Notes on the Fermi Paradox

I recently realized I hadn’t written much in this blog about the Fermi Paradox, though I do write about it elsewhere. So here is a quick note.

The Fermi Paradox, sometimes paraphrased as “where are they?” is a question about the apparent lack of intelligent alien life in the universe. The universe is extreme old relative to the speed of light at galactic scales. Light has been able to cross our galaxy a thousand times since the dinosaurs died out, and that was relatively recent (less than 2%) compared to the age of the Earth. So if life is common in the universe (it seems like it might be) and if intelligent life is an eventually winning strategy of evolution (it seems to be) and technological life follows from this (it has at least once) and technology leads relatively quickly to space travel and visible technosignatures (this is probably not the hard part) then why isn’t the universe teeming with alien life?

There are a bunch of potential solutions to this puzzle. I’ll mention a few before I get to my preferred one.

We haven’t looked very carefully.

Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.
Douglas Adams, The Hitchhikers Guide to the Galaxy.

We have only relatively tiny telescopes on one tiny planet looking out into a vast darkness. We have found only a few thousand exoplanets, of which just a handful might be able to support life. We have not a single spectra from an exoplanet atmosphere. Our nearest star Proxima Centauri has planets and we know almost nothing about them. For all we know, there’s already an advanced civilization there and we would not be able to see it. We’ve run various SETI searches for a few decades but again, barely scratched the surface. We could build much larger telescopes but even one the size of the Earth would hardly rule out intelligent life in our galaxy – much of which is obscured by dust.

At our current rate of technology, we’re not going to discover intelligent aliens unless they’re very close by and sending us very powerful radio signals, or they visit us directly.

Interstellar travel might be impossible.

The galaxy might be only 100,000 light years across and nearly 10 billion years old, but you and I typically travel at perhaps a meter per second, while light covers the same distance in just 3 nanoseconds. That is, the galaxy is relatively small if you’re a photon, and impossibly enormous otherwise. Our fastest space probes would take nearly 100,000 years to reach the nearest stars. Antimatter might be energetic enough to accelerate to close to light speed, but that doesn’t mean that interstellar travel is possible – colliding with a single dust grain would be very bad news. Perhaps the galaxy has a million technological civilizations, and they’re all trapped in their respective solar systems by the enormous gulfs of space.

The Great Filter.

Maybe intelligent alien life is rare because there’s some filter or set of filters that kills off life forms that get too advanced. This filter could be in our past (multicellularity, asteroid extinction, solar flares) or in our future (nuclear war, hostile aliens killing upstarts, AIs starving us to death, depopulation, loss of culture of exploration). But you only need one very powerful alien species to overcome these filters and then they can fill up the galaxy relatively quickly. As far as we can see, the galaxy is not full.

Near light speed travel is hard to observe for people at the destination.

This is my preferred explanation at present.

The most interesting stuff I’ve read about the Fermi Paradox is Robin Hansen’s work on Grabby Aliens, which uses the fact that the universe appears to be empty and that cultural selection on expansionist aliens would lead to their rapid spread if they did occur to conclude that intelligent life must actually be very rare (fewer than one species per multiple galaxies) or that evolution must be very slow.

There is an observational subtlety to alien observations, which is that when we look out into the universe we are observing only our past light cone. If grabby aliens were expanding at a high fraction of the speed of light (c), the light carrying information of their coming would be only just ahead of them. So even though aliens might be quite close, we wouldn’t see them until just before they arrived. In fact, there is quadratically more available space further away from Earth, so while a nearby alien species might reach us with their slower, first generation starships, any starships that get here from more distant parts of the galaxy are almost certainly the fastest, latest tech ones which overtook the slow ones on their way here.

The universe could be in three different states, observationally. What we observe (no aliens), aliens seen but not here yet, and aliens among us. But if the aliens we see are traveling at high speed toward us, the intermediate state (seen but not met) is unlikely to be longer than a handful of weeks. Choosing our present time at random, there is almost zero chance for humanity to find itself in a time where we’re aware of alien intelligences but haven’t yet met them. That is, Earth is 4.5 billion years old (no aliens), then one day the Vera Rubin Observatory sees a flash that turns out to be an alien spacecraft departing to meet us from 100 light years away, traveling at 99.9% of c. They arrive just five weeks later. For the remaining billions of years of Earth’s existence, we are in the world of aliens among us.

I think it’s physically possible to reach 99% of c with current human technology, so there’s no reason to suppose aliens with better technology would fly slower than this, and they could fly much faster.

I put together this chart a year ago. If relativistic aliens are flying towards us, we won’t see their launch until the light gets here, and if they’re right behind the light, they’ll be here soon after. For example, reading this chart, if they’re traveling at 99% c, we will see them only when they’re 99% of the way here. If they’ve traveled 1000 light years to visit us, we’ll see them (at best) 10 years before they arrive. We might not see them at all – 1000 light years is far enough away that some stars are too dim to see with the naked eye. Meanwhile, 1000 light years is a long way to go, so it’s fortunate that at high speed, relativistic time dilation kicks in and helps to pass the time. This is shown with the yellow curve. At 99% c, the 1000 light year trip only feels like 142 years. This is still a long time, so perhaps they will travel to us at 99.9% c. In that case, the trip will feel like only 45 years to them, and we will get a whole year of warning, assuming we see them launch 1000 light years away.

I think this factor is under-estimated when discussing the Fermi Paradox. If most of the planets in the universe are too far away for us to see alien life, then if we see it at all we’ll be seeing their space ships as they come to us. But we won’t even see them launch to us, even with perfect telescopes staring out into the galaxy, until they’re almost here. In practice this means that, in the grand scheme of human history, the phase between becoming aware of aliens and meeting them is vanishingly short.

This quirk is intuitively obvious in the context of supersonic planes – whose sound arrives after the plane.

How to use this chart: Select your speed on the horizontal axis, and decide on your travel distance. Then run up vertically to read off the distance-time multiplier (blue line) for visible travel time on the ground and (orange line) the apparent travel time for the traveler due to time dilation. For example, let’s say we’re doing 99.5% c over 500 light years. Then we’re going 0.5% slower than c, so the delta t multiplier is 0.005*500 = 2.5 years, while the subjective travel time is 0.1*500 = 50 years. We will be in flight for 502.5 years, we will arrive 2.5 years after our light, and on board we’ll feel just 50 years pass by.

The Grabby Aliens hypotheses points out that expanding alien civilizations appear as circular regions in the night sky where, for example, we can observe spectral changes in stars or their planets, given a sufficiently powerful telescope. For an expansion speed that’s small compared to c, this gives the correct intuition. But, at higher speeds, the apparent angular size before contact shrinks. You might think that you’d see the alien sphere expand through stars in your field of view until it surrounded you, but in fact the light from their arrival at nearby off-axis stars is still on its way to you when they arrive. So the apparent shape of their expanding sphere, looking into our past light cone, is a cone whose narrowness increases with flight speed. In the extreme case, we would see nothing even with a perfect telescope. It’s quite hard to see things thousands of light years away!

There are a couple of other aspects to the Fermi paradox. It seems to me that the Fermi paradox can be at least partly explained if either relativistic interstellar travel is relatively easy, or any kind of interstellar travel is basically impossible. I think the intermediate case is ruled out quite well by even our limited observation.

I favor the first explanation. The implication is that the night sky is not full of alien civilizations because they’re expanding so fast that the period of time between our feeble telescopes being able to detect expansion and them actually arriving is extremely short. This does, however, imply that no traveling aliens could have occurred in our galaxy in the past billions of years, right up to barely 100,000 years ago, when our ancestors first started leaving Africa. There is still no good reason for this to be true, other than the anthropic principle.

Accordingly, when we look up and wonder where are the intelligent aliens, we can know two things for sure.
1) Our telescopes are bad and we should feel bad.
2) They could be passing Betelgeuse (700 ly away) right now on their way here and we would still not have seen their departure. If they’re going fast enough they could be closer and brighter than Alpha Centauri and we still wouldn’t have seen them yet.

And if we could only detect them at Betelgeuse, picking up a thruster signature with blue-shifting indicating 0.99 c travel speed, they’d be here in about 7 years (running just behind the light announcing their arrival) – an even more laughably ridiculously short period of time for us to know we’re not alone and have not yet shaken tentacles.

With Vera Rubin telescope up and running, we’d have a chance of detecting incoming relativistic spacecraft out to maybe 1000 LY, which means 10 years warning at most. If they can hit 0.99c, why not 0.999c?

37 thoughts on “Notes on the Fermi Paradox”

  1. Not so much discussed is the distinction between “intelligent alien life” and “intelligent aliens with detectable technology”. It’s not a foregone conclusion that intelligent life leads to technological life.

    I’ve not seen any modelling on what combination of planetary factors are necessary to produce industrial technology that requires atmospheric conditions and suitable ore deposits to do any kind of metallurgy and from there get to radio broadcasts, let alone rocket technology.

    Imagine aquatic planets with intelligent creatures unable to produce fire (or even to conceive of it as a technology) – octopuses and their like, or at a science fictional extreme, the ocean of Solaris. What would it take for us to detect them short of close contact? The universe could be teeming with intelligent life filtered by chemical limitations from advancing to even 19th century Earth technology.

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  2. I am not at all convinced that there are any space-colonizing aliens in our galaxy. Our planet is billions of years old, still AFAWK life has appeared only once. Probably around a billion species have existed here, many of them intelligent in different ways and dependent on technology, yet only one species has ever bothered to invent metals-based tech and consider space travel. I am also not sure that every species capable of inventing space travel would actually do it. It’s costly, boring, uncomfortable and dangerous. The chances of finding a planet where you’ll feel better than on the one where you’ve evolved are pretty much zero. Moreover, it is suicidal because the risk of running into a more advanced species that will exterminate you is too high. If there is life outside Earth (and that’s a huge if), it likely has better things to do. The reason we have to even consider leaving Earth is that we are smart enough to invent destructive technology but not enough to curb our population and live sustainably. Other intelligent species don’t behave this way. No species of hominid other than H. sapiens is known to have caused mass extinctions, for example – in fact, there is hardly any evidence of them causing any extinctions.

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    1. There are additional reasons for humans to consider leaving Earth.
      1. Avoiding celestial calamity : impacts, solar events

      2. Curiosity

      3. Social missions (or personal missions of the ultra-wealthy)

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      1. I can’t imagine a technology that would allow any significant portion of our population to leave. Curiosity is great but there will be no practical way of reporting back, so why would be those staying behind bear the huge expense? I sure hope we’ll never have individuals wealthy enough to afford interstellar travel.

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      2. Spreading risk doesn’t require moving a sizeable proportion of the population, only a sustainable breeding population (augmented probably by gene banks).

        Reporting back doesn’t have to be in real-time

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  3. Relatavistic travel does not explain why nobody advanced to this step in our galaxy before us. Nature favors explosive radiating growth, so the galaxy should be full of the first race to figure out near light travel.

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    1. Yeah, as he mentions it’s still relying on the anthropic principal, that by sheer coincidence, this hasn’t happened yet, which is why we’re here with no aliens. Or that it did happen but we missed it, and it didn’t leave any record in the stars or on earth (queue ancient aliens guys).

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    2. Nature doesn’t always favor unlimited explosive growth. Some species have this strategy, but they usually go through boom-and-bust cycles. Most others have some kind of inbuilt population control.

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      1. I’m not sure what is meant by in-built population control. The boom and bust cycles are factors of the environment, as far as I can tell. eg: deer populations explode when predators dissappear or if food becomes plentiful. I don’t think they have any kind of genetic limiter to their population, but let’s say for argument that deer did have a genetic limiter. In that case, if something caused an explosion in the food supply, the deer would be out-competed by some other species that did not have the population limiter. in this way, evolution favors exposive growth. Species that can do this tend to win when ecological niches present themselve.

        This matters in the fermi paradox because explanations of the paradox often require that all members of some alien species behave the same way without exception. But evolution will tend to favor the individuals that break out and reproduce like crazy. e.g. A civilization of billions might enact some self-preservation law to never travel to the stars, but all it takes is a single individual to break out (or some other less restrained civilization to arise) and the galaxy fills up.

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      2. “Some species have this strategy, but they usually go through boom-and-bust cycles. Most others have some kind of inbuilt population control.”

        Thats usually due to environmental boundaries. Their population exceeds the food supply, or they cause an explosion of a predatory animal that reduces their numbers.

        In space, the boundaries are the edges of the galaxy

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      4. If nature really did favor unlimited growth, our planet would be sterile by now. Just look what is happening because one species learned to escape its population controls: every country in the world is now grossly overpopulated, all ecosystems are severely degraded, we are well into a mass extinction and are on course to kill the planet completely or at least erase all multicellular life.

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      5. I’m not talking about over crowding. Those same rats that limit their own population, if I placed them in some location where there is plenty of food an no other rats, their population would explode geometrically. If I placed two species of rat in that same empty space, and one of the species reproduced more quickly, they would likely out-compete the other. (Not always, but usually)

        The galaxy is essentially unpopulated and full of resources. It will be rapidly filled by the first star-travelling species (or robot) that wants to grow explosively.

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      6. It’s not that simple. Rapid expansion from a small founder population has its own risks. Also, different species have different strategies, and fast breeders do not always outcompete slow breeders. We are the slowest breeders among mammals and have outcompeted them all. Interstellar travel for colonization purposes, if possible at all (I don’t think it is), is outstandingly costly and has uncertain benefits. It is extremely difficult, if at all possible, to find a planet so similar to your own that it would be colonizable without terraforming, and terraforming requires hauling over a lot of weight. A truly intelligent species would not even consider such a ridiculous gamble unless absolutely desperate and obsessed with perpetual survival, which a truly intelligent species shouldn’t be. If your star is about to go nova or something, the smart thing to do is to cut the birthrate so everybody dies in comfort before the inevitable happens, rather than make some poor individuals forever wonder around the galaxy in search of another place to repeat the same pointless cycle.

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  5. Im not convinced of this argument at all.

    It assumes that the only bit of high energy activity these aliens do, is travel towards earth at >99% the speed of light, while skipping all other high energy activities.
    If they travelled anywhere else, we would see them.
    As they develop this technology, they would leave signatures hard to miss.
    They would also be doing other things that would emit large amounts of energy.
    To get enough anti-matter to make a trip like this, will waste far more energy than the energy being put into that trip.

    This argument essentially says that these aliens go from a low space faring civilisation, to being able to travel at 99% the speed of light, and only doing it towards earth, in one step.

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    1. “If they travelled anywhere else, we would see them.”

      They would have to travel somewhere else first. And that trip would have to end up fairly close by or we might not see them even when they arrive at their destination.

      High energy activities? Again, if they’re 1000ly away, it’s hard to see them.

      Assuming they’re fast, there is a point where our ability to see them crosses over with their light. Assume they start 1000ly away. It would be quite hard to see their launch even if it’s quite energetic. But when they’re 500ly away the light (assuming constant emission) is now 4x as bright.

      How far away could we detect e.g. a Saturn launch from an airless moon with our current tech?

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      1. Accelerating to 10% the speed of light would output more energy over the acceleration period than our sun puts out. (depending on acceleration, mass, efficiency ext) Saturn V is to this scenario as a flies wing floating in the water is to an oil tanker.

        And going from 10% the speed of light to 99% the speed of light requires over 1000 times more energy. This will be the brightest thing in that region of space over the full acceleration period. Getting to 99% C is months of acceleration.

        We can assume that they will be learning to push things fast at ~10% C, often, and in multiple targets, long long before they are sending colonisers 1000 light years away. Most probably to their closer stars first. A generation ship, is order of magnitude more complex than sending out hundreds of observation sats to nearby systems. Each one of those will be visible. And if each one of those wants to stay and observe those systems, they will need to dump an equivalent amount of very visible energy to stop.

        This kind of civilisation would be observable from across the galaxy.

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      2. Since accelerating to something like 80% of c requires you to convert a mass equal to the mass accelerated completely to energy, there would have to be external engine involved, or new physics.
        But the waste energy involved (unwasted energy by definition isn’t radiated to us) depends heavily on just how much mass gets accelerated.
        How visible this energy is also depends on the frequencies involved. As far as I know we don’t see the visible light emitted by whatever’s at our galaxy core, and we didn’t know about it until we could observe X-rays from space. If it’s just a lot of infrared that might be missed, or mistaken for something else.

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  6. Wouldn’t any such spaceship moving towards us using jet propulsion be completely invisible because the jet stream (or whatever the proper term is) would be moving in the opposite direction? Also, wouldn’t some kinds of such ships be invisible even from the side because they would use, for example, backward-aimed particle streams with no scattering? Of course, no sane civilization would launch such a dangerous thing from their planetary surface – they would all be assembled in orbit, preferably around a distant moon or another planet in the system, and their launch would be as invisible as their cruising flight. So if there is a vermin eradication team heading in our direction, we wouldn’t see them until they deploy their payload pods during the flyby.
    AFAIK there is still possibility of natural sources of antimatter. What a sight it would be, antistars used as gas stations by ships from all over the galaxy, lowering magnetic traps into their atmosphere and sucking up the antijuice…

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    1. While I’d like to believe that anti-suns exist, I doubt their existence. Let me know if you take issue with any part of this argument.

      If an anti-suns does exist, then obviously the protoplanetary disc is also composed of antimatter. That means all the objects in that solar system are antimatter.

      interstellar objects appear to be common, and if any of the three objects that we’ve detected so far had been composed of antimatter, they would have behaved quite differently when encountering our solar system! So much so that I feel safe in inferring we’ve never encountered an antimatter object.

      Ok, so maybe each galaxy has to be composed of either matter or antimatter. But we do see galactic collisions, and if we saw entire galaxies made of antimatter colliding with matter, we’d notice it!

      So, unfortunately, I don’t think anti-suns exist.

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      1. Three is a very small sample size. If there are just, say, ten antistars in our galaxy, what are the chances?

        I am also skeptical about antistars, but for a different reason: I just can’t imagine a process that would result in matter and antimatter segregating themselves that way. I find it a bit easier to believe in, like, one half of the universe being matter and the other antimatter.

        But in any case, if a piece of rock or a comet from an antistar system entered ours, how would we know? The trajectory would be exactly the same. It would probably light up if being hit by a CME, but what about the solar wind? Would it destroy a small meteor as soon as it crosses the heliopause? Would we be able to detect such an event?

        Also, is there any way to tell if a crater was made by a meteor or an antimeteor? Has anyone checked the thousands of craters in the Solar system for such a signature?

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    2. An antistar would generate a lot of X-ray and hard gamma via interaction with the interstellar medium (or rather the border between matter and antimatter), which is not an absolute vacuum.
      Which is why our sun is also similar in composition to the medium in our vicinity since we don’t see that in our neighborhood.

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      2. An actual anti-galaxy might be too far away. I was under the impression that matter-antimatter annihilation gave off pretty distinctive radiation, but I’m no expert.

        Though that runs into other problems of why matter-antimatter distribution ended up so lumpy.

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      3. The only scenario I can imagine that would leave us with a few antistars among stars or a few antigalaxies among galaxies is that initially there was orders of magnitude more matter and antimatter with slightly lumpy distribution and slight excess of matter, then almost everything annihilated and only excessive matter and antimatter remained in scattered fragments. But such a process would leave humongous relict x-ray and gamma background radiation, wouldn’t it?

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      4. I’m not up on the latest theories of our early universe, but one of the puzzles when I last looked at it was how we ended up with so much matter compared to antimatter, since particles form in pairs. Or at least the ones that aren’t their own anti-particle (not sure about that).

        So one theory was that there was a slight imbalance, which is what was left over.

        Not sure how this interacts with dark matter/dark energy, since we really don’t know what those are – we have some ideas of what properties they must have to make our theories work, but last I heard we have no idea how they were formed or what relation they have to visible energy/matter.

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  7. The enormous cost of high speed travel is an economic incentive to send tiny von-neumann machines, no? Evolution will favor machines that grab resources at destination stars to build as many high-speed machines as possible. We should be able to see signatures of expanding type II bubbles in distant galaxies long before any such species reached us.

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  8. I think that before aliens come for a visit, they would send a probe or two. Then have active surveilance for a while.

    If fusion was possible, then these probes could be built and sent practically for free – using fusion to power them.

    Does this mean that either there is no intelligent alien life or that controlled fusion is not possible?

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  9. While I think the Fermi Paradox is a great thought experiment about how we got where we are and how alien worlds might do the same, by the time you get to SETI, it becomes a religion, where excuses are made for every failed prediction, just like Doomsday predictions, but more expensive.

    I subscribe to the rare earth hypothesis, where the universe is teeming with life–microbial life. We should be able to see if this theory is true with powerful enough telescopes.

    I don’t see anything wrong with taking the slow road to colonize the galaxy. The earth itself has about a billion years left; the galaxy has 100 billion stars. Just send out 100 colony ships per year at 0.001c (as can be done with current technology)!

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  10. Unless you’re launching that colony ship out of a gun barrel the size of the Oort cloud, or using tachyons as remass, or the payload is a stabilized wormhole, Tsiolkovsky rocket equation plus cargo economics says there’s no point going much faster than about 60% of the speed of light. For any finite construction budget, trying to squeeze out total delta-V of more than 1.2 times the exhaust velocity gets you a worse mass ratio, thus less colony-stuff delivered to the destination – and since, by definition. a colony ship can’t count on pre-existing infrastructure there to catch them, half that delta-V budget has to go toward slowing back down.

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