Is Earthly life premature from a cosmic perspective?
phys.org
phys.org
As far as this paper goes, the most interesting implication isn't mentioned in the phys.org article. It seems to me that main idea of the paper is to connect the development of life to the cosmological evolution of the universe. We find that we live in a somewhat special time, when the universe is just starting to be dominated by dark energy after having been dominated by matter since ~300,000 years after the big bang. Anytime you see that we live at some sort of a special time, the Copernican principle leads one to wonder if there is some underlying reason for this.
The main argument of this paper is that as the universe becomes more dominated by dark energy, the expansion of the universe accelerates and this shuts off the flow of gas onto galaxies. As galaxies stop accreting matter, star formation slows, making it less likely that life will develop in the far future. However, low mass stars are more abundant and have a longer lifetime, so even if it takes a longer time for life to evolve in that environment, it's still, on balance, more likely that life evolves around low mass stars in the near future ("near" still being tens to hundreds of billions of years). So we would expect that the typical life form will evolve around a low mass star, far in the future. Thus, life on Earth seems to have developed early, but maybe not unusually so, and it's maybe not a coincidence that we happen to live a little after the time at which the universe became dark energy dominated. I would file this away in the "interesting if true" category.
I find that premise to be false, given our (limited) knowledge about exoplanets already. The limited Goldilocks zone for low mass stars usually results in planets being tidally locked, leaving life to develop in narrow bands or in the hostile zones with direct sunlight or lack thereof.
Not all exoplanets around faint stars may be tidally locked, but with increasing distance it also pushes them out to the limits, where there may not be enough energy for life to develop (at near freezing ocean temperatures) With more information we may find that perhaps there's also an optimal star brightness, which provides just enough energy at just the right distance for life to develop, disqualifying most dwarf stars.
In either case, whatever you believe, given our extremely limited information, any papers produced can be labeled as speculative, especially when we're talking about something where N = 1.
I will propose such a puzzle. The moon has been moving away from the Earth at a certain rate every year. Yet it is only in the time frames of the humans that we find the moon and the sun roughly the same size, so they can form full solar eclipses etc.
Is this a coincidence? Surely there can't be any physical explanation for this.
Coincidence seems like a perfectly adequate explanation to me.
It will take 1billion years for the moon to move 10% farther away. (200K miles 0.1 / (4cm/yr))
https://www.wolframalpha.com/input/?i=(distance+from+earth+t...
Not sure how long back the 4cm/yr rate has been a good estimate.
That is outrageous. How many eclipses have you seen in your life? There are 2-5 solar eclipses every year. Most people will never see one cross the piece of dirt they inhabit over their whole lives. The idea that a transient celestial phenomenon that lasts around 5 minutes, and that a large number of organisms in the species will never even see, is going to somehow stimulate a cognitive awakening is absurd.
Why wouldn't a storm create this 'cognitive kick start'? There are more of them and they require more wiles to survive.
I specifically said it's the corona, not the eclipse, that's spectacular. The corona only lasts seconds, not anywhere near 5 minutes.
> and that a large number of organisms in the species will never even see
That's based on the present geographic distribution of humans. When humankind was making the transition to intelligence, they were distributed differently over the earth, over a zone more likely to produce eclipses with a corona.
> How many eclipses have you seen in your life?
Only one with the full corona. There's three naturally occurring time periods in nature: days, years, and inter-coronal intervals.
Check it out, accounts for Big Bang and dark energy:
http://m.phys.org/news/2015-02-big-quantum-equation-universe...
Some processes in the universe seem to take longer than 13B years
http://www.dailygalaxy.com/my_weblog/2010/06/you-couldnt-mak...
This seems like the time that Lord Kelvin calculated the Earth to be no more than 100 million years and it took a century for scientists to challenge that. Becauss they discovered radioactivity but mostly because the old generation receded in power.
While life appeared very early in the history of the earth, technologically advanced life took billions of years to evolve. You require a planet with an extraordinarily stable climate to allow a technological society to evolve and I predict this is the filter.
Just one factor contributing to this stability is the need for a very large moon to keep the axis of rotation stable over billion of years. If the Earth didn't have its moon the Earth's axis would tip over creating massive climatic change. The more we look at the universe the more unlikely the stability of Earth's climate looks.
First I've heard of this, do you have any more information?
1. http://www.nature.com/nature/journal/v361/n6413/abs/361615a0...
2. http://www.astrobio.net/news-exclusive/the-odds-for-life-on-...
3. http://www.space.com/12464-earth-moon-unique-solar-system-un...
I actually think you would use a series of nested Dyson bubbles to capture the energy via a cascade much like the electron transport chain [2] uses a multi-protein redox cascade to capture the energy of metabolism.
What about undetected energy aka dark energy.
> Just one factor contributing to this stability is the need for a very large moon to keep the axis of rotation stable over billion of years. If the Earth didn't have its moon the Earth's axis would tip over creating massive climatic change. The more we look at the universe the more unlikely the stability of Earth's climate looks.
I have heard this argument but I don't buy it compared to the need for asteroid protection. Things can get various like climate living underground/water. One of the critical things about our solar system that made life happen with out having a mega earth planet with crushing gravity is the presence of Jupiter and Saturn.
Jupiter and Saturn were in our solar system at the right place and right time. This theory is called the "Grand Tack" [1] and was recently covered in Scientific America.
Needless to say Jupiter continuously protects us from asteroids and various other objects from colliding with us on a continuous basis and not our axis. [2]
Continuous asteroid collisions vaporize oceans and destroy atmospheres along with other various debilitating things.
Those are two different things. "Common" is something up for interpretation, especially with the huge numbers concerned. Intelligence being uncommon could still mean hundreds of civilizations active in the galaxy right now. The term "uncommon" allows us to have somewhat meaningful discussions about probabilities. "Unique" on the other hand is an unsubstantiated, almost supernatural, claim of us being the only intelligence in existence anywhere.
> No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light.
That is also a big claim. Yes, a civilization pursuing a maximum energy strategy would probably harness stars, at least from our current technological perspective that seems reasonable. However, very few people looking for signs of this would actually claim that all civilizations more advanced than us must do this. So originally at least, this hypothesis was proposed under the more reasonable premise that for purposes of detection it would be enough if one such civilization pursued this strategy. Again, the claim that all of them would necessarily do this seems needlessly extremist.
> You require a planet with an extraordinarily stable climate to allow a technological society to evolve and I predict this is the filter.
There is a good chance you're at least somewhat correct, but then again I can easily imagine planets with a way more stable history than Earth had.
A sample size of one is always going to be a problem, especially if you lack the means of even knowing what to look for. Making absolutist claims a priori seems to me like an argument to stop looking. Historically, claims of uniqueness and specialness have had a tendency to be wrong. It seems to me like a good default stance is to assume that whatever we see in front of us has a chance of occurring that is significantly higher than zero.
The more we learn about other planets the more unusual Earth looks from a climatic stability perspective. There really is a good chance that Earth is unique in its level of stability - at least within the visible universe.
If you look at the rocket equation, a slightly different planet would make launching rockets extremely difficult.
I just imagine an intelligent beaver: "There can't be any other intelligence out there, just look at all these rivers and streams!"
Meanwhile, humans are enjoying fusion-power and will take a little while to notice suspiciously-well-engineered beaver dams showing up everywhere.
Could you please explain that to the World Bank and governments in Africa, SE Asia, China, and Oceania that insist on damning the last free-flowing rivers on Earth?
When it comes to hydroelectric power and titanic concrete structures, we look for very different water-situations than beavers do.
In order to justify ignoring all the burning stars, it needs to be easier to tap than solar and much much more energy than all the stars will ever generate. I think that requires new physics.
Really the energy source must be infinite. If you can see that it will run out in a trillion years and you can survive for another billion by harvesting suns, somebody will.
We have nuclear power but still dam rivers, and I think beavers notice.
OTOH If there is a source of unlimited free energy, maybe some of those bright spots in space aren't stars at all.
Even if we move onto mars a GRB would cook all of us.
And even if it's something considerably more boring then expanding to other planets or even star systems might not save a civilization from collapsing.
The argument is really more why is there only one technological species on the planet - the answer is the first one to arise will displace all the other potential competitors. This is exactly what has happened on Earth and why any other species that has a chance of creating a technological civilisation is not doing so well.
The same argument applies to the universe where the first civilisation that chooses to take over the universe will displace any competitors that choose not to. All it takes is one civilisation to spread and the whole universe goes dark.
Orang-Utan: 50,000
Chimpanzee: 200,000
Bonobo: 35,000
Homo Neandertalensis: 0
Homo Habilis: 0
Gorilla: 100,000
Homo Sapiens: 7,500,000,000
Note that ants don't directly compete for a niche with humans.
And megacolony species such as the Argentine ant are displacing other ant species.
2. Ant biomass is about the same as human biomass on Earth.
3. Typical human mass is 60kg, typical ant mass is 3mg.
4. Do the math.
Arguing that smelting steel is the key thing is the typical, we're good at this therefore this is what matters argument we've been making since time immemorial.
Humans can't sneak into other organism's bodies and take over their DNA (yet, to be sure) but viruses can, so humans just don't have a real shot.
Isn't this implying that any civilization more advanced than us must be able to take advantage of much more of a star's energy? Like say... humans in 6 months?
I see no reason for the band of possible civilizations between current humans and Dyson sphere building ones must be narrow at all. In fact, the vast majority of all possible civilizations might lie in that band, simply because it might be objectively impossible to actually build a Dyson sphere.
Then you've got the logistics problem: it's easy to build a modern CPU (for example) if you're assuming all the infrastructure already exists. If you're assuming it doesn't, it takes much more energy to produce the first one than any subsequent ones. And it may be the case that in order to build an energy-positive Dyson sphere/von Neumann probe network you'd need to already have consumed a couple stars prior. And to do anything on that scale, you're already talking about coordinating activities across multiple planets or solar systems, which comes with it's own massive overhead.
There's a lot of ground to cover between burning fossil fuels and consuming stars, and little reason to assume a successful civilization must do the latter. You could potentially go quite a while syphoning fuel off a gas Giant, for example.
Unless it is not worth the trade off. Material is rare and might be more useful for data storage than for harnessing energy. It's not difficult to imagine a civilization that consists of a single Dyson swarm, perhaps using the star as both an energy source and propulsion system. When they use up the star, they can move on to another star.
Detectable, but just barely, and only if we happen to look in the right direction.
Let's just posit that every single advanced civilization ends up building lots and lots of Dyson spheres (which I'm totally unconvinced of). Now, can you prove that there are no such spheres out there? No, you can't. You can only prove that a whole lot of stars have no sphere, but maybe we suck at detecting them and there's one right next to us.
The only conclusion the argument draws if that advanced aliens are not yet nearly finished enclosing all the stars in spheres, and you might try to determine an upper limit to the rate of stars with Dyson spheres without us noticing.
It only takes one civilisation in the universe to do this and all stars will cease to shine. The fact that we can see stars in the night sky means that they don’t have Dyson spheres around them hence there are not civilisations ahead of us - at least within the visible universe.
1. https://en.wikipedia.org/wiki/Self-replicating_spacecraft
An interesting thought is if a new von Neumann spewing civilisation arose in the galaxy we would not have much warning before it reached us. The probes would be travelling just behind the light front and so would reach us only a little before the information that the stars between the probes origin and us went out. About the only way you could detect such an event is to look at very distant galaxies for examples that had spherical chunk taken out of them.
I should get around to writing this all up in a blog post because it is such an interesting topic :)
Also, your thought on the probes travelling behind the light front implies that they are going to be travelling at relativistic speeds. Is that realistic?
There have been actual studies searching for the waste heat of Dyson spheres[0]. You basically image an "empty" part of the sky in the visual spectrums and look for infrared waste. (The studies turned up nothing but were also limited in scope.)
It would be really interesting to do a survey of galaxies looking for these spherical chunks. It should be possible to just use existing data and some machine learning to scan for possible candidates. If anyone is interested in doing this with me my contact details are in my profile.
It's like arguing about building a computer that can simulate reality. It just keeps getting bigger and more complicated and consuming more power and running slower as you learn about the new things it has to do, and soon the "possible in theory" becomes larger than the observable universe.
However, in case there actually is a Dyson sphere around a star, it will probably emit energy in the form of infrared radiation and we should be able to detect it. Especially if there are billions of stars like that.
1. https://en.wikipedia.org/wiki/Cosmic_microwave_background
Doesn't a pluto orbit sized energy collection unit leave a lot of energy on the table? Something about inverse squares..?
1. https://www.tillett.info/2016/08/03/carnot-efficient-dyson-s...
Recent news on this via studies.
Significant evidence of insufficiently advanced Dyson Spheres seems somewhat compelling.
What's the internal temperature of an efficient Dyson sphere? Or conversely, what's the minimum size of such a sphere around various star classes to maintain a goldilocks-zone internal temperature? What are the mass requirements of such a sphere? How do the nonrotating elements of that maintain themselves against stellar gravity?
At some point in the thermal life of the sphere, it's got to maintain a steady-state flux of energy out to energy in. At a galactic scale, that's probably going to be detectable.
It's also quite possible that there are astral objects around which one would prefer not to build Dyson spheres. Black holes, say, or neutron stars, or binary or multiple star systems (a sphere around a single star is likely complex enough). Etc.
I've never found the Dyson sphere concept particularly compelling myself. Interesting concept, but challenging to pull off.
But then, what do I know, best I can claim is to have blown up a single planet. And that was with the assistance of alien technology.
I think the confusion might be you are thinking about a classical Dyson sphere and not what would be built by a civilisation trying to most efficiently use the resources available. An efficient system would be composed of a set of independent (but co-ordinated) energy collectors arranged in a series of shells (for lack of a better name lets call this structure a Tillett Onion). The inner shells will be quite hot, but the outer shells will be very cold. The emission spectra of the Onion will be determined by the outermost layer, which if it is out near the orbit of Pluto, will be close to the background temperature of interstellar space.
If these collectors are coordinated they can use some of the energy they capture to maintain their orbit. These energy collectors don’t need to use much matter (they can be one atom layer thick) since all they are doing is capturing the energy released by the inner shells.
This really needs a more detailed write up than a post buried on HN :)
You have: 4 * pi * 40au^2 * 0.0077g/m^2
You want: million gigatonne
* 86.618649
/ 0.011544858
You have: earthmass / 86.6 million gigatonne
You want:
Definition: 68986.619
Turns out that there's not all that much carbon on Earth -- it's 730 ppm, but that's still a total abundance of about 50x what would be needed for our Pluto-diameter shell.
On the other hand, that means that at best, using an entire Earth's worth of carbon, you could create a Pluto-orbital-radius graphene Dyson sphere 50 atoms thick.
But Pluto's kinda way out there. What if we make the sphere the size of Earth's orbit instead. Surely that's going to save us a few orders of magnitude of matter, right?
Not quite. About 1.5 OOM. We go from ~90 million gigatonne to 2 million gigatonne. The difference between squares and cubes matters. So, on the positive side, you get about 2,000 layers of graphene, but you're still using up all of Earth's available carbon to do that.
There are more abundant minerals -- silicon or magnesium or iron, for example. But you'd still be using planetary-mass quantities for any substantial structure (or multiples of structures orbiting independently and forming a sphere).
Which is what Dyson's also proposed. And my thought is that a Dyson "sphere" comprised of, say, multiple flat sheets of stuff orbiting a star, would tend to have a few issues, such as solar wind, and occasional emissions from incomplete closure. You've also got the question of how you're going to move all that captured energy from where it's been captured to where you plan to use it.
And you still need a cold end to your hot end.
https://en.m.wikipedia.org/wiki/Abundance_of_the_chemical_el...
http://www.geek.com/science/geek-answers-what-is-graphene-15...
http://www.mathopenref.com/spherearea.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solar/soldata3.ht...
That is interstellar space @ 2.7K :)
I actually just went through the calculations on my blog and to achieve a 99% Carnot efficient Dyson sphere (emitting at 58K) it would need to have a radius of ~7 AU - somewhere between Jupiter and Saturn [1]. There is plenty of mass to achieve that in the solar system and it would be also be undetectable in the infrared.
1. https://www.tillett.info/2016/08/03/carnot-efficient-dyson-s...
Any civilisation advanced enough to harvest dark energy isn't going to bother with backwards, inefficient starlight.
And any civilisation that has found the loopholes in the Second Law of Thermodynamics isn't going to waste time dealing with barbaric resources like dark energy.
Even if you assume that dark energy can be harvested then any advanced civilisation is still going to be energy constrained - Bremmermann's limit far exceeds any energy that can be harvested from dark energy [1].
Look at this diagram of the size of the space that could pick up on our radio broadcasts with regards to the Milky Way: http://www.planetary.org/blogs/emily-lakdawalla/2012/3390.ht...
If we get at all pessimistic about the likelihood of life happening, it doesn't feel like much of a stretch to imagine that we are some of the first ones. I would love to be proven wrong in this by having some advanced Elder Race show up and try to help us get out of this short-sighted, self-destrictive capitalist spiral we're in, but I'm not gonna hold my breath.
Don't you think?
*Common in the sense of "easy enough to create", i.e. the macrostate corresponding to "life" can be made up of a very large number of microstates.
Even if aliens chose to leave Earth alone for some strange reason, we still haven't observed any evidence of aliens elsewhere. No dyson spheres, no signals from space, no stars arranged in perfect geometrical patterns. Let alone visits from aliens, either to help us or hurt us.
The Milky Way is about 100K light years in diameter.
Our sample size of one on Earth supports that. We have millions of species, but only one has evolved higher intelligence, and there's no reason to think that it's a successful evolutionary strategy v.s. say being a parasite.
> [...]by having some advanced Elder Race show up
> and try to help us[...].
> Equidistant to the other two tribes sat the Historians. They didn't have too many thoughts on the probable prevalence of intelligent, spacefaring extraterrestrials— but if there are any, they said, they're not just going to be smart. They're going to be mean.
>> They're going to be mean.
Using the history of humanity as a reference, we will be the villains in the plot.
"One possibility is we're premature. Another possibility is that the environment around a low-mass star is hazardous to life."
Wow that is some hardcore "anthropic" logic. Is the question really well-posed enough to lead to the latter conclusion?
Imagine if you go to a bus stop in a completely foreign city/country, and the bus shows up within 1 minute. There are 2 possibilities. Either
A) The bus usually takes a very long time to come (30+ minutes) and you just happened to get very lucky, or
B) The bus runs at a very frequent interval, so your 1-minute-wait is somewhat normal
Statistically, B is much more likely to be true than A. Which is pretty much what the author is alluding to.
This depends entirely on what you mean by "statistically". Having travelled in many cities around the world, I have encountered many bus lines with long intervals, and not that many bus lines with very short intervals, so I would rather say A is statistically more likely than B. Discarding all previous data and then doing statistics is a great way to be wrong often IMO.
If the bus shows up within 1 second of your arrival then there can be no discussion of "frequent buses", you were definitely lucky, but only because the prior for buses with frequency of Order-of (1 second) is so low.
The analogy here is time-to-life and we have 1 data point for that, not enough to establish credible priors for minimum\mean time-to-life under any circumstances.
If you have a single data-point of an event E, taking time T to occur, then there are 2 possibilities:
A) Event E usually takes much much longer than T (>100T), and what you just witnessed is an anomaly, or
B) Event E usually does take roughly time T to occur, and what you witnessed is within the range of what usually occurs.
Both conclusions have an extremely high likelihood of being wrong, since the sample size is so small. But between the 2, conclusion B is more likely to be true than conclusion A.
Is that life itself is relatively "easy" and is likely to be common.
It seems more likely to me that the crucial "filter(s)" are for a species of lifeforms to cross the boundary into a global, technological civilization. There appear to be very unique, perhaps somewhat accidental set of circumstances that led to homo sapiens crossing that rubicon that aren't just evolution doing a "dial intelligence to 11", but more about an ability for intelligent individuals to share and store their thoughts and build shared abstractions and culture over periods much longer than individual lifetimes (e.g. capability for complex vocalizations and extremely precise control with hands and fingers; not living in water)
It's either that, or life is so hard that it can only appear during a short time-span.
Life appears common, but multicellular life appears to be extremely uncommon and appears to have evolved only once on earth, at least according to Nick Lane's excellent (though inadvertently depressing) book The Vital Question: Energy, Evolution, and the Origins of Complex Life (https://www.amazon.com/Vital-Question-Evolution-Origins-Comp...). I can't gauge the accuracy of his claims but the book does not appear to have been rebutted, at least from what I've found. Given all the discussion about biology on this thread I'm surprised no one else has mentioned it.
The common explanation is that once life evolved and began to spread, that it would prevent other life from appearing by out-competing it. But there is plenty of raw materials and sunlight to go around, so I don't buy that explanation.
The only thing that seems to fit, is that life is hard to evolve, but has extra-terrestrial origin. If that is the case then life should exist (at least in bacterial form) wherever a suitable habitat is found.
There's whatever fueled the Cambrian explosion of diverse animal lifeforms (some say "multicellular life" but I'm not sure about that as multicellularity has evolved independently "at least 46 times" according to Wiki) - this took billions of years so we can classify that as "hard".
Then there's "intelligent enough to solve problems and be self-aware" - dog level intelligence. This is relatively common on Earth, so we can call that "easy".
Then, as you say, there's "culture and language" (language implying at least a certain level of abstract thought), which are not unique to humans (other primates, corvids, orcas, and elephants spring to mind) but is quite rare. Still, it's evolved several times independently (and there were pack hunters even among dinosaurs that may have qualified), so perhaps we can file that under "not numerically common, but inevitable given time".
Then, at last, there's "technological" - it's not at all clear what the qualitative adaptation is there, though I would suspect some sort of runaway sexual selection fixated on abstract thought beyond what is strictly required for survival. That's kind of a freak event, but on the other hand sexual selection as a whole isn't that rare and it fixates on all kinds of things. And of course, it happened a few hundred million years after the Cambrian explosion, and maybe only a few tens of millions of years after evolution got far enough to give us culture and language etc. So maybe this is also a "give it time" kind of thing. A plausible stat would be 1% of cultured, language-using species ending up technological. Not super common, but neither a unicorn event. We just happen to be the first.
Now bearing in mind that the lifespan of a star like ours is roughly 10 billion years, it seems to me like overwhelmingly the hardest thing in all this is making the jump from just slime growing on everything, to little animals running around eating each other, before the star dies. That jump took billions of years for us, and everything after that has happened stupendously quickly by comparison.
My personal opinion for the "great filter"? A combination of "life-bearing planets are rare", "animals are rare", and "technological civilizations don't make it to space for reasons unknown" (either lack of will or more likely self destruction).
> The "Dark Ages" span a period during which the temperature of cosmic background radiation cooled from some 4000 K down to about 60 K. The background temperature was between 373 K and 273 K, allowing the possibility of liquid water, during a period of about 7 million years, from about 10 to 17 million after the Big Bang (redshift 137–100). Loeb (2014) speculated that primitive life might in principle have appeared during this window, which he called "the Habitable Epoch of the Early Universe" .
https://en.wikipedia.org/wiki/Chronology_of_the_universe#Dar...
http://journals.cambridge.org/action/displayAbstract?fromPag...
Note that the other, Avi Loeb, is the same from the OP article.
The cosmological dark ages lasted from photon decoupling (when photons started free streaming and the cosmic microwave background was born) to a few hundred million years later when the first stars begin to form. Loeb's suggestion is a narrow window of a few million years when the CMB itself would be the source of warmth, rather than stars. I'm not sure how matter was distributed on small scales during that time period.
http://www.slate.com/articles/health_and_science/mysteries_o...
> will expire before life has a chance to evolve.
Protip: if you are looking to hide the existence of your civilization, a good place would be in the vicinity of one of these.
I still don't see this as a solution to the Fermi paradox, considering the billions (more?) of planets in our galaxy.
Speed of light is a barrier for intergalactic colonization. It is not a barrier for an advanced civilization trying to colonize a galaxy. Even a relatively large galaxy such as our own.
The distance to Andromeda is about 20x times the diameter of Milky Way. But of course this is massively longer that hopping from a solar system to the next.
Point of interest, we'll meet Andromeda in about 3 billion years or so.
Point of additional interest. Due to the expansion of the universe, it is impossible even in theory to travel to most of the galaxies that we can see.
Point of amazement. Our telescopes can pick out more galaxies in the universe than individual stars!
The galaxy - and the speed of light limit - are also trivial distractions to any low-energy civs that evolved to run at a much slower metabolic rate, or adapted themselves to same.
With Fermi, the smart money is on the galaxy being full of life - but it's either avoiding Earth completely, or it's here already and doesn't want to be seen.
Besides - the goals and technology of >10^6 civs are literally unimaginable from a human perspective.
We assume that exploration and travel are major interests. They may not be.
There may be other pastimes we're completely unfamiliar with - ones which make getting into metal spaceships and banging atoms together to make them move through spacetime in a linear way seem about as interesting as sleeping in tree branches are to us.
The first argument is a fairly weak argument/theory but about the same strength as most of the other common arguments/theories.
To me the strongest argument that there is a paradox is the idea of probes that can replicate. That it would be fairly easy with some sort of advance but not FTL technology to easily deal with the size and expansion of the universe (of course I could be full of it on this as I haven't looked into this idea in some time).
Basically why haven't we run into a probe yet?
(As you might have already guessed I have views on astrobiology vastly different from those of the late Carl Sagan.)
It's only been a few years since we have been able to even begin to survey the solar system and that would mean that a bracewell probe could have been burrowed in an asteroid for millenia observing the development of the human race.
Perhaps it choses to not make contact with us because we have not yet crossed a threshold that it is programmed to observe before making contact/annihilating us.
Which indicates that the UFOs are an intelligent civilization that is adapting to observe us in secret. Whenever we develop means of tracking them, they hide better.
1. The civilization was bordered on all sides by other interstellar civilizations, which stopped the probes from spreading.
2. Our civilization is bordered on all sides by other interstellar civilizations (or perhaps is in a remote backwater of a civilization, like uncontacted indigenous tribes in the Amazon), which stopped the probes from reaching us.
There's even a third, somewhat stranger and less likely option:
3. Some civilization is profoundly annoyed by Neumann probes, and has devoted themselves to destroying them everywhere. They made their own stealthy fighter probes, which wander deep space destroying any probes they may find. Perhaps they are keyed in to one specific model of probe that annoyed them, or perhaps will target any unmanned spacecraft with certain properties.
That could mean as simply as getting stuck like in local minimum in the hill climbing algorithm or something extremely complex as a probe like civilization (aka Star Trek 1 but in this case no longer cares about space exploration).
I think you missed the pint of my comment. I mean a probe that can replicate itself and send its offspring to other solar systems.
But even replication alone would be very difficult on a planet with no prior life. a bacteria can't just harvest light, rocks and dirt to reproduce.
"The vast majority of search space remains unexplored. Search space does not just mean the three-dimensional volume of the Galaxy. As the Jet Propulsion Laboratory’s Michael Klein explained, it is a multidimensional space that includes source location, signal frequency, power level, time of arrival, signal modulation, and polarizations.[9]"
And this assumes that electromagnetic transmissions are what we need to look for. Anyhow the Fermi paradox is like seeing just one straw in an enormous haystack, observing that it is not a needle, and then quitting.
Anyhow, the assumption here is that doing so is desirable. I deal with the desirability of doing so (openly) elsewhere in the thread. It's not clear that doing this is the best idea.
Really? I find that surprising. On a related note:
This, BTW, is why I am strongly in favor of optical SETI.
> According to this line of thinking, the Earth should have already been visited by extraterrestrial aliens. In an informal conversation, Fermi noted no convincing evidence of this, leading him to ask, "Where is everybody?"
I think a better way to phrase this is that abiogenesis is stupendously picky, but also incredibly eager. My understanding is that current life developed about as quickly as it possibly could have, at least on a geological scale. It's possible that worlds we're finding that look "ideal for life" from the perspective of what we can detect from this distance may be missing something vital that Earth had.
This is commonly quoted, but actually not at all implied by the data, at least as long as you allow for the very real possibility that the road from inert matter to conscious observers required multiple improbable steps. For counter-intuitive probabilistic reasons, you can't infer the "improbableness" of a step from the amount of time it took to occur when several such steps occur.
The great filter theory implies that the last two terms are also low, but again, my point was mainly focused on going from no life to basic life. Essentially I think that while there are improbably steps between inert matter and consciousness, abiogenesis itself does not seem to be one of them, though the suitability for abiogenesis likely is.
[0] https://en.wikipedia.org/wiki/Drake_equation#Equation [1] the average number of planets that can potentially support life per star that has planets [2] the fraction of planets that could support life that actually develop life at some point [3] the fraction of planets with life that actually go on to develop intelligent life (civilizations)
> My understanding is that current life developed about as quickly as it possibly could have, at least on a geological scale.
does not actually imply that abiogenesis is "eager". Our evidence is consistent with it being being so improbable that even if the universe were filled with young Earth's with the appropriate conditions, there would still be less than one such event to date per observable universe.