Is it possible that an advanced extraterrestrial intelligence (ETI) would have knowledge of the astronomical development of more primitive societies, and hence pre-emptively manufacture artificial megastructures around stars within their domain in an attempt to make their presence known? This is precisely the question that astronomer Luc Arnold sought to answer in his paper which laid down the groundwork for the idea that such structures would be detectable by modern astronomical instruments. Several years in advance of the deployment of the Kepler space observatory, a landmark mission which aimed to quantify the frequency of Earth-like planets orbiting Sun-like stars using a detection technique called “transit photometry,” Arnold posited that the high precision photometric monitoring of stars afforded by Kepler would be sufficient to distinguish between artificial structures embodying a variety of geometries and extrasolar planets, which are approximately spherical. If an ETI had the desire to reveal themselves, they would take advantage of the fact that societies with emerging science would perform routine astronomical observations of stars (in an attempt to detect worlds orbiting them) and hence place something less obviously natural in front of them! Arnold examined the possibility of three geometries of objects that may serve this purpose: a pure equilateral triangle, a double-screened object, and a series of screens on a louver. In all three cases, by subtracting the best fit circular aspect (assuredly that of a planet) from the artificial lightcurves of these geometries, the residuals were above the \textit{Kepler} photometric sensitivity and hence theoretically distinguishable. The louvre system may be actuated in such as a way that it could also convey information, and so Arnold quantified the effectiveness of a megastructure signaling system by examining its spatial data rate, which he showed to be comparable to that of laser (but without the requirements and shortcomings that come with laser signaling, such as precise knowledge of the system’s future position at the time of receipt). Therefore he concludes that such a signal system is feasible. However there are some problems that would have to be addressed, such as perhaps the scale of the engineering project. Even granting the alien intelligence the benefit of the doubt and ascribing to them an advanced knowledge of astroengineering, I still had a few concerns. Wouldn’t the triangle have to not significantly rotate along the transit arc in order to maintain its projected equilateral aspect from our vantage point? Would such an object be three-dimensional or 2D planar, and in either case, what would happen if it spun on its axis? One could imagine that these structures could be statites, objects that are stationary with respect to the host star supported by radiation pressure. If the triangle was composed of solar sail material, then every time it minimizes its aspect during rotation (i.e. when it is parallel to our line of sight) then wouldn’t it fall inward towards the star? Setting these problems aside, this is nonetheless a brave submission by Arnold and worth taking into consideration as more and more photometric data becomes available.
Tag: New Methods
Papers describing new search methods for SETI
Is It a Megastructure? (No)
The G-HAT papers have definitely been dense reading. I think Part IV (the paper I’ll be discussing in this blog post) has been the most accessible one to me so far.
My favourite table/figure in this work is simply Table 1, so I’m going to talk about it a lot.
Firstly, I think it encapsulates the fundamental challenges in artifact SETI: 1) the artifact has to exist and 2) we have to be able to tell that there’s an artifact. This sounds simple, but I think is a useful comparison to the fundamental challenges in communication SETI: 1) the (intentional!) signal has to exist and 2) we have to be able to tell that it’s a signal.
Secondly, it’s good to keep perspective in SETI research: not every anomaly is ETI. In fact, all of them so far are not. The table illustrates that for every megastructure-y looking object, there are reasonable (and plausible) “natural confounders”. This makes sense; though we’re trying to wring as much information out of a single lightcurve as we can (and we’re quite good at it – we can even tell the full 3D stellar rotation and planetary orbit geometry) it’s still just a lightcurve, and there are many inputs that produce the same output. Perspective is vital, especially in a paper that is ~hunting for alien megastructures~
Thirdly, I just love lists, as you can probably tell by the way I write my blog posts. The section following the table goes through the six physical “Distinguishing Features of Megastructures” (ex. anomalous masses, aspects, or orbits). The section after that talks about the nine physical “Confounding Natural Sources of Megastructure Signatures” (ex. starspots, ring systems, or non-transited stars in the field of view aka. “blends”). I’m italicizing the word physical to illustrate exactly what it is that I like about the structure of this section: it shows what distinguishing properties of the systems being observed would be visible to an observer within the system. But we are not within the system – we’re working from lightcurves. And that’s where Table 1 comes in: showing exactly which of the physical properties discussed in (perhaps agonizing) detail would cause which of the 10 lightcurve anomalies in the Table.
I will now briefly summarize the rest of the paper, which I found generally less interesting to me. The next section talks about a few objects in particular that show some of the transit anomalies discussed in the previous section. The section after that discusses how to distinguish a signal beacon from a constant source from an information-rich signal by doing statistical analyses in both the frequency and time domains. The authors quantify it with a “normalized information content statistic”. I’ll admit that the methods in this section were mostly over my head, but I think (hypothetically) that the uniform application of them to future SETI studies would be a fruitful pursuit.
Is that a ringed planet or a spiffy pyramidal satellite?
Ground- and space-based photometric missions have proven that observing the light from a star can reveal exciting orbital companions, such as extrasolar planets. Luc F. A. Arnold, an astronomer at the Observatoire de Haute-Provence, also thinks photometry can reveal artificial, transiting objects. In his 2005 paper, Arnold proposes a new method to detect alien mega-structures by examining transiting light curves from space-based photometry for deviations from the expected transit of a spherical body (see Figure 1). Arnold has proclaimed that:
“Artificial structures may be the best way for an advanced extraterrestrial civilisation to signal its presence to an emerging technology like ours”
Arnold considered the capabilities of space-based missions, such as ESA’s Corot telescope and NASA’s Kepler telescope, and assumed a photometric precision of 10-4h-0.5, where h is the per point integration time in hours. It is important to note that this was not an analytical treatment of the transit signal for an arbitrarily shaped objects as Arnold did not predict the functional form such a transit would follow. To estimate how the signals would vary for various shapes (see Figure 2), Arnold performed simulations on a strip of solar surface. The strip would vary depending on the impact parameter of the transiting object, but in each case the limb darkening parameters were interpolated to create a realistic stellar surface. In each simulation, the transiting object was assumed to have a semi-major axis of 1 [AU], a cross-section of 1.16 Jupiter radii, and orbited HD 209458, a sun-like star. The artificial shapes considered in the paper were (i) a triangle, (ii) a louvre-like two-screen object, and (iii) a louvre-like six-screen object. The louvre shape was intended to mimic that of known man-made objects.
The results from Arnold are encouraging for proponents of artifact SETI. The different shapes, when compared to that of a planet, produced noticeable residuals. If the different shape rotates, the transit shows further differences (see Figure 3). Furthermore, if the host star were a smaller, cooler dwarf, there would be a stronger transit signal. Arnold does note the degeneracies between the curve of a ringed, transiting planet and a non-rotating, artificial triangular object. Given the precision of Kepler and Corot, this would be a concern for future, more sensitive photometric missions. Things get easier to distinguish if we assume a louvre-shaped satellite, as each screen can be considered as a single object transiting and would collectively alter ingress and egress.
Perhaps the most important thing to note is the implication of using transits as beacons to signal others. A set of close objects would transit quasi-simultaneously and, if they were the same size, the light curve would show variable depth. Reminiscent of the prime number sequence a Bracewell probe would send, Arnold posits the time between the altering transits could be used to encode information as a message. Most importantly, any observer to the transit would receive the message. This blogger maintains skepticism about the applicability of this analysis to transit searches. For reference, the Kepler mission has produced 40,726,580 light curves. For the robust approach presented by Arnold, this would require carefully scrutinizing all light curves for anomalies and then properly vetting such signal. Perhaps future data scientists will properly address this problem and the search for a transiting triangle will become feasible.
Just stick with lasers
Arnold (2005) is the first mention (I think) of using transits as a potential form of communication. Arnold suggests that advanced civilizations could embark on ridiculous engineering adventures and launch one or multiple crafts into orbit as a way to long-term communicate their existence. A civilization could launch a single, large object that would block out a substantial amount of the star (like a Dyson sphere), or they could launch objects far from circular, whose transit curves would have different ingresses and/or egresses.
Personally, I doubt this would ever happen with the intent of communication. Such a project would take so long to complete, and so many resources, that I doubt a civilization would bother. If they were to bother, I don’t think the intent would be as a form of communication.
Arnold mentions that civilizations could group multiple objects into prime numbers (his example is 11 objects with 1,2,3, then 5 objects). This just seems like pure fiction to me. I haven’t done any math or simulations, but I’m skeptical that it is possible to keep this many objects gravitationally stable while still maintaining transit alignment. And even if this could be done, it would quickly go unstable, with orbits deteriorating.
If people search trasist data for megastructures, which they have and will hopefully continue to do, I feel that they should just look for odd or anomalous transits, possibly even swarms. But I don’t think people should bother with looking for messages in transits.
Fitting Megastructures Into Lightcurve Holes
In Arnold (2005), the possibility of finding evidence of ETI by looking for transits of unnatural shapes is proposed (a new search method!). Several possible examples are discussed and their transit depths and shapes are calculated. In addition, the effectiveness of using these structures as a way to communicate is evaluated and compared to other proposed, direct communication methods.
Arnold singles out equilateral triangles and a louver-like screen (repeated, aligned rectangles, like a window screen) as his examples of choice to analyze. He simulates the stellar flux during the transits of these objects by taking a simulated image of the star and setting the pixel values to be equal to zero when the object being tested in the intermediate line-of-sight.
Actually identifying the signatures of these shapes in transit signals, is surprisingly difficult. Not only are the differences between the transits caused by the investigated shapes and best fit sphere only on the order of 10^-4, but the differences you can see may be indistinguishable from a totally natural cause like rings around a normal planet.
This paper was important as it was one of the first to popularize the idea of searching transit studies for possible signals imparted by ETI megastructures. Since this paper was published, the Kepler space observatory was launched and the ideas from this paper were used in a Kepler data search (Wright et. al. 2016), where some interesting (aka weird) transit candidates were found!
We are too young for that!
Nikolai Kardashev, in his seminal 1964 paper, classified civilizations into three broad categories:
- a civilization with technological level close to that presently attained on the earth.
- a civilization capable of harnessing the energy radiated by its own star, and
- a civilization in possession of energy on the scale of its own galaxy.
Even since this publication, people have hypothesized different methods of detecting alien megastructures (see Figure 1 for examples). A feat of extensive galactic engineering (e.g. through many Dyson spheres) would be readily discernible in the spectrum of an object. At least this was the hypothesis for the Fermilab astrophysicist James Annis when he undertook a search for class III Kardashev civilizations. Annis argued the “most direct way to obtain power” for type II and III civilizations would be to harness stellar power. This would imply the interruption or redirection of starlight which, when performed on a galactic scale, should produce an observable change in a galaxy.
Figure 1: The Hunt is on!
The search for alien mega structures! Some have argued we should use the transit method to detect transits inconsistent with a planet. Others have argued artifact SETI should focus on the thermodynamic evidence. This often means one Dyson sphere covering a star or a galaxy full of Dyson spheres. Annis himself was looking for galaxies modified by type III civilizations, such that the redirection of optical light would result in significant dimming which would present itself as an outlier on scaling relationships. Source: New Scientist
Annis made a distinction between natural galaxies and those that have been modified by a type III civilization. Galaxies are bound by their masses such that there exist gravitational-thermal scaling relationships. To derive the fundamental scaling relationships, Annis used the virial theorem and assumed the temperature of the galaxy was defined by the random velocities of its stars (e.g. similar to an ideal gas). He derived a generate relationship between the radius of a galaxy, R, the surface density, I, and the temperature, T, such that T=CIR, where C is a constant. In practice, the relationship he derived is not that simple but does emphasize that (i) self-gravitating systems have a simple relationship for R, I, and T and (ii) a natural, unperturbed galaxy shows such relationship. The relationships he uses are the Tully-Fisher relationship for spiral galaxies (L∝IR2) and the fundamental plane for normal elliptical galaxies (R∝T0.68I-0.85) and he notes a small scatter of ~10%.
Given that the relationship between these parameters was relatively consistent, this allowed Annis to look for outlier galaxies in each trend. His argument was that a type III Kardashev civilization would be “an outlier on R-I-T relations in the sense of anomalously low I, probably with a thermal IR excess, and possibly with a low surface brightness in absolute terms”. The results of his search through 31 spiral galaxies and 106 elliptical galaxies is shown in Figure 2. In this available sample, Annis found no candidate type III civilizations where at least 75% of the light would be dimmed. Annis attributed the null detection to various factors: his limit for outlier classification, instrumental effects, and potential bias against modified galaxies within current catalogs.
To this blogger, the most important thing is the limits Annis was able to place on the formation of type III civilizations. He presents one of the first attempts to statistically evaluate our isolation in the Universe. As a thought experiment, we can assume the appearance of a type III civilization follows a Poisson distribution (p=e-rT) and could occur at any point in time. Given that, on order, the galaxy’s age is 10 billion years, a 99% probability of a null detection would suggest an occurrence rate of r=4.61×10-10 per year or, equivalently, that it takes at least 2.1 billion years for a type III civilization to occur by chance. If each galaxy represents an independent realization of the thought experiment, then the upper limit on the occurrence rate suggests ~300 billion years must pass for a type III civilization to exist. Ergo, the Universe is too young for these civilizations.
The work by Annis suggests future searches for type III civilizations would be illogical. Future attempts have tried to address this. Most recently, Wright et. al. argue against the temporal argument raised by Annis. Wright et. al. agree with Annis in that his sample, derived from optical catalogs, may be biased against galaxies which theoretically have little to no optical emission. They also criticized the use of a random Poisson process and independent of time as this would presumably contradict the time-dependent evolution of life (e.g. from no life, to type I, type II, and finally type III civilization). Wright et. al. argue that a much larger sample, preferably in the IR, would serve as a better search, although any outlier found would still require significant vetting. Annis himself has pondered:
“Life, once it becomes spacefaring, looks like it could cross a galaxy in as little as 50 million years, and 50 million years is a very short time compared to the billion-year timescales of planets and galaxies. … Maybe spacefaring civilizations are rare and isolated, but it only takes just one to want and be able to modify its galaxy for you to be able to see it. If you look at enough galaxies, you should eventually see something obviously artificial. That’s why it’s so uncomfortable that the more we look, the more natural everything appears.”
As of now, there is no unambiguous detection of a type III civilization. This blogger is not particularly surprised by the null detection. Perhaps future searches will bear fruit, but for now the only thing for certain is that the Universe holds one type I civilization.
19 Years Ago We Didn’t Find Aliens
Surprise! Bet you didn’t know that.
In 1999, Annis performed a simple “search” for alien life. He hypothesized (probably correctly) that a type III Kardashev civilization, one that controls power comparable to an entire galaxy, would be an outlier on R-I-T relations, where R is the size of the galaxy, I is the intensity, and T is the temperature. Both spiral and elliptical galaxies have such relations, Tully-Fisher for spiral and the fundamental plane for elliptical galaxies. He also theorized that such galaxies would have an excess of IR radiation and would have low surface brightness. Using data on 106 elliptical galaxies and 57 spiral galaxies taken more than a decade before he published his paper, Annis looked for any outliers in the above relations. He defined his outliers to be objects more than 1.5 magnitudes dimmer than the emission predicted by the respective relation. This seems fairly safe, and like a sound argument for detecting life. Personally, I don’t see any civilization becoming a type III civilization, so I consider this and related searches to be a decent waste of time (for SETI; some of the searches lead to important science).
Not surprisingly, Annis found no objects that met his outlier criteria. I say this isn’t surprising for two reasons. First, I feel that a galaxy with anomalously low emission would have been flagged prior to this search given how long the data were available. Second, this search was of so few galaxies. (A weak third argument would simply be that a civilization capable of eating up energy on the order of their galaxy would have ways of compensating for this fact, like Kipping 2015 suggested covering transits with lasers.)
This search to me just seems a bit useless and like he had a free afternoon and wanted to just quickly write up and publish *something*. Even if he were to find a galaxy with too low emission, it would require additional data in some other field to confirm it had anything to do with ETs. That being said, I would be interested in seeing someone do this with all the data we have nowadays.
Annis (1999) Summary
This paper presents an empirical way to find type 3 Kardashev civilizations, which is to identify outliers in the Tully-Fisher relation for spiral galaxies and in the fundamental plane for elliptical galaxies.
The author begins by arguing that it is possible to observe galaxy scale interruption of starlight. Then the author derives the scaling relation for galaxies argues that the scatter seen in empirical data around the scaling relation is as low as 10 percent.
This tight relation could be used to identify outlier galaxies which have very low surface brightness, probably with a infrared excess if the type 3 Kardashev civilizations use technologies such as Dyson sphere.
Then the author describes the empirical effort to find such outliers in 31 spiral galaxies and 106 elliptical galaxies. However, no significant outlier has been detected in those galaxies. The author argues that this could be due to setting a too strict cut on the surface brightness deviation. Newer samples could reduce the cut down to 50 percent. However, the author leaves it for another time. Further, the author discusses the limitation of photographic plates not being able to detect low surface brightness galaxies which could give us biases towards regular galaxies than galaxies harnessed by type 3 Kardeshev civilizations.
Finally, the author discusses how long it would take for a type 3 Kardashev civilization to rise and the upper limit is 304 billion years which is much longer than the age of the universe. There are several reasons which could lower this upper limit. First, the sample is incomplete and they do not have any very low surface brightness galaxies. Second, not all type 3 Kardashev civilizations have to be star-fed. They could harness other types of energy such as dark matter and dark energy. Third, the time for those civilizations to develop may be much shorter than 10 billion years. One thing I do not understand is that why it is an upper limit than a lower limit because if those civilizations do not exist, the upper limit should be infinite.
The Case for Artifact SETI
Olaf Stapledon’s 1937 science-fiction novel, Star Maker, pushed the imagination by featuring an advanced civilization that built a spherical shell around its host star to capture the radiation and meet its energy requirements. This spherical shell would later become known as a Dyson sphere (see Figure 1), after the astrophysicist who contemplated the mid-IR excess such an object would release. While the concept originated in fiction, it has since gained a niche as a potential alien mega-structure that could be observed. It also emphasizes that the search for extraterrestrial intelligence (SETI) is difficult due to the limitations of our imagination. While SETI is fundamentally a search for a society capable and willing to communicate, there exist other approaches that make the SETI more feasible and draw on the concept of waste-heat from alien activities, such as the Dyson sphere. Jason Wright and fellow scientists recently published a four-part series to motivate and present the Glimpsing Heat from Alien Technologies (G-HAT or Ĝ) survey. Wright et. al. argue the sensitivity of a waste-heat search would be greatest for a civilization which satisfies a physicist’s definition of intelligent life. This defined ETI as a species that (i) “processes resources and energy to produce more of itself” and (ii) “overcome[s] local resource limitations through the application of energy”. They surmise that “if a species is spacefaring, then its level of intelligence is such that there is no practical resource limitation that it cannot overcome, except that of energy”. The authors note there may be other intelligent life excluded by this definition of intelligent life, but argue such a society would not be easily detected through the Ĝ survey.
The first paper introduces the philosophies of SETI, with particular emphasis on the Hart argument that the dearth of ETI encounters implies we must be the first intelligent species in our galaxy. Hart stated there were four categories of solutions to this problem: (i) physical, (ii) sociological, (iii) temporal, and (iv) ETI has visited, but he denounced each solution and conclude we were alone in our galaxy. Wright et. al. review each of the categories with insights from our current understanding of astrophysics and reinforce the temporal and sociological reasons presented by Hart. An order-of-magnitude calculation show a colony of ships traveling at 10-4 c (comparable to the velocity of Voyager 2 or Pioneer) in a rotating disk (i.e. our galaxy) should populate the Milky Way in at most a billion years. With regards to the extinction theories, the authors aptly note this must hold for all colonies of a civilization that has spread throughout its galaxy. A species confined to one planet can go extinct but as long as there is a self-sufficient colony somewhere away from a gamma ray burst or interstellar war (anything lethal), the species can always repopulate the galaxy. The authors state that sustainability arguments do not hold because, before all stars are colonized, there is no limit in the relevant resource (stars) and there is no reason galactic hegemony must be explicitly driven by a lack of resources.
While Hart’s argument was pessimistic, it only considered one galaxy. Therefore, if each galaxy is considered to be an “independent realization” of his experiment, there could exist galaxy-spanning ETIs. Wright et. al. then describe the previous searches for Dyson spheres and discuss the promise of NASA’s Wide-Field Infrared Explorer (WISE). Wright has stated that “WISE was launched by NASA for pure, natural astrophysics; it just happened to be perfect Dyson sphere finder.” In the case of a Dyson sphere, if a star were perfectly encased by a shell at roughly 1 AU, the resulting spectrum could be approximated with a few-hundred-Kelvin blackbody. The results of searching for this heat-waste, this artifact of ETI activity, would at worst put an upper limit on alien activity.
The Ĝ survey strives to address some of the issues inherent in typical SETI, namely the assumption that ETI is emitting a signal amenable to radio detection. Instead of forcing ETI to behave this way, artifact SETI seeks to detect the thermodynamic consequences of galactic-scale colonization. In this first paper, the Wright et. al. briefly note something like the Ĝ survey will be “hard pressed to prove that an unusual source is artificial”. IR observations are prone to contamination from dust and must be disentangled form other, astrophysical processes. The survey analyzed roughly 100,000 galaxies to determine the reddest sources and concluded “no galaxies resolved by WISE contain galaxy-spanning supercivilizations with energy supplies greater than 85% of the starlight in the galaxy”. Others have introduced possible corrections to the treatment of data, but nothing so far suggests galaxy-spanning civilizations exist.
Movie 1: Above is a video showing KIC 8462852 and a possible alien mega-structure (around the 1:00 mark) explaining the decrease in flux. Source: The Washington Post
While the Ĝ survey is more scientific and data driven than conventional SETI, it is important to carefully vet candidates and, at the very least, apply the law of parsimony. One such example of artifact SETI gone awry is KIC 8462852 (see Movie 1). Perhaps to Wright’s chagrin, he has been “credited” with fomenting the idea that an alien mega-structure is to blame for the dips in KIC 8462852 (see here and especially here). Additional observations suggest optically thin dust may cause the dips. The biggest damage is to the credibility of SETI, as it degrades the science behind these papers to nothing more than a form of sensational pseudoscience. The support from the Templeton Foundation, often criticized as having a history of supporting controversial and speculative research (see here for physicist’s opinion), is another issue that may affect the credibility of the Ĝ survey. Regardless of one’s prior on SETI, Dyson provides some keen insight:
“If there are any real aliens, they are likely to behave in ways that we never imagined. The WISE result shows that the aliens did not follow one particular path. That is good to know. But it still leaves a huge variety of other paths open. The failure of one guess does not mean that we should stop looking for aliens.”
This blogger, while having concerns with SETI as a whole, firmly believes more research should be done with both conventional SETI and artifact SETI. Until there is more data, it is unscientific to completely reject the premise of SETI, however flawed its premise may be.
Dyson Spheres: Fiction or Fantasy? (definitely not vacuums)
I don’t mean to just flat-out say that Dyson Spheres are impossible (even though they are). Instead, I separate their possibility into two categories: “fiction” and “fantasy.” Here, fantasy is something thought up by the imaginative yet impossible (think wizards and dragons) but fiction is something thought up that *could* be real. Maybe.
In 1960, Dyson wrote an article (letter?) to Science, stating a fun mind experiment that he had come up with: that advanced civilizations could build a “biosphere” (later called a Dyson sphere) around their star out of some outside matter (e.g. Jupiter), successfully harnessing all of the power of the star and solving over-population problems. This biosphere would block radiation from the star to outside viewers, except in the IR. Therefore, Dyson argued, we should begin a search for objects with strong IR emission and not much else.
It’s hard to discuss this paper without mentioning some of the published responses he received and his rebuttal to them, mostly because some other people basically mentioned my arguments to this idea nearly 58 years ago. Maddox points out that a Dyson sphere is physically impossible. To keep this shell in orbit around the Sun at any distance in the Habitable Zone would require some force counteracting gravity and pushing outwards on the sphere. In stars, this is radiation pressure. Maddox points out that radiation pressure wouldn’t really work, but doesn’t go into any specifics beyond that (it was only a short response). I wonder if there is any material strong enough that we know of or even theorize that would maintain its structure, so that the rigidity itself was counteracting gravity. Probably not, but still fun to think about. Dyson counters that by biosphere he imagined was obviously not whole as that would be impossible, but instead made of “a loose collection or swarm of objects traveling independent orbits around the star.” Wait, what? So even though it is a sphere, it’s not actually a sphere. But it is a lot of artificial bodies around the star..would that be bodies going the same velocity so having the same semi-major axis? Or are people just going to build a bunch of planets with different inclinations to fully cover a sphere? I can understand what s sphere around a star would look like, and agree that it probably is not mechanically possible, but I cannot figure out how “a loose collection or swarm of objects” would work at all. Personally, I feel this clarification terribly backfired. I had also always thought of and heard of Dyson spheres as being, well, spheres, so it seems that this clarification wasn’t read or remembered by most anyways.
Anderson then argues that such a sphere could not even be constructed. Since it would take so long to construct (several thousand years), there is basically no way that a civilization would bother continuing it. Sure, they could start, but after a few generations someone would come up with a better solution to over-population or for energy usage that would require so much less work and time. He argues that the only way this could work would be to go all Brave New World and condition people to accept the continuation of the project, in which case people could just be conditioned reproduce at sustainable rates! I completely agree with this argument, although I’m not sure how relevant it is to Dyson’s paper. It is true that a Dyson sphere is so unlikely to be produced that no civilization would do it, but Dyson spheres themselves aren’t really possible, so this seems like a waste of ink. I, however, do not agree with Anderson’s conclusion that “astronomical discovery of infrared sources won’t prove anything about the inhabitants of other planets.” I mostly don’t agree with this because we have no way of knowing for sure what we can learn from an IR signature until we actually start looking at them. Only slightly related to this, we can *detect* planets using their IR signatures (kind of, direct imaging in the IR is easier due to the slightly less horrible contrast issues); besides, as Maddox pointed out, an IR search would still be astronomically valuable, even if it wouldn’t lead to the discovery of Dyson spheres.
I personally believe that Dyson spheres under the definition of a solid sphere somehow encasing a star are impossible. The sheer force required to keep the sphere together would be insane, and no civilization would bother putting that much time and money into such a feet (we can’t even get funding to put man back on the Moon!). I can see the appeal of a Dyson sphere as it would lead to plenty of space for people and solar panels and such, but I don’t think it would solve over-population problems nor do I think anyone would bother making it.