Where Are All The Aliens? We Ranked Every Explanation 

So fun! Fraser this is the most enjoyable tier list I have ever watched. I found myself smiling through the entire 2 hour journey and was disappointed that it had to end. Excellent hosting, great colleague insights, a true RU-vid gem! Thank you so much.

Rare Earth is definitely my number 1 choice.... Now throw in the vast distances between Earth like planets + Drake equation and civilizations are just rare and too far apart (causally disconnected).

I agree with Fraiser on one point for Dark Forest, any species powerful enough to be a threat already knows we’re here. Hiding is pointless

People try to pick "a solution" when i think it's lots of them combined, with each one lowering the chances of us seeing intelligent life near us.

my only disappointment is that you didn't include Isaac Arthur, President of the National Space Society, considering he has an entire youtube series covering fermi paradox solutions.

With regard to Dakotah Tyler's comments on the simulation hypothesis, he had two criticisms: 1. to simulate every atom in the universe, would require a computer larger than the universe. 2. It makes no sense to simulate an entire universe, and then only put one planet of life in it. But what if these two ideas are complimentary... what if to get around 1, they only simulate the parts of the universe at the detail and scale they need for the people on the single planet to have a compelling view of the rest of the universe, without simulating all the atoms of every star out there. In fact, that's how many modern computer games work, simulating finer grained details only as you get close enough for those details to become relevant to the view (Or whenever you zoom in on them with a telescope etc). In that case, the simulation of only one world with conscious observers, might help them to solve the issue with the information theory and computational power issues involved. I give it a C+. Only a little higher, but still.

a simulation would not have to simulate the whole universe, it would have to simulate only our perception of the universe

I also like the idea of 'dumb aliens'. Consider if the dinosaurs weren't interrupted this planet would likely still be theirs.

We are one of many cilvizations, but due to the size of Universe, our Light Cones will never intersect :(

I remember reading a story about a hiker who got lost in Japan. This kind of baffled me, because surely if you walk in a straight line in Japan for a few kilometres you're bound to bump into a farm, city, railway, highway... I mean, it's pretty densely populated. But you can't see a city 25km away if you're in the woods. Space may well be the same, but we haven't really got the ability to see all that far. In 100+ years when we've surveyed the whole galaxy with future super telescopes and haven't found anything, then it gets interesting.

The answer to the Fermi Paradox is so simple (as per Occam's Razor): space is so much bigger than we can comprehend, if there are any others out there, it’s unlikely we’d ever see them. Using the needle in the haystack apology, the haystack is so unimaginably big, and the needle so small, even if the aliens were super advanced, the time it would take to statistically find one would probably be thousands or millions of years

If we live in a simulation, our ENTIRE experience of the entire universe is still INSIDE the simulation and we’d have zero idea of the real size and power abilities of entities outside this simulation.

A simulation of the universe doesn't require a computer with the power of the universe, like Dakotah says. You don't need to simulate every atom. Only things that are observed need a supply of data, which, compared with the size of the universe, is miniscule. Open world video games don't load the entire map into memory the entire time, just what is active on and around what's on the screen. This actually ties in fairly neatly with quantum mechanics and measurement and wave functions. One could take it a step further and apply the holographic principle to make the data requirement much smaller/compressed, since the perceived three-dimensionality of space is contained in only two dimensions. I'm not convinced regarding the simulation hypothesis, but it is certainly a compelling idea. I'd put it in A tier.

The nearby star flyby in 1.3 million years is Gliese 710. I've already made plans to visit.

I miss the weekly space hangout, but thank you for helping me find Moiya again! And thanks for promoting new and amazing talent. Found Dakotah a while back thanks to you as well. Amazing everyone! Thanks!

My problem with many of these is that they have to account for ALL life. Any scenario where a species has to choose an option (go ethereal, stay home, don't spend extravagant energy, ...) may apply to SOME species, but not ALL. For me any solution has to be caused by nature/physics or there will be exceptions that we might detect. Anything else goes straight in the D-bucket for me. The most plausable to me is a combination of rarity and not having looked wide enough - in other words, they exist, but are far and few between and we just need to keep looking (possibly for millennia to come).

This was great, everyone seemed to have a good time

I think the grabby alien's hypothesis is pretty compelling to us being among the first. I guess you could call this one Rare Intelligence too. I liked Dakotah's answer here about the hard steps from going from simple life to complex. It really is one of the single best pieces of data that we have towards the answer to the Fermi Paradox. That combined with the null results from all our searches, the other big piece of data we have, is basically the grabby aliens hypothesis. I think both of these should be S-tier. If you are reading this and haven't heard of the grabby alien's hypothesis, search for the video from PBS Spacetime. It is a good summary. You can also find the paper titled Grabby Aliens where they walk through their full argument and the math behind it. It is pretty easy to read in terms of science papers, and is very compelling.

I'm only 10 minutes into this but I already feel like you should do this format again but with Isaac Arthur as a guest. That guy has a lot of really clear and in-depth thoughts about this subject matter that might make a video like this very informative to a lot of people.

Time and distance is the biggest filter: Odds are against any peoples being at level of sophistication that they can find and begin communication with another peoples of the same mindset. There might be someone thinking almost the same as we do but they are three galaxies away and the odds of us finding each other are next to zero.

I think the true answer is a combo of a lot of these. I think that the most potent options is the idea that we are one of the firsts and expansionist civilizations kill themselves, and non expansionist civilizations don't feel the need or find is selfish to take the whole universe to themselves.

I vote for a combo; "Intelligence" ( the kind that gives a damn about the sky in any way ) is EXTREMELY rare, and we're temporally alone.

The best objection to the "simulation hypothesis" was not the ones they settled on but one they glossed over - it's not a hypothesis at all because it's not falsifiable. I'm 100% with Moiya on the simulation hypothesis.

I think our tech is the most important bottle neck when we talk about detecting other intelligent life forms. Because light year travel time prohibits us from observing planets that are light years away in our current present time, due to the vast distance and the speed of light, any planet we observe would be a past version, not a current/present version of that planet. So complex life could have developed but all we can see is just the planet before any activity happens.

Would we be able to detect life on Earth if we were like 20 lightyears or more away from Earth with our contemporary state of the art technology? I highly doubt it. And that's your answer. Our technology simply isn't sufficient enough to detect alien life in even our local part of the galaxy. So what paradox?

I personally think the Fishbowl hypothesis is most accurate. Life is common; even multi-cellular life is common. Even intelligence is not all that rare. Yet, civilization with technology and access to the cosmos is extremely rare. Most life will evolve with water inside an ice bound planet or moon. And even if intelligence happens and creates a form of civilization, the chances to discover space faring tech is so extremely improbable. It took humans 2 million years to get to this early point of space travel.

I think you guys got it completely wrong with the simulation. Like, first, why do you assume every atom in the universe would have to be simulated? Do you now how video games/ virtual environments are rendered? Computer only renders in detail the thing we are interacting with, everything else kinda doesnt exist. Sounds quite quantum, doesnt it? :D Also, the if the base universe is much larger, complex then our universe, as real as it seems, could be their equivalent of Minecraft. Does Steve in Mincraft say it would be impossible to have a computer simulate all that Minecraft wolrd because, according to his Minecraft physics, it would take a computer bigger than the entire Minecraft universe?

Regarding the game theory argument you made on the Dark Forest Hypothesis, it is only valid for a game with only one interaction. When you have a game with multiple interactions, strategies with good behaviour outperform detrimental ones.

If this zoo hypothesis should work it must mean there actually are a bunch of intelligent aliens and all of them are both in agreement and 100% disciplined, over long time? What are the chances? Are they not even slightly similar to us, where some guy jump in to the bear pit?

Isn't it conceivable that complex intelligent life and technological civilizations are possible in the Universe, but extremely, extremely unlikely? And we happen to be it, the only winning lottery ticket in the galaxy or the Universe?

We're the first. It's the perfect null hypothesis that doesn't require us to imagine anything that may or may not be happening elsewhere.

My favourite solution is the evidence horizon. So civilisation spreads across the galaxy like ripples in a pond, falling apart afterwards because of natural processes and the problems of maintaining that civilisation, like distance and time. So in short, we’re either existing before the first wave, or we missed the last wave and we’re surrounded by ruins and the dead, slowly falling apart all evidence disappearing. Also, I think the great filter is that little girl!

Great guests and chemistry and exciting discussion. I watched for 2 hours without a break.

When you were talking about "sending a bad idea" to a civilization; I'm pretty sure this is the plot in the 1995 film Species. Good stuff.

Octopi have arms. I can see them evolving into something formidable given another billion years.

Considering the vastness of space, the diversity of life just on this one planet, I can't imaging any solution that involves everyone responding to the same problems in the same way. Humans don't even do that within our own species. There's just far too many unimaginable ways to be.

I'm a video game programmer. The behavior of photons (wave to particle on interaction) is the same way we optimize physics in games. Gravity can also be explained by processing speed. each tick of time is the length of time needed to run the simulation in that given area. The more calculations needed, the slower time moves. So the more matter in an area, the more calculations needed per tick. It would work this way if each Plank Length is a processor of fixed processing speed. As far as simulations being visibly different is incorrect, that's not what we do as game developers. We find a simulation that gives the same results with lower resolution. A fractal calculation will give as much detail as you can observe and all that accuracy will be correct. WE CAN MAKE THIS NOW., but In a much simpler way. Looking at the universe from the perspective of high accuracy optimized simulation, the simulation theory is a B or an A.

Great filter is natural selection. It is also an argument for why we should find a few, very intelligent species, because every impact, mass extinction, cosmic conditional filter, will prune the dumb life and select for intelligent / adaptable life. That filter doesn't need to mean nobody at all, just nobody anywhere near us (at least very unlikely) -- the most likely answer by this logic is that we're the first/only in our galactic neighbourhood, but not the universe.

Personally, I think a combination of the following factors makes the most sense: 1. Vast Distances(I feel like no matter how educated you are, it's really easy to handwaive the absolutely massive distances between planets, star systems, galaxies, and so forth) 2. Narrow scope of the places and methods we've looked for intelligent life(We really are just shining a laser pointer into the corner of a dark forest and assuming no life exists because the light didn't land squarely on Sasquatch's forehead) 3. Narrow definition of what we would consider "intelligence" (i.e. assuming intelligence = technological expansion across space at ever increasing power demands) 4. Timing (For instance, there could have been alien species relatively close to or greater than human intelligence, but say, lived 100 million years ago and went extinct after 100 thousand years) 5. Great Filters (I am more inclined to believe most of the filters are behind us, but some like the conditions needed to grow technologically are specific enough that there are probably tons of planets with life thats somewhere between single celled bacteria and the dinosaurs, but almost none that got to the "mining the planet for ancient stores of fossil fuels if they even have that equivalent to power their technology" stage) 6. We're the First/There are no advanced civilizations(tied in with 1 and 2 - It could just be that there hasn't been any other advanced intelligent life that developed in the relatively small amount of space we've looked at so far. Doesn't mean there isn't 100 star faring civilizations in Andromeda, but just that there aren't any Galactic Empires nearby) In a nutshell, it's a lack of scope combined with just the vast distances/timescales and what we consider intelligence being heavily dependent on our own evolution that answers the Fermi Paradox. I don't think there are any invisible civilizations "hiding the truth" from us, but instead that life is possibly prolific, and space is just like, really, really big on timescales that don't mesh well with our current standards.

Nice. I had waited to listen to this until i had a nice drive and it was well worth it. Though, I was wondering about the 420 hypothesis where it is the scenario that they show in movies when someone gets "altered" mentally and wonders if our solar system isnt just an atom in a fingernail of a larger being 😅

the solar system has a timeline you need to expand out of it before the solar area becomes uninhabitable.

Loving Moiya's constant Stargate references. ONE OF US!

This was the highlight of my day thank you for making this video and having these conversations in a way that everyone can understand them.

I'm really glad you were the voice of reason in this, Fraser! I would've been screaming at my screen way more if you hadn't been there! ❤

Usually not a SETI guy but was so much fun! Great trio!

I was surprised that the panel dismissed Fraser's zoo/simulation hypothesis. One of the huge advantages of that type of hypothesis is that it completely bypasses the Copernican principle/mediocrity principle, which is arguably the main reason that the Fermi paradox is a paradox to begin with. Where is everybody? if life is so mediocre and common, as it should be based on the sheer numbers, where is everybody? answer: they are common in the universe but we are being hidden from them and nobody is bothering to look for us just like nobody on earth is bothering to find an invisible grain of sand located somewhere on one of the beaches of the earth. Also, the simulation hypothesis doesn't require anything like a simulation of the entire universe, as others have pointed out.

Fascinating subject, I love the insight that the guests provide, and you all cooperate beautifully. Nice one, it's my favourite science video in a fairly long time. Long time patreon supporter here, keep up the exceptional work :)

This episode was awesome! Thank you guys for spending all this time on such a great subject! I’m also very excited to finally have a question worth asking for the question show! We’ve spent a lot of time studying the possible generations of stars and galaxies, but has anyone spent time studying the generations or life cycle of planets? Like if we separated Jupiter from the Sun and left it to its own devices how long would it last? Would it churn with storms forever? Or get so cold it would shrink and freeze? Could there be planets formed at the earliest times of the universe that are pure hydrogen and that’s how they stay forever?

42:32 - I really have to disagree. “Life forms really early” is not some universal fact, it’s just what we observed in our particular case. After all, we only have a sample size of one. Until we see evidence of life forming equally early elsewhere in the universe, it’s much more correct to say “OUR life formed really early”, and to use that to *support* the “we’re first” argument, rather than to refute it.

My solution to the Fermi Paradox : Interstellar travel and interstellar communication have NEVER been possible due to the laws of physics. Since the universe has stabilized, millions of civilisations have thrived, reached an advanced state then withered away, having depleted all their available resources, trapped in their own solar system, just as we are. These civilisations have built radio-telescopes to listen to the void, but to no avail. Artificial signals have always been, and always will be too weak not to fade away in the Oort clouds/gas clouds surrounding each star. Like we did, they built space station, reached their moon(s), maybe their closest planet, but that's all. Probes were sent, but were just too insignificant to be noticed, and died. The best aspect of this solution is that it does not contradict anything we witness now : No signals, no signatures, no megastructures, nothing.

Great vid, excellent audio from all guests, fun topic. Looking forward to more like this. :)

Awesome episode! This one is definitely source material for the history books.

Easiest solution: We are not looking Well, we have looked at a couple stars. A couple stars is not even a dip.

Woot! Dr. McTier is back! Always good to see her! Great to see Dakota Tyler back too!

This is one of my favorite episodes. Loads of fun and provided a new perspective on the Fermi Paradox. It's also nice to see really young scientists who seem to have fun in this activity. Thanks so much for all you do, Frazier.

Fun and informative video - thanks Fraser! Also, you and your panelists touched on something that has fascinated me for decades. Namely, how popular culture, including science fiction, can influence how we view the world. I’ve loved science fiction since I was a child and now as an adult I can see its influence, both positive and negative. On the plus side, there are countless examples of people who watched sci fi shows like _Star Trek_ who were inspired to become scientists, engineers, doctors, artists, and so forth. On the minus side, there are a lot of people, and one in particular, who say we’ll have millions of people living on Mars by 2050, which I think comes from taking some sci fi too literally. Anyway, I think it would be a great topic for a video. 😀

A very enjoyable two hours watching this. Two great guests

Anyhow, when we confirm we have a working warp drive technology we also know in the same moment we are alone in the universe

I have had decades to think about the Fermi paradox. 49:we are alone. Uniformity of physics in the background of space says if we find life on the earth, life should have regenerated for billions of years before the oxygenation event. We find life began as a singularity. The Miller Urey experiment and tunneling chemistry in the cold of space show that amino acids are found in asteroids and moons. We dont have a theory or definition of life that predicts life. We find life by comparison. Even if it's panspermia you only push the origin back in time. That's why life is not cause and effect of uniform physics. Life has not been detectable in space as complex proteins for 10.2 billion years. Life on earth is a paradox. You can't seed the earth without complex proteins in Kuiper belt objects. Life is a thin layer on the surface of earth. Levinthal paradox of proteins. The human body 37,000 billion cells, 37.5 trillion symbiotes, Andromeda is only a trillion stars, 80 billion neurons, about as many as the clouds of Milky Way stars, our dna can stretch twice across the solar system, dendrites are 1,000 to 10,000 trillion, Our own thoughts may occur because of Feynman paths and Penrose microtubules. If life is cause effect there would be swarms of Boltzman brain comets. You are a singularity from every generation 37 trillion cells accurately reproduced from one cell. Physics does not predict free will from a swarm of particles. By the same logic life doesn't begin from inert metallicity. It is as though the universe does not have the degrees of freedom. See Y Chromosome Bottleneck at the dawn of civilization, Cambrian Explosion of Life, Origins Revisited by Richard Leakey Jr. Just the 1s2 tip of discovery

For me I tend toward the interstellar travel is impossible (or just so inefficient to be useless) but also with the grand filter removing civilizations at various points, with self destruction being one of these filter possibilities.

I'm with Fraser on the "We're Alone" being in S-tier. No indication that the formation of life is an easy step that just "must happen" on all the other planets that have the right ingredients. That seems like a logical fallacy where people see a large number of potential planets out in the universe and since they can't comprehend that number, assume that somewhere one of those has to be life bearing, when the step to generate life could be 1 over 1000 times that number for all we know. Non-life -> life transition is still too poorly understood to make a claim that it surely happens across other planets.

I’d like to think Earth is an anthill in the middle of a dense remote forest . I mean, would **you** look for a specific anthill in the middle of the huge forest in Oregon?

This discussion is awesome, thanks :D

Thoroughly enjoyed this and would love to see more videos in this format. Btw, I've been a huge fan of Dr. McTier since watching her contributions on The Weekly Space Hangout!

People just don't appreciate how rare it is for life to exist, even when all the data tells you that.

cool video idea

There's quite a bit of overlap between many pairs here. I wonder if something like a principal component analysis of the attributes would be helpful if you ever do an update to this.

4 sittings and finally finished this really fun convo! thanks Frasier and fab experts!

"Interstellar travel is too difficult" is the obvious answer. It's not impossible, but without FTL all of those sci-fi dreams of interstellar civilizations are SUPER silly. The whole "If one in a thousand makes it then someone has to explore everything" argument doesn't make sense. It's a massive amount of energy, resources, and time and nobody has unlimited versions of any of those. One doesn't follow the other at all.

As an animal, I'd say it's simple. The transformation of very intelligent animals to a wise species , can't happen fast enough to make it off any planet successful.

"We are first" is one of my favorite because it's 100% true for some alien species out there. Maybe us, but definitely for someone.

From what I have gathered about the JWTS is that we are actually looking through 2 universes and that is a total trip !

My thoughts on the simulation hypothesis is that we are in stasis on a colony ship, and the simulation is training us how to build a civilization so that when we arrive, we have learned how to live together and not destroy ouselves.

40:32 this is what i believe, we are the first intelligent civilization, and the universe are ours to conquer

Awesome interview!! 1:34:22 approx.. thank you again, that answers another question I recently asked.. there's an obvious long term benefit to sending missions to objects that are speeding towards us, but none planned?

Fraser's "Ferry Boat" hypothesis is very cool. Just wait for a star to naturally drift close and hop aboard!

Reality is a simulation and the sysadmins haven't purchased the "Xenotools" license packs yet. Management keeps vetoing the purchase requests.

Maybe we aren't advanced enough to see them... Maybe they aren't advanced enough for us to see.

Regarding the simulation hypothesis, you need to talk to a programmer, specifically a game developer. You don't have to simulate every atom in every celestial body in the universe, you only have to have a good model for each to simulate the relatively few photons that reach us from all of those objects.

Actually some of the crazy things we see and hear about in quantum physics seems to support the simulation hypothesis. Heisenberg's uncertainty principle, the double slit experiment, quantum entanglement, speed limits not exceeding 186,000 miles per second, hidden code in string theory, etc. It's very fascinating and top scientists are taking it very seriously these days

Fell in love with Dr Moiya when she brought up Daniel Jackson ascending in Stargate lol This was such an adorable episode to watch these scientists have so much fun engaging in this exercise.

Thank so much guys! I really feel like there is a shortage of long form astronomy podcasts. Maybe I'm just to hungry to learn and I particularly enjoy these conversation sort of videos. So yeah. Thank you.

I love this format; especially the legible labels on the chart.

Prime Directive - “Again with the fictional TV series. Is that where you get all your life lessons?” - Dennis Taylor, For We Are Many (Bobiverse Book 2)

My personal fav is that FTL tech works and when developed aliens use it to vacate dangeous galaxies.

Very good discussion. I think the answer combines almost everything in tiers S through C. The exception is interstellar travel being impossible. I think it's more difficult than we understand, perhaps to the point of being impractical.

Very engaging discussion, thanks for presenting it. Cheers🎉

This is the best content imaginable. Thank you guys! 🙏 A Tier List of "what an alien life could look like" could be next. Including: silicon-based life, balls of energy, squid-like creatures, gray aliens with big black eyes and etc. 😊

After having watched the whole video I have to say this is my all time fave of yours. That said, I don’t find any of these compelling as an S tier by themselves. Even though I agree Self Destruction is most likely, if there are tons of civilizations, I think that enough would survive to make it through. Unless there aren’t a ton of civilizations…which means we have to also consider Rare Intelligence. And I think that Expansion is Inefficient is another big factor, though if you can wait for Star ferries, it’s not a major impediment. But that probably does preclude the existence of Type III civs, which means that Tech Isn’t Good Enough becomes the primary factor. So I think the Occam’s Razor solution with the fewest assumptions is the combination of essentially Great Filter X Expansion is Inefficient X Tech Isn’t Good Enough. But the exciting thing is that we are on the cusp of our Tech actually being good enough. We just don’t have the data for how many planets actually get life yet. I’m starting to worry that even though planets are so common they’re essentially 1 in the Drake equation, if we don’t find Earth sized planets with atmospheres around M dwarfs soon, it’s going to reduce the possible candidates to K and G dwarfs. My specialized concern is that if it turns out complex life requires active plate tectonics, and it turns out the reason we have that is because Theia smashed into us, then just having an Earth sized planet in the habitable zone of a sun like Star won’t suffice.

At what minute in the video is the rating table discussed, which I see on the video’s intro? I think I watched the entire video, but missed the table. Thank you in advance!

I think it'll turn out to be one of two things. The Dark Forest idea is plausible. Assuming there have been advanced civilizations out there, there has had to be at least one case of one civ wiping out another one. This would mandate that all of the remaining civs assume a Dark Forest scenario approach. But my first choice for explanation is that we just don't know what to look for. Everyone says we've been looking and haven't detected anything. We've detected lots of things in the universe. We might not understand what we're truly seeing. Some animals don't realize they are looking at themselves when they look in a mirror. They lack the capability to grasp what a reflection is, or maybe they don't have any sense of self. Maybe we humans are also lacking a key element that is needed to recognize extraterrestrial life when we detect it.

I like the spy game variant of the prime directive hypothesis, where it's not that they aren't "allowed" to interact, but that they think it would be a bad idea for one or both of us, so they hide.

Fascinating discussion -- loved it! I agree with most of the rankings, though personally I'd have "We're alone" and "We're the first" much higher, perhaps even S tier. I don't know if that makes me a pessimist -- I'm a lifelong Trekker, and would love for the universe to be populated with humanoids with different bumps on their heads and weird ears, but I think it's pretty clear that at least our galaxy isn't lousy with spacefaring aliens.

I found this paper "Asymptotic burnout and homeostatic awakening: a possible solution to the Fermi paradox?" quite compelling. Oh and thanks for a great discusion.

Absolutely a blast to listen to . Loved it.

Group question What are the science forums do you enjoy. I don't know any -- hence why I'm asking

I love it how the "scientists" think that our technology would see through much more advanced technology

Great video! I think about the drake equation and the fermi paradox many times throughout every year. Love the assumptions brought in which tilted my thought a bit: - different evolved intelligence will likely have evolved their own view making certain solutions less likely. Thanks for the fun conversation!!!

@9:00 for the Star Trek prime directive theory when Dakota was talking about we have to be careful about sci-fi and projecting ourselves, our ideas, our feelings, it reminded me of the US and Russia during the Cold war, the us was doing some practice training missions and Russia thought it was a the beginning of an attack and they were very close to launching nukes, that was just here on our planet just between different countries, we can't imagine what another species will do

I have listened through the duration ! Very thought provoking discussions.