# Astronomy, space exploration and the Great Filter

Astronomical research has what may be an under-appreciated role in helping us understand and possibly avoiding the Great Filter. This post will examine how astronomy may be helpful for identifying potential future filters. The primary upshot is that we may have an advantage due to our somewhat late arrival: if we can observe what other civilizations have done wrong, we can get a leg up.

This post is not arguing that colonization is a route to remove some existential risks. There is no question that colonization will reduce the risk of many forms of Filters, but the vast majority of astronomical work has no substantial connection to colonization. Moreover, the case for colonization has been made strongly by many others already, such as Robert Zubrin's book "The Case for Mars" or [this essay by Nick Bostrom](http://www.nickbostrom.com/astronomical/waste.html). 

Note: those already familiar with the Great Filter and proposed explanations may wish to skip to the section "How can we substantially improve astronomy in the short to medium term?"

  

### What is the Great Filter?

There is a worrying lack of signs of intelligent life in the universe. The only intelligent life we have detected has been that on Earth. While planets are apparently numerous, there have been no signs of other life. There are three possible lines of evidence we would expect to see if civilizations were common in the universe: radio signals, direct contact, and large-scale constructions. The first two of these issues are well-known, but the most serious problem arises from the lack of large-scale constructions: as far as we can tell the universe look natural. The vast majority of matter and energy in the universe appears to be unused. The Great Filter is one possible explanation for this lack of life, namely that some phenomenon prevents intelligent life from passing into the interstellar, large-scale phase. Variants of the idea have been floating around for a long time; [the term was first coined by Robin Hanson in this essay](http://www.webcitation.org/5n7VYJBUd). There are two fundamental versions of the Filter: filtration which has occurred in our past, and Filtration which will occur in our future. For obvious reasons the second of the two is more of a concern. Moreover, as our technological level increases, the chance that we are getting to the last point of serious filtration gets higher since as one has a civilization spread out to multiple stars, filtration becomes more difficult.  

### **Evidence for the Great Filter and alternative explanations:**

At this point, over the last few years, the only major updates to the situation involving the Filter since Hanson's essay have been twofold:

First, we have confirmed that planets are very common, so a lack of Earth-size planets or planets in the habitable zone are not likely to be a major filter.

Second, we have found that planet formation occurred early in the universe. (For example see [this article](http://www.universetoday.com/118510/oldest-planetary-system-discovered-improving-the-chances-for-intelligent-life-everywhere/) about [this paper](http://arxiv.org/abs/1501.06227).) Early planet formation weakens the common explanation of the Fermi paradox that the argument that some species had to be the first intelligent species and we're simply lucky. Early planet formation along with the apparent speed at which life arose on Earth after the heavy bombardment ended, as well as the apparent speed with which complex life developed from simple life,  strongly refutes this explanation. The response has been made that early filtration may be so common that if life does not arise early on a planet's star's lifespan, then it will have no chance to reach civilization. However, if this were the case, we'd expect to have found ourselves orbiting a more long-lived star like a red dwarf. Red dwarfs are more common than sun-like stars and have much longer lifespans by multiple orders of magnitude. While attempts to understand the habitable zone of red dwarfs are still ongoing, [current consensus is that many red dwarfs contain habitable planets](http://www.centauri-dreams.org/?p=22313). 

These two observations, together with further evidence that the universe looks natural  makes future filtration seem likely. If advanced civilizations existed, we would expect them to make use of the large amounts of matter and energy available. We see no signs of such use.  We've seen no indication of ring-worlds, Dyson spheres, or other megascale engineering projects. While such [searches have so far been confined to around 300 parsecs and some candidates were hard to rule out](http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm), if a substantial fraction of stars in a galaxy have Dyson spheres or swarms we would notice the unusually high infrared spectrum. Note that this sort of evidence is distinct from arguments about contact or about detecting radio signals. There's a very recent proposal for [mini-Dyson spheres around white dwarfs](http://arxiv.org/abs/1503.04376)  which would be much easier to engineer and harder to detect, but they would not reduce the desirability of other large-scale structures, and they would likely be detectable if there were a large number of them present in a small region. One recent study looked for signs of large-scale modification to the radiation profile of galaxies in a way that should show presence of large scale civilizations. They looked at [100,000 galaxies and found no major sign of technologically advanced civilizations](http://phys.org/news/2015-04-advanced-civilizations-earth-obvious-galaxies.html) (for [more detail see here](http://arxiv.org/abs/1504.03418)). 

We will not discuss all possible rebuttals to case for a Great Filter but will note some of the more interesting ones:

There have been attempts to argue that the universe only became habitable more recently. There are two primary avenues for this argument. First, there is the point that  early stars had very low metallicity (that is had low concentrations of elements other than hydrogen and helium) and thus the universe would have had too low a metal level for complex life. The presence of old rocky planets makes this argument less viable, and this only works for the first few billion years of history. Second, there's an argument that until recently [galaxies were more likely to have frequent gamma bursts](http://science.slashdot.org/story/15/01/27/2250253/gamma-ray-bursts-may-explain-fermis-paradox). In that case, life would have been wiped out too frequently to evolve in a complex fashion. However, even the strongest version of this argument still leaves billions of years of time unexplained. 

There have been attempts to argue that space travel may be very difficult. For example, [Geoffrey Landis proposed that a percolation model](http://www.geoffreylandis.com/percolation.htp), together with the idea that interstellar travel is very difficult, may explain the apparent rarity of large-scale civilizations. However, at this point, there's no strong reason to think that interstellar travel is so difficult as to limit colonization to that extent. Moreover, discoveries made in the last 20 years that brown dwarfs are very common  and that most stars do contain planets is evidence in the opposite direction: these brown dwarfs as well as common planets would make travel easier because there are more potential refueling and resupply locations even if they are not used for full colonization.  Others have argued that even without such considerations, [colonization should not be that difficult](http://www.fhi.ox.ac.uk/will-we-eventually-be-able-to-colonize-other-stars-notes-from-a-preliminary-review/). Moreover, if colonization is difficult and civilizations end up restricted to small numbers of nearby stars, then it becomes more, not less, likely that civilizations will attempt the large-scale engineering projects that we would notice. 

Another possibility is that we are underestimating the general growth rate of the resources used by civilizations, and so while extrapolating now makes it plausible that large-scale projects and endeavors will occur, it becomes substantially more difficult to engage in very energy intensive projects like colonization. Rather than a continual, exponential or close to exponential growth rate, we may expect [long periods of slow growth or stagnation](http://www.gwern.net/the-long-stagnation). This cannot be ruled out, but even if growth continues at only slightly higher than linear rate, the energy expenditures available in a few thousand years will still be very large. 

Another possibility that has been proposed are variants of the [simulation hypothesis](http://en.wikipedia.org/wiki/Simulation_hypothesis)— the idea that we exist in a simulated reality. The most common variant of this in a Great Filter context suggests that we are in an ancestor simulation, that is a simulation by the future descendants of humanity of what early humans would have been like.

The simulation hypothesis runs into serious problems, both in general and as an explanation of the Great Filter in particular. First, if our understanding of the laws of physics is approximately correct, then there are strong restrictions on what computations can be done with a given amount of resources. For example, [BQP](http://en.wikipedia.org/wiki/BQP), the set of problems which can be solved efficiently by quantum computers is contained in [PSPACE](http://en.wikipedia.org/wiki/PSPACE),  the set of problems which can solved when one has a polynomial amount of space available and no time limit.  Thus, in order to do a detailed simulation, the level of resources needed would likely be large since one would even if one made a close to classical simulation still need about as many resources. There are other results, such as [Holevo's theorem](http://en.wikipedia.org/wiki/Holevo%27s_theorem), which place other similar restrictions.  The upshot of these results is that one cannot make a detailed simulation of an object without using at least much resources as the object itself. There may be potential ways of getting around this: for example, consider a simulator  interested primarily in what life on Earth is doing. The simulation would not need to do a detailed simulation of the inside of planet Earth and other large bodies in the solar system. However, even then, the resources involved would be very large. 

The primary problem with the simulation hypothesis as an explanation is that it requires the future of humanity to have actually already passed through the Great Filter and to have found their own success sufficiently unlikely that they've devoted large amounts of resources to actually finding out how they managed to survive. Moreover, there are strong limits on how accurately one can reconstruct any given quantum state which means an ancestry simulation will be at best a rough approximation. In this context, while there are interesting anthropic considerations here, it is more likely that the simulation hypothesis  is wishful thinking.

Variants of the "[Prime Directive](https://en.wikipedia.org/wiki/Prime_Directive)" have also been proposed. The essential idea is that advanced civilizations would deliberately avoid interacting with less advanced civilizations. This hypothesis runs into two serious problems: first, it does not explain the apparent naturalness, only the lack of direct contact by alien life. Second, it assumes a solution to a massive coordination problem between multiple species with potentially radically different ethical systems. In a similar vein, Hanson in his original essay on the Great Filter raised the possibility of a single very early species with some form of faster than light travel and a commitment to keeping the universe close to natural looking. Since all proposed forms of faster than light travel are highly speculative and would involve causality violations this hypothesis cannot be assigned a substantial probability. 

People have also suggested that civilizations move outside galaxies to the cold of space where they can do efficient reversible computing using cold dark matter. [Jacob Cannell has been one of the most vocal proponents of this idea](/r/discussion/lw/m2x/resolving_the_fermi_paradox_new_directions/). This hypothesis suffers from at least three problems. First, it fails to explain why those entities have not used the conventional matter to any substantial extent in addition to the cold dark matter. Second, this hypothesis would either require dark matter composed of cold conventional matter (which at this point seems to be only a small fraction of all dark matter), or would require dark matter which interacts with itself using some force other than gravity. While there is some evidence for such interaction, it is at this point, slim. Third, even if some species had taken over a large fraction of dark matter to use for their own computations, one would then expect later species to use the conventional matter since they would not have the option of using the now monopolized dark matter. 

Other exotic non-Filter explanations have been proposed but they suffer from similar or even more severe flaws.

It is possible that future information will change this situation.  One of the more plausible explanations of the Great Filter is that there is no single Great Filter in the past but rather a large number of small filters which come together to drastically filter out civilizations. However, the evidence for such a viewpoint at this point is slim but there is some possibility that astronomy can help answer this question.

For example, one commonly cited aspect of past filtration is the origin of life. There are at least three locations, other than Earth, where life could have formed: Europa, Titan and Mars. Finding life on one, or all of them, would be a strong indication that the origin of life is not the filter. Similarly, while it is highly unlikely that Mars has multicellular life, finding such life would indicate that the development of multicellular life is not the filter. However, none of them are as hospitable to the extent of Earth, so determining whether there is life will require substantial use of probes. We might also look for signs of life in the atmospheres of extrasolar planets, which would require substantially more advanced telescopes. 

Another possible early filter is that planets like Earth frequently get locked into a "snowball" state which planets have difficulty exiting. This is an unlikely filter since Earth has likely been in near-snowball conditions multiple times— once very early on [during the Huronian](http://en.wikipedia.org/wiki/Huronian_glaciation) and later, about [650 million years ago](http://en.wikipedia.org/wiki/Snowball_Earth). This is an example of an early partial Filter where astronomical observation may be of assistance in finding evidence of the filter. The snowball Earth filter does have one strong virtue: if many planets never escape a snowball situation, then this explains in part why we are not around a red dwarf: planets do not escape their snowball state unless their home star is somewhat variable, and red dwarfs are too stable. 

It should be clear that none of these explanations are satisfactory and thus we must take seriously the possibility of future Filtration. 

How can we substantially improve astronomy in the short to medium term?
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Before we examine the potentials for further astronomical research to understand a future filter we should note that there are many avenues in which we can improve our astronomical instruments. The most basic way is to simply make better conventional optical, near-optical telescopes, and radio telescopes. That work is ongoing. Examples include the [European Extreme Large Telescope](http://en.wikipedia.org/wiki/European_Extremely_Large_Telescope) and the [Thirty Meter Telescope](http://en.wikipedia.org/wiki/Thirty_Meter_Telescope). Unfortunately, increasing the size of ground based telescopes, especially size of the aperture, is running into substantial engineering challenges. However,  in the last 30 years the advent of [adaptive optics](http://en.wikipedia.org/wiki/Adaptive_optics), [speckle imaging](http://en.wikipedia.org/wiki/Speckle_imaging), and other techniques have substantially increased the resolution of ground based optical telescopes and near-optical telescopes. At the same time, improved data processing and related methods have improved radio telescopes. Already, optical and near-optical telescopes have advanced to the point where we can gain information about the [atmospheres of extrasolar planets](http://en.wikipedia.org/wiki/Extraterrestrial_atmospheres#Extrasolar_planets) although we cannot yet detect information about the atmospheres of rocky planets. 

Increasingly, the highest resolution is from space-based telescopes. Space-based telescopes also allow one to gather information from types of radiation which are blocked by the Earth's atmosphere or magnetosphere. Two important examples are x-ray telescopes and gamma ray telescopes. Space-based telescopes also avoid many of the issues created by the atmosphere for optical telescopes. Hubble is the most striking example but from a standpoint of observatories relevant to the Great Filter, the most relevant space telescope (and most relevant instrument in general for all Great Filter related astronomy), is the planet detecting [Kepler spacecraft](http://en.wikipedia.org/wiki/Kepler_(spacecraft)) which is responsible for most of the identified planets. 

Another type of instrument are [neutrino detectors](http://en.wikipedia.org/wiki/Neutrino_detector). Neutrino detectors are generally very large bodies of a transparent material (generally water) kept deep underground so that there are minimal amounts of light and cosmic rays hitting the the device. Neutrinos are then detected when they hit a particle  which results in a flash of light. In the last few years, improvements in optics, increasing the scale of the detectors, and the development of detectors like [IceCube](http://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory), which use naturally occurring sources of water, have drastically increased the sensitivity of neutrino detectors.  

There are proposals for larger-scale, more innovative telescope designs but they are all highly speculative. For example, in the ground based optical front, there's been a suggestion to [make liquid mirror telescopes with ferrofluid mirror](http://arxivblog.com/?p=525)s which would give the advantages of liquid mirror telescopes, while being able to apply adaptive optics which can normally only be applied to solid mirror telescopes.  An example of potential space-based telescopes is the [Aragoscope](http://www.colorado.edu/news/releases/2015/01/23/new-space-telescope-concept-could-image-objects-far-higher-resolution) which would take advantage of diffraction to make a space-based optical telescope with a resolution at least an order of magnitude greater than Hubble. Other examples include placing telescopes very far apart in the solar system to create effectively very high aperture telescopes. The most ambitious and speculative of such proposals involve such advanced and large-scale projects that one might as well presume that they will only happen if we have already passed through the Great Filter.

What are the major identified future potential contributions to the filter and what can astronomy tell us? 
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Natural threats: 

One threat type where more astronomical observations can help are natural threats, such as asteroid collisions, supernovas, gamma ray bursts, rogue high gravity bodies, and as yet unidentified astronomical threats. Careful mapping of asteroids and comets is ongoing and requires more  continued funding rather than any intrinsic improvements in technology. Right now, most of our mapping looks at objects at or near the plane of the ecliptic and so some focus off the plane may be helpful. Unfortunately, there is very little money to actually deal with such problems if they arise. It might be possible to have a few wealthy individuals agree to set up accounts in escrow which would be used if an asteroid or similar threat arose. 

Supernovas are unlikely to be a serious threat at this time. There are some stars which are close to our solar system and are large enough that they will go supernova. Betelgeuse is the most famous of these with a projected supernova likely to occur in the next 100,000 years. However, at its current distance, Betelgeuse is unlikely to pose much of a problem unless our models of supernovas are very far off. Further conventional observations of supernovas need to occur in order to understand this further, and better  neutrino observations will also help  but right now, supernovas do not seem to be a large risk. Gamma ray bursts are in a situation similar to supernovas. Note also that if an imminent gamma ray burst or supernova is likely to occur, there's very little we can at present do about it. In general, back of the envelope calculations establish that supernovas are highly unlikely to be a substantial part of the Great Filter. 

Rogue planets, brown dwarfs or other small high gravity bodies such as wandering black holes can be detected and further improvements will allow faster detection. However, the scale of havoc created by such events is such that it is not at all clear that detection will help. The entire planetary nuclear arsenal would not even begin to move their orbits a substantial extent. 

Note also it is unlikely that natural events are a large fraction of the Great Filter. Unlike most of the other threat types, this is a threat type where radio astronomy and neutrino information may be more likely to identify problems. 

Biological threats: 

Biological threats take two primary forms: pandemics and deliberately engineered diseases. The first is more likely than one might naively expect as a serious contribution to the filter, since modern transport allows infected individuals to move quickly and come into contact with a large number of people. For example, [trucking has been a major cause of the spread of HIV in Africa](http://www.cam.ac.uk/research/news/research-recommends-how-to-tackle-spread-of-hivaids-by-african-truckers) and it is likely that the recent Ebola epidemic had similar contributing factors. Moreover, keeping chickens and other animals in very large quanities in dense areas near human populations makes it easier for novel variants of viruses to jump species. Astronomy does not seem to provide any relevant assistance here; the only plausible way of getting such information would be to see other species that were destroyed by disease. Even with resolutions and improvements in telescopes by many orders of magnitude this is not doable.  

Nuclear exchange:

For reasons similar to those in the biological threats category, astronomy is unlikely to help us detect if nuclear war is a substantial part of the Filter. It is possible that more advanced telescopes could detect an extremely large nuclear detonation if it occurred in a very nearby star system. Next generation telescopes may be able to detect a nearby planet's [advanced civilization purely based on the light they give off](http://www.space.com/13514-alien-city-artificial-lights-extraterrestrial-planets.html) and a sufficiently large  detonation would be of the same light level. However, such devices would be multiple orders of magnitude larger than the largest current nuclear devices. Moreover, if a telescope was not looking at exactly the right moment, it would not see anything at all, and the probability that another civilization wipes itself out at just the same instant that we are looking is vanishingly small. 

Unexpected physics: 

This category is one of the most difficult to discuss because it so open. The most common examples people point to involve high-energy physics. Aside from theoretical considerations, cosmic rays of very high energy levels are continually hitting the upper atmosphere. These particles frequently are multiple orders of magnitude higher energy than the particles in our accelerators. Thus high-energy events seem to be unlikely to be a cause of any serious filtration unless/until humans develop particle accelerators whose energy level is orders of magnitude higher than that produced by most cosmic rays.  Cosmic rays with energy levels  beyond what is known as the [GZK energy limit are rare](http://en.wikipedia.org/wiki/Greisen%E2%80%93Zatsepin%E2%80%93Kuzmin_limit).  We have observed occasional particles with energy levels beyond the GZK limit, but they are rare enough that we cannot rule out a risk from many collisions involving such high energy particles in a small region. Since our best accelerators are nowhere near the GZK limit, this is not an immediate problem.

There is an argument that we should if anything worry about unexpected physics, it is on the very low energy end. In particular, humans have managed to make objects substantially colder than the background temperature of 4 K with temperature as on the order of 10^-9^ K. There's an argument that because of the lack of prior examples of this, the chance that something can go badly wrong should be higher than one might estimate (See [here](http://aleph.se/andart2/risk/existential-risk-risk/cool-risks-outside-the-envelope-of-nature/).) While this particular class of scenario seems unlikely, it does illustrate that it may not be obvious which situations could cause unexpected, novel physics to come into play. Moreover, while the flashy, expensive particle accelerators get attention, they may not be a serious source of danger compared to other physics experiments.  

Three of the more plausible catastrophic unexpected physics dealing with high energy events are, [false vacuum collapse](http://en.wikipedia.org/wiki/False_vacuum#Vacuum_metastability_event), black hole formation, and the [formation of strange matter which is more stable than regular matter](http://en.wikipedia.org/wiki/Strangelet#Dangers).  

False vacuum collapse would occur if our universe is not in its true lowest energy state and an event occurs which causes it to transition to the true lowest state (or just a lower state). Such an event would be almost certainly fatal for all life. False vacuum collapses cannot be avoided by astronomical observations since once initiated they would expand at the speed of light. Note that the indiscriminately destructive nature of false vacuum collapses make them an unlikely filter.  If false vacuum collapses were easy we would not expect to see almost any life this late in the universe's lifespan since there would be a large number of prior opportunities for false vacuum collapse. Essentially, we would not expect to find ourselves this late in a universe's history if this universe could easily engage in a false vacuum collapse. While false vacuum collapses and similar problems raise issues of observer selection effects, [careful work has been done to estimate their probability](http://arxiv.org/abs/astro-ph/0512204). 

People have mentioned the idea of an event similar to a false vacuum collapse but which occurs at a speed slower than the speed of light. Greg Egan used it is a major premise in his novel, "Schild's Ladder." I'm not aware of any reason to believe such events are at all plausible. The primary motivation seems to be for the interesting literary scenarios which arise rather than for any scientific considerations. If such a situation can occur, then it is possible that we could detect it using astronomical methods. In particular, if the wave-front of the event is fast enough that it will impact the nearest star or nearby stars around it, then we might notice odd behavior by the star or group of stars. We can be confident that no such event has a speed much beyond a few hundredths of the speed of light or we would already notice galaxies behaving abnormally. There is a very narrow range where such expansions could be quick enough to devastate the planet they arise on but take too long to get to their parent star in a reasonable amount of time. For example, the distance from the Earth to the Sun is on the order of 10,000 times the diameter of the Earth, so any event which would expand to destroy the Earth would reach the Sun in about 10,000 times as long. Thus in order to have a time period which would destroy one's home planet but not reach the parent star it would need to be extremely slow.

The creation of artificial black holes are unlikely to be a substantial part of the filter— we expect that small black holes will quickly pop out of existence due to [Hawking radiation](http://en.wikipedia.org/wiki/Hawking_radiation).  Even if the black hole does form, it is likely to fall quickly to the center of the planet and eat matter very slowly and over a time-line which does not make it constitute a serious threat.  However, it is possible that black holes would not evaporate; the fact that we have not detected the evaporation of any primordial black holes is weak evidence that the behavior of small black holes is not well-understood. It is also possible that such a hole would eat much faster than we expect but this doesn't seem likely. If this is a major part of the filter, then better telescopes should be able to detect it by finding very dark objects with the approximate mass and orbit of habitable planets. We also may be able to detect such black holes via other observations such as from their gamma or radio signatures.  

The conversion of regular matter into strange matter, unlike a false vacuum collapse or similar event, might  be naturally limited to the planet where the conversion started. In that case, the only hope for observation would be to notice planets formed of strange matter and notice changes in the behavior of their light. Without actual samples of strange matter, this may be very difficult to do unless we just take notice of planets looking abnormal as similar evidence. Without substantially better telescopes and a good idea of what the range is for normal rocky planets, this would be tough.  On the other hand, neutron stars which have been converted into strange matter may be more easily detectable. 

Global warming and related damage to biosphere: 

Astronomy is unlikely to help here. It is possible that climates are more sensitive than we realize and that comparatively small changes can result in Venus-like situations.  This seems unlikely given the general variation level in human history and the fact that current geological models strongly suggest that any substantial problem would eventually correct itself. But if we saw many planets that looked Venus-like in the middle of their habitable zones, this would be a reason to be worried. Note that this would require detailed ability to analyze atmospheres on planets well beyond current capability. Even if it is possible Venus-ify a planet, it is not clear that the Venusification would last long. Thus there may be very few planets in this state at any given time.  Since stars become brighter as they age, so high greenhouse gas levels have more of an impact on climate when the parent star is old.  If civilizations are more likely to arise in a late point of their home star's lifespan, global warming becomes a more plausible filter, but even given given such considerations, global warming does not seem to be sufficient as a filter. It is also possible that global warming by itself is not the Great Filter but rather general disruption of the biosphere including possibly for some species global warming, reduction in species diversity, and other problems. [There is some evidence that human behavior is collectively causing enough damage to leave an unstable biosphere](http://www.sciencemag.org/content/347/6223/1259855). 

A change in planetary overall temperature of 10^o^ C would likely be enough to collapse civilization without leaving any signal observable to a telescope. Similarly, substantial disruption to a biosphere may be very unlikely to be detected. 

Artificial intelligence

AI is a complicated existential risk from the standpoint of the Great Filter. AI is not likely to be the Great Filter if one considers simply the Fermi paradox. The essential problem has been brought up independently by a few people. (See for example [Katja Grace's remark here](https://meteuphoric.wordpress.com/2010/11/05/light-cone-eating-ai-explosions-are-not-filters/) and my blog [here](http://religionsetspolitics.blogspot.com/2011/07/fermi-paradox-great-filter-and.html).) The central issue is that if an AI takes over it is likely to attempt to control all resources in its future light-cone. However, if the AI spreads out at a substantial fraction of the speed of light, then we would notice the result. The argument has been made that we would not see such an AI if it expanded its radius of control at very close to the speed of light but this requires expansion at 99% of the speed of light or greater. It is highly questionable that velocities more than 99% of the speed of light are practically possible due to collisions with the interstellar medium and the need to slow down if one is going to use the resources in a given star system. Another objection is that AI may expand at a large fraction of light speed but do so stealthily. It is not likely that all AIs would favor stealth over speed. Moreover, this would lead to the situation of what one would expect when multiple slowly expanding, stealth AIs run into each other. It is likely that such events would have results would catastrophic enough that they would be visible even with comparatively primitive telescopes.

While these astronomical considerations make AI unlikely to be the Great Filter, it is important to note that if the Great Filter is largely in our past then these considerations do not apply. Thus, any discovery which pushes more of the filter into the past makes AI a larger fraction of total expected existential risks since the absence of observable AI becomes  much weaker evidence against strong AI if there are no major civilizations out there to hatch such explosions. 

Note also that AI as a risk cannot be discounted if one assigns a high probability to existential risk based on non-Fermi concerns, such as the [Doomsday Argument](https://en.wikipedia.org/wiki/Doomsday_argument). 

Resource depletion:

Astronomy is unlikely to provide direct help here for reasons similar to the problems with nuclear exchange, biological problems, and global warming.  This connects to the problem of civilization bootstrapping: to get to our current technology level, we used a large number of non-renewable resources, especially energy sources. On the other hand, large amounts of difficult-to-mine and refine resources (especially aluminum and titanium) will be much more accessible to future civilization. While there remains a large amount of accessible fossil fuels, the technology required to obtain deeper sources is substantially more advanced than the relatively easy to access oil and coal. Moreover, the [energy return rate](http://en.wikipedia.org/wiki/Energy_returned_on_energy_invested), how much energy one needs to put in to get the same amount of energy out, is lower.  Nick Bostrom has raised the possibility that [the depletion of easy-to-access resources may contribute to making civilization-collapsing problems that, while not  full-scale existential risks by themselves, prevent the civilizations from recovering.](http://www.nickbostrom.com/existential/risks.html) Others have begun to investigate the problem of rebuilding without fossil fuels, such as [here](http://aeon.co/magazine/technology/could-we-reboot-civilisation-without-fossil-fuels/).

Resource depletion is unlikely to be the Great Filter, because small changes to human behavior in the 1970s would have drastically reduced the current resource problems. Resource depletion may contribute to existential threat to humans if it leads to societal collapse, global nuclear exchange, or motivate riskier experimentation.  Resource depletion may also combine with other risks such as a global warming where the combined problems may be much greater than either at an individual level. However there is a risk that large scale use of resources to engage in astronomy research will directly contribute to the resource depletion problem. 

Nanotechnology: 

Nanotechnology disasters are one of the situations where astronomical considerations could plausibly be useful. In particular, planets which are in the habitable zone, but have highly artificial and inhospitable atmospheres and surfaces, could plausibly be visible. For example, if a planet's surface were transformed into diamond, telescopes not much more advanced beyond our current telescopes could detect that surface. It should also be noted that at this point, many nanotechnologists consider the classic "grey goo" scenario to be highly unlikely. See, for example, [Chris Phoenix's comment here](http://www.reddit.com/r/Futurology/comments/2z1mj4/hi_im_nanotechnologist_chris_phoenix_ama/cpewm4g). However, catastrophic replicator events that cause enough damage to the biosphere without grey-gooing everything are a possibility and it is unclear if we would detect such events. 

  

Aliens:

  

Hostile aliens are a common explanation of the Great Filter when people first find out about it. However, this idea comes more from science fiction than any plausible argument. In particular, if a single hostile alien civilization were wiping out or drastically curtailing other civilizations, then one would still expect the civilization to make use of available resources after a long enough time. One could do things like positing such aliens who also have a religious or ideological ideal of leaving the universe looking natural but this is an unlikely speculative hypothesis that also requires them to dominate a massive region, not just a handful of galaxies but many galaxies. 

  

Note also that astronomical observations might be able to detect the results of extremely powerful weapons but any conclusions would be highly speculative. Moreover, it is not clear that knowing about such a threat would allow us at all to substantially mitigate the threat. 

  

Other/Unkown: 

  

Unknown risks are by nature very difficult to estimate. However, there is an argument that we should expect that the Great Filter is an unknown risk, and is something so unexpected that no civilization gets sufficient warning.  This is one of the easiest ways for the filter to be truly difficult to prevent. In that context, any information we can possibly get about other civilizations and what happened to them would be a major leg-up.

Conclusions 
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Astronomical observations have potential to give us data about the Great Filter, but many potential filters will leave no observable astronomical evidence unless one's astronomical ability is so high that one has likely already passed all major filters. Therefore, one potential strategy to pass the Great Filter is to drastically increase the skill of our astronomy capability to the point where it would be highly unlikely that a pre-Filter civilization would have access to those observations.  Together with our comparatively late arrival, this might allow us to actually detect failed civilizations that did not survive the Great Filter and see what they did wrong.

  

Unfortunately, it is not clear how cost-effective this sort of increase in astronomy would be compared to other existential risk mitigating uses. It may be more useful to focus on moving resources in astronomy into those areas most relevant to understanding the Great Filter.