The great filter is the sum total of all of the obstacles that stand in the way of a simple dead planet (or similar sized material) proceeding to give rise to a cosmologically visible civilization. As there are 280 stars in the observable universe, and 260 within a billion light years, a simple dead planet faces at least roughly 60 to 80 factors of two obstacles to birthing a visible civilization within 13 billion years. If there is panspermia, i.e., a spreading of life at some earlier stage, the other obstacles must be even larger by the panspermia life-spreading factor.
Yeah. Running with it a little... if it's not intelligent life that's difficult but intelligent land-based-life that's difficult, that definitely has implications for the projected quantities of spacefaring civilizations. We ourselves have dolphins and whales, both of which have large, evolved brains, but not necessarily intelligence much greater than that of a dog.
Actually this might add a new dimension to the debate: it may be that it typically takes aquatic lifeforms much longer to evolve into a spacefaring civilization (they can't invent fire early on in their development for example) and therefore there simply are no billion year old civilizations anywhere near the Milky Way galaxy. There then might be some that are a few million years older than us but haven't yet conquered local space.
The same rules still apply to a billion year old civilization: they need to have a physical presence very near to Earth for us to detect them. I know your assumption is that such civilizations would (successfully) send self-replicating probes to galaxies other than the one they evolved in, everything kind of stands or falls with that (and with the probability of intelligent life elsewhere evolving a long time before us).
Of course. That has been the case for many decades; we see new things, we puzzle over them, and eventually explain them satisfactorily in terms of dead stuff.
I'm not talking about visibility of a tech civ when it first appears. I'm talking about the visibility of its descendants after a billion years of continued evolution.
Visibility will most likely be limited to a few light years beyond their actual physical presence. There's now way in hell we'd be able to detect a civilization in even the nearest galaxy (actually even a civilization 1000 light years away in our own galaxy would most likely be invisible to us) unless they sent a physical object to our galaxy. And there are a lot of galaxies within a radius of 1 billion light years... I just cannot see how this would not bug you when thinking about the great filter.
My assumption is just that there is a substantial chance that a tech advanced civ would become visible. The smaller that chance, the larger that last filter step. There are many reasons that civs might do things besides what they might learn as a result.
Here's an arxiv link showing that gamma rays seem to make 90% of the galaxy uninhabitable. That's part of the fermi paradox right there and a massive, massive filter for complex life (bacteria would likely survive it). Perhaps it already happened once before on Earth in the past. http://arxiv.org/abs/1409.2506
Your implicit assumption in discussing this topic always seems to be that technologically advanced civilizations would be interested in being "cosmologically visible." Why? There's nothing to learn from traveling the stars that can't be learned from simulations. (This point holds even apart from your ancillary assumption that technologically advanced civilizations would have the wayfaring instincts of humans.) Nor is there any clear reason to think that the energy or resource demands involved in exploration via simulation would be visible at great distances.
The post just below makes half of this argument via implication.
You're very welcome. I very much enjoy your blog so I'm glad to be able to contribute.
Thanks for the link - I made it a center of my next post.
It will, nevertheless be extremely interesting to find out exactly what large solar system scale infra-red emitting objects could be.
Yeah. Running with it a little... if it's not intelligent life that's difficult but intelligent land-based-life that's difficult, that definitely has implications for the projected quantities of spacefaring civilizations. We ourselves have dolphins and whales, both of which have large, evolved brains, but not necessarily intelligence much greater than that of a dog.
Attack of the space octopi ;-)
Actually this might add a new dimension to the debate: it may be that it typically takes aquatic lifeforms much longer to evolve into a spacefaring civilization (they can't invent fire early on in their development for example) and therefore there simply are no billion year old civilizations anywhere near the Milky Way galaxy. There then might be some that are a few million years older than us but haven't yet conquered local space.
The same rules still apply to a billion year old civilization: they need to have a physical presence very near to Earth for us to detect them. I know your assumption is that such civilizations would (successfully) send self-replicating probes to galaxies other than the one they evolved in, everything kind of stands or falls with that (and with the probability of intelligent life elsewhere evolving a long time before us).
Of course. That has been the case for many decades; we see new things, we puzzle over them, and eventually explain them satisfactorily in terms of dead stuff.
Actually Penn State researchers are looking exactly for large artifacts in other galaxies and they have some candidates that need explaining away.
http://arxiv.org/pdf/1408.1...
Yeah. Good point.
I'm not talking about visibility of a tech civ when it first appears. I'm talking about the visibility of its descendants after a billion years of continued evolution.
Visibility will most likely be limited to a few light years beyond their actual physical presence. There's now way in hell we'd be able to detect a civilization in even the nearest galaxy (actually even a civilization 1000 light years away in our own galaxy would most likely be invisible to us) unless they sent a physical object to our galaxy. And there are a lot of galaxies within a radius of 1 billion light years... I just cannot see how this would not bug you when thinking about the great filter.
My assumption is just that there is a substantial chance that a tech advanced civ would become visible. The smaller that chance, the larger that last filter step. There are many reasons that civs might do things besides what they might learn as a result.
It could also mean most civilizations are aquatic...
Here's an arxiv link showing that gamma rays seem to make 90% of the galaxy uninhabitable. That's part of the fermi paradox right there and a massive, massive filter for complex life (bacteria would likely survive it). Perhaps it already happened once before on Earth in the past. http://arxiv.org/abs/1409.2506
Your implicit assumption in discussing this topic always seems to be that technologically advanced civilizations would be interested in being "cosmologically visible." Why? There's nothing to learn from traveling the stars that can't be learned from simulations. (This point holds even apart from your ancillary assumption that technologically advanced civilizations would have the wayfaring instincts of humans.) Nor is there any clear reason to think that the energy or resource demands involved in exploration via simulation would be visible at great distances.
The post just below makes half of this argument via implication.
I'm not arguing for or against goals here. I'm trying to draw inferences from our data of no visible civilizations.