We can't detect life at our stage of development from afar, or from earlier stages, but things that extremely advanced civilizations build like dyson spheres can't be hidden really.

The issue is that the universe is extremely old compared to how long it took for earth to develop life. It's ample time for civilizations to develop technologies extremely far more advanced than ours. So why isn't there a single civilization that's so advanced that it builds these things?

So there must be some "filter" that prevents civilizations from doing that, whether they are alive or dead. Maybe they just don't want to, but then ALL of them have to not want to do it, which would be weird.

If we detect life close to us, it makes it more likely that there is tons of life around the galaxy, which makes it more likely that the filter is ahead of us. If we detect no life close to us, it makes it more likely that the filter is past us.

See this Kurzgesagt video about the topic: https://www.youtube.com/watch?v=UjtOGPJ0URM

I don't like that theory, it's completely limited by our current understanding of how a "more advanced" civilization would look like. If we haven't seen signs of what we think more advanced civilizations would look like, the simplest explanation is that what we think advanced civilizations are like is wrong

I agree. Saying that a Dyson Sphere is something that an 'extremely advanced civilization would build' presumes that we can project the course of civilization. Seems presumptuous and short sighted to me.

Yeah a Dyson Sphere contains the presumption that an advanced alien civilization's population would grow exponentially like earths population did in the 20th century.

I don't know how you square a utterly stupid idea like exponential population growth with the word advanced in advanced civilization. Instead this sounds like blinkered mid twentieth century thinking. Or the old science fiction trope of seeing the future as like the present but more of it.

> The issue is that the universe is extremely old compared to how long it took for earth to develop life

I wouldn't call the universe very old. It's only like 13.x billion years old. If you take into account the amount of time it takes for heavier elements to be created, the time for the universe to settle down I'd say we are in the first cohort. It took us 4.5 billion years to evolve and if humanity got wiped out life on Earth would most likely continue on as it has for millions of years before with megafauna.

Looking at how life appeared immediately after the planet cooled down and had liquid oceans most likely indicates abiogenesis is common, multicellular may be much more rare and intelligence probably is exceedingly rare given how expensive big brains are. I just don't think there was enough time in the early universe for intelligent civilizations to have evolved.

My preferred solution to the great filter is to assume that things like Dyson spheres can't feasibly be built, at any level of technological development.

So there may be millions of civilizations that can get into orbit with chemical or nuclear rockets, but not a single one that can carry out a megaproject that could be seen from Earth.

I've always thought it made more sense that microbial life with took billions of years to develop, and originated someplace other than earth. The reason we don't see hyper-advanced civilizations is because intelligent life is only just starting to pop up alongside us, right now, and perhaps we're one of the earlier manifestations.

It's not my domain, but there are apparently some markers of genetic complexity that give some weight to this theory: https://arxiv.org/ftp/arxiv/papers/1304/1304.3381.pdf

Personally I think life originated on earth. We shouldn't look at contemporary prokaryotes to determine how complex prokaryotes were at the start of life. They had billions of years to develop as well and if the more complex ones are fitter than the less complex ones, they will win. E.g. you think rotting plants is a normal thing but there was a time when plants wouldn't and just form giant heaps, forming the coal reservoirs of today.

Anyways, let's assume life originated at some other planet and it has started life at half of the galaxy at the time it reached earth (so we weren't lucky to be "close" to the origin).

Earth is 4500 million years old, and life is assumed to have existed here since 4400 million years. So intelligent life took 4400 million years to "develop" here on earth. Light speed allows you to colonize the milky way comfortably within a million years. What if it takes 100 million years less on some other planet? Or it took just 10 million years less?

Either earth is very close to the real origin (or has been at the start of life), or the filter is in the past, or some other explanation.

This theory is actually proposed by Nick Lane et al - it says that microbial life is probably ubiqutuous and that complex life is not and was a happy accident on Earth, which means its very improbable on other planets. The prereqs for it happened only once on Earth, when one microbe swallowed another and the smaller one became mitochondria enabling complex cell structure and shape. Given that microbial life exist in extreme environments on Earth and that its prereq is just a hot molecular soup, its probable it exists everywhere. On the other hand, the 'happy incident' didn't repeat on Earth to create another complex cell form - all higher life uses the same cell structure.

That's what the great filter theory means when they talk about it being behind us -- we don't see life in other places because developing to where we are now is almost impossible.

I don't buy it though -- we're in an average planet on an average star, nothing about our environment seems to be particularly rare.

I think the statement that we are on an average planet deserves more scrutiny. We have yet to observe another planet like ours.

There are a number of Earth's attributes that we don't understand and have poor extra-solar data on. Particularly, the following:

Technically active plates after 4.5 Byrs. Effective magnetic core after 4.5 Byrs. Large amounts of surface water, presumably from our late heavy bombardment. Small enough to leave with a chemical rocket while meeting the above.

All those 'limitations' are too much anthropomorphic for my taste - life can probably exist in far more niches.

Also, at our current tech level, planets are very hard to even observe for existence, let alone their geographical features.

I tend to agree with both your points, but was addressing the specific claim that Earth-like conditions are average, or at least not particularly rare.

We simply don’t have data to support that claim, and the parameters required to make earth habitable for life (as we know it) are vastly more numerous than most people acknowledge.

Lets start with Oscam Razor then: its wast universe. Even on Earth, there are untouched places. All other theories compared to this one, which is easy to understand and also already known to be in effect on this planet, are as close to the truth as theory that great filter are evil pink elephants in the center of the galaxy.

> I've always thought it made more sense that microbial life with took billions of years to develop, and originated someplace other than earth. The reason we don't see hyper-advanced civilizations is because intelligent life is only just starting to pop up alongside us, right now, and perhaps we're one of the earlier manifestations.

This explanation does not work, because of the timescales.

Five thousand years ago, we built the Pyramids. Fifty years ago, we landed on the Moon.

In contrast, it took four billion years to go from single-celled life, to the pyramids.

The odds of us being first by happenstance are astonishingly small. It's much more likely that either we are unique, or that civilizations don't leave much of a trace outside of their solar systems, regardless of whether or not they die out in the blink of a cosmic eye.

We simply haven't developed enough yet. It wouldn't be good game design to encounter other civs at this stage.

Tutorial level civilization.

You must complete energy collection on current planet for XP to advance to level 2.

> If we detect life close to us, it makes it more likely that there is tons of life around the galaxy, which makes it more likely that the filter is ahead of us.

Isn't is the other way around? I though that the main argument for the filter ahead of us is that we haven't found any civilization, which means civilizations cannot develop past some level at which they can be detected by us. But if we'll have found one, this means that such level of civilization can be passed, and therefore reduces the chances that there is a filter.

I think by "life" the parent was talking about microbes on Mars or waterbears on comets or something. Technology developing life is a different thing entirely. If we find primitive life nearby, then there probably is primitive life everywhere, and very little technology developing life (because of the "filter").

Yeah, detecting life could mean many things:

- The filter is ahead us

- There is no filter

- The filter is behind us, but ahead of the other planet

- Both planets have successfully got past the filter

We'll still be debating :). Then we'll go crazy trying find evidence of life in more planets.

An addition to there is no filter (yet):

- We're one of the first nearby and we're going to help build a filter

The various AI we give birth to over the next two centuries are going to dominate several galaxies. There are many filters - competing standards - throughout the universe, our galaxy doesn't yet have one. Humans will cease to exist as we think of them now, within those two centuries (and not due to climate change or any other similar event).

that said, the likelihood of there being _no_ filter is still lower than the others.

if we detect life so near to us and _yet_ we still have yet to encounter life from another planet means there's likely still _some_ kind of filter. So I don't think the likelihood of no filter is high if we detect life

Rather then apocaliptyical filter, the most probable reason is that space is so wast that even if advanced life was common, chance of meeting/observing one would be next to 0. Think about our civ living on a sand grain in the desert - the chance is almost 0 to encounter another civ living on sand grain on random desert location no matter how advanced your tech is.

yikes - that's true, we could still move life beyond earth and still never meet another civ. I didn't realize this at first but I was conflating the two

just learned about the "great filter", very interesting. I think it does mean that our observation tech is not good previously - it also sounds like if we did start observing life, the fact that we have yet to be visited / colonized by that life form means only re-affirms the unlikelihood of interplanetary travel..

according to [wikipedia](https://en.wikipedia.org/wiki/Great_Filter):

> So by this argument, finding multicellular life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2–6 are easy, and hence only 1, 7, 8 or 9 (or some unknown step) could be the big problem.[4]

Imagine that, knowing there's intelligent life out there, but knowing we will never meaningfully interact with them because it's impossible to travel any useful distance or communicate in any real way. Even if we could travel vast distances, It seems likely if we discover any life out there it may be long dead by the time we get there.

that's really sad :(. 300 light years ... gonna take a while. maybe the filter is space travel - no civilization is able to amass enough energy to go those distances

We have an enormous, almost inexhaustible source of energy just 1 AU away.