A recent paper, astro-ph/0907.3432 by Milan Cirkovic, reviews the arguments surrounding Fermi's Paradox. There is a good discussion of how quickly the galaxy would be completely colonized by any spacefaring civilization -- a negligably short time span in cosmological terms -- and that Earth is if anything a late comer to our galaxy, and there has been billions of years available for other "Earths" to beat us to the spacefaring punch. The author states that Fermi's Paradox is "even disturbing" nowadays. He gives what I presume are all the major arguments on the topic.
However, two points are entirely missing. I give them here. One point is that of the chance of life spontaneously arising on an Earth-like planet. Barring the scenario of life being seeded from outer space, we have no indication of how likely life is to arise. Earth itself gives us no clue, as life must arise here for us to be in a position to consider the question, thus it is a necessary prerequisite to the problem. Put another way, we don't know if half of all Earth-like planets generate life, or if it is just 1 out of a googol. Therefore, if the chances of life arising are indiscernably close to zero, then that explains why we are the only ones in the galaxy. This is not "disturbing".
The second point is the difficulty of becoming an interstellar spacefaring civilization. The significant step in colonizing space is not to reach it, but to generate an external-to-Earth presence which is viable and growing, in other words, a space civilization that does not need Earth at all. This is as far beyond us, at this point, as we are from the apes. As a matter of fact, it is not yet clear that this can in fact be done, beyond our belief that it can be. We don't yet have the technology, nor do we have a direction to achieve it.
So these two points are conspicuously absent from M. Cirkovic's paper. Responents welcome.
Eric Flesch
[[Mod. note -- One piece of evidence which does at least tangentially bear on how likely it is for life to arise, might be the time scale on which life arose on Earth. That is, if life arose very quickly, that might suggest that it was "easy", and hence would be relatively likely to happen elsewhere.
That is, we're quite confident (from dating lunar rocks) the Moon was formed by the impact of a Mars-sized protoplanet with the Earth 4.5 GYr ago, which would certainly have wiped out any Earthly life that existed before that. We know from the Marble Bar microfossils that there was life on Earth 3.465 Gyr ago, i.e. around 1Gyr after the Moon-forming impact. There's some circumstancial evidence (carbon isotope ratios) for life on Earth 3.9 Gyr ago, i.e., 0.6Gyr after the Moon-forming impact.
These time scales (at most 1Gyr) are fairly "short" on a cosmological time scale.
Of course, the flaw in this argument is that one could imagine a scenario where life on a given planet either arises vairly quickly, or never at all, with the former scenario very rare. Arguing from n=1 data points is hard. :( -- jt]]
>A recent paper, astro-ph/0907.3432 by Milan Cirkovic, reviews the > arguments surrounding Fermi's Paradox.
[..]
> However, two points are entirely missing. I give them here. One > point is that of the chance of life spontaneously arising on an > Earth-like planet. Barring the scenario of life being seeded from > outer space, we have no indication of how likely life is to arise. > Earth itself gives us no clue, as life must arise here for us to be in > a position to consider the question, thus it is a necessary > prerequisite to the problem. Put another way, we don't know if half > of all Earth-like planets generate life, or if it is just 1 out of a > googol. Therefore, if the chances of life arising are indiscernably > close to zero, then that explains why we are the only ones in the > galaxy. This is not "disturbing".
But this assumes we are special in some way. The standard assumption that we are not unique in anyway is made because, otherwise, the results are far too subjective.
For example, (and I'm sorry if this is too obvious) if we did detect an extraterrestrial civilization, we would initially assume we had about the same basic level of intelligence because any other assumption would require some justification, and we have nothing to base it on.
This means we should be concerned that most civilizations may die out before achieving "cosmological greatness." (Rees, IIRC, puts the odds of mankind making to the 22nd century at about 50%).
On Tue, 11 Aug 2009, "Craig Franck" <craig.fra...@verizon.net> wrote: >"Eric Flesch" wrote >>... we have no indication of how likely life is to arise. >> Earth itself gives us no clue, as life must arise here for us to be in >> a position to consider the question, thus it is a necessary >> prerequisite to the problem. Put another way, we don't know if half >> of all Earth-like planets generate life, or if it is just 1 out of a >> googol.
>But this assumes we are special in some way.
No it doesn't. I state only that our presence here is required for us to consider this question. A banal tautology. Very unspecial.
>The standard >assumption that we are not unique in anyway is made because, >otherwise, the results are far too subjective.
Anyone considering the question must a priori exist, i.e., "I think, therefore I am". You incorrectly jumped to the conclusion that it confers uniqueness.
Eric Flesch wrote: > One > point is that of the chance of life spontaneously arising on an > Earth-like planet.
Unless it spread through space. One found some bacteria in a camera that was left on the Moon for a few years, and they could be cultivated.
> The second point is the difficulty of becoming an interstellar > spacefaring civilization.
Science fiction has a wide range of answers to that. One question is if it is possible to travel faster than c - look up the speed of instantity. Without it, there is not going to be much travel for short lived humanoid life.
> [[Mod. note -- One piece of evidence which does at least tangentially > bear on how likely it is for life to arise, might be the time scale > on which life arose on Earth. That is, if life arose very quickly, > that might suggest that it was "easy", and hence would be relatively > likely to happen elsewhere.
First life was methane based. Though it produces oxygen, in early Earth it becames first bound to iron. So it took the order of a billion years to produce free oxygen. Oxygen is more energetic, and is though to be a requirement for higher, thinking, life. It is also poisonous to methane based life. So when free oxygen first becomes abundant, life takes a sharp decline until animal life has been come by.
> That is, we're quite confident (from dating lunar rocks) the Moon > was formed by the impact of a Mars-sized protoplanet with the Earth > 4.5 GYr ago, which would certainly have wiped out any Earthly life > that existed before that. We know from the Marble Bar microfossils > that there was life on Earth 3.465 Gyr ago, i.e. around 1Gyr after > the Moon-forming impact.
So this may not have made any difference, since there would not have been any free oxygen around at that time anyhow.
> These time scales (at most 1Gyr) are fairly "short" on a cosmological > time scale.
Assume that life spreads from planet to planet by material leaking out into space from the places that already have it, and that the speed of the spread is say 15 km/s, or about 0.00005 c. In a billion years, the spread is 50000 light years. The Milky Way has diameter about 100000 light years.
So a billion year would then suffice for a large spread within the galaxy. Then life would emerge pretty much as soon as it is possible. That is what seemingly happened on Earth.
> "Craig Franck" wrote: >>"Eric Flesch" wrote >>>... we have no indication of how likely life is to arise. >>> Earth itself gives us no clue, as life must arise here for us to be in >>> a position to consider the question, thus it is a necessary >>> prerequisite to the problem. Put another way, we don't know if half >>> of all Earth-like planets generate life, or if it is just 1 out of a >>> googol.
>>But this assumes we are special in some way.
> No it doesn't. I state only that our presence here is required for us > to consider this question. A banal tautology. Very unspecial.
If all you are doing is making an existential point, then you are correct. But then there would be no paradox since there would be no contradiction between high estimates for extraterrestrial civilizations and the lack of evidence for their existence.
You have solved the paradox by demonstrating there wasn't one in the first place. From a philosophical POV, that is all that is required.
But from a scientific Fermi paradox perspective, if Earth were the only planet in the universe with life, I believe that would make us special; however, I'm no expert on this topic, so please forgive me if I just didn't "get" your post.
-- Craig Franck craig.fra...@verizon.net Cortland, NY
To elaborate just a little, a "paradox" means there's something wrong or missing in our belief system. A very simple example is the "paradox" of knowing the sky is yellow but looking up and seeing it is blue. So this can be resolved by changing our minds about the sky.
The idea that the galaxy is teeming with life is not based on observation. We have only a single data point, ourselves, and that was required in the formulation of the problem, so we in fact have no data pont at all.
I think it's quite clear to anyone who thinks about it, that the current prevalence of belief in the common-ness of life out there, is the result of social trends, not of rational thought. People think they are being rational in adopting the belief, but it isn't actually rational thought.
We're looking at an Earth which has been an oxygen-bearing planet for 200,000,000 years, but nobody ever came to colonize it. We can be pretty sure they're not going to be coming in the forseeable future either.
If we sort ourselves out and make it into interstellar space, I expect an empty galaxy. This is not religious thought, I'm not a religious person, but my thinking has come full circle. The evidence is clear. We are alone. cheers.
Eric Flesch <e...@flesch.org> wrote: > If we sort ourselves out and make it into interstellar space, I expect > an empty galaxy. This is not religious thought, I'm not a religious > person, but my thinking has come full circle. The evidence is clear. > We are alone. cheers.
-- -- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu> Dept of Astronomy, Indiana University, Bloomington, Indiana, USA "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral."
> I think it's quite clear to anyone who thinks about it, that the > current prevalence of belief in the common-ness of life out there, is > the result of social trends, not of rational thought. People think > they are being rational in adopting the belief, but it isn't actually > rational thought.
It depends on what you consider "rational thought." The broadest definition might be "the felt need to justify your beliefs or actions to a group of your peers." A cleaver person might try to argue that *your* conception of rational thought was *itself* a negative social trend of some sort or another.
The problem with your restricted conception of rationality is then most people simply are not rational. Whatever you think of medieval Scholasticism or the belief that Jesus was an extraterrestrial, they are at least making appeals to a rational framework.
> If we sort ourselves out and make it into interstellar space, I expect > an empty galaxy. This is not religious thought, I'm not a religious > person, but my thinking has come full circle. The evidence is clear. > We are alone. cheers.
It seems to me the most rational response to the issue might be to say we simply cannot tell one way or the other.
It strikes me as absolutely incredible that life would form on Earth within the first few hundred million years of a suitable environment, and halfway thru the life of the sun a species arises that at least has the potential to populate the entire galaxy -- but life only happened once, at least in our home galaxy.
So even with a data set of one (it seems ones existence is at least a single data point -- if it was zero, then what would it be if you didn't exist?) you can reason about your situation. A person on a deserted island who can only remember growing up there alone would be justified in believing there are or were other humans, or he simply would not exist.
-- Craig Franck Citizen of the Galaxy craig.fra...@verizon.net Cortland, NY
On Fri, 14 Aug 2009, "Craig Franck" <craig.fra...@verizon.net> wrote: >It strikes me as absolutely incredible that life would form on >Earth within the first few hundred million years of a suitable >environment, and halfway thru the life of the sun a species >arises that at least has the potential to populate the entire >galaxy -- but life only happened once, at least in our home >galaxy.
I must reiterate: To you, this is "absolutely incredible" because you are working from a false premise. You are giving yourself the status of "detached observer" and in so doing, promoting the data point of Earth to a fully usable prototype. This is wrong. You are not a detached observer, because Earth's existence is a prerequisite to your model, else you and your model would not exist. Earth is not a data point, it is an observing point. You simply cannot extrapolate from Earth's example. Once you take this on board, there is no more "absolutely incredible".
Sorry if this is a repeat of what I wrote before, but it is the key, the missing element, in most discussions of this kind. Earth is not a data point.
(Eric Flesch) writes: > However, two points are entirely missing. I give them here. One > point is that of the chance of life spontaneously arising on an > Earth-like planet. > [[Mod. note -- One piece of evidence which does at least tangentially > bear on how likely it is for life to arise, might be the time scale > on which life arose on Earth. That is, if life arose very quickly, > that might suggest that it was "easy", and hence would be relatively > likely to happen elsewhere.
> That is, we're quite confident (from dating lunar rocks) the Moon > was formed by the impact of a Mars-sized protoplanet with the Earth > 4.5 GYr ago, which would certainly have wiped out any Earthly life > that existed before that. We know from the Marble Bar microfossils > that there was life on Earth 3.465 Gyr ago, i.e. around 1Gyr after > the Moon-forming impact. There's some circumstancial evidence > (carbon isotope ratios) for life on Earth 3.9 Gyr ago, i.e., 0.6Gyr > after the Moon-forming impact.
> These time scales (at most 1Gyr) are fairly "short" on a cosmological > time scale.
Of course, it's not enough for life to arise; one needs intelligent life as a prerequisite to space travel.
What we do know is that life arose relatively early on Earth, but also that until about 600 million years ago, i.e. for most of the duration of life on Earth, nothing more complicated than algae was around. I think that this argues against "higher" life forms being "inevitable" once life itself has arisen.
On Mon, 17 Aug 2009, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---remove CLOTHES to reply) wrote:
>What we do know is that life arose relatively early on Earth, but also >that until about 600 million years ago, i.e. for most of the duration of >life on Earth, nothing more complicated than algae was around. I think >that this argues against "higher" life forms being "inevitable" once >life itself has arisen.
Well, the complex life forms required an oxygen atmosphere, so the question boils down to whether photosynthesis is an expected development. I'd guess it is, because it makes sugars.
Others on this thread have mentioned that life started early in Earth's history, implying it was easy. But another possibility is that only catastrophic events would provide the catalyst for life, and that these were available only in the early days. Just sayin'.
> Of course, it's not enough for life to arise; one needs intelligent life > as a prerequisite to space travel.
Or the opposite: perhaps only very simple life is capable of space travel. :-)
> What we do know is that life arose relatively early on Earth, but also > that until about 600 million years ago, i.e. for most of the duration of > life on Earth, nothing more complicated than algae was around. I think > that this argues against "higher" life forms being "inevitable" once > life itself has arisen.
And some wonder if intelligent may arise on the planet Earth. :-) (This is also a theme treated in SF; for example, in the movie "Forbidden Planet", the most intelligent humans are about as intelligent as a three-year old Krell.)
But let's do some rough estimate computations for detecting EM radiations from other civilizations (I'm skipping details here, like that higher life perhaps cannot happen in any region of the galaxy, and taking a rather ad hoc percentage for the chance of higher life to emerge). Assume that a civilization radiates detectable EM fields for about hundred years, then shifting towards other means of communications (which could be more focused and less energetic EM fields in combination with in frequencies that do not spill out much into space). Assume there takes a few billion years on a planet for such a civilization to emerge. The Milky Way has a couple of hundred billion stars. Suppose 1% of them has a planet capable of developing such higher life. Then at any time there will be 100/3E9*300E9^0.01 = 100 such EM emitting civilizations.
Then the Milky Way is about 100000 light years in diameter. So the average distance between those civilizations, if scattered evenly throughout the galaxy will be of magnitude (skipping some factor) 100000/sqrt(100) = 10000 light years.
Now, if there is 10000 light years to the nearest one, what is the possibility that we should detect it with our current EM technology? My guess is that chances are low. Current SETI searches do not reach out that far. http://en.wikipedia.org/wiki/SETI
So the conclusion is quite interesting:
Even if giving quite favorable numbers for higher civilizations to emerge, if they fairly quickly shifts away from primitive forms of EM communication, they will be quite hard to detect, at least with the methods that we have now.
Glycine is also found in interstellar space (Interstellar Glycine, Kuan, Yi-Jehng; Charnley, Steven B.; Huang, Hui-Chun; Tseng, Wei-Ling; Kisiel, Zbigniew URL: http://adsabs.harvard.edu/abs/2003ApJ...593..848K)
This means that the basic ingredients of life are found in the galaxy itself. If this is not a strong hint at a galaxy teeming with life I do not know what could be.
Another point is that we haven't explored much of the galaxy, not even our own system.
From our system, we know already that there are strong hints of life in the following planets/satellites:
Mars: Methane emissions are confirmed by a NASA team. This methane is destroyed by the Marsian conditions, hence it must be continually replenished.
Europa (satellite of Jupiter) We have discovered there the first extraterrestrial ocean. Conditions there are optimal for life to exist: Abundant water, shielded from radiation from Jupiter by a thin layer of ice, and with good sources of energy from the ocean floor. There were unexplained colors seen by the Galileo probe in the cracks of the ice cover.
Titan (satellite of Saturn) Thick atmosphere containing millions of tons of organic molecules and compounds that give it an orange color. We have there with high probability life forms.
Encedalous (satellite of Saturn) The second extraterrestrial ocean to be discovered. It spews geysers of hot water into space. Analysis by the Cassini probe revealed organic compounds.
This means that in our own system, with only the most primitive methods of exploring, we have already at least 4 places where life probably exists.
NOTE: The Kepler observatory that is dedicated to discovering extra-solar planets has started its work only recently. We will see very soon what new discoveries it will bring to us. Obviously, the scientific exploration of the galaxy searching for life hasn't even STARTED.
But even with the primitive methods at our disposal, we have strong hints that life as we do not know it is present everywhere.
Conclusion:
Two ants, at the top of an anthill in the middle of the amazonian forest, wonder if there are any explanations for the absence of any civilization in this planet:
"We sent our chemical signals 176 days ago. No answer came with the wind. We are completely alone in this planet. If there were any civilizations, they would surely have noticed us"
On Wed, 19 Aug 2009 23:58:24 EDT, "Dave" <dst...@sbcglobal.net> wrote: >Is the premise that"absence of evidence is not evidence of absence" taken >into consideration in this debate?
No. Pink Elephant. Seen one? Absence of evidence is indeed evidence of absence.
>Is the fact that we cannot know what, if any, ethical considerations our >would be spacefaring neighbors hold taken into account?
No, because that implies that all spacefaring civilizations would share the ethical stance of not spreading geometrically. We have no grounds to make such a sweeping generality.
I think this cartoon http://xkcd.com/638/ is quite relevant to any discussion of the Fermi paradox.
(The cartoon shows two ants on a tiled floor, one saying to the other "We've searched dozens of these floor tiles for several common types of pheromone trails. If there were intelligent life up there, we would have seen its messages by now." the caption reads "The world's first ant colony to achieve sentience calls of the search for us.")
-- -- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu> Dept of Astronomy, Indiana University, Bloomington, Indiana, USA "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam
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