I read the book "Alone in the Universe" by John Gribbin recently.  He argues that the conditions that produced intelligent (sort of) life on earth are incredibly rare, so this explains the "Fermi paradox":  if there are aliens out there, why don't we see them?   (assuming as seems reasonable, that we don't). 

He gives a lot of plausible reasons for why the rareness of life.  Like, only a certain part of the Milky Way is a good home for life.  Our planet has a magnetic field that shields it from the solar wind - charged particles that stream out of the sun.  Continental drift may not be common, and it's crucial for regulating the concentration of carbon dioxide in the atmosphere, which is a kind of natural thermostat for Earth.  He thinks there were serendipitous mass extinctions of life forms that were unlikely to become intelligent, like the Ediacarans.  They were a weird kind of multicellular life that were around before the Cambrian.  Strangely shaped things.  And so on. 

Some of it is very questionable to me.  Like, he thinks that land life is much more likely to become technological, and that the reasons are obvious.  I'm not sure what the obvious reasons are and I looked online and didn't find them.  I wondered if part of the reason was that aquatic creatures would be less likely to develop hands, but then I found that octopi can manipulate objects with their tentacles.   A lot of technology wouldn't work underwater, but being underwater would make some things easier, too.  Aquatic intelligent creatures might be very good at spatial thinking since they live in 3 dimensions.  He mentions the idea of an advanced civilization putting self-replicating machines out into the universe that would colonize everything in time, and report back what they found to the home planet.  So why aren't there self-replicating alien machines on Earth, if there are alien civilizations?  But I'm not convinced that an advanced civilization would necessarily want to do this, or that it would be at all practical or feasible. 

He talks about a dinosaur called Troodon, which lived right before a 6-mile wide rock (probably) smashed into the earth, causing the end-Cretaceous extinction.  He says based on its brain size, Troodon was about as smart as a small baboon.  So the dinosaurs were on their way to evolving intelligence when they were wiped out. 

It would be interesting to speculate what an intelligent creature that Troodon could have evolved into, would have been like.  They might have been more warlike than us (NOT a happy thought) since they were carnivorous and we evolved as omnivores.  Carnivores generally seem to be rather aggressive towards others of the same species.

There are obviously a lot of pitfalls with arguing that life is rare, based on lucky accidents that produced us.  Evolution is very creative, and examples of convergent evolution show how the same solution to a problem that life is faced with, are found over and over.  Why shouldn't intelligence be a solution that convergent evolution finds over and over, on different planets, even?  And, different solutions to different problems posed by different circumstances, could be found by alien life, and the limitations of our imagination are no argument against this. 

To me, the slow speed of light compared to the size of the cosmos seems a good reason why we'd be left alone by Them.  The Milky Way galaxy is about 100,000 light years in diameter.  One could perhaps make a good argument that other intelligent life would exist on a roughly similar time scale to us, so that 100,000 years would look like a long time to Them, too. 

If it's true that conscious life is very rare, then it returns the specialness to us that science took away so much, by discovering the hugeness of the universe, that the sun and stars don't revolve around us, etc.  The wonderful nature of our world is partly explained by the fact that we are here to see its wonders - which is made possible by the enormous size of the universe. 

For me, thinking about this kind of spacey thing is a mind-refreshing relief from the humdrum forced on me by health problems.  And you can enjoy exploring the cosmos too in your imagination.

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That sounds like an interesting book. We really have no idea at this point if life is rare or not. The Fermi Paradox applies only to life more advanced than we (capable of interstellar travel). The ubiquitous nature of life on Earth even in extreme environments suggests to me that life may well be common in the galaxy, though technologically advanced civilizations may be rare or unique to us. The speed of light seems like a daunting barrier, but the basis of the Fermi Paradox is that even if interstellar travel were limited to, say, one tenth of one percent of the speed of light, it would only take 100 million years for life to spread across the galaxy. No individual civilization would embark on that journey; instead one planet would send a colony ship to the nearest habitable planet (10 light years is a likely distance), and that civilization would spawn its own colonies, so that eventually the galaxy would be spanned by this civilization in a time that is considerably shorter than the age of the galaxy. Of course, even at 10 light years it's going to take a very long time and a large amount of energy to make the trip. So another resolution to the Fermi Paradox is simply that civilizations don't get to the point (either voluntarily or involuntarily) of interstellar colonization.

I dunno, this sort of stuff seems to be wildly speculative.  He's speculating about why intelligent life is so rare, without establishing that intelligent life is actually all that rare.  How many assumptions deep do you have to go, before you're just being silly and making things up at random?

What makes our life and our technology possible is the fact that our solar system is the result of the blow-off of a previous gigantic star's supernova.  We need those heavier elements.  Considering that the universe has been around in it's current state for over 13 billion years, that's a lot of time for that series of events to happen.

oop. my previous thoughtful reply didn't make it....

thanks for the topic, this looks like a must read for me, i LOVE this kind of thing.

it makes me wonder what constitutes "life?" i mean, i realize there are obvious characteristics, but i think that even the Curiosity mission has a specific model for what would qualify. our instruments and theoretical models are calibrated by humans, after all.

for that matter, what is "intelligence?" what about "technology?" i mean, these are assumptions, to a certain extent, innit? as a mythographer, i tend to think of our human view of the possibilities for "intelligent life" in the universe is filtered through our projection of ourselves onto that universe. we want to see our own future selves out there. UFOs are, in my view, symbolic of our own path on this linear trajectory we imagine for ourselves, the more complicated the gadgetry, the more intelligent the beings, thus justifying our own techno-fetishism. (the mini iPad? your iPad was too big, but your iPhone was too small?)

for that matter, Sasquatch is the marker for the other end of the spectrum, eh? Bigfoot at one end, E.T. on the other, and stuck in the middle with you...

the vastness of just our own galactic neighborhood is almost inconceivable, let alone beyond our galaxy group. who knows what kind of conditions exist elsewhere and what kind of stuff they produce? all the more reason to return some of the funding to the space program, IMHO...

Scientists are discovering exoplanets hand over fist now. Projecting on what they have discovered in a relatively small area compared to the size of the universe there must be hundreds of billions of them. As such, the probability is not low but very high that some of them are capable of sustaining life. Life to the degree of our intelligence could permeate the universe without being detectable to other such life. Life that is much more advanced than ours might not be interested in interfering with us. It might have the ability if it developed the capacity to travel through worm holes. However, in this case it wouldn't need our resources because it would have the whole universe to access in serving its purposes. It certainly wouldn't view a species of our development as a threat. As such, I don't think the Fermi Paradox is a paradox at all. To the contrary I think it is the case that we haven't noticed other life so far because we haven't been sophisticated enough to have done so.     

Yes, one resolution to the Fermi paradox is that aliens follow the Prime Directive.

I think a far simpler explanation is the distances involved.  Intelligent life could be fairly common, and we still might not have gotten a visit, yet.

Really, how long have we been detectable from more than 10 or 20 light-years away?  We've only had an effect on the radiation output of our planet for maybe 150 years, with the first wireless communication.  The first intelligible transmissions were much later than that.

I don't think another species would have randomly discovered us, unless they were in the habit of cruising around searching for any possible life, on every planet.  We're kind of out on the ass-end of nowhere, in a very sparse part of the galaxy.  Of course, the backwaters are a more likely place for the development of intelligent life, since the galactic core and anywhere else high-traffic is going to cause problems from a stability perspective.  Intelligent life needs a long time to develop.

But, assume that the speed of light holds and that even approaching the speed of light causes survival issues, due to the blue shifting of radiation into dangerous, high-energy, lethal rays.  How long is it going to take for the nearest space-traveling race to each us, after we announced our presence with our high radio-wave output?

great point about our relative invisibility until recently (unless all the carbon here made us obvious, radioactively speaking). for me Fermi is essentially combining the size of the observable universe, the basic statistical likelihood for the existence of other civilizations, especially given our young star, and the complete lack of evidence of any contact. we should be lousy with aliens by now, but where are they? one potential answer is that, in spite of our ability to imagine interstellar travel possibilities such as warping space-time, or that our galactic neighbors are mostly liquid like jellyfish, traveling at even a tenth of the speed of light is simply an impossibility. or it takes so long that they just haven;t gotten here yet because we have only been formed as a planet for a fraction of the time it takes them to travel here.

I think it's something like 40% or 50% of the speed of light, at which point some of the higher-frequency background-radiation will blue-shift into cosmic rays that will penetrate pretty much any hull and kill the occupants.

The point of the Fermi paradox is that the distances are irrelevant because the length of time for interstellar travel is so great. It takes only a tiny fraction of the age of the galaxy to traverse the galaxy at even a very small fraction of the speed of light. So: IF life develops anywhere in the galaxy at least once to an interstellar-faring civilization, then it should spread to every habitable planet in the galaxy. Stars are the ultimate energy source, and energy is the ultimate resource, so advanced civilizations presumably would colonize every stellar system. That has not happened. The discussion in this thread touches on many possible explanations for why. Some are rather depressing, such as life not making it to interstellar capabilities, others less so (such as voluntary restraint from interstellar colonization for one reason or another), but the size of the galaxy and the speed of light are not explanations for the lack of contact. They are, quite the contrary, the cause of Fermi's initial query: "where are they?". 

I don't particularly buy the Fermi paradox.  I think there are a lot of details left out.

For one thing, the age of the universe gets trimmed down significantly, if you want to look at the fraction of the existence of the universe in which the existence of intelligent life is probable.

The first stars could not have developed intelligent life.  They didn't have the requisite elements for ... well, anything, really.  Any element higher on the periodic table than hydrogen was forged in the heart of a star.  Then, the initial stars went supernova, blowing off material to later condense into daughter stars.  The planets that formed around those stars would then have the higher elements necessary to develop life.  The planets around those stars would then have to progress sufficiently through their cycle to develop life.

We're already down to much less than half of the time in the universe.  How much less, I don't even know.  I have a good background in physics, but not that good.  I'd call it about 8 or 9 billion years along.

The distances involved are still significant.  The Milky Way Galaxy is over 100,000 light years across, and over 1,000 light years thick.  Space is three-dimensional.  This isn't like exploring the "flat" surface of a planet.

If you crunch the numbers:

14,000,000,000 - 9,000,000,000 = 5,000,000,000

5,000,000,000 / 100,000 light years wide / 1,000 light years thick = 50 times the time necessary to explore everything linearly.

Now, that's not perfect, since you'll save some time in the outward expansion.  We'd be expanding in more of a spherical pattern.  However, there's also a lot of time lost in redundancy.  You're not going to have a perfect leap-frog pattern, with new colonies immediately launching colonization missions to the next star outward.  I'd say that if anything, we'd have a delay of several times the travel time, for consolidation and further exploration, before colonization.

On top of that, if we find that we're restricted to a third or a fourth of the speed of light, due to the blue-shifting I mentioned above, I can easily believe that large potions of the galaxy have not been colonized or even thoroughly explored, particularly out on the ass-end of nowhere, on a small spur of one of the lesser spiral arms, where we are.

To sum it all up, I think Fermi left out a large number of the potential problems with and realities of galactic exploration and colonization.  I don't think the ratio of the time to the distances involved is anywhere near sufficient to say that it's irrelevant.

Joshua, thanks for referring to the Fermi paradox.  I don't remember hearing of it before, and so far, it sounds interesting and reasonable.  It's changing my thinking on the subject.

The title "alone in the universe" is misleading - Gribbin doesn't know the size of the universe, nobody does. Very likely we're effectively alone in the universe if we're alone in the Milky Way. Our galaxy has about a trillion stars. A civilization that started in a galaxy would have plenty of stars to expand to within the galaxy. There would be little incentive to travel a million light years to the next galaxy.

John Gribbin's contention is that there are a lot of factors that make us rare enough that likely, we would be alone in the Milky Way. Once you start multiplying together probabilities, it doesn't take many unlikely coincidences to make a species capable of interstellar travel, one in a trillion. Michio Kaku has said the same sort of thing although he didn't give as extensive a list of reasons.

Gribbin also thinks that many types of galaxies are unlikely homes for life, the stars are too close together and would be too disruptive; there's a supermassive black hole at the center that would bathe everything in radiation or disrupt the stars gravitationally, and so on, I don't remember very well.

He gives a lot of non-obvious reasons why our development is rare, that's the interesting thing about the book. So what I would like to hear about is if anyone has comments on how valid those reasons are. I mentioned some things I was skeptical about, like why would it be difficult for intelligent technological marine life to develop? I didn't mention a lot of his reasons, one would have to read the book for that.

Life is endangered a lot by objects colliding with the Earth, relatively small impacts have caused mass extinctions in the past and a big impact would sterilize the Earth.

Joshua Colwell writes:

The ubiquitous nature of life on Earth even in extreme environments suggests to me that life may well be common in the galaxy
Even the extreme environments on Earth are still on Earth - not bathed in a lot of radiation, near-circular planetary orbit, with the thermostat provided by continental drift, etc. Even environments in our solar system are still in a favorable part of the galaxy.

one planet would send a colony ship to the nearest habitable planet (10 light years is a likely distance)

10 light years seems very optimistic.  Sure it would be easier to survive in a place than for people to evolve there - but what would be needed is a planet that isn't just habitable, but a place where a civilization can thrive well enough so it's capable of interstellar travel on its own, and has necessary resources for interstellar travel. Somalia is extremely habitable relative to most of the universe, but they aren't going to be sending a mission to Mars anytime soon. Our highly technological society can easily be disrupted enough to stop missions to outer space.

another resolution to the Fermi Paradox is simply that civilizations don't get to the point (either voluntarily or involuntarily) of interstellar colonization.

That is grim but plausible. With us, technological change has outrun our biological evolution, so we're maladapted. For example our estimates of risk are dramatically wrong. We evolved primarily to look out for our immediate interests and those of the people we're close to, not the interests of the human species. There seems to be such a thing as group selection, where traits are selected for that benefit a group as a whole, even though they may be anti-survival for individuals. But individual selection seems to have determined much more who we are.

Technology has protected us from many of the risks our ancestors faced, but the human species as a whole is subject to new dangers, like nuclear wars and global warming, which we cope with very badly. Most people spend little of their time thinking about what the human race as a whole could do.

We are aggressive towards other groups of people, which may have been important in developing intelligence, but we could wipe ourselves out with too much aggression.

People develop high technology in corporations that are in economic competition, but cooperation is important to cope with large-scale risks like wars.

So to what extent would these things be generally true of an intelligent evolved species? I don't know, but evolutionary exobiologists might have thought about it. I don't feel optimistic about humanity's next 100 years.

one resolution to the Fermi paradox is that aliens follow the Prime Directive.

That's likely just science fiction. If we even found sasquatches, no way would we leave them alone! There would be a Sasquatch Journal, a Sasquatch Reservation where people would parachute in if they didn't allow Sasquatch tourism, motel strips would develop near Sasquatch migration routes, Sasquatch impersonators would attempt to infiltrate the sasquatches, unfortunate sasquatches would be kept in labs and studied.
Even a sasquatch would be the zoological find of the century - but that's NOTHING to the fascination an alien intelligent species would have!
We are insatiably curious if our immediate survival needs are met. Likely true of any intelligent technological species. If you believe the UFO stories (they seem very fishy to me), aliens are insatiably curious about us, at least they do nasty experiments on individual people.
I wrote a review of a book that I liked very much - Jeffrey Kargel's  Mars: a warmer, wetter planet.  He writes a lot about the possibilities for microbial life on Mars, he's a planetary geologist. 

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