of ten.
    Personally, I wouldn't even begin to expect interstellar travel until the Oort clouds were heavily exploited and colonized, but that's me. This volatile rich outer fringe is the motherload of stellar resources to my considerations.

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    Quote Originally Posted by IsaacKuo View Post
    This assumption is, to me, dubious to say the least. But certainly Landis does address it explicitly.
    Once one removes the dubious assumptions the whole percolation effect dissolves. Colonisation patterns may be complex, but there is no reason to assume that they will be self-limiting.

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