title: Notes on the Fermi Paradox
author: cjhandmer
contenttype: article
publication: Casey Handmer's blog
published: 2026-03-03T18:47:20+00:00
sourceurl: https://caseyhandmer.wordpress.com/2026/03/03/notes-on-the-fermi-paradox/
word_count: 1914
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 0.005500 = 2.5 years, while the subjective travel time is 0.1500 = 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?