Astronomers Detect a Possible Signature of Life on a Distant Planet

This is your daily reminder that if we do indeed discover life so close (~120 LY) to Earth, it's an incredibly bad sign for us. This is an exercise in Bayesian reasoning.

Imagine there are 2 planets in the Milky Way where life has developed. The odds are incredibly low they're next to each other, assuming a random distribution. So it's way more likely that there are more than 2. Imagine a sphere of radius 60 LY (120/2). Our Earth is the center of one. This planet is another. That's a volume of 10^6 LY^3. The Milky Way volume (from Google) is ~17T LY^3 so there'd be roughly 170M such spheres in our galaxy.

Now imagine if the odds of simple life becoming intelligent life that we could detect and could become spacefaring is 1 in 1 million. There'd be ~170 such civilization in the Milk Way.

We have absolutely no evidence of this So simple life is a lot less common, intelligent life is a lot less likely or, and this is the scary part, something tends to wipe out sentient civilizations and that's likely in our future.

In Fermi Paradox terms, we call this a Great Filter.

On a related note, Birth-Death Formalism connects Jaynes' Prior Probabilities in a paper, "Do SETI Optimists Have a Fine-Tuning Problem?"

https://arxiv.org/abs/2407.07097

One of the authors posted an approachable video explaining how the ideas connect:

https://www.youtube.com/watch?v=b6-9Hq8dV_4

There’s no evidence yet, but we also haven’t looked very carefully or in very many places. And we only know to look for planetary life as we know it.

The middle of the galaxy could have a wildly different composition and habitat for life than the outer reaches where we are.

Time is also another factor. Maybe the galaxy is perfect for complex life, we just happen to be early on the timescale and in another billion years or two life will be clearly abundant everywhere.

For some perspective, if you take the size of the Milky Way (100k light years) and relate it to the size of NYC (~30 miles), then 120 light years ends up being ~190 feet in NYC, or less than a city block.

If you assume life is very likely then the Great Filter has to be an almost perfect filter to explain the Fermi Paradox. With fewer civilisations in the galaxy (<50) you might be able to assume that if 95% of them destroy themselves it would be unlikely we'd ever find life. If we assume 170 civilisations then the great filter would have to be something akin to a natural law.

My gut says that the Great Filter is real but probably doesn't filter more than 99% of civilisations. Were I to guess complex life is uncommon, and intelligent life very uncommon. The majority of planets which do develop intelligent life destroy themselves fairly quickly.

There's only the fact that we probably just CANNOT perceive extremely advanced civilizations.

An insect crawling around a 75 year old brick house that is covered in ivy and moss will have NO idea that the object it is walking upon is NOT part of it's natural environment. That brick house seems as natural to the environment as the grass, and trees, and rocks, and streams nearby it -- to the bug at least.

Similarly we take our telescope out and see what looks like a natural organic universe with organic galaxies and normal looking stars etc...

Because we don't have solar system sized brains and billion year life spans we are absolutely hopeless to realize that theres' a lot of massive artificial structures in this universe. We're too bug-like to even be able to perceive them from our natural environment.

*we do know of massive cosmic structures like filaments, voids, and the great wall. So it is possible we as humans are starting to notice the "house" in the woods since our theories of physics cannot really explain why these structures exist at these massive scales (we would expect uniformity at those scales). See [here](https://en.wikipedia.org/wiki/List_of_largest_cosmic_structu...)

extremely advanced civilization would be living around black holes because they are perfect heat dumps (they get colder as you add matter!) and would enable fundamentally better technology.

The middle of the galaxy is unlikely to be conducive to forming intelligent life simply because it's not stable enough for long enough.

In our part of the galaxy, the mean distance between stars is around 4-5 LY (at a guess) in terms of nearest neighbours and ignoring binary (and up) systems. At the galactic center it's a few light days.

We've had many events that have caused mass extinction. There are many more that could end all life as ew know it (eg gamma ray bursts, a sufficiently close supernova. We have ~10 stars within 10 LY of us. Imagine if that were millions instead. I find it hard to believe conditions would be stable enough for the millions or billions of years necessary to create and sustain complex life.

As for our galaxy being "perfect" this touches on a coupole of concepts, notably the Anthropic Principle. But again we return to Bayesian reasoning. If there was going to be 1000 spacefaring civilizations in our galaxy, what are the odds that we are first? And while the Sun is <5B years old, there are stars up to 14B years old. There's been a ton of extra time for civilizations to develop elsewhere.

A lot of assumptions are made here about intelligent life with the great filter ideas. Intelligent life is not a fundamental phase of matter. It took a random walk over billions of years to end up there, with any point potentially leading to a deviation and entirely different outcome. Take the number of bacteria dividing over the last 4 billion years on earth to one and there’s your rate of intelligent life in the universe based on what we’ve seen on earth. Are there even that many planets or bodies in the galaxy for what a billion to the power of a billion to one odds or whatever absurd number this is?

This idea that can't perceive sufficiently advanced civilizations doesn't hold up to scrutiny. It comes down to energy and mass (ie resources). Both of them essentially necessitate a civilization becoming large. You can argue that not every civilization will be expansionist but those that aren't tend to get swallowed up by those that are, at least based on Earth's history.

So for energy, a likely path will be the Dyson Swarm, meaning a cloud of orbitals. Many mistakenly think a Dyson Sphere was a rigid shell around a star. It never was. There's no material, actual or even theorized, that has the rigidity to sustain that. Because of that confusion, many now prefer the nomenclature of "Dyson Swarm" over "Dyson Sphere".

Dyson Swarms have the advantage of creating incredible amounts of living room and solving energy needs with relatively low tech (ie solar). They can also be built incrementally. A cloud of orbitals that capture the Sun's energy with orbitals between Venus and Mars will (IIRC) have a mean distance between them of ~100,000km.

Why is this important? Because the only way to get rid of heat in space is by expelling mass or, more likely, radiating it away into space. You can reuse waste heat to some degree but it's not perfect (because thermodynamics) and you can't totally avoid radiating heat away totally anyway. The wavelength of such radiation is entirely dependent on the temperature. At any likely temperature, that means infrared ("IR") radiation.

So a Dyson Swarm around our Sun would stick out like a sore thumb with a massive IR signature. There's really no hiding it. And we're capable of detecting it.

Conversey, there's really no hiding from any civilization capable of such feats of engineering. Plus any such civilization would be capable of sterilizing the galaxy out of any competition.

Mass follows on from this.

  Now imagine if the odds of simple life becoming intelligent life that we could detect and could become spacefaring is 1 in 1 million.

Why not assume the odds are 1 in 200 million, so that there's only one such planet in the whole galaxy and there's no Great Filter to worry about? It seems just as valid, except "200 million" is slightly less aesthetically pleasing.

We already know that human-type technological civilization is extremely unlikely, since the planet was full of somewhat intelligent bipedal dinosaurs for about 100m years, yet none of them seemed to engage in mining, large-scale construction, severely disruptive extinctions, nuclear power... (humans will leave a thick geological layer of concrete and pollution, along with plentiful unique minerals coming from slag, plastic, etc.)

I really have no patience for this sort of p(doom) nonsense. If you make up numbers you can come to any quantitative conclusion you want. Mildly tweaking totally unfalsifiable odds makes the Fermi "paradox" go away entirely.

Anything that acts to lower the odds by 200x so that we're the only likely intelligence in the galaxy is the definition of a "great filter".

I don't understand why people fixate on Dyson spheres. Why would a Kardashev type 2 civilization rely on something that a type 0.7 civilization could come up with? Surely in the intervening 1.3 they could come up with something we can't. Dyson spheres are a type 0.7 imagination exercise. A type 2 civilization wouldn't just extend our technology - it would transcend it. Maybe they use dark energy, maybe they gather energy from simulations. We can't understand.

This argument is fairly dated now, we no longer need alien ruins to prove that humanity will probably destroy itself.

> it's an incredibly bad sign for us.

No reason to expect other lifeforms to be as bellicose and destructive us humans.

That's not right, the great filter comes after the development of intelligence

A Great Filter could come before or after intelligence.

For example, predation could be a Great Filter. We made it past the threat of predation, but perhaps most life forms in the universe don’t. In this example, life could be pervasive in the universe but it’s optimizing for defensive attributes like speed and armor rather than intelligence. That could explain why we don’t see aliens colonizing the galaxy. With the Great Filter behind us, nothing would be stopping us from colonizing the galaxy ourselves, we just might be among the first to do it.

Putting the Great Filter in front of us postulates that many life forms have achieved our same accomplishments but haven’t colonized the galaxy. It could be because of doom and gloom scenarios like war, aliens, cosmic catastrophes, etc. Or it could be a beneficial Great Filter like enlightenment and the lack of desire to propagate and consume endlessly, or the ability to survive in deep space, leaving stars and planets untouched.

That last bit is the part I always think about. You could easily hide an entire civilization in space, and there’s a lot more “space” than “planet”

There are billions of engines sitting around the universe; might as well just use those.

The more advanced versions tend to simply use more radiative heat. they might arrange stellar nurseries, modify stars to be more efficient, or use black holes for engines, but even then you’re still just feeding them stars anyways.