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AABoyles comments on [Link] Study: no big filter, we're just too early - Less Wrong Discussion
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This research doesn't imply the non-existence of a Great Filter (contra this post's title). If we take the Paper's own estimates, there will be approximately 10^20 terrestrial planets in the Universe's history. Given that they estimate the Earth preceded 92% of these, there currently exist approximately 10^19 terrestrial planets, any one of which might have evolved intelligent life. And yet, we remain unvisited and saturated in the Great Silence. Thus, there is almost certainly a Great Filter.
Surely "unvisited" is insignificant. There's no current science suggesting any means of faster-than-light travel. So, if you assume that extraterrestrial life would have lifespans grossly similar to terrestrial lifespans, we ought to remain unvisited.
"Saturated in the Great Silence" seems like a far more significant point.
Human beings spread all over the globe on foot 75000-15000 years ago, despite the fact that no single human probably walked all the way from Africa to Australia. It's a fairly trivial assumption that an expanding interstellar civilization would not be limited by the lifespan of its inhabitants.
The galaxy may be big, but it is very small compared to the time-scales involved here. At walking pace (~5 km/h), you could travel 61 light years during the time from when the milky way formed up to now. At speeds easily achievable using chemical propulsion (~15 km/s), you could travel the circumference of the milky way --- twice!
http://www.fhi.ox.ac.uk/intergalactic-spreading.pdf
You didn't actually do the math on that. According to this paper by the Future of Humanity Institute (Nick Bostrom's group), if life evolved to the point of interstellar travel 3 billion years ago and could travel at 50% of c, then you would expect it to travel not just to this galaxy, but the nearest million. If you go back five billion years and assume travel speeds of 99% of c, it could reach a billion galaxies. 75% of stars in the Milky Way that could support life are older than our Sun. It really is an enigma.
Are talking about civilization/life lifespan or individual organism lifespan?
Civilizations can send out long lived probes, and individual lifespans are somewhat irrelevant, especially for post-biological civilizations.
If life is as plentiful as it appears to be, then due to the enormous numbers we should expect to have been visited in our history unless there is alot of future filtering somewhere in num civs * avg civ 'active' lifespan * fraction of civs that explore.
FTL travel isn't necessary at all. The natural easy way to travel around the solar system is to use gravitational assists, which allows for travel at speeds on order of the orbital speeds. The sun orbits the galaxy at a respectable speed of around 251 km/s or 0.1c, and some stars such as Schol'z star travel in the opposite direction. So it should only take about a million years for even a slow expanding civ to expand out 1,000 lyrs. And very small scout probes could more easily travel at faster speeds.
Basically we should expect the galaxy to be at least fully visited, if not colonized, by at most one galactic year after the birth of the first elder space civ. Earth is only about 18 gyrs old, whereas the galaxy is 54 gyrs old.
Do you really think the probability that aliens have visited our system over it's history is less than say 10^-9?
The 10^19 or so planets that could have independently evolved civilizations generates an enormous overwhelming prior that we are not the first. It would take extremely strong evidence to overcome this prior. So from a Bayesian view, it is completely unreasonable to conclude that there is any sort of Filter - given the limits of our current observations.
We have no idea whether we have been visited or not. The evidence we have only filters out some very specific types of models for future civs - such as aliens which colonize most of the biological habitats near stars. The range of models is vast and many (such as cold dark models where advanced civs avoid stars) remain unfiltered by our current observations.
Taking the Bayesian view further, our posterior likelihood is the prior times the likelihood inferred from observations. You're right that the prior must consist of very strong belief in the existence of aliens. However, an expanding alien civilization would be a very large, obvious, and distinctive spectacle, and we have seen no evidence of that so far. Thus it is not clear what our posterior belief must be.
An expanding stellavore civ would be very obvious, and the posterior for that possibility is thus diminished.
However there are many other possibilities. An expanding cold dark civ would be less obvious, and in fact we could already be looking at it.
There also the transcendent models, where all expansion is inward and post singularity civs rather quickly exit the galaxy in some manner - perhaps through new universe creation. That appears to be possible as far as physics is concerned, and it allows for continued exponential growth rather than the unappealing cubic growth you can get from physical expansion. Physical expansion would be enormous stagnation from our current growth perspective.
After updating on our observations the standard stellavore model becomes low probability relative to other future civ models.
Why couldn't a civilization lead to both expanding and universe-exiting threads of evolution? Taking life on Earth as an analogy, it's clear that life expands to fill all niches it can. A particular thread of evolution won't stop occurring just because another thread has found a more optimal solution. In other words, it's not a depth-first search, it's a breadth-first search. Unless there's a good reason for a civilization to not expand into space, it will probably expand into space.
It would seem very strange, then, that no expanding interstellar civilization has occurred.
Sure, but we are uncertain about everything, including what the niches for postbiological civs are. Physics suggest that computation is ultimately primarily entropy/temperature limited (rather than energy limited), and thus the niches for advanced civs could be in the cold dark interstellar material (which we know is more plentiful than the hot bright stuff). We don't see stellavores for the same reasons that humanity isn't interested in colonizing deep sea thermal vents (or underwater habitats in general).
So the stars could be the past - the ancient history of life, not it's far future.
In the cold dark models, the galaxy is already colonized, and the evidence is perhaps already in front of us . ..
In this model the physical form of alien civs is likely to be in compact cold objects that are beyond current tech to image directly. The most likely chance to see them is during construction, which would be more energetically expensive and thus could take place near a star - perhaps the WTF star is a civ in transition to elder status.
The WOW signal was an alien radar ping, similar to what aliens would see from the radar pings that we use for planetary radar imaging with arecibo.
Aliens most likely have already visited sol at various points, but for them it is something like the ocean floor is to us - something of minor interest for scientific study.
On that note, it's starting to look like the emDrive and kin are real. If that is true, it is additional evidence for aliens. Why? Because the earliest and most credible modern UFO reports - such as the Kenneth Arnold sighting - are most consistent with craft that is vaguely areodynamic but does not rely on areodynamic principles for thrust. The arnold report contains rather specific details of the craft's speed and acceleration, lack of contrail, etc. As we know more about future engineering capabilities for atmospheric craft, that report could become rather strong evidence indeed (or not).
In the transcendent models, civs use all available resources to expand inward, because that allows for continued exponential growth. Transcendent civs don't expand outward because it is always an exceptionally poor use of resources. Notice that that is true today - we could launch an interstellar colony ship for some X trillions, but spending those resources on Moore'ls Law is vastly preferred. In the transcendent model, this just continues to be true indefinitely - likely ending in hard singularities, strange machines that create new universes, etc.
Finally, the distribution over various alien civs are not really statistically independent, even if they developed independently. Our uncertainty is at the model level in terms of how physics and future engineering works. The particular instance variables of each civ don't matter so much. So if the cold dark model is correct, all civs look like that, if the transcendent model is correct all civs look like that, etc.
The hypothesis that dark matter could be comprised of cold clumps of matter has been considered (these objects are called MACHOs) and as far as I know this hypothesis has been largely ruled out as they have properties that aren't consistent with how dark matter actually behaves.
I also think you're making an unfounded assumption here - that advanced civilizations could be stealthy. But what we know suggests that there ain't no stealth in space. There are a number of difficulties in keeping large energy-consuming objects cold, and even if you succeeded in keeping the brains themselves cold, the associated support equipment and fusion reactors that you mention would be pretty hot. And the process of constructing the brains would be very hot.
Unrelated. There is baryonic and non-baryonic dark matter. Most of the total dark matter is currently believed to be non-baryonic, but even leaving that aside the amount of baryonic dark matter is still significant - perhaps on par or greater than the baryonic visible matter. Most important of all is the light/dark ratio of heavier element baryonic matter and smaller planets/planetoids. There are some interesting new results suggesting most planets/planetoids are free floating rather than bound to stars (see links in my earlier article - "nomads of the galaxy" etc).
There is a limit to how big a giant computing device can get before gravitational heating makes the core unusable - the ideal archilect civ may be small, too small to detect directly. But perhaps they hitch rides orbiting larger objects.
Also, we don't know enough about non-baryonic dark matter/energy to rule it out as having uses or a relation to elder civs (although it seems unlikely, but still - there are a number of oddities concerning the whole dark energy inflation model).
Well we are talking about hypothetical post-singularity civs . . ..
There doesn't appear to be any intrinsic limit to computational energy efficiency with reversible computing, and practicality of advanced quantum computing appears to be proportional to how close one can get to absolute zero and how long one can maintain that for coherence.
So at the limits, computational civs approach CMB temperature and use negligible energy for computation. At some point it becomes worthwhile to spend some energy to move away from stars.
Any model makes some assumptions based on what aspects of engineering/physics we believe will still hold into the future. The article you linked makes rather huge assumptions - aliens civs need to travel around in ships, ships can only move by producing thrust, etc. Even then from what I understand detecting thrust is only possible at in-system distances, not light year distances.
The cold dark alien model i favor simply assumes advanced civs will approach physical limits.
The CMB temperature (2.7 K) is still very warm in relative terms and it's hard to see how effective large-scale quantum computing could be done at that temperature (current crude quantum computers operate at millikelvin temperatures and still have only very miniscule levels of coherence). The only way to get around this is to either use refrigeration to cool down the system (leading to a very hot fusion reactor and refrigeration equipment) or make do with 2.7 K, which would probably lead to a lot of heat dissipation.
You would absorb a large amount of entropy from the CMB at this temperature (about 1000 terabytes per second per square meter); you'd need to compensate for this entropy to keep your reversible computer working.
The CMB is just microwave radiation right? So reflective shielding can block most of that. What are the late engineering limits for microwave reflective coatings? With superconducting surfaces, metamaterials, etc?
Some current telescopes cool down subcomponents to very low temperatures without requiring large fusion reactors.
If the physical limits of passive shielding are non-generous, this just changes the ideal designs to use more active cooling than they otherwise would and limit the ratio of quantum computing stuff to other stuff - presumably there is always some need for active cooling and that is part of the energy budget, but that budget can still be very small and the final device temperature could even be less than CMB.
The Great Filter isn't an explanation of why life on Earth is unique; rather, it's an explanation of why we have no evidence of civilizations that have developed beyond Kardashev I. So, rather than focusing on the probability that some life has evolved somewhere else, consider the reason that we apparently don't have intelligent life everywhere. THAT's the Great Filter.
"They do exist, but we see no evidence" is an alternative theory to the Great Filter, and I believe what Jacob Cannell is using wrt the cold dark model.