Right, because that’s what or current techniques mostly allow us to find. Big planets close to their primaries are most favorable for techniques based on transiting such as light curve analysis. Drawing conclusions based on limited data with primitive tools is a poor notion though, and it tells us nothing about the typical makeup of extrasolar systems.
I think that plate tectonics is absolute prerequisite for life to evolve. Not only that, you need to have plate tectonics for a very long time for life to evolve. You need sharp temperature gradients stable for very long time on geologic scales and you only get that with plate tectonics. Without plate tectonics and with presence of atmosphere and voluminous bodies of water (other requirements) everything just gets obliterated quickly and the entire planet becomes flat without much features or temperature gradients. At later stages of evolution I think geographic features are important as they create niches which promote specialization and differentiation.
I am currently reading on origins of political order (https://www.audible.co.uk/pd/The-Origins-of-Political-Order-...). I think it might be possible large brain and high intelligence are not inevitable evolutionary results. The author speculates, and very convincingly, that the intelligence is result of human and chimpanzee males and females having different goals that cause them to deceive each other. The one who could better deceive would have evolutionary advantage and this caused humans to grow in intelligence as a result. But this level of intelligence is not inevitable as we see other species dealing with the problem in different ways. Large brain is a huge liability to an animal as it consumes a lot of energy and causes lots of other issues. It is true that animals use large brains to avoid predators or, being predators, to be more effective at the task. But it also seems that most animals don't seem to be growing their brains more with relation to their body size as it seems there are diminishing returns at some point and bigger brain costs exponentially more than it is worth.
As other responders have pointed out, brains are just one possible substrate for intelligence. We can't account for other systems (biological or otherwise) that could lead to the development of intelligence, so of necessity any probability estimate based on us has to be a lower bound of possibility.
One bit of our earth history that is worth noting is that the conditions when life presumably arose are hostile to multicellular life as we know it. Earth's early oceans were hot, anoxic and reducing, and under heavy pressure from far thicker atmospheres. The great oxidation event essentially turned earth's atmosphere into a bacterial waste dump, the waste being oxygen produced by early cyanobacteria. Life had to evolve ways to cope with oxygen or learn to hide. This is still true of our own cells. While we need oxygen to survive, it is still and will always be toxic. Free radical damage such as from reactive oxygen species is a constant threat, and is the ultimate form of irrecoverable information death when we die.
Might it be that the conditions under which life can arise, and under which intelligent life can arise, are different? We got lucky that earth traversed from the first to the second. Add this to the list of a million things that had to go right for us to exist at all. The same might well be true of any other intelligences out there. And the odds of two arising close enough in time and space to contact each other would be far less common than lonely civilizations looking out into the dark.
It took 2 billion years for the hard-working unicellular cyanobacteria to produce enough oxygen to first oxidize the rocks and minerals on the Earth's surface, and to build up enough oxygen in the atmosphere for the birth of multicellular life to be possible.
But if the planed had only supported one half of the cyanobacterial biomass it did, or if the amount of rocks to oxidize, or the mass of the atmosphere, was larger, that 2 billion years could have as well taken e.g. 4 billion years. And the Sun would have already become too hot, and multicellular life would never have evolved.
Life is not necessary for intelligence. As a counter-example, intelligent robots are certainly possible. What other substrates might support intelligence?
Life does not require proper axial tilt or distance from supernovas. There is life on the bottom of the ocean that survive on the heat and chemicals from hydrothermal vents, a metabolic pathway that does not depend on the sun. What other possibilities are there that don't happen to be present on earth?
We are biased towards human size, time scales, and biochemistry. And time and again, we have been surprised by forms of life that work differently.
If you've only had time for 3 thoughts (to simplify) since evolving intelligence... a civilization of your beings probably isn't doing too well.
The fastest object we've sent outside of the gravitational field of the Sun would take about 75,000 years to reach the nearest star (if we pointed it at it). For perspective, the last glacial advance started in earnest about 75,000 years ago. We don't have any technology that can maintain an automated craft for that long, let alone life support for sentient creatures. Hell, our current technology is insufficient to permit a self-sustaining colony on any other planet or satellite.
Ultimately, the Fermi Paradox is based on the assumption that, given enough chances, an unlikely event is basically certain. While this is true, to do a proper analysis, you do need to look at orders of magnitude to see if there really have been enough chances. And we have so little information to base the computations on that the dominating term is your priors: if you think we ought to have seen alien life by now, you'll pick priors that suggest life is quite likely and find that we ought to have seen it; if you think we oughtn't, your prior will reflect that.
"Later studies indicate that the top cruise velocity that can theoretically be achieved are a few percent of the speed of light (0.08–0.1c).[17] An atomic (fission) Orion can achieve perhaps 9%–11% of the speed of light. A nuclear pulse drive starship powered by fusion-antimatter catalyzed nuclear pulse propulsion units would be similarly in the 10% range and pure Matter-antimatter annihilation rockets would be theoretically capable of obtaining a velocity between 50% to 80% of the speed of light. In each case saving fuel for slowing down halves the maximum speed. The concept of using a magnetic sail to decelerate the spacecraft as it approaches its destination has been discussed as an alternative to using propellant; this would allow the ship to travel near the maximum theoretical velocity.[18]
At 0.1c, Orion thermonuclear starships would require a flight time of at least 44 years to reach Alpha Centauri, not counting time needed to reach that speed (about 36 days at constant acceleration of 1g or 9.8 m/s2). At 0.1c, an Orion starship would require 100 years to travel 10 light years. The astronomer Carl Sagan suggested that this would be an excellent use for current stockpiles of nuclear weapons"
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
Not true: https://en.wikipedia.org/wiki/Kepler-90
I am having trouble finding the reference that I found that claim in. But note that Kepler-90 does not have rocky planets that look like the Earth. It has super-earths, and gas giants. A gas giant will do a worse job of protecting a super-earth than it will of protecting small rocky planets.
So Jupiter still seems rare.
Incidentally while looking for the article that I saw before, I stumbled across https://www.reddit.com/r/askscience/comments/4ohkkn/how_uniq... which pointed out something I had not thought of. Our Sun is an unusual combination of old and metallic. It is likely that a neutron star collision happened near the stellar nursery where the Sun is born. This is important for a couple of reasons. The first is that metallic stars give rise to more planets. The second is that metallic stars can give rise to planets with richer chemistry. This is another way in which we are weird, which could be potentially important for the development of intelligent life.
There is a huge amount of observational bias in exoplanet research and given that we've "only" found a few thousand planets, it's naive to think our Solar System is unique. It's practically certain that with better instruments we'll find new planets in currently known systems.
For example HD_10180 (7 planets currently) also has two potential gas giants. See also HD 219134.
Also: stats of 100 billion to one, those are reasonable odds. That would make two species in the Milky Way, let alone the other billions-trillions of galaxies in the Universe.
The point of the Fermi paradox is that an advanced technological system should be able to colonize the whole galaxy in a geologically short time scale using Von Neumann machines. So if it had happened anywhere else in the galaxy, we'd expect to know about it because they would be already here. But they are not here. So we're probably the first in this galaxy.
Given that there are around 250 billion stars in the galaxy, if the odds were above 1/100 billion per star, the observed outcome would be unlikely. So it is probable that the real odds are below 1/100 billion.
I was offering potential requirements under which real odds on the order of 1/trillion could be justified. These odds are low enough that the observation "we're the first technological civilization in the Milky Way" is not a surprise and the Fermi paradox stops being a paradox.
Neither the Fermi paradox nor my argument suggests that there aren't lots of other galaxies which contain advanced technological civilizations. The time for an advanced galactic civilization to cross intergalactic space is NOT a geological eye-blink. And almost all galaxies that we see are in other clusters, which are much, much farther off.
The Milky Way is 105,700 light years across. At voyager probe speeds that’s ~1.8 billion years. Stopping requires 4x as much energy, and cutting that time in half requires 4x as much energy. Worse slowing down to build a probe adds more time as does however long it takes to select targets, gather resources, and build several more probes.
On top of a huge number of issues could slow things down such as lost probes, or needing to avoid the center of the galaxy resulting in an even longer trip. Not to mention collisions, at a minimum you need to survive grain of sand sized collisions, but interstellar space may contain significant numbers of much larger objects.
Finally these probes would both come here, and stick around for us to see. A true space based civilization might have little interest in planets like ours, or have sent multiple probes to our solar system that failed.
It seems possible for hundreds of civilizations to all be expanding across the galaxy even if we have seen none of them.
Note that with the magic of exponential growth, a few probes sent long distances and then spreading out locally should be able to reach every star in a galaxy with surprisingly little extra time over simply making the trip.
Even hand waving that we don’t have force fields to survive single gram impacts at 1% light speed. At those velocities anything is going to act like a massive shaped charge. Space is fairly empty, but when you start talking about the volume swept up across light years that’s really not a safe assumption.
70 years ago people generally assumed fusion was right around the corner. It’s only when you start building these things you get some idea of all the real limitations.
Now it is absolutely true that we could not survive any significant impacts at that speed. However we also have data indicating that the interstellar medium is essentially empty.
Here is a sanity check. Per http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht... the density of dark matter (that is, all the stuff that we can't see) is around 6x10^(-28) kg/cm^3. Since you arbitrarily said a gram, let's switch that to g/m^3, and we get 6x10^(-19) g/m^3. The distance to Alpha Centauri is on the order of 4x10^16 m. So a 1000 square meter surface should run into about a gram of "stuff". According to our best theories, most of it is currently unknown stuff, 99% of the rest is single atoms, and the vast majority of the rest is dust particles with sizes on the scale of waves of blue light.
Dealing with that dust and hydrogen is a serious issue. But the odds of running into pebble sized objects at random are pretty low. Heck, even microbe sized objects will be vanishingly rare. True, it will be a disaster if we do. But that translates in practice into, "There will be a failure rate for probes."
The size of a probe to bootstrap manufacturing seems likely to be a lot smaller than a billion tons of cargo. Don't forget, this is going to be encoded using technology that is as far beyond us as we are beyond building the pyramids. I think we can assume rather efficient storage/bootstrapping mechanisms to be available for us.
As to your sanity check, Alpha Centauri our closest star, if we are sending multiple probes then you need to targets much further out. Especially if we are trying to get somewhere approaching a straight line in 3D space. The 20th closest star system is 12 light years from earth and on the order of distances we need. Though this gets worse if you need specific kinds of star systems.
That’s pushing 3+ grams under your estimate and your 30 ton payload would need truly massive shielding. Or, you can have a 30 ton shield being activly maintained in front of a postage stamp sized cargo and possibly survive the trip.
As to that billion tons of cargo, it’s almost unimaginably better than current tech could do. We are talking either self sustaining habitats for hundreds of people or safe replicating near full AI on million year timescales. It’s got to gather energy and resources, build CPU’s, engines, etc to gather even more resources. It’s got to travel hundreds of years meaning redundancy and self repair on all critical systems etc etc the list just keeps going.
In terms of engendering from ancient Egypt to now the tallest structure is just under 6x as tall. Some things have improved vastly more and others even less. We simply can’t assume magic even on 10,000 year time scales.
> As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely.
Plus some claims about the uniqueness of our solar system which are not necessarily true, given the small sampling of planet's we've found.
There is so much bias in what we observe that I don't think you can make those statements, whether or not you're implying that a species would be galaxy-conquering.
Given these biases I think the odds may well be much better than 100 billion - though I'm not current enough to judge whether that's an appropriate estimate based on curent astrophysics. There's so much uncertainty that it could be orders of magnitude different in either direction.
In the same way game theory makes guesses about nuclear standoff strategy.
I'm not sure this is particularly useful. It could be like a medieval European saying land to the west of the Atlantic doesn't exist, because they've never once seen it through their looking glasses.
How far could we detect intelligent life, say on a similar level of current humans? If it is based off of radio ways, we would only be able to detect life within a 100 light year radius of earth, which is merely 2% of the way across our galaxy.
However it is not arbitrary, and not an assumption. When you run simulations you can test different combinations of factors and see what difference they make. See, for example, https://www.space.com/31577-earth-life-jupiter-saturn-giant-... to see how important gas giants are for blocking large impacts on small rocky inner planets.
That said, there may be another way to get long-term stability. But if so, then we haven't found it. And yes, we've looked.
What do you mean? 'You have to replicate the solar system to come up with intelligent life' is a gigantic assumption, on top of the 'solar system-like planetary system are rare' assumption. Is this a conceivable narrative for the Fermi paradox? Sure. But it's hardly supported enough to make a reasonable probabilistic argument out of it.
We also have reasonably compelling evidence that both factors are important for intelligent life to arise. Or, more precisely, good reason to believe that extreme axial tilts and cometary impacts are really, really bad for the biosphere. So the importance of those factors is itself not exactly arbitrary.
It is obviously an assumption other intelligent life will be like us, we do not know of any other intelligent life to be able to say otherwise. But if I'm looking for life, especially intelligent life, I will look for planets having conditions similar to ours. This is because our sample size is just one, us.
It could be that NOT having gas giants around is how intelligent life ACTUALLY exists in 99.99% of the cases of intelligent life in the milky way galaxy and Earth is the 0.01%. But if I have looked a lot and still just have Earth to guess with, I will continue looking for Earth-like planets. There is no way for me to actually know, but I don't want to waste my time.
I don't think we know the former and we kind of sort of maybe know the latter for the Solar system right now. As to the rest, I don't follow the relevance, we're not talking about searching for intelligent life at all.
You're right about the former not being certain, but it's definitely rare amongst the planets we've found? And the latter is quite certain.
What else are we talking about?
I mean this thread was not about how we search for life. It's about an estimate of the prevalence of intelligent life - an 'explanation' for the Fermi paradox. I'm saying it's basically a story.
Not really. The amplitude of the Sun's vertical motion is about 100 parsecs (~ 300 light years), but since the main ("thin") stellar disk has a vertical exponential scale height of about 300 parsecs, the local density of stars at the top/bottom of the Sun's orbit is only about 30% less than when the Sun is crossing through the midplane. (And this isn't at all an unusual orbit.)
What makes a real difference is being relatively far out in the disk from the galactic center: if we were ten times closer to the center, the local density would be almost 100 times higher, and you might indeed have to start worrying about nearby supernovae.
This is compounded by:
1) a lot of planets are not hospitable for a variety of factors. A society might resent the limit for many years if it was possible to implement such a limit.
2) the technology era is short-lived and not ongoing. A new civilization starts 10,000 years later. How will they even know homo-sapiens existed if a plague wipes out homo-sapiens tomorrow? A few tiny satellites in the vast emptiness of space?
One of the scariest thoughts I have ever had is that we truly are alone, but the sheer size of the universe makes me feel slightly better about that prospect.
As someone quite new to this discussion, can I ask why? My only immediate reaction is a mild one going the other way: that being alone is reassuring as it means there is not this extra potential threat out there.
Another thing making earth rare is its high proportion of iron ore. That and our abnormally large moon are thought to be the result of Earth colliding with another large rocky planet. The tidal force of an extra large moon + our extra ironey core give us an extra strong magnetic field to keep the nasties out and hold our atmosphere
Intervening material does very little to cut out light. If it did, we couldn't see distant stars.
However the intensity of radiation falls off as the square of the distance from the source of the radiation.
Therefore the primary determinant of risk is the number of stars that could be about to go boom within the critical distance of the Sun. If we're in a less densely populated neighborhood, then our odds of seeing a dangerous supernova go down. Full stop.
The fact that there are patches of dust doesn't change the fact that we are seldom in the middle of such patches.
And even when we are, it is a lot like the difference between being in light mist versus seeing it from a long ways away. We are, in fact, in the middle of a patch of "mist" called the "local fluff". See https://en.wikipedia.org/wiki/Local_Interstellar_Cloud for more. This does not noticably cut out light between us and stars that are under 100 lightyears away.
There are a few planets and moons in our system that have or had the potential for life, but upon which there's been no evidence for intelligent life, if any life is to be found on them at all: Mars, Europa, Ganymede, among others. While this would seem to increase the probability of life in other solar systems, seeing observable life on only one of the worlds in our system really has a negative impact on the overall odds of life in the universe.
Whether a moon is necessary or not for life on Earth [2], it remains that a stable tilt is currently thought to be critical for the development of complex life. We also had the benefit of an asteroid that obliterated the dominant species of the planet and changed the environment sufficiently to allow for new species to take their place. Hundreds of millions of years of flourishing life also provided us with the abundant, cheap energy sources in the forms of coal and oil that we needed to build civilizations.
There's also an open question as to whether evolution necessarily leads to intelligent species. Earth had large, complex lifeforms for 165 million years, and none of them appear to have developed civilizations. Human societies have existed for just 50,000 years if you take a very liberal definition of "society". Research has found an evolutionary fluke that fed lots of oxygen-rich blood to hominid brains [3]; other research has suggested that intelligence is not only not an evolutionary advantage, it's a disadvantage, due to the much higher calorie diet required to sustain it.
And finally, our universe may not have been hospitable to advanced civilizations until only (geologically) recently. It took a billion years for the universe to form galaxies and stars and planets [4]; it was a more chaotic, hot, radiation-rich universe before our solar system was born. Any civilizations which developed much before our own solar system may have met a premature end by radiation or cosmic debris.
Given the empirical lack of observable life in the universe so far, and that continuing research keeps growing rather than reducing the number of special cases that led to human civilization on Earth, it seems likely to me that even if there are other intelligent civilizations in the cosmos, they are approximately as young as we are, too far away and technologically primitive to make contact.
[1]: A good paper on this from last year: https://arxiv.org/pdf/1806.02404.pdf [pdf]
[2]: https://www.astrobio.net/news-exclusive/the-odds-for-life-on...
[3]: https://theconversation.com/how-our-species-got-smarter-thro...
[4]: https://en.wikipedia.org/wiki/Chronology_of_the_universe
Especially when, in this context, we're talking about a rather peculiar type of intelligence, ie. the propensity to construct spacecraft and broadcast high-powered message-carrying radio signals. It seems very difficult to argue that there's any evolutionary pressure in favour of these behaviours, just given the time-lag from the development of our large brains to this.
Start looking at things that are weird about our Solar System and the Earth. Figure out which ones are probably necessary for intelligence. Add those into the Fermi calculation. And see that it is likely that we're the first intelligent life in this galaxy.
What are some of those special things? It is not enough to have a planet that has the right ingredients to start life. It is necessary that it's progress not be constantly wiped out by major disasters over a very long time scale. What kinds of disasters?
You don't want a supernova going off too close. We have been lucky to avoid that. Our odds were greatly improved by the fact that we spend most of our time out of the galactic plane, away from other stars that could be about to go boom. This is an unusual orbit.
You don't want to be hit by too many comets. We've been hit by some, but far, far less than we would have without Jupiter acting like a sweeper to clean up dangerous stuff in our neighborhood. How rare is that? We have cataloged planets in hundreds of other solar systems. We see ones with only rocky planets. Ones with only gas giants. But we're the only one we know of with both gas giants and rocky planets. The only one where the rocky planets wind up protected from most of dinosaur killer kinds of impacts.
Oh, and get this one. Without the Moon, the tilt of the Earth's spin is unstable. Without it, in simulations we would wind up with one pole aimed at the Sun and the other not once every few tens of millions of years. Probably not good for the development of intelligent life. Again we don't know how rare this is, but we suspect that it is rather uncommon.
Suppose that each of these only happens to one star out of a thousand. Suppose further that there is one more, as yet unidentified, special factor about us that is also required. Again make that a 1/1000 coincidence. That would make the odds good that of the ~250 billion stars in our galaxy, we're the only ones with intelligent life.
Or if you do the back of the envelope just with the three that I named, and made them 1/10,000 coincidences, you get the same result.
As a sanity check, the fact that no other intelligent life has been observed is evidence that it is unlikely. And given Fermi's argument, it is probably unlikely at least on the scale of 100 billion to one against.