imagine nasa announced todaythat they found aliens. bacteria on mars, weird alien fishin the oceans of europa, and also ancient alien ruins on titan. wouldn't that be great? well, no. it would be horrible news, devastating even. it could mean that the end of humanityis almost certain and that it might be coming soon. why?
why would the most excitingdiscovery of our lifetime be bad? let us imagine the development of life, from its inception to us today, as a flight of stairs. the first step is dead chemistrythat needs to assemble itself into self-replicating patterns,stable and resilient, but also able to change and evolve. the second step is for our early life tobecome more complex,
able to build more complicated structures, and use the available energymuch more efficiently. on the next step, these cells combineto become multicellular beings, enabling unbelievable varietyand further complexity. the step above sees the speciesevolve big brains, enabling the use of tools, cultureand shared knowledge, which creates even higher complexity. the species can now become thedominant lifeform on its planet, and change it according to its needs.
first shy attempts to leave itsplanet are happening. this is where we are now. it's in the nature of life as we know it to reach out, to cover everyniche it can. and since planets have a limitedcarrying capacity and lifespan, if a species wants to survive, it will look for more placesto spread to. so the steps above the currentones seem logical: colonize your own solar system,
then spread further to reachother stars, to the possible final step:becoming a galaxy-wide civilization. it's very likely that this is a universalprinciple for civilizations, no matter where they're from. if a species is competitive and drivenenough to take control over its planet, they'll probably not stop there. we know that there are up to 500 billionplanets in the milky way, at least 10 billion earth-like planets. many have been around billionsof years longer than earth.
but we're observing zerogalactic civilizations. we should be able to see something... ...but there's nothing. space seemsto be empty and dead. this means something is preventing livingthings from climbing the staircase, beyond the step we're on right now. ...something that makes becominga galactic civilization extremely hard, maybe impossible. this is the great filter.
...a challenge or danger sohard to overcome, that it eliminates almost everyspecies that encounters it. there are two scenarios: one means we are incrediblyspecial and lucky, the other one means we are doomedand practically already dead. it depends on where the filteris on our staircase: behind, or ahead of us? scenario 1: scenario 1:the filter is behind us.
we are the first. if the filter is behind us, that means that one of the steps we passed is almost impossible to take. which step could it be? is life itself extremely rare? it's very hard to make predictionsabout how likely it is for life to emerge from dead things. there is no consensus.
some scientists think it develops everywherewhere the conditions are right; others think that earth mightbe the only living place in the universe. another candidate is the stepof complex animal cells. a very specific thing happenedon this step, and as far as we know, it happenedexactly once. a primitive hunter cellswallowed another cell, but instead of devouring it,the two cells formed a union. the bigger cell provided shelter,
took care of interacting with theenvironment and providing resources, while the smaller one used itsnew home and free stuff, to focus on providing a lot of extraenergy for its host. with the abundant energy, the host cellcould grow more than before and build new and expensive thingsto improve itself, while the guest became thepowerhouse of the cell. these cells make up everyanimal on the planet. maybe there are billions of bacteria-covered planets in the milky way, but not a single one, apart from us,
has achieved our level of complexity. ...or intelligence. we humans feel very smart andsophisticated with our crossword puzzles and romantic novels. but a big brain, is first and foremost, a very expensive evolutionary investment. they are fragile, they don't help in a fistfight with a bear, they cost enormous amounts of energy,
and despite them, it took modern humans, 200,000 years to get from sharp sticksto civilization. being smart does not mean you getto win automatically. maybe intelligence is just not so great, and we're lucky that it worked out for us. scenario 2: scenario 2:the filter is ahead of us. plenty of others died already. a great filter before us is orders of magnitude more dangerous
than anything we encountered so far. even if a major disaster killed most of us or threw us back thousands of years, we would survive and recover. and if we can recover, even if it takes a million years, then it's not a great filter, but just a roadblock to an eventual galactic civilization. on universal timescales, even millions of years are just the blink of an eye. if a great filter really lies before us, it has to be so dangerous, so purely devastating and powerful,
that it has destroyed most, if not all, advanced civilizations in our galaxy over billions of years. a really daunting and depressing hypothesis is that once a species takes control over its planet, it's already on the path to self-destruction. technology is a good way to achieve that. it needs to be something that's so obvious, that virtually everybody discovers it,
and so dangerous, that its discovery leads almost universally to an existential disaster. a large-scale nuclear war, nanotechnology that gets out of control, genetic engineering of the perfect super bug, an experiment that lights the whole atmosphere on fire. it might be a super-intelligent ai that accidentally (or purposely) destroys its creators. or things that we can't even see coming right now.
or it's way simpler: species competitive enough to take over their planet necessarily destroy it while competing with each other for resources. maybe there are runaway chain reactions in every ecosystem that once set in motion, are not fixable. and so once a civilization is powerful enough to change the composition of its atmosphere, they make their planet uninhabitable 100% of the time. let's hope that that's not the case.
if the filter is ahead of us, our odds are really bad. what we can hope for. this is why finding life beyond earth would be horrible. the more common life is in the universe, and the more advanced and complex it is, the more likely it becomes that a filter is in front of us. bacteria would be bad, small animals would be worse,
intelligent life would be alarming. ruins of ancient alien civilizations... would be horrible. the best case scenario for us right now is that mars is sterile, that europa's oceans are devoid of life, and the vast arms of the milky way harbor only empty oceans hugging dead continents. ...that there are billions of empty planets waiting to be discovered
and to be filled up with life. billions of new homes... waiting for us... to finally arrive. how likely is it that we'll find life outside of earth that is similar to us? well, that depends on how many planets there are out there in their star's goldilocks zone-- the area around the star where water can be liquid. because stars come in all sizes and configurations,
this zone is different for every star system and requires a little bit of physics to figure out. if that sounds like fun to you, this quiz from brilliant helps to break down the maths for exactly how this is calculated. brilliant is a problem-solving website that teaches you to think like a scientist by guiding you through problems. they take concepts like these break them up, into bite-sized bits present clear thinking in each part, and then build back up to an interesting conclusion.
if you visit brilliant.org/nutshell or click the link in the description, you can sign up for free and learn all kinds of things. and as a bonus for kurzgesagt viewers, the first 688 people will also get 20% of their annual membership. and if you do find life on other planets, it may be wise to leave them alone for a while.
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