I think that the great filter/fermi paradox is a combination of two facts,

1. Our entire radio output (the only example we have to go by) is pitiful compared to the sun, like a candle in front of a flood light, you’ll only be able to see it so far before it’s completely drowned out. After a few dozen light years our radio output is less than the margin of error of a stars detectable radio output.
2. As a civilization advances it must reduce radio leakage. As data gets more important, it gets more important that you’re not wasting energy moving it around. Narrow beamed radio transmission becomes the norm and even less radio signals escape the system than when radio was messy and overpowered.

They’re not missing or gone, they’ve just moved beyond messy radio signals. Even we tightened up our radio emissions in a little over a century. Most of what we watch or listen to comes to us via fiber, cable, or short range transmissions like cell phone towers and Wi-Fi.

An alternative is we are among the first. Third generation stars are the ones that have planets with enough heavy elements to allow for complex chemistry. Sol (our star) is thought to be among the first batch of third generation stars in our gallexy.

Light speed does seem to be the upper speed limit for the universe. Talking that into account we probably haven’t had a chance to see other early life as it would likely be spread pretty thin right now.

Honesty, I don’t think that there is a Great Filter. The Fermi Paradox strikes me as not very well-reasoned. A whole hell of a lot of things would have to go exactly right for civilizations to make contact, rather than it being the default assumption. There are lots of filters, not just one Great one.

But the closest to a Great Filter is that space is really, really. stupendously big. The chances of even detecting each other across such distances is vanishingly small, much less traversing them. Add in the difficulty of jumping the metabolic energy gap to become complex life, and that could reduce the density of civilizations down to a level that they’re just not close enough to each other in spacetime to admit even the possibility of contact. And we’re hanging our hat on some highly-speculative concepts like alien mega-structures harnessing whole solar systems to allow detection.

I think a lot of persnickety, smaller filters combine to make interstellar contact between civilizations against long odds. Perhaps the best we’ll get is spectral signatures from distant planets that are almost-conclusive proof of some sort of life.

We’re currently in it. Failing to create a clean, renewable, and scalable energy source powerful enough to run a society that is ever increasing in both population and technology without destroying their only inhabited planet has got to be the most common great filter.

Asteroids strikes, super volcanoes, solar CMEs, and other planetary or cosmetic phenomena that exactly line up in both severity and timing are too rare IMO.

Every society that attempts to progress from Type 1 to Type 2 has to deal with energy production. Most will fail and they will either regress/stagnate or destroy themselves. Very few will successfully solve the energy problem before it is too late.

I don’t think there is a single filter. My personal gut feeling however is that the jump to “specialised generalists” would be a major hurdle.

Early human civilizations are very prone to collapsing. A few bad years of rain, or an unexpected change of temperature would effectively destroy them. Making the jump from nomadic tribal to a civilisation capable of supporting the specialists needed for technology is apparently extremely fragile.

Earth also has an interesting curiosity. Our moon is extremely large, compared to earth. It also acts as a gyroscopic stabiliser. This keeps the earth from wobbling on its axis. Such a wobble would be devastating for a civilisation making the jump to technological. Even on earth, we are in a period of abnormal stability.

I suspect a good number of civilizations bottleneck at this jump. They might be capable of making the shift, but get knocked back down each time it starts to happen.

It may very well be republicans

My thought is the evolution of intelligent life itself. If you think about it, intelligence is contrary to most of the principles of evolution. You spend a shit ton of energy to think, and you don’t really get much back for that investment until you start building a civilization.

As far as we can tell, sufficient intelligence to build technological civilizations has only evolved once in the entire history of the Earth, and even then humans almost went extinct

Howabout a reasonably advanced civilization destroying itself and its homeworld after exploiting and then running out of petroleum?

Probably too optimistic and unhinged, but maybe a species advanced enough for interstellar travel, building mega structures etc. are advanced enough to ascend to a higher plane of existence or alternate dimensions or whatever. Maybe there’s some alternative to this reality that will be unlocked by advanced technology that made all advanced life prefer that, to here.

Everyone is talking about society or physiology stuff. That is just things that might get humans.

Stars going super-nova is the real great filter. Our sun is 4.6 billion years old. Life started 4 billion years ago. In 4 billion years, the sun goes supernova. We are halfway to the end of the earth.

Smaller stars last longer, but have smaller ranges that life can exist in - and planets tend to move in or out in their orbits. Bigger stars have giant habitable zones - but some large stars born when humans took their first steps are in their last decades of life. You couldn’t get from the pyramids to NASA in that time, never mind the 4 billion years it took to get to humans.

I would say it’s the size of the universe and the fact that it is still expanding at an accelerated rate.

If the speed of light is really the “top speed” of the universe, it is inadequate for interstellar travel. It is barely good enough for timely communication, and not really even that.

Life can be as likely as it wants to be, but it seems to me that we’re all quite divided, to the point of not being able to communicate at all with other potential intelligent species.

We have had Millions of years of (presumably) intelligent Dinosaurs on this planet, but only 200.000 years of mankind were enough to create Civilization IV, the best Strategy game and peak of life as we know it.

So clearly, Civilization™ is what sets us apart.

Jokes aside, the thing evolution on earth spend the most time on is getting from single celled life-forms to multicellular life (~2 billion years). If what earth life found difficult is difficult for all, multicellular collaboration is way harder than photosynthesis, which evolved roughly half a billion years after life formed.

For a technological civilization like ours, I think it’s just that Earth/humans are weird and we’re past the main ones (like going from single-cell to multi-cellular organisms).

Having to overcome the physical obstacles on other planets rules out the type of spacefaring technological civilizations like ours. No matter how intelligent a civilization on a water world is, it’s not starting fires, much less building rockets. Just getting out of the water would be their space program. Even a totally Earth-like planet that’s a bit bigger and has an intelligent species wouldn’t be able to get to space with chemical rockets.

And also, humans are weird. It could be as basic as “we have hands for building complex tools.” We have a seemingly insatiable need to compete and explore, even beyond all logic—maybe no other intelligent species wants to strap someone to a rocket and send them to space because it sucks up there. We’re violent: without WWII and the Cold War, do we even have a space program?

So many things had to come together to create an intelligent, tool-building species with hands that lives on a planet with the right balance of land and water. As far as we know, it never even happened on Earth before and even then, we had thousands of years of civilizations before anyone was dumb enough to strap themselves to a rocket just to see what would happen.

• space

• time

We’ve been producing noticeable radio waves for a matter of decades. We’ve been capable of detecting even super-powerful, super-deliberate, super-targeted broadcasts for even less time.

And on top of that, it doesn’t look as though our civilisation is going to exist for more than a handful more decades, in any detectable-from-light-years-away form.

The chances of that onionskin-thin slice of lightcone intersecting with that of any other civilisation out there seems ludicrously remote.

Honorable mention: we haven’t detected alien probes, because intelligent alien societies haven’t begun consuming the galaxy with exponential numbers of self-replicating robotic probes, because that’s just a really bad idea:

Simple workarounds exist to avoid the over-replication scenario. Radio transmitters, or other means of wireless communication, could be used by probes programmed not to replicate beyond a certain density (such as five probes per cubic parsec) or arbitrary limit (such as ten million within one century), analogous to the Hayflick limit in cell reproduction. One problem with this defence against uncontrolled replication is that it would only require a single probe to malfunction and begin unrestricted reproduction for the entire approach to fail – essentially a technological cancer – unless each probe also has the ability to detect such malfunction in its neighbours and implements a seek and destroy protocol (which in turn could lead to probe-on-probe space wars if faulty probes first managed to multiply to high numbers before they were found by sound ones, which could then well have programming to replicate to matching numbers so as to manage the infestation).