Monday, February 08, 2016

The Fermi Paradox: Are we alone in the universe?

All available evidence suggests that we are quite likely the only advanced life form in the Milky Way galaxy. Maybe our planet harbors the only life in the entire universe.



56 comments :

  1. "All available evidence" is only a minute sampling of the accessible evidence and even the accessible evidence is far from sufficient to draw grand conclusions.

    The best answer we have to this question is that there isn't sufficient evidence to know either way.

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  2. Larry, your post conflates life with advanced civilization. The available evidence and the Fermi paradox concern only the latter, if that.

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    1. I did not conflate "life" with "advanced civilization." Maybe I just got too far ahead of you and you got confused. :-)

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  3. Given what's known about organic chemistry related to the origin of life, I personally think that life may be inevitable on all sufficiently warm planets with liquid water. Some equivalent of bacterial life.

    Intelligent beings with technology? That's another story. We're so improbable that we might be the only species like that in the galaxy. And if there are other such species in other galaxies, I think we'll never know.

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    1. Given what's known about organic chemistry related to the origin of life, I personally think that life may be inevitable on all sufficiently warm planets with liquid water.

      Given what I know about physics, chemistry, and biochemistry, I think that the origin of life on Earth was a highly improbable event. I see it as more like a lucky accident than something that follows inevitably from the laws of physics and chemistry.

      I think my view is much more defensible than yours because if the origin of life were so easy we would probably have a better hypothesis than we do now.

      That's another story. We're so improbable that we might be the only species like that in the galaxy.

      I agree with this part. Given that life begins somewhere it's highly improbable that it will produce a technologically advanced society.

      I don't agree with any of those people who think that technologically advanced societies will commit suicide. That just seems ridiculous to me.

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    2. So you haven't been reading the papers, then.

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    3. I don't agree with any of those people who think that technologically advanced societies will commit suicide. That just seems ridiculous to me

      Why? I think it follows directly from the logic of the evolutionary process.

      But note that I do not have in mind the usual scenarios for self-destruction of a technological civilization, which always invoke a technological disaster. The reality is probably a lot more mundane and has to do with ecology and resources.

      When our current civilization falls apart over the course of this and next one or two centuries, this will be the end of advanced technological civilizations on this planet, at least for a few hundred million years because we will have extracted and dissipated all the easily available concentrated mineral resources thus whoever/whatever in the future might try to restart the process will not have the resource base to jumpstart it the way we did. Which given the future projections for how long the climate of the planet will be able to maintain a certain homeostasis against the background of increasing solar illumination does most likely mean a real end and I am probably being optimistic regarding ores (while most oil and gas reserves date to the Mesozoic, many of those were deposited deep in Precambrian).

      There are very good reasons to think exactly the same happens on every planet where intelligent life appears because the reason we are doing it is not so much because we are uniquely bad, but because of the logic of the evolutionary process. Having access to material resources confers a quite strong selective advantage, which is why first, behavior is oriented towards obtaining access to such resources for oneself and one's kin, and second, intraspecific competition is largely based on maximizing one's control over material resources (or at the very least, the perception of it) relative to others. Which is what drives the engine of the capitalist expansionist economy and the ecological catastrophe we're generating in the process, and the same was true for all previous civilizations that collapsed.

      This might be a preventable process in principle, but that requires sufficiently deep knowledge and understanding of one's biological nature and the relationship between humans and the environment to overcome the innate behavioral factors and exercise some restraint when it comes to reproduction and resource consumption. The problem is that obtaining knowledge is hugely expensive for the individual in terms of time and effort expenditure, which is why most people do only the bare minimum that will enable them to not be at severe disadvantage in society and prefer to spend their time elsewhere, in activities more directly related to short-term evolutionary success. The end result is that society as a whole behaves exactly as a yeast culture in a petri dish.

      I've thought a lot about this and I have hard time seeing how an extraterrestrial intelligent species would be able to both develop a technological civilization and avoid falling in this trap. One can give examples of societies on Earth that had cultures more harmonized with nature. But precisely because of that, those are also societies that would have never developed science and technology.

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    4. Dr. Moran, I hope life (very basic life) is more probable than that, but hoping doesn't make things true. Of course, bacteria-like cells aren't exactly the life forms most people hope to find.

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    5. Given what I know about physics, chemistry, and biochemistry, I think that the origin of life on Earth was a highly improbable event. I see it as more like a lucky accident than something that follows inevitably from the laws of physics and chemistry.

      Prof. Moran. don't you think the fact that life arose so quickly implies that it must have arisen easily?

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    6. Quick and probable are not the same thing. It could have arisen very quickly (in fact, if you ask me, it has to have been that way -- any intermediates we can imagine are quite fragile, thus quick evolution to a more robust state is preferable) yet it could also have been extremely improbable (it is highly improbable that you will be struck by lightning, yet if it happens it occurs almost instantaneously).

      What would imply that it arose easily is a definite demonstration that it arose more than once.

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    7. Georgi.. By quick, I'm not talking about the amount of time it took to get from the first replicators to life as we know it.

      By quick I'm referring to the fact that a mere few hundred million years separate the collision with Theia and the formation of our moon from the first evidence of life appearing in the geological record.

      If life were something exceedingly unlikely, we shouldn't expect it to arise as soon as conditions become favorable.

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    8. Given that life begins somewhere it's highly improbable that it will produce a technologically advanced society.

      In the only example we have, life has branched out in all directions with regard to complexity, limited only by the fact that at some point less complexity = non-living. (Gould's metaphor was a bush planted near a wall.) Life seems to be exploring all possible spaces with regard to complexity here on Earth. I don't see any reason that, once established, it would not do the same anywhere else.

      The other piece of this is the "once established" probability. Given the billions of galaxies and the millions or billions of stars in each, and all the planets around those stars, it would be mind-bogglingly improbable, nigh on impossible really, for life *not* to have established itself on an uncountably vast number of planets.

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    9. @Aceofspades,

      You seem to think that because it took several hundred million years (at least) for life to arise on Earth that this is "quick."

      I don't agree.

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    10. judmarc says,

      Given the billions of galaxies and the millions or billions of stars in each, and all the planets around those stars, it would be mind-bogglingly improbable, nigh on impossible really, for life *not* to have established itself on an uncountably vast number of planets.

      Let's imagine two possible scenarios.

      1. It's easy for life to arise so there are millions of planets with life on them even though we have no evidence that they exist.

      2. Life arose only once in the entire universe and we're it.

      We do not have enough evidence to distinguish between these possibilities. All we can say for now it is that our galaxy doesn't seem to be teeming with advanced civilizations. We can also say that there doesn't seem to be any life on Mars even though it is suitable.

      You are not being logical when you say that scenario #2 is mind-bogglingly improbable.

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    11. Larry says: All we can say for now it is that our galaxy doesn't seem to be teeming with advanced civilizations.

      Well, no. We can't see the galactic core (where most of the stars are) or the further rim. If there were a civilization even 300 light years away, we'd have no way of knowing. Our galaxy is huuuge and other galaxies are larger.

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    12. @ProfMoran

      Life couldn't have survived the Theia collision which turned the surface of this planet into magma and created our moon. So let's take that as our starting point: 4.5bya

      I don't know when the crust began to form but by some estimates it wasn't until 4.4 - 4.45bya

      We see the first evidence of life appearing in zircons from Western Australia which appear to contain biologically-produced carbon: 4.1bya (paper)

      So this life had between 300 and 350 million years to establish itself.

      Now consider how long this planet should be capable of supporting life for: Our sun will begin to run out of hydrogen roughly 5 billion years from now. So altogether, our planet should have 9.5 billion life bearing years available to it from start to finish.

      If the history of this planet were a 24hr day, life would appear in the first hour. If life were unlikely, I wouldn't expect that at all.

      There are three possible explanations I could reach for:

      1) Life is virtually inevitable and will arise within a few hundred million years wherever the conditions are right

      2) Life is exceedingly unlikely and it came from elsewhere (panspermia)

      3) It was only possible for life to arise in the first few hundred million years of our planet's existence because of some unknown special conditions that existed around that time.

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    13. Our sun will begin to run out of hydrogen roughly 5 billion years from now. So altogether, our planet should have 9.5 billion life bearing years available to it from start to finish.

      This is not exactly correct.

      The planet will be most likely sterilized long before that -- remember that the Sun's luminosity increases continuously and that 4.5 billion years ago it was only 70% of what it is now. The planet should have been frozen back then but it wasn't because the greenhouse effect must have been much stronger back then (lots of methane and CO2 in the atmosphere). In fact the temperature of the ocean might well have been unthinkably hot from a modern perspective. The methane was oxydized during the Great Oxygenation event, which most likely resulted in the Huronian glaciation 2.5 billion years ago and the most severe Snowball Earth episode in the history of the planet. Since then there have been a few other Snowball Earth events and some extreme global warming episodes but overall the planet has maintained a state of climatic homeostasis hospitable to life. The problem is that this has been achieved by burying carbon in the ground. It was 6000 ppm at the beginning of the Cambrian but there were no foraminiferans, coccolithophorids or land plants back then. Between the Cambrian and now those appeared and huge amounts of carbon were buried underground as a result of their activity. So CO2 was less than 200ppm at the end of the last ice age. It can approach zero, but once it does (although there will be a huge problem prior to that -- plants need CO2 for photosynthesis), the limit of the planetary thermostat will have been reached and there will be a runaway greenhouse effect (oceans evaporating and the planet being sterilized). That will happen at around a billion years from now (although multicellular life will probably be gone by then anyway)

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    14. @Georgi That makes sense. By some estimates we only have another 300 million years for complex, vulnerable, intelligent life forms such as ourselves on this planet.

      If that's the case, had life arisen much later, we might not have been around to ask this question.

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    15. "Given what I know about physics, chemistry, and biochemistry, I think that the origin of life on Earth was a highly improbable event. I see it as more like a lucky accident than something that follows inevitably from the laws of physics and chemistry. "

      What part of it? It seems to me the big hurdles to overcome is the origin an evolvable entity and of the translation system, but we know so little about how any of this came about, I don't see how one can swing either way on these questions.

      With regards to the mostly lucky accident route, at some point it just stops having any compelling explanatory power. If we're going to relegate the origin of life to mostly luck, where does it end? What was the entity that happened to luckily emerge? A fully functioning cell with a complexity comparable to E coli? Maybe just a primitive ribosome, some tRNA's, some aaRS's and the mRNA that codes for all of them? All of these seem to me so improbable as to defy rational belief, so maybe something even simpler?

      This brings up another question. If life is really just some extreme statistical fluke, how could we ever know it was so? Wouldn't all arguments to that effect constitute appeals to ignorance? At what point do we stop bothering trying to figure it out?

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    16. @Diogenes

      You are assuming that advanced civilizations will be confined to their home planet and not advertise their existence. I prefer to go with the assumptions in the video

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    17. I'm not convinced by a view that considers life vanishingly improbable, yet advanced civilisations given life to be probable. The second is obviously dependent on the first, but a great deal else besides.

      There are many combinations of the two respective probabilities that would give the pattern we see (ie ... "hello -ello -ello -ello!"). Life has as long as it needs to start, while communicating civilisations last (on present evidence) a few hundred years, which requires a substantially improbable local simultaneity for contact. Aliens looking for us in 1850 would have concluded: "We Are Alone". There aren't many stars within 100 light years of us, still fewer habitable planets.

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    18. @Aceofspades

      There's no good evidence for cellular life before 3.5 billion years ago. It took one billion years for life to form on this planet. Admittedly, conditions might not have been favorable for much of that time but that still leaves hundreds of millions of years for an event you think is inevitable.

      There's a fourth possibility in your list. Life formed spontaneously on this planet but it was a higly improbable event that's unlikely to occur very frequently in the rest of the universe.

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    19. unlikely to occur very frequently in the rest of the universe

      Considering the number of planets in the "rest of the universe," even just those that would be considered hospitable to the forms of life we know of - trillions, or perhaps as few as many, many billions - something that is merely extremely unlikely has got to be occurring millions of times over, and something vanishingly unlikely must still happen many thousands or a few million times.

      Possibly a better explanation for why we haven't yet heard from "out there" is that the places with most stars and planets are full of electromagnetic radiation likely to swamp anything we'd recognize as a signal.

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    20. @ProfMoran

      I've revised my position. Even if life arose within 300MY, it would still require 4 billion years (in our case) for it to reach the point where self-aware entities could begin asking the questions that we are asking now.

      If we consider that we might only have another 300MY left then it doesn't appear as though life arose early at all. If you consider this image, the best we could do is move the green bar left or right a little into the blue areas bounding it on either side.

      If life arises 300MY too late, there likely wouldn't be time for self-aware beings to ponder this question and so a type of anthropic principal takes effect.

      Only planets where life arises quick enough develop to the point where their life forms are able to ponder these questions.

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    21. Life taking 300 million years to arise here, if it were the average of a probabilistic process, would not make it an unlikely event among planets that have 300 million years to play with - that would be most of 'em. I think intuitions on likelihood don't deal well with enormous time frames.

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  4. If any civilization were more than 100 light years away, there'd be no way for us to know. The galaxy is 100,000 ly across, has 100 billion stars, we can't see the core or far side very clearly, and there are > 100 billion galaxies, some enormously larger than ours.

    The probability of life originating spontaneously can't really be estimated, given our knowledge of both astronomy and chemistry. The probability could be 1 in a billion, or life might appear in 90% of all planets with liquid water, carbon, nitrogen, iron and tectonic forces. The probability has huge error bars. We don't even know the probability within even two orders of magnitude. Let's admit the limitations to our knowledge.

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    1. There are two possible pieces of evidence that would mostly refute the idea that it is extremely improbable:

      1) Discovery of microbial life in the Solar of clear independent origin from that on Earth
      2) Solid evidence that life on Earth originated more than once. People have found 13C signatures in zircons dating back to before the LHB compatible with the presence of life. So the scenario of life originating prior to the LHB, the LHB then sterilizing the planet, and then life originating again is still viable. The problem is that the LHB itself is not rock solid, and that it sterilized the planet and that there was life on it prior to that even less so.

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    2. P.S. Such a discovery would be very bad news by the way, because it would imply that there should be a lot of life out there. And we don't see it, which means our own future becomes even less likely to be of the rosy sort we like to dream of.

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    3. "because it would imply that there should be a lot of life out there. And we don't see it,"

      Well, I'm not sure why we'd see it. We have, what, two planets in the habitable zone and humans have only been on one of them........

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    4. "because it would imply that there should be a lot of life out there. And we don't see it,"

      Well, I'm not sure why we'd see it. We have, what, two planets in the habitable zone and humans have only been on one of them........

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  5. Admittedly I am not a biochemist, but like Aceofspades I find it telling that life arose so quickly after the formation of the planet. And it seems obvious to me that whatever happens on this planet can just as easily happen on others that have a similar chemistry and temperature.

    As with other evolution-related questions, the problem could be that one looks at a bacterium and goes "that is very improbable" even as every single step between a proton gradient in a deep sea rock fissure (or whatever) and the final bacterium was minuscule and may well have followed necessarily and unavoidably from the previous one.

    As for the Fermi paradox, people seem to under-estimate the distances between stars as much as the deep time and number of stars that provide lots of "tries" at getting life to start. A good argument can be made that it is impossible in principle to travel to other stars and survive, so that alone explains the absence of alien intelligences across the galaxy.

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  6. Why isnt it possible for new life forms to be "generated" constantly/continuously?

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    1. Darwin was first to address that. Basically any interesting and juicy collection of organic chemicals that arises gets eaten by bacteria before they can form a new form of life.

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    2. Thanks Joe. Then the earth is so awash in bacteria now, that for these events to occur now, is essentially impossible?

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    3. Not just bacteria, but free oxygen. It's biogenic, and not particularly friendly toward biological molecules. It seems reasonable on historic and biochemical grounds to suppose that life arose in the absence of oxygen. There are anaerobic regions, but far fewer than in early history, rendering the result, to the extent that it has a probabilistic element, less likely.

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    4. Oh thats interesting! The occurrence and subsequent evolvement of life then made it much more difficult for new life forms to arise ...

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  7. I still have problems thinking this through. If life did originate deep under the sea in white smokers, what evidence is there that it isn't continuing to do so? It seems likely that new life would be gobbled up rapidly by organisms of our lineage. So, why would we see evidence of it anywhere except in the immediate vicinity of the white smokers-- which is not a place where humans regularly hang out.

    Is there other, independent evidence that life can no longer be forming anew?

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  8. What does it mean to say it took life some amount of time to "establish"? What is included in this process?

    There's a difference between emergence of life and life taking a strong foothold (establishing?) in all the environments of a planet such that it is hard to eradicate.

    It seems to me that going merely for emergence, it could take as little as a few weeks for the chemical reactions to end up with something like a self-replicating metabolism. But for that life to evolve and conquer the oceans might very well take tens of millions of years.

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  9. Wasn't the Fermi paradox supposed to be about the inevitability of life being contradicted by the absence of aliens? There's only a paradox if we think life inevitably means intelligence that will produce detectable signs visible from Earth (or even visitors.) I think the the safe generalization from Earth history is that however probable life is, intelligence is much less probable. And technology that is easily visible from other stars is unknown, intrinsically improbable. There is no paradox, in other words.

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    1. I think you missed the point. Given that we will likely colonize most of the Milky Way in the next million years or so then if the evolution of advanced technological beings is a highly probable event, where are the aliens?

      They should be here by now.

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    2. Given that we will likely colonize most of the Milky Way in the next million years or so

      This seems to be wildly optimistic. FTL speeeds probably won't ever be possible. It will take thousands of years to reach nearby stars, these will be one-way trips and we won't even know what to expect before we leave.

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    3. I would add to Aceofspades and bwilson295's replies that I can't think of any civilization which would be willing to embark on such journeys (assuming FTL is impossible) given they would last millennia at least, with no assurance, even with suspended animation technology, of arriving alive at some habitable location, with the single exception of a civilization inhabiting a planet orbiting a dying star. These folks, *if* successful, would be motivated to find one or perhaps a handful of worlds and stop there. How many such civilizations have there been, given the age of the universe and the type of star necessary to give life a chance to get started?

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    4. "with the single exception of a civilization inhabiting a planet orbiting a dying star." -- That's true. That would be an incentive!

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    5. "Given that we will likely colonize most of the Milky Way in the next million years or so..."

      But I thought the real point is that this is not a given. Current theory says any FTL will take more energy than available in a single solar system. We have no idea how to devise a stable self-contained ecosystem for a century long journey to another star.

      If we did, I do not see how a civilization can be built in an environment hostile to our type of life. Even if there weren't inimical life forms like bacteria, different chemical compositions of atmosphere and lithosphere could have devastating long term consequences. Limited to only the resources brought from Earth, there is no margin for errors, unexpected problems and random disasters.

      And invoking "terraforming" simply wishes away the difficulty of a centuries long project done with imported resources. The biggest long term project I can think of offhand is the US interstate highway system.

      And as far as von Neumann automata go, I'm much more skeptical of the possibility of the technology. Even if a self-replicating probe were possible, most of these figures assume that the resources are easily available, making the time for exponential growth to fill the galaxy rather short, astronomically speaking. The availability of trace elements or fissionable materials suggest that self-replicating probes will spend a very long time collecting them, especially in the outer most reaches of a solar system. But landing on rocky planets still raises difficulties of transport. The gravity that collected more materials relatively close together also raises the physical costs of transporting them even short distances.

      The funny thing is, for all we know, alien von Neumann automate have come, replicated and moved on, leaving waste heaped up on some ice dwarf we haven't even located yet. Or a mere 300 000 years ago there was omdeed an expedition to Earth.

      Sorry, Fermi was wrong, even though he's smarter than me.

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    6. From a discussion of a proposed FTL drive being worked on by NASA:

      There are numerous problems with an Alcubierre drive — such as whether you’d be able to survive inside the bubble, or my personal favorite: annihilating the entire star system when you arrive at your destination — but the sheer amount of energy required to reach the speed of light, let alone surpass it, is probably the main drawback.

      Last year, Sonny White revealed a new design (pictured top) for the Alcubierre drive that reduces the energy requirement from the total mass-energy of a planet the size of Jupiter, down to the mass-energy of Voyager-1 (700 kilograms). We say “mass-energy,” because no one quite knows how to fuel an Alcubierre drive, with some research suggesting that it might require more energy than the mass of the observable universe, or possibly negative amounts of energy.


      So it's not established whether an energy source exists that could power an FTL drive. And this is not from an academic paper, it is a PR release, where as we know, all things are usually possible and breakthroughs fly like snowflakes in the Yukon.

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    7. Expanding on something I said above: Assume it takes 4 or 5 billion years for intelligent life to evolve. This requires stable, long-lived stars like Sol. Then go forward another few billion years until the inhabited planet's star begins to die, and you have the first appreciable numbers of intelligent beings who are motivated to take on the extreme risks of interstellar travel, and who may have the necessary technology even to think of trying it. Compare this to the current age of the universe, allowing time after the Big Bang to form the necessary long-lived stars. It's not that vast a difference.

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  10. I think a central problem is that we simply lack the technology for detecting life in the galaxy and/or universe and necessarily pin all of our perceptions on detecting messages purposely sent by intelligent entities.

    But based upon a sample size of one, life on this planet has consisted for its entire existence (and even today) as cellular forms unable to either send or receive messages – save one lousy species out of the untold billions that have ever existed. And for this one species, its ability to purposely send or receive messages can be measured in decades, a mighty narrow slice of time in which to hope to intercept a message from another intelligent life form, now itself quite possibly extinct for millions of years.

    I’m betting that life might be quite common, relatively speaking. But intelligent life is another matter and the chances of such civilizations overlapping with each other in communication-relevant ways yet rarer.

    Sheesh, what if the asteroid said to have struck the earth 65 million years ago occurred a few million years earlier or later. I guess I don’t know, but life on earth would be different than it is now and could quite likely not include a species capable of sending or receiving messages.

    And as for humans perhaps experimentally figuring out how life arose if it was a common/easy phenomenon, well, all of test tubes and flasks ever produced by humans is a pittance to say the least compared to the potential number of natural “test tubes” available in the galaxy.

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    1. I think a central problem is that we simply lack the technology for detecting life in the galaxy and/or universe and necessarily pin all of our perceptions on detecting messages purposely sent by intelligent entities.

      No, the central problem is why haven't advanced alien civilizations come to visit us? Why didn't they colonize Earth a few million years ago, or Mars?

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    2. "why haven't advanced alien civilizations come to visit us?"

      I think probably they can't. If there are faster-than-light methods of travel and it's possible for intelligent organisms to figure out how to used them, then civilizations could have colonized us. But what if there aren't? Will organisms choose to send out settlers that they never hear from again? Settlers the stay-at-homes can't profit from in their life-times or their great-great-great-great grandchildren's liftimes? And if they did try, how often would they do it? (Discounting the question of whether they can build ships capable of keeping the settlers safe for the thousands and thousands of years required.)

      I long expected that humans and other technological societies would be willing and able to colonize planets of other stars, and I'm saddened to think it probably won't happen.

      So anyway, I don't think that the central question is why advanced alien civilizations haven't visited us.

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    3. No, the central problem is why haven't advanced alien civilizations come to visit us? Why didn't they colonize Earth a few million years ago, or Mars?

      Assuming interstellar travel is a feasible accomplishment by some intelligent aliens before going extinct, it would first be put to use for reaching the closest inhabitable star system, so as to escape their own finite planet. Some small fraction of these accomplished species might even go on, in time, to use interstellar travel for curiosity-driven exploration.

      But it’s a big galaxy possibly full of many life forms to study and planets to colonize– nothing special about reaching earth.

      Perhaps your doubts are a little like some perplexed peoples in the year 1200 sitting in present-day Mexico saying: “If there really are other people on distant lands, why haven’t they visited us? Why haven’t they colonized our land?”
      I don’t suppose I need to complete the analogy.

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    4. SRM says,

      Perhaps your doubts are a little like some perplexed peoples in the year 1200 sitting in present-day Mexico saying: “If there really are other people on distant lands, why haven’t they visited us? Why haven’t they colonized our land?”
      I don’t suppose I need to complete the analogy.


      No you don't have to complete the analogy.

      Let me do it for you. The argument in Tenochtitlan back in 1200 was about the existence of civilizations that were very much more advanced. If they existed then it was reasonable to ask why they hadn't visited.

      Turns out they were right. There weren't any civilizations at that time that were advanced enough to pop over to Mexico for a visit.

      300 years later it was a different story when Hernán Cortés and his merry band of tourists stopped by to see the Temple of Huitzilopochtli.

      But it’s a big galaxy possibly full of many life forms to study and planets to colonize– nothing special about reaching earth.

      Did you watch the video? Do you understand the Fermi Paradox?

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    5. I dont understand why we need to assume that there are other civilizations more advanced then us?

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    6. Did you watch the video? Do you understand the Fermi Paradox?

      Yes and yes, more or less. I'm less sure that the assumptions leading to the paradox are necessarily sound. But who knows.

      In any case, I'm pretty sure the technology for reaching the "new world" did in fact exist by 1200 having arisen independently (probably) in several cultures. Indeed the norse landed some 200 years before that (albeit by a lesser distance than it would take to ultimately meet the Aztec several hundred years later).

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    7. I dont understand why we need to assume that there are other civilizations more advanced then us?

      I don't think that is much of a stretch... we are far more advanced than "we" were just a hundred years ago, and doubtless "we" will be considerably more advanced 100 years into the future than we are now. But I think it is a valid point that this doesn't imply a never-ending increase in technological capabilities. And even if this were so, there is that nasty reality of extinction before such extraordinary advances are ever realized.

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  11. WE are the alien civilization.

    WE will populate the universe.

    WE will seed life elsewhere.

    Fermi's Paradox. Solved.

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  12. "The probability of life originating spontaneously can't really be estimated, given our knowledge of both astronomy and chemistry."

    Yeah, I think we probably gave up too early on alchemy.

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