# What exactly is the “Great Filter” theory, and how does it work?

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What exactly is the “Great Filter” theory, and how does it work?

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The universe is huge. The observable universe, less so, but still quite huge. It’s been around for quite some time. It is speculated that life probably existed before us.

Why, then, have we not seen evidence of such life?

We would expect to see evidence of any radio-using spacefaring life – especially a Kardashev-2 or -3 civilization. However, we don’t see any. Why not? What barrier stands between primordial life and kardashev-2 civilization, which has prevented any alien race before us from reaching it?

This barrier is ‘the great filter’.

It is possible that ‘the great filter’ is math, or language, or abstract thought, and we humans have already been lucky enough to pass it.

It is possible that the great filter is nuclear weaponry, and we are still teetering on the edge of oblivion.

It is possible that the great filter is something we have not yet discovered.

To explain the idea behind the Great Filter, I first have to tell you about the Fermi paradox.

The idea was first brought up by Enrico Fermi, one of the scientist behind the creation of atomic bomb.

Fermi pointed out that the universe was very, very big both in terms of size and age.

There a re billions of stars in out galaxy alone and even if only a small fraction of them ever ended up developing intelligent life, there should be some intelligent civilization that had been around for a very long time and had enough time to visits every star in the galaxy.

There are however no easily noticeable aliens around here.

That means that something weird is going.

Another physicist called Drake came up with a a way to quantify the problem. He basically summed up all the fractions involved.

If only 1% of all stars in the galaxy have rocky planet and only 1% of those can support life and only 1% of those will develop life and only 1% of those will develop intelligent life and so on.

Obviously we don’t actually know the percentages but we can make guesses. If you multiply all those fractions and then multiply the tiny fraction that results with the number of stars in our galaxy that tells you if there should be spacefaring civilization out there.

We are starting to get an idea of some of the numbers. We for example have found out in recent years that planets seem to be very common at least.

One way to look at the whole problem is to imagine that to go from a star to an interstellar empire, the potential alien civilization needs to pass though a number of filters that take out some of them and allow others to pass.

For example we might imagine that life is incredibly common but that the jump to intelligent life is really, really rare. So developing life would be a small filter that only diverts a small faction of candidates but developing intelligent life is a big one that stops most of them.

Since there are no signs of an alien civilizations out there and from what we know about physics, it would be hard to hide evidence of one, we can imagine that at some point all potential alien civilizations pass though a number of filters that filter most of them out.

It could either be a number of small filters that add up or a really big filter that catches almost all of them or a combination of those.

We humans are already most of the way there to colonizing the galaxies. So it would be nice to assume that there was a great filter at some point in out past that we passed though successfully.

It is a bit scary to imagine that a great filter is still ahead of us. The idea that plenty of alien civilizations make it to where we are now and almost all are killed of before they can reach Star Trek level of advancement is really scary.

Since this whole thing first came up during the cold war, one of the popular filters proposed was that many advanced civilization kill themselves of with nuclear war or an equivalent. It seemed natural at that time to consider that a possibility.

Currently people are eyeing some future tech that might go catastrophically wrong, like AI going skynet on us or nanobots going out of control or some bio engineered plague taking us all out. Those seem less likely scenarios.

Especially the robot takeover suffers from the problem that if that was a common fate of civilizations we would expect to see interstellar robot empire out there in equal numbers to the biological ones they replaced. It only shifts the problem.

There might be something ahead that we can’t even think of yet that will kill us all.

That is a really scary thought.

Sci-fi writers have embraced this idea in various forms. A common theme is that one civilization came first didn’t want any competition and thus periodically wipes out any civilization that reaches a certain point of advancement.

It makes for good sci-fi but seems overly complicated to explain real life. (Why not wipe out all planets with life on them?)

In any case as far as we know great filters are real. We don’t know if we passed them all a long time when life first began on our planet and that life is extremely rare or when we first developed intelligence or when we avoided destroying ourselves with out technology (so far) or if there is something still waiting ahead that has a 99.9% chance of killing us all.

Doing things like searching for extra-solar planets to see how common they are (apparently very), searching for signs of life on Mars and Venus (maybe) and looking at the history of life on our own planet and the intelligence of other animals gives us some clue to which numbers we should use for the stuff that may have filtered us out in the past.

If everything we know of our past doesn’t add up to a 1 in 100 billion chance, we may have to worry a bit more for our future.

We don’t see any live anywhere in the universe and started speculating whether before any life is able to ‘explore space’, might there be just too many barriers in nature which destroy a species. Too many ‘filters’ which together form a Great Filter.

We thought of a number of possible causes (filters) that could kill us before our society becomes advanced enough to really travel out of or solar system. We imagine this could apply to alien species as well. It’s possible that our social nature is too narrow-minded to overcome issues like greed, fear for other people and we nuke ourselves before we are wise enough to collaborate and start a space oriented society. A planet in general might just not have enough resources to make this possible, or at the time ant society is a advanced enough they’ve always used up too much already. A giant meteorite might statistically always crash into planets before they reach maturity.

All of these are examples of filters we, or any other species, have to overcome before we can really travel through space. As the universe seems kinda ‘dead’ to us, maybe it’s just not possible to overcome these filters for any creatures that might arise. There’s a lot of unknown unknowns in this of course.

I think of the great filter as the huge barrier that must be overcome to go to other planets in other stellar systems . With current tech and the fastest rocket ship it would take thousands of years (close to 100,000 years) to reach just our nearest star system Alpha Centauri.

There’s also the radiation, extreme temps, and who knows what else.

ELI5: The universe should be teeming with spacefaring civilization, all reaching out. It isn’t. Why?

* Is intelligence rare? This idea is that Humans are unique and that the universe could be full of non-intelligent life.
* Could a species be wiped out or wipe themselves out? War, pandemic etc takes out a species even if they are advanced.
* Could there be a combination of factors or an unknown factor that prevents a species from ever leaving their home planet?

That’s the great filter in a nutshell.

Some people said the universe is so gigantic with so many places, there should be a lot of other intelligent civilizations.

Then some other people critiqued this by saying “ok, if there are so many places, where are all these proposed civilizations, why is nobody here to greet us?”

Then some other people again tried to reconcile this by proposing a “great filter”: Every civilization which has obtained technology to just go into space but hasn’t spread out into it yet, could use this technological might to destroy its home planet.

The great filter is now that maybe overcoming this obstacle – to spread into space before you destroy yourself into oblivion – is universally for all lifeforms quite insurmountable.

Thus you can have many places where apes develop, and as soon as they have nukes and/or climate change sets in, they extinct themselves, so that none of these every visited us as they don’t make it to flying a starship Enterprise.