What makes a planet a good host for life? That is, what does a planet need for life to originate there and then evolve to something at the human level? Astronomers today say a planet at least needs a star that 1) lasts long enough, 2) has enough heavy elements, and 3) is not too often hit by nearby supernovae or gamma ray bursts. Using such criteria, several astronomers (mentioned below) have tried to calculate “galactic habitable zones,” i.e., galactic distributions of good-for-life planets, in both space and time. Such calculations are far more important than I had realized – they can help say how common are aliens! Let me explain.
"Why? If no extinctions occur knowledge will be preserved."
Really? I think the Romans would disagree.
"Metals don't disappear when you use them, you can recycle them so again."
Second Law of Thermodynamics? Yes, you can recycle, but each time you do so, some of the source metal will be lost.
"Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there"
True. But we can also make synthetic gasoline. The reason we don't is because it takes more energy to make gas that way than you get back from burning it. So it's not economical.
I'm not saying that interstellar colonization is impossible - just that nothing is a simple as you think. Plus, the Fermi Paradox + Von Neumann's work together seems to support the notion that interstellar colonization is extremely extremely unlikely. Else...Where is everybody?
"Over that period of time, everything will be learned and forgotten again and again."
Why? If no extinctions occur knowledge will be preserved.
"But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today's energy requirements - those resources are close to exhaustion. And we've been using them only a century or so. No. It's not clearly sustainable."
Metals don't disappear when you use them, you can recycle them so again, if there are enough of them in year 1 there will be enough of them in year 100 million. Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there. In the near future there will be a transition from the current state of scarce energy and abundant minerals to one of abundant energy and scarce minerals, that's when space mining will kick-off.
Over that period of time, everything will be learned and forgotten again and again. By speciation, I meant the technological cybernetic sort - not the biological kind. But that's a bigger side discussion on the technological singularity.
Fusion and Solar energy sources are more or less limitless, yes. But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today's energy requirements - those resources are close to exhaustion. And we've been using them only a century or so. No. It's not clearly sustainable.
There won't be technological advancement lasting millions of years, in a couple of thousand years all that can be learned will be learned. Speciation won't happen if the population keeps mixing it is possible that over millions of years populations in different solar systems become speciated but that would only happen if physical contact between the solar systems were impossible and in that case there can be no eugenicist movement or interstellar war either and even if one of those things did happen it's possible one side would simply emerge victorious and continue the civilization.
The available energy sources (mostly stars) are constant over billions of years. It won't be the same as with fossil fuels on Earth: an energy source available in year 1 will still be available in year 100 million.
Physical changes to their biology wouldn't necessarily doom the civilization. However, constant technological advance - required for an interstellar civilization - stresses the social structures which underpin the civilization. At least for humans, it has for the last two centuries. It's not at all clear that any technological civilization can survive the continuous upheaval for a thousand years - not to mention a million.
Similarly, biological change (like speciation) would also stress the civilization. Combine both speciation and technological advance (e.g. Neo-luddites vs. Kurzwellian eugenicists or some such) and you can see where the society could be stressed.
But even if we concede that biological evolution isn't going to doom the civ, a technological civilization requires a steady supply of *enormous* amounts of energy. The more advanced the civ, the more energy it requires (ala the Kardashev scale). That energy has to come from somewhere. Can an energy appetite that big be sustained for megayear timeframes?
No species remains constant. Even the coelacanth undoubtedly underwent numerous changes to its immune system during its existence, it just happens to look much the same on the outside as it did 300 million years ago (and it's not like a "species" is a neat discrete category either). I also fail to see how physical changes to an intelligent species would necessarily spell the doom of their civilization. As long as they don't become less intelligent or die out the civilization should continue to evolve with them.
You are right. I'm not speaking to civilizations composed of sea turtles etc just the species that have remained constant over tens or hundreds of millions of years. I think it's much more plausible that an intelligent species will survive for say 5 or 10 million years (e.g. in a stone age) than a civ surviving for millions of years.
I agree. And on a side note, the stellar density near the core would also cause many more orbital disruptions planets those stars may have. Evolution of complex life would be much more difficult under those less stable planetary conditions. But microbial life could still be common.
Here's an interesting question; Suppose that we just concede that intelligent species (defined as using radio technology) will be detectable for an *average* of 10-20M years. If the Milky Way has produced 10,000 of these civilizations over it's history, what would be the probability of another such civilization currently being detectable by us?
If you convert Lineweaver's space-time distribution of habitable planets into a probability distribution for the space time location of intelligent life you could answer this question by randomly sampling 10,000 planets from that distribution.
The answer is, if you want a reasonable chance of detecting a civilization this way, the AVERAGE civilization needs to remain detectable for timespans measuring tens or hundreds of millions of years. But if that average drops into the 10,000 year range, the odds are that humans are alone in the Milky Way.
But on the other hand, if you assume that MORE planets could develop simple life which could last billions of years, the chances become very good that there is simple life out there right now.
Do the crocodile, shark or sea-turtle's civilizations require 474 exajoules of energy per year to support? Nope. Not the same thing.
It's *possible* (in theory) that human beings could remain biologically the same for 65 million years. But our civilization right now isn't sustainable on the scale of hundreds of years not to mention millions - and sustaining a technological civilization capable of interstellar travel is the issue at hand.
Is it possible that *some* civilization could have done this? Possible - but very unlikely.
In general you are right, but there are in fact outliers. The crocodile, shark, coelecanth and sea-turtle are all examples of creatures that have remained the same since at least the dinosaur era.
There are somewhere on the order of 1 supernovae happening every 50 years. There are approximately two supernovae capable of causing a mass extinction on Earth every billion years. Earth is significantly further away from most of it's surrounding stars than stars closer in to the center of the galaxy. From that we may surmise that extinction level supernovae occur more frequently close to galactic center due to both proximity and population. How much? From Ohio State dept of astronomy (http://www.astronomy.ohio-s... we have this:"The estimated number of stars within a parsec from the center is 10 million per cubic parsec. In Earth's region of space, the density is a mere 0.2." I make that to be 50 million more stars per unit of volume. If we are able to use that as a metric, then all else being equal we have somewhere between 2 and 100 million mass extinction causing supernovae every billion years increasingly linearly towards the center. If we further hypothesize that life will form on equivalent star systems with an equal probability then there is a 50 million times greater chance of life forming on the stars at the center than out here in the boonies. Thus.... joining the dots in my shaky conjecture, vastly more life forms in the center only to be wiped out every ten million years. If there's only 100 or 1,000 human-level candidates, how many of them could possibly make it (from e.g. lizard equivalents all the way up to mammal equivalents over a period of several hundred million years) with a mass extinction happening every ten million years knocking them back to rodent scale? It strikes me as a long shot to say the least. That said, I suspect there is very likely viruses and to a lesser extent bacteria/archae equivalent all over the place.
But really - on a timescale of millions of years there are so many other issues that come up. Evolution for example. Just looking at the Earth, there isn't a single species that has remained constant over that kind of timescale. So suggesting that the conditions required for a technological civilization to even be possible could exist for that time period is VERY speculative. It certainly has never happened on Earth.
And there is a large range between ten thousand and millions. Human civilization is only 10,000 years old. Modern humans are only 100,000 years old. Talking about a technological civilization capable of interstellar travel surviving for time-spans that are many tens of times older than our species is just ludicrous.
"Why? If no extinctions occur knowledge will be preserved."
Really? I think the Romans would disagree.
"Metals don't disappear when you use them, you can recycle them so again."
Second Law of Thermodynamics? Yes, you can recycle, but each time you do so, some of the source metal will be lost.
"Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there"
True. But we can also make synthetic gasoline. The reason we don't is because it takes more energy to make gas that way than you get back from burning it. So it's not economical.
I'm not saying that interstellar colonization is impossible - just that nothing is a simple as you think. Plus, the Fermi Paradox + Von Neumann's work together seems to support the notion that interstellar colonization is extremely extremely unlikely. Else...Where is everybody?
"Over that period of time, everything will be learned and forgotten again and again."
Why? If no extinctions occur knowledge will be preserved.
"But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today's energy requirements - those resources are close to exhaustion. And we've been using them only a century or so. No. It's not clearly sustainable."
Metals don't disappear when you use them, you can recycle them so again, if there are enough of them in year 1 there will be enough of them in year 100 million. Abundant amounts of metals exist on every asteroid, planetoid and rocky planet out there. In the near future there will be a transition from the current state of scarce energy and abundant minerals to one of abundant energy and scarce minerals, that's when space mining will kick-off.
Over that period of time, everything will be learned and forgotten again and again. By speciation, I meant the technological cybernetic sort - not the biological kind. But that's a bigger side discussion on the technological singularity.
Fusion and Solar energy sources are more or less limitless, yes. But extracting and transmitting that energy also require other natural resources (e.g. precious metals and semi-conductors, rare elements). And even on scales close to today's energy requirements - those resources are close to exhaustion. And we've been using them only a century or so. No. It's not clearly sustainable.
There won't be technological advancement lasting millions of years, in a couple of thousand years all that can be learned will be learned. Speciation won't happen if the population keeps mixing it is possible that over millions of years populations in different solar systems become speciated but that would only happen if physical contact between the solar systems were impossible and in that case there can be no eugenicist movement or interstellar war either and even if one of those things did happen it's possible one side would simply emerge victorious and continue the civilization.
The available energy sources (mostly stars) are constant over billions of years. It won't be the same as with fossil fuels on Earth: an energy source available in year 1 will still be available in year 100 million.
Physical changes to their biology wouldn't necessarily doom the civilization. However, constant technological advance - required for an interstellar civilization - stresses the social structures which underpin the civilization. At least for humans, it has for the last two centuries. It's not at all clear that any technological civilization can survive the continuous upheaval for a thousand years - not to mention a million.
Similarly, biological change (like speciation) would also stress the civilization. Combine both speciation and technological advance (e.g. Neo-luddites vs. Kurzwellian eugenicists or some such) and you can see where the society could be stressed.
But even if we concede that biological evolution isn't going to doom the civ, a technological civilization requires a steady supply of *enormous* amounts of energy. The more advanced the civ, the more energy it requires (ala the Kardashev scale). That energy has to come from somewhere. Can an energy appetite that big be sustained for megayear timeframes?
No species remains constant. Even the coelacanth undoubtedly underwent numerous changes to its immune system during its existence, it just happens to look much the same on the outside as it did 300 million years ago (and it's not like a "species" is a neat discrete category either). I also fail to see how physical changes to an intelligent species would necessarily spell the doom of their civilization. As long as they don't become less intelligent or die out the civilization should continue to evolve with them.
You are right. I'm not speaking to civilizations composed of sea turtles etc just the species that have remained constant over tens or hundreds of millions of years. I think it's much more plausible that an intelligent species will survive for say 5 or 10 million years (e.g. in a stone age) than a civ surviving for millions of years.
I agree. And on a side note, the stellar density near the core would also cause many more orbital disruptions planets those stars may have. Evolution of complex life would be much more difficult under those less stable planetary conditions. But microbial life could still be common.
Here's an interesting question; Suppose that we just concede that intelligent species (defined as using radio technology) will be detectable for an *average* of 10-20M years. If the Milky Way has produced 10,000 of these civilizations over it's history, what would be the probability of another such civilization currently being detectable by us?
If you convert Lineweaver's space-time distribution of habitable planets into a probability distribution for the space time location of intelligent life you could answer this question by randomly sampling 10,000 planets from that distribution.
The answer is, if you want a reasonable chance of detecting a civilization this way, the AVERAGE civilization needs to remain detectable for timespans measuring tens or hundreds of millions of years. But if that average drops into the 10,000 year range, the odds are that humans are alone in the Milky Way.
But on the other hand, if you assume that MORE planets could develop simple life which could last billions of years, the chances become very good that there is simple life out there right now.
Do the crocodile, shark or sea-turtle's civilizations require 474 exajoules of energy per year to support? Nope. Not the same thing.
It's *possible* (in theory) that human beings could remain biologically the same for 65 million years. But our civilization right now isn't sustainable on the scale of hundreds of years not to mention millions - and sustaining a technological civilization capable of interstellar travel is the issue at hand.
Is it possible that *some* civilization could have done this? Possible - but very unlikely.
Yup. I did the calculation of roughly back of the envelope how much better off we are somewhere else in the comments.
In general you are right, but there are in fact outliers. The crocodile, shark, coelecanth and sea-turtle are all examples of creatures that have remained the same since at least the dinosaur era.
There are somewhere on the order of 1 supernovae happening every 50 years. There are approximately two supernovae capable of causing a mass extinction on Earth every billion years. Earth is significantly further away from most of it's surrounding stars than stars closer in to the center of the galaxy. From that we may surmise that extinction level supernovae occur more frequently close to galactic center due to both proximity and population. How much? From Ohio State dept of astronomy (http://www.astronomy.ohio-s... we have this:"The estimated number of stars within a parsec from the center is 10 million per cubic parsec. In Earth's region of space, the density is a mere 0.2." I make that to be 50 million more stars per unit of volume. If we are able to use that as a metric, then all else being equal we have somewhere between 2 and 100 million mass extinction causing supernovae every billion years increasingly linearly towards the center. If we further hypothesize that life will form on equivalent star systems with an equal probability then there is a 50 million times greater chance of life forming on the stars at the center than out here in the boonies. Thus.... joining the dots in my shaky conjecture, vastly more life forms in the center only to be wiped out every ten million years. If there's only 100 or 1,000 human-level candidates, how many of them could possibly make it (from e.g. lizard equivalents all the way up to mammal equivalents over a period of several hundred million years) with a mass extinction happening every ten million years knocking them back to rodent scale? It strikes me as a long shot to say the least. That said, I suspect there is very likely viruses and to a lesser extent bacteria/archae equivalent all over the place.
But really - on a timescale of millions of years there are so many other issues that come up. Evolution for example. Just looking at the Earth, there isn't a single species that has remained constant over that kind of timescale. So suggesting that the conditions required for a technological civilization to even be possible could exist for that time period is VERY speculative. It certainly has never happened on Earth.
You mean no alien civilizations? Because there is probably alien bacteria somewhere. Otherwise I agree.
And there is a large range between ten thousand and millions. Human civilization is only 10,000 years old. Modern humans are only 100,000 years old. Talking about a technological civilization capable of interstellar travel surviving for time-spans that are many tens of times older than our species is just ludicrous.