For some context, Avi Loeb has a certain ... reputation in the astrophysics community. He's a super smart guy, but he has a habit of writing a ton of short, very speculative papers. One gets the impression that he's throwing a lot of ideas out there and seeing what sticks. It can be as valid a strategy in astronomy as in the startup world, but the downside is that it can be hard to believe anything he writes.
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.
> 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.
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.
Maybe. Many hot Jupiters are probably tidally locked, but it's not clear to me whether rocky planets will be tidally locked. Though given that they're considering timescales of 100s of Gyr maybe they will end up tidally locked by that point. At any rate, I don't think our understanding of tides is good enough to claim that they will or won't be tidally locked one way or another. But needless to say, the authors didn't consider details like this in their paper.
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.
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.
The Sun and Moon appearing the same size means there's a corona during a solar eclipse. I don't think the corona appears if the Moon is larger than the Sun. It's the corona that's spectacular, and it could even have jolted life to become intelligent life in some way.
> and it could even have jolted life to become intelligent life in some way.
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.
> The idea that a transient celestial phenomenon that lasts around 5 minutes [...] is going to somehow stimulate a cognitive awakening is absurd
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.
The moon causes both annular and total solar eclipses. That is, it is precisely far away enough to both completely cover the Sun, and to incompletely cover it.
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.
The big argument against intelligent life being common (and us not being unique) is the existence of stars. No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light. If you assume von Neumann probes and Dyson spheres are possible, then star light is incompatible with the existence of advanced technology.
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.
I hate to break it to you, but we have already dammed most of the rivers. Any beaver who thinks that there are no dams around is just one that hasn’t tried to swim far downstream :)
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.
Have you read City? (Not that it's seriously making an argument the other way.)
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.
Exactly. I like this analogy especially because our needs as a more advanced civilization are drastically different from the function performed by beaver dams. It's not even about energy per se. We could dam all the streams, but that's not a meaningful pursuit given our interests. Likewise, wondering about damming all the stars assumes that solar energy requirements are both the bottleneck and the ultimate purpose of advanced civilizations.
Live in the universe is necessarily finite, because the useable negentropy is finite. It is a plausible assumption that an advanced race would want to keep on living as long as possible. This means managing the negentropy as well as possible, which means dismantling stars and using them. So unless you disagree with my assumption, or add magical energy sources outside of known physics, starlight seems to indicate that there is no race capable of harnessing all available energy.
"We could dam all the streams, but that's not a meaningful pursuit given our interests"
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?
Humans dam lots of small streams too for agricultural and recreational uses. They don't get much news, but if you look at aerial surveys of places where people live you will see lots of small dams.
But now you're hypothesizing Aliens + some extremely convenient energy source.
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.
Aren't you hypothesizing aliens + making human whims seem like universal goals? I don't think there needs to be an easier source of energy than solar to justify not tapping stars, just lack of desire.
This is kind of like how we currently have 8tb drives, but many systems are shipping with 128 or 256gb drives. In that case, the trade-off is for the speed of SSDs.
Or lack of ability if FTL is completely impossible relativity puts a limit on the size of a civilization.
And the universe can easily out a time limit on it as well because extinction events can happen.
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 big argument against intelligent life being common (and us not being unique) is the existence of stars. No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light. If you assume von Neumann probes and Dyson spheres are possible, then star light is incompatible with the existence of advanced technology.
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.
Yes the presence and size of Jupiter and Saturn are another unlikely factor contributing to extreme stability of the Earth’s climate. Another one is earth seems to have escaped being exposed to a supernova going off very nearby [1].
> The big argument against intelligent life being common (and us not being unique)
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.
You don’t need most advanced civilisations to want to spread, just one. The first one that choose to spread will occupy the universe and capture all the energy that the stars are currently emitting into space.
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.
You have to postulate that all civilisations fail to spreed, not just some. On the economics once you can make the first von Neumann probe the cost of taking over the universe is zero.
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.
The argument is once you have one civilisation that can create von Neumann probes [1] then that civilisation will spread through the universe wrapping each star in a Dyson sphere [2] that will capture all the light.
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.
Sure, but are we sure these probes are instantaneous? What if it takes more than zero time to build a Dyson sphere? As I said earlier, you're only proving that they have been building spheres for less time than it takes to finish up the whole universe, which they say is big.
The probe’s spread and and dyson sphere building does not need to be instantaneous, just that it had to have happened in the past. For example, if we were to create von Neumann probes and have them spread across the galaxy, it would take less than 100,000 years for the probes to reach every star. When you compare this to how long it took for our civilisation to evolve on Earth it is a blink of an eye.
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 :)
> 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.
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.)
Yes I am aware of this study. From memory the temperature limit was 50K which no civilisation capable of building a Dyson Sphere would let leak out. Also the study was limited to our galaxy. The final issue is that if the von Neumann probes are moving at close to the speed of light we would not be able to detect these Dyson spheres anyway.
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.
There was a quote yesterday in HN (either here or through wiki) saying it would take 500,000 years for von Neumann probes to spread throughout the Galaxy. Not meaningfully different on cosmic timescales, but still.
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?
Yes they could be travelling just under the speed of light if the probes are very small and they are being accelerated by an energy source (say laser) at the origin.
If they are sent as a swarm you could bounce the laser off one to slow a second. This way you would not need to transport fuel. Also since you are aiming at stars you could use a hub and spoke model which would speed up the takeover process since you could bypass the build stage for most stars.
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.
I agree with you and I also don't think that existence of intelligent life will necessarily lead to all stars being surrounded by Dyson spheres. Maybe there are other sources of energy that are easier to harvest for an advanced civilization.
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.
You can only detect a Dyson sphere in the infrared if the Dyson sphere is relative close into the star. If the outer sphere is at the orbit of Pluto then the emission would be close to the cosmic microwave background [1].
Where are you going to find the material to build a sphere around a star the size of Pluto's orbit? How are you going to arrange this material into energy harvesting equipment in a way that results in a net positive energy outcome?
The energy collector sphere only needs to be one atom thick and spaced just below the wavelength you want to capture. There is more than enough matter in our solar system to do this at the orbit of Pluto if you use a series of shells.
And you do this why? You're making the assumption that capturing all of a star's energy is somehow a good thing. Something worth devoting goodness knows what effort to. And whoops another comet destroyed a ludicrously big section of it.
The study was limited to looking for inefficient Dyson spheres that leaked a lot of energy in the infrared. Efficient Dyson spheres would be emitting at just above the cosmic microwave background level and would thus be undetectable.
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.
>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?
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 :)
So, using some values off the Web, notably 0.0077g/m^2 for the weight of graphene, and computing the mass of a spherical shell at Pluto's radius, roughly -- 40 AU from a central star, I get (using GNU Units):
You have: 4 * pi * 40au^2 * 0.0077g/m^2
You want: million gigatonne
* 86.618649
/ 0.011544858
How's that compare to the mass of one typical rocky planet?
You have: earthmass / 86.6 million gigatonne
You want:
Definition: 68986.619
So, 1/70,000th an Earth Mass.
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.
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.
>No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light.
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.
I did assume that it was possible to build a Dyson sphere, or some structure functionally equivalent. If you have von Neumann probes then Dyson spheres are by comparison simple.
Even a functionally equivalent system will have problems. For one, if you're absorbing a significant portion of the sun's radiation, that's going to generate a lot of heat that you've got to dissipate somehow. Then you've got to transport that energy in a way that's more efficient than the way the sun is already doing it.
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.
From what little I understand, a Dyson Sphere would require so much material in terms of heavy elements that you'd have a great deal of trouble building a significant number of them even if you were raiding the entire universe for these materials. But like I said, I understand very little (and would love clarification).
There are alternatives to a classical Dyson sphere (Dyson bubble is one) that can be constructed to capture all the energy released by a star and which do not require much material [1].
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.
> No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light.
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.
99.999% of all civilisations could have no interest in doing this, but all it takes is one in the universe and all the stars will be converted (over time).
"No civilization more advanced than us is going to let stupendous amounts of energy go to waste in the form of star light."
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.
If you assume the laws of physics hold even for advanced civilisation then they are not going to be able to find a loophole in the second law of thermodynamics :)
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].
I was idly looking at "age of universe", "age of earth", and "age of life" numbers and wondering something like this just the other day. The universe is only about 3x as old as life on Earth, and we've just barely gotten to the point where we can make noise nearby worlds can conceivably hear. Radio's a hundred years old. That's nothing on those time scales.
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.
To me the algae vs. alumnae solution to the Fermi paradox seems appealing. I.e. that life is common, but that intelligent life isn't.
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[...].
Right, "help". I'm just going to let the history of how advanced civilizations have treated primitive civilizations here on Earth speak for itself.
We could also quote Blindsight by Peter Watts: http://www.rifters.com/real/Blindsight.htm
Search for "Once there were three tribes." and read 10 paragraphs :) The key quote is:
> 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.
In the lists of "once's", we have: 1) proto-life, 2) single-cellular life, 3) multi-cellular life, 4) neurons (which eventually form brains), 5) enough intelligence to launch rockets. Now it is possible (maybe even likely) that other life on earth will develop human-level intelligence (not only other primate, but thinking of crows and dolphins too). But intelligence is rare enough that it takes a long time to achieve.
"Capitalism" may be a likely equilibrium for a system made up of interacting H. sapiens, but that doesn't generalize even to other somewhat comparable species on earth, let alone to entities we can't yet imagine. Why aren't the cells in our bodies "capitalist"?
If we assume that life is common enough* that it will evolve independently multiple times in the future, then the chance we are "the first" is closer to that of being "the last", unless you have a convincing prior I'm unaware of. So it should be a pretty low probability.
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.
The first intelligent life to evolve will be able to colonize it's entire galaxy in a relatively short amount of time. The fact that Earth hasn't been colonized is pretty strong evidence to me that we are the first. To me at least.
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.
100k years is tiny compared to the age of the universe. Even if their ships only travel a tenth of the speed of light, they could have colonized the whole galaxy in a million years. That's a third of the time it took humans to evolve from chimpanzees, and only a tiny fraction of Earth's total history.
Only if the intelligent species survives long enough to actually achieve those things. The more we advance, the easier it becomes to destroy ourselves. How many years till 3D printers are able to make nuclear bombs.
Well, yeah that's a plausible hypothesis. I tend to think the great filter is behind us. Most animals on Earth aren't intelligent, and it took a long time for humans to appear. But I did read something today about how a nuclear war almost happened at least a few times. Though I think that would only be a temporary setback, it would take an awful lot to exterminate us completely.
If I were reviewing this paper, I would be quite surprised that there is no mention of how long it took life to emerge on Earth after there was liquid water. While the earth is around 4.6 By old, my understanding was that liquid water was not present until about 3.8 Bya, and that life probably emerged in the by 3.5 Bya. So, 300 million years for life to emerge in on Earth with liquid water. I realize it's only one data point, but it seems like it should inform the probability calculation.
You are using priors to decide that B is more likely than A, that is, you are saying that the frequent bus hypothesis is likely because buses with frequent intervals are common.
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.
The point of the thought experiment is to avoid using any priors.
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.
> 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.
What seems to me obvious if one looks at:
a) the age of the universe
b) the age of the earth
c) the age of life on earth
d) the age of aware-enough-to-look-at-stars-and-wonder-about-aliens life on earth
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)
Is that life itself is relatively "easy" and is likely to be common.
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.
Also, as far as we know, single-celled life also evolved exactly once. This is one thing I haven't seen a good answer for -- if life came about so early, that would indicate that it is easy. But since it occurred only once, that would indicate that it is hard.
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.
I like this style of analysis, but I disagree with your conclusion. There's a few intermediate levels I'd also put in there, each of them a filter in their own way.
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).
Wasn't there a time for some time after big bang when the space was on average a lot more warm everywhere and conditions for life were incredibly more abundant than now? I remember reading something like this here which would seem to be an antithesis to this article.
> 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" .
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.
Does Loeb address whether there would have been oxygen and carbon 10-17 million years after the Big Bang? I thought that the heavier elements formed much later, and had to be forged in the center of stars. Our first generation of stars appeared 560million years after the Big bang. (Reference is the same wiki article you linked)
He talks about it in the paper. Basically, depending on your assumptions about nongaussianity of the primordial perturbations, there were at least a few stars simply by accident in the early universe.
Definitely. No need to be worried about what orientation you are in relation to a hypothetical observer either. Then again, you could sit in a molecular cloud, or near something really exotic like a neutron star of some variety to REALLY hide over longer time scales.
The Fermi paradox really isn't an issue though to begin with. The following is an excerpt from the excellent Contact with Alien Civilizations: Our Hopes and Fears About Encountering Extraterrestrials:
"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.
The Fermi paradox isn't "why aren't we seeing alien civilizations at the specific stars/times/wavelengths/etc we happen to look at". It's "why haven't alien civilizations colonized the entire galaxy by now, using conceptually simple things like von Neumann probes which even our none-too-sophisticated primate brains can come up with?"
I suppose there are now different formulations of the Fermi paradox but if I remember Fermi's original words they were about the apparent radio silence (possibly minus the Wow! signal).
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.
The formulation Wikipedia cites is "Where is everybody?"
> 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?"
Again, though, it doesn't need to be the best idea, it just needs ONE civilization somewhere in the galaxy to have done it ONCE. We can't be the only species with a tendency to derp on occasion.
But present day human civilization on earth would in fact be entirely invisible to a human-level civilization even one star over. To be surprised that radio transmission is not ubiquitous, you have to say that "louder" civilization should evolve but we so-far have no real proof such civilization are possible at all, since we are not at such a level - and indeed the idea that civilizations would take this trajectory is entirely hypothetical.
I'm too lazy to google a source right now, but actually, yes, we could detect Earth out to a few tens of lightyears. Not far in the scale of things, and the only thing we'd really be able to detect are the most powerful radars, but it is theoretically possible to detect ourselves at nearby stars.
Radio energy dissipates by square of the distance to a location - essentially as a transmission expands, the same amount of energy is spread evenly around a sphere of increasing area. Area of a sphere centered at the sun and encompassing the nearest star is vast.
Oh yeah, true that, I thought the signal would be faint several light years out for omnidirectional signals, like FM radio and TV, I just didn't think it would be undetectable. I mean I assume that Contact was pretty scientifically tenable and that had aliens sending back transmissions of Adolf Hitler on television and that was over a much greater distance, but I'm willing to change this belief.
This, BTW, is why I am strongly in favor of optical SETI.
Perhaps, but I doubt it. Very dependent on a very specific environment to reproduce, and can't really spread itself to other star systems without evolving intelligent species and hoping they come up with von Neumann probes themselves.
Highly adaptable though, unlike the mechanical probes. This might be more important in the long run, and might be a good reason to hold the mechanical probes back for the sake of diversity.
There was a discussion thread a while back about this on HN [1].
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).
The Neumann probe idea has one flaw: it's assumed the probes will propagate and spread largely unimpeded. However, any interstellar civilization which encounters a probe is going to capture it and analyze/destroy it. They would not allow it to float though their territory and replicate. At the least they will intercept any that enter their territory; if the civilization gets annoyed they may embark on a campaign to wipe out as many as they can. Which leads us to two possible explanations, assuming a civilization has already sent out probes:
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.
4. I would add an even stranger 4th option in that given the probes are sufficient intelligence they perhaps "evolve" into an uncontrolled state.
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).
5. The von Neumann probes are microscopic and densely packed with a chemical structure encoding the information needed to reproduce in a variety of environments. Occasionally some of them mutate and get stuck in a planet's gravitational well, where they keep reproducing and mutating into who-knows-what. The urge to explore the cosmos is never completely lost, and eventually they reach a state where it's possible to continue the mission.
I'm not convinced such a probe can be much smaller than a large global civilization. What makes us think they can be compact? Just magical nano technology?
You mean a probe capable of self-replication? They can absolutely be quite small. As a motivating example, a bacteria is capable of self-replication despite being microscopically small! Human beings are capable of self replication, along with being able to construct anything they want out of raw materials.
Issuing von Neumann probes like this seems like a flashing neon advertisement of your presence in space: not necessarily the best strategic choice. On the other hand it's possible that such a species of probe already exists but is more or less of the Fred Saberhagen Berserker variety and desires to operate stealthily. Perhaps the crews of these vessels like to sing Das Lied vom außerirdischen U-Boot Mann.
(As you might have already guessed I have views on astrobiology vastly different from those of the late Carl Sagan.)
My personal belief is that we haven't run into them yet because they are quietly observing us from the asteroid belt.
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.
There used to be thousands of UFO sightings every year. Now with the arrival of smartphones and ubiquitous cameras, there are almost none.
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.
Simple solution - relativity holds up. Our light cone is much smaller than the galaxy. The universe could be teaming with intelligent life but not so full that you'd expect any to overlap the period where we're both using radio waves to communicate.
Once you get to civs that last >10^6 years, the galaxy becomes a trivial size.
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 thing is that all told, humans pick more fruit than other primates, and have more visible effects on forests than other primates. It's not unreasonable to think something similar applies to space-capable supercivilisations.
The Milky Way is about 100k light years across. If colonization proceeded at 1% of the speed of light, it would take 10 million years to colonize the galaxy. That is roughly 0.2% of the time that the Earth has existed. And a bit under 2% of the time since the Cambrian that we've had complex life forms.
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.
> Speed of light is a barrier for intergalactic colonization.
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.
I prefer the "Lots of life, utterly separated by time and space, running into some version of the 'Great Filter'." Space is just too damned big, and the timescales involved in two way communication are too nasty. Maybe the solution is that any sufficiently advanced species either blows up, or stays focused entirely on what it can realistically communicate with 2-way (so incredibly close by, cosmically speaking).
The OP argument isn't about the Fermi paradox. To see this, first note that those words don't appear in it. Second, consider the possibility that abiogenesis is just stupendously rare. This solves the Fermi paradox (regardless of whether you think this solution engenders problem of its own, such as why life seemed to develop so soon after Earth initially formed), but the anthropic issue raised by the OP argument would persist.
> Second, consider the possibility that abiogenesis is just stupendously rare
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.
> My understanding is that current life developed about as quickly as it possibly could have, at least on a geological scale.
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.
I actually mean solely abiogenesis. From the perspective of the Drake Equation [0], I think it's likely that ne [1] is quite low, and fl [2] is quite high. I don't think we can speculate on fi [3], given a sample size of 1. Though as we seem to be somewhat of an anomaly across several great extinction events, a low estimate there seems fair as a starting point.
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.
The universe may only be 20B years old. It takes our galaxy 250M years to rotate once. So the universe is only something like 80 galactic years (days?) old. It is still early innings.
I thought the very fact that complex organisms can be traced back to the Archean hints at cosmic origins of live on earth. Also no signs of a 'RNA-world' on this planet?
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.