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[Link] Study: no big filter, we're just too early
"Earth is one of the first habitable planets to form - and we're probably too early to the party to get a chance to meet future alien civilisations."
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"Earth is one of the first habitable planets to form - and we're probably too early to the party to get a chance to meet future alien civilisations."
Comments (44)
The article is unclear in its terms. At the top is says "92 percent of the Universe's habitable planets have yet to be born" and at the bottom it says "Earth is in the first 8 percent". Those two statements can only both be true if no habitable planets were formed between the formation of the earth and now (which is, of course, not the case). If the former is correct, earth might be significantly higher than top 8%.
I still don't see how this escapees the Fermi paradox though. Even if we're top 1%, that still means there must be great, great many potential alien civilizations out there. A factor 100 isn't going to significantly affect that conclusion.
The conclusions of this paper do not take into account empirical results indicating that most baryonic mass will probably be unable to form stars if trends that have held for the history of the universe so far continue to hold. I'm fairly convinced conclusions made on this basis do not resemble what will actually happen in our universe. I hereby link to a previous time I examined this study on this site and found it wanting:
http://lesswrong.com/lw/mpa/september_2015_media_thread/cpz6
http://lesswrong.com/lw/mpa/september_2015_media_thread/crhi
In short, they don't take into account that star formation often completely shuts down over time in galaxies despite there still being plenty of gas around, especially as they merge into large galaxies which is a continuing ongoing process, and that if you empirically look at star formation rates over time in the universe we are actually probably in the latter fractions of stars ever formed due to formation constantly declining at a precipitous rate. Their conclusion that we are early (8th percentile) in planet-formation order is based on the fact that something like 8% of baryonic mass in galaxies has become star systems [EDIT: Oops, more than 8% of gas, they have a metallicity cutoff that excludes gas that formed stars early in the history of most galaxies], not a projection of how much WILL eventually form stars, using uniformitarian assumptions rather than what I would consider more realistic models.
When you look at the empirical data, our position in planet-formation order seems likely unremarkable, and probably somewhere not far from the middle.
I don't see why you think they didn't take those factors into account; the article clearly says that:
In addition, your claim that "Their conclusion that we are early (8th percentile) in planet-formation order is based on the fact that something like 8% of baryonic mass in galaxies has become star systems" doesn't seem to be true; their conclusion instead seems to be more nuanced and based on taking into account metallicity and empirical rates of habitable planet formation: http://mnras.oxfordjournals.org/content/454/2/1811
What you say after the quote is correct and I will edit my parent comment accordingly, they do include metallicity cutoffs which decreases the contribution from star formation in the early universe, and most of the star formation of old giant ellipticals.
However, they do pretty much explicitly state that their conclusions are based on all potentially star-forming gas within the dark matter halos of galaxies eventually forming stars. This is not a good assumption since galaxy-quenching may be more or less permanent, and if you integrate fits of measured star formation rates over time in the universe into the future they converge towards total final numbers of stars that aren't THAT much larger than today. Frequent shutdown of star formation after galaxy mergers bring spirals and dwarf galaxies together into large galaxies, other less dramatic quenching events that apparently happen while being poorly understood, and the presence of many galaxies with large amounts of gas that nonetheless have failed to form stars for many gigayears (http://www.dailygalaxy.com/my_weblog/2014/02/giant-elliptical-galaxies-why-are-they-red-and-dead.html , http://mnras.oxfordjournals.org/content/439/3/2291.full.pdf), and empirical studies showing universal rates of star formation are declining very rapidly (I could point to a couple papers, but this one http://arxiv.org/abs/1006.5751 even though it isn't exactly ABOUT star formation rate has the prettiest graph I've seen in figure 1) are not taken into account. See second comment I link to.
<NOTE: you may notice that star formation rate is indicated as solar masses per cubic megaparsec per year in most studies and worry that the expansion of the universe throws that off; it is not because they are generally using what is called 'comoving volume' which is volume times the cube of the scale factor of the universe at the time. As a general rule, star formation rate per comoving volume is seen to be declining rapidly for more than five billion years when you look across studies.>
EDIT: I've gone back into the empirical fits of universal star formation rates found in two recent papers that actually have equations, and after doing some very bothersome math to convert between redshift and universe age, it would appear that the fits not only both converge to a finite number of stars going forward in time but also agree that in terms of TOTAL stars today the universe is between 85 and 95% of the way through the total complement that will ever exist.
The sun then shows up at the ~75th percentile of stars in star-order for both fits (as in 75% show up behind us at the end of time). I am unprepared to rigorously normalize this with metallicities on my own to deal with the fact that the huge early batches of stars 10+ gigayears ago were probably unsuitable for terrestrial planets without putting way more time than I am currently likely to have into the effort, unfortunately.
EDIT 2: I can do a little VERY naive normalization. Read at your own risk:
The study that gives the ~85 percent figure for current existing stars (Yuskel et al 2008) also gives the Sun's position as about the 85th percentile in currently existing stars and ~75th percentile in total stars ever. The linked study that started this whole conversation (Behroozi & Peeples 2015) gives the Earth's location as about the 50th percentile amongst currently existing terrestrial planets after their metallicity normalization. If we assume star formation rate since the Earth's formation is roughly fixed relative to terrestrial planet formation rate (heavy elements having polluted most places) then we get that the Earth formed after 0.5 / (1+0.5*0.15/0.1) = 29% of terrestrial planets.
Examining the paper (Sobral et al 2012) indicating 95% of eventual stars exist, and the sun is currently in the 82nd percentile, and eventual 75th percentile in the same way indicates that the Earth formed after 0.5/(1+0.5*0.05/0.13) = 42% of terrestrial planets. I trust the other number a bit better given it has a better estimation of star formation rates longer ago. I also caution that nobody really understands cutoffs for terrestrial planet formation, and that there could be other important factors, and these numbers only mean so much.
There's been some rounding here and there as I did all these calculations. May redo them later, in the hopes of making sure I didn't accidentally tune these numbers or propagate errors.
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.
Actual paper: http://hubblesite.org/pubinfo/pdf/2015/35/pdf.pdf
Note that "Earth is one of the first habitable planets to form" seems to be an overstatement. It is one of the first 8% to form. But nonetheless...
Even ignoring uncertainty in their estimate, being in the first 8% is hardly statistically unusual or 'early'. It's just what we'd expect from the mediocrity principle. Unusual would be say 10^-20, like some physical constants which are apparently under significant observational selection pressure and have extremely improbable values.
Knowing that we are not improbably early does perhaps suggest that some alien models are unlikely - for example it is unlikely that aliens are very common and very aggressive/expansive in our region of the multiverse, because if that was the case then observers like us would always tend to find ourselves on an unusually early planet. But we aren't.