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Glimpsing Heat from Alien Technologies

An assistant professor of astronomy and astrophysics at Penn State, Jason Wright is well known to the Centauri Dreams community because of his continuing work on the search for extraterrestrial intelligence through detection of its waste heat rather than directed communication. The discipline widely known as Dysonian SETI is receiving more and more attention, and given Dr. Wright’s prominence in the field, I was delighted to receive the essay below, which offers background on the subject at large and an overview of his current project. Dr. Wright is a member of the Center for Exoplanets and Habitable Worlds and the Penn State Astrobiology Research Center (part of the NASA Astrobiology Institute), as well as being a member of the California Planet Survey consortium. His AstroWright blog is essential reading for anyone interested in SETI and the process of science at work. He also maintains the Exoplanet Orbit Database and Exoplanet Data Explorer at exoplanets.org.

by Jason T. Wright


My colleagues and I have begun the Glimpsing Heat from Alien Technologies (G-HAT) SETI program, which has been written about here on Centauri Dreams and in other places, like in this nice summary article. I describe some of the foundations of the search here on my blog, but I have written up this short primer for Centauri Dreams to collect much of what is there into a single post.

“Dysonian” SETI

The benefits of expanding beyond “communication” SETI have been discussed on Centauri Dreams before (for instance, here) and the argument was made forcefully by Bradbury, Ćirković, and Dvorsky here.

The essence of Dysonian SETI is that one is searching for the passive signs of an alien civilization, instead of the deliberate communication from them. Freeman Dyson’s original articulation of this principle remains the simplest: search for the energy that a civilization has used for its own purposes after it expels that energy. The disadvantage to this approach is that it may be difficult to distinguish such waste heat from natural sources, or it may be that advanced technologies do not emit large amount of waste heat.

Energy supply as sign of intelligent life

The term “energy supply” in the context of humanity refers to the total annual production of energy for human use. We generate this primarily through fossil fuel extraction and combustion, but it also includes energy generated through the collection of solar power.

Most species on Earth collect energy passively (through photosynthesis or collecting heat from the environment) or through consumption of other species. Intelligence is sometimes defined in terms of tool use, which involves the application of energy to objects to achieve some goal. More generally, we might use a “physicist’s” definition of intelligence to be the capacity of an organism or species to apply energy to its environment to collect additional resources for survival or other purposes. For instance, an intelligent species like humanity can build solar panels to heat its homes or burn gasoline to run farm equipment to harvest more food.

This is not to say that there cannot be other forms of intelligence, but this specific form of intelligence would allow a species to overcome resource limitations and grow. This intelligent application of energy can, in principle, be used to expand a population, and an energy supply, to fundamental physical limits (see for instance, my “TED-style” talk here.)

If this has ever happened — if an alien civilization has ever used its intelligence to create an energy supply that rivals the output of stars — then their waste heat would probably be detectable with today’s astronomical instrumentation.

“Waste heat” does not imply inefficiency or “waste”

Conservation of energy means that when one is done using energy for some purpose, one must expel it or else store it (though in the long run you can’t keep storing more and more energy). One might object that an arbitrarily advanced alien civilization could overcome this limitation, and it’s true that if alien civilizations inevitably violate conservation of energy, then our search will fail. But conservation of mass-energy is as fundamental a physical law as we have, and if we cannot assume that then we cannot have a meaningful, physics-based discussion about advanced civilizations at all. So it is reasonable from a physics perspective to search for the energy in waste heat, which should exist if alien civilizations do.

The term “waste heat” may seem to imply some sort of unnecessary inefficiency that an advanced civilization would be able to overcome. Not so. The confusion here is that when most people say or hear “energy” they are really thinking of “free energy” — the amount of work that can be done with a certain amount of energy.

For instance, when you drive to the supermarket the energy stored in the chemical bonds of your gasoline is converted to useful work that accelerates your car. When you are done with this energy — when you are ready to slow down at the supermarket parking lot — you press the brakes which dissipate the energy into the brake pads as heat, which then ultimately gets radiated away as mid-infrared radiation. This energy coming out of the brake pads now has a higher entropy than the energy that was in the gasoline— this means the energy has less “free energy” than before, so you can’t use that energy to make your car move again. In a regenerative braking system (like in electric or hybrid cars) your car attempts to collect this energy and put it back into the battery, but the second law of thermodynamics puts an upper limit on how efficiently this can be done — some of the free energy is lost with each braking cycle. Also, losses to friction with the ground and the air during your trip cannot be recovered efficiently.

So conservation of energy says that on the whole, an alien civilization that has a very large energy supply must expel as much energy as it collects or generates, and the second law of thermodynamics says that this expelled energy will have high entropy (very little free energy). We call this high-entropy expelled energy “waste heat”, even if the alien civilizations that uses it is very efficient and not at all “wasteful”. In fact, the more efficient the civilization is, the higher the entropy of the expelled energy, and the more it will have the properties of the sort we expect to see from waste heat.

One way around this limit is to emit the heat at a lower temperature. This is not possible on the surface of the Earth, where you cannot radiate heat away at a temperature lower than your surroundings (if you try, the opposite occurs — your surroundings heat up your apparatus). But in principle we could build huge, cold radiators in space that could operate as part of a heat pump, extracting more free energy from our waste heat to do more useful work. The difficulty here is that the radiators must be huge to get even a small benefit — the size of the radiators scales as the fourth power of the efficiency you gain, so improving the maximum theoretical efficiency of sunlight collection on Earth from 95% to 99.5% would involve building radiators with a surface area 10,000 times the size of that of the Earth, which hardly seems worth the effort. This means that we should expect alien waste heat from starlight to never be orders of magnitudes cooler than the surface of the Earth, because the engineering difficulties make the task pointless.

Detecting waste heat with telescopes

Waste heat at these temperatures will be apparent at mid-infrared wavelengths. The IRAS mission in the 1980’s surveyed the sky at these wavelengths, but did not have the sensitivity to detect most galaxies or stars because of the higher-than-expected background emission from dust in the Milky Way. The WISE satellite has much better resolution and sensitivity, and so does not suffer from this problem over most of the sky.

Most galaxies and many stars have “infrared excesses” — they give off much more mid-infrared emission than one would expect from stars alone. Today, we understand that this is because of astrophysical “dust” — a very fine smoke of organic molecules that is produced from the ashes of supernova explosions, in the atmospheres of giant stars, and in the disks of forming planetary systems. This dust glows brightly in mid-infrared wavelengths when it is illuminated by starlight — just as we expect alien civilizations to do. Now that we have sensitive mid-infrared surveys, distinguishing mid-infrared emission from dust and alien civilizations is the primary obstacle to detecting alien waste heat.

For now, the best we can do is to put upper limits on these civilizations. We can show, for instance, that there are no nearby galaxies filled with alien civilizations using all of their starlight — and we can do this for about 1,000,000 galaxies! We can also rule out civilizations using about 50% of the starlight — even the dustiest galaxies do not have so much dust that half of the starlight is being reprocessed by it. Going forward, we will continue to lower this limit down to around 20% (or even lower for some galaxies, such as ellipticals which host almost no dust).

Going any lower will require careful observation to see if the mid-infrared morphology or spectrum of a galaxy is characteristic of dust, or if it is anomalous in some way. Looking to individual stars in the Milky Way will actually be somewhat difficult, because many things that look like mid-infrared-bright stars are actually distant galaxies that are red for other reasons, dusty giant stars on the other side of the Galaxy, or young stars still forming planets in dusty disks. When the GAIA satellite finishes its survey, it will give us distances to most of the stars in the mid-infrared surveys. This will allow us to search for those that are mid-infrared bright, not giants, and not associated with star-forming regions filled with dusty young stars. If we find a star that is very mid-infrared bright, about the luminosity of the Sun, and not part of a stellar nursery, that will be a dead giveaway that something very strange is going on with that star.

A process of exclusion

Even if we find something anomalous, as scientists we must always reach for the naturalistic explanation first. Finding mid-infrared-anomalous objects is scientifically interesting in its own right and so a worthwhile scientific endeavor. If we can find no scientific explanation for an anomalous object, we must continue to search for new explanations and not immediately jump to the conclusion of “aliens,” lest we commit an “aliens of the gaps” fallacy. Only if we see an unambiguous sign of intelligence — if the Allen Telescope Array, for instance, detects complex and obviously meaningful radio signals from the object — will we be able to say SETI has succeeded. The G-HAT search, then, will have two implications for SETI: we will put an upper limit on the size of energy supplies being emitted as waste heat in nearby stars and Galaxies, and our best candidates will inform a target list for communications SETI efforts. In this way, the Dysonian and communications SETI approaches are strongly complementary.


Comments on this entry are closed.

  • Bob February 7, 2014, 15:53

    We are taught that the Second Law is inviolable but current work suggests there may be boundaries, loophopes or limits to it which a super advanced civilization may be able to achieve. If the idea’s of physicist Daniel Sheehan of USD have merit, an advanced civilization may be able to recycle most of their energy almost endlessly which means very little waste heat or at least a different heat signature than assumed. Even though the Second Law ultimately triumphs, our current ideas of efficiencies, based on primitive heat engines, may be far off what an advanced society may achieve. So the idea that radiating at or near the backgroud may not be pointless after all. Perhaps we should consider looking for regions with anomalous unique signatures around the background radiation in otherwise dark and sparse regions.

  • Michael February 7, 2014, 17:04

    We can barely detect the heat signatures of exo-planets, what chance do we have of detecting the waste heat of alien civilisations with more than likely more efficient engines or systems. We are more than likely be detecting radio and higher frequency emissions before we see the heat.

  • Doug M. February 7, 2014, 17:08

    GAIA — currently undergoing shakedown out at Earth’s L2 point, with nominal five-year mission scheduled to begin in May — is probably the most underappreciated major space mission of the last decade. It’s going to do fantastically precise astrometry, which will have all sorts of uses across a wide range of fields. But it gets pretty much zero attention in the space forums, never mind the mass media.

    Anyway: once GAIA’s work is done, we should be able to know with a high degree of certainty whether there’s a Kardashev II level civilization anywhere remotely near us. I suspect we won’t see anything, but it’s certainl worth looking.

    Doug M.

  • ljk February 7, 2014, 17:29

    An advanced society might use black holes as power stations. I presume they also have a means to protect their stations from the dust and other debris falling into the black hole plus all the generated radiation.


    Some might even live INSIDE supermassive black hole:


  • David Messerschmitt February 7, 2014, 17:51

    It is great to pursue alternative ideas about signatures of intelligent life. Nice work. Your observation about the entropy of waste heat being high as a sign of an efficient energy producer and consumer is interesting to me, because there are parallels to electronic communication signals.

    In the presence of white Gaussian noise (such as the cosmic microwave background), one characteristic of a communication signal that is operating near the theoretical limit on power and rate is that it will itself be white and Gaussian (within a certain bandwidth constraint). Such Gaussian signals have the highest possible entropy. Thus, high entropy is a signature of efficiency in communications, and presumably another signature of a more advanced civilization (one aware of and able to design to fundamental limits).

    Entropy in signals and communications is a little different from entropy in thermodynamics, but they share the same formulas and are a measure of uncertainty or randomness in both cases.

    Also, from the receiver perspective a signal that is optimally distinguishable from local sources of radio frequency interference also looks like Gaussian noise, and thus has the highest possible entropy. This was discussed here on Centauri Dreams awhile back. See


  • Enzo February 7, 2014, 19:27

    @Doug M.
    I agree with you on Gaia, but that tends to happen a lot to non US missions in general. And ESA is pretty bad at PR.

    Anyway, there’s a SETI talk about detecting heat with a 70 m telescope built for this purpose :

    If I understand it well, it should be great for exoplanet in general as it is designed to block the light from the main star.
    Anyway, he uses an hopelessly optimistic number for habitable planet in HZ (given the Kepler results as incomplete they might be). He uses 0.5 planet/star instead then a more realistic number (< 0.001 ?). See at 9:00 minutes. Keep in mind that a planet to have generated a civilization must be not just habitable with microbes just surviving)
    Of course, had he used a realistic number his whole project would have been pretty certain to detect nothing.

  • David February 8, 2014, 4:44

    I heard that talk too and to put it extremely diplomatically, I was totally
    unconvinced and not just for the reason you mentioned although
    that is a big one. First, nothing was said about potential geological
    false positives. For example, how does one distinguish between the heat
    from an Alien city from say the heat from a geological feature like a hot spot
    which may have no terrestrial analog and since
    the telescope can not resolve the surface, you would not know which was which.
    The city of Paris was said to have a temperature 4 degrees warmer than the surrounding
    countryside, one could imagine a hot spot which would have the same size and temperature
    differential with the surrounding area.

    This is even assuming that any aliens would choose to live in anything that we
    would recognize as a city and who knows what the waste heat would be even if they
    did. Again, not knowing anything about the geology of the planet, I can’t see how one could be
    certain that anything artificial was not because of some natural feature.

    As you said Enzo, assuming that half of all planets that develop life will develop
    a technological civilization is ridiculously optimistic. If one accepts that number,
    than we are talking about the Milky Way having hundreds of millions, if not billions of technological civilizations which really makes the Fermi Paradox very acute. I recall
    that the Sun gets new neighbors on a time span of a few hundred thousand years so that over the last billion years, hundreds, if not thousands of technological civilizations would be
    within a few dozen light years of the Earth. If so, one might think that our Solar System would be swarming with aliens or their artifacts.

    As far as I have been able to determine, nothing about there project has hit the refereed
    journals. Sure, I can understand keeping the instrumentation details private since
    patents are likely to be involved, but I find it strange that there are no journal papers
    about the project such as goals or methodology and I suspect is for the reason that you pointed out, namely that the project is very unlikely to find anything that could be unambiguously

    My suspicion is that these guys realize all this and have other motivations for the project.
    For example, soliciting funds from rich people to directly detect alien civilizations is probably a lot easier than soliciting funds to construct a telescope that would be powerful enough to
    do radial velocity observations of all the planet candidates that the Kepler mission has found.
    What I do worry about, is that once at it is determined that nothing alien has been found,
    then it may well be harder to raise funds for much better designed SETI projects such as the one discussed here by Jason Wright.

  • Doug M. February 8, 2014, 11:46

    @Enzo, GAIA also suffers from a couple of handicaps. One, it’s doing very pure science — stuff that is hard to make interesting or accessible to a general audience. Two, it won’t produce any “New York Times” photographs or milestones that are likely to catch the public imagination. And three, most of its data won’t be accessible for years to come. In fact, if I understand correctly, most of its data won’t be accessible until after its five year mission has ended in 2019! So while it’s easy to grasp what Curiosity or Cassini are doing, GAIA probably seems like a very expensive thing that’s been shot into space to do obscure strange science stuff.

    It’s a pity, because its work really is very important and interesting. (And also, it’s in a Lissajous orbit around Earth’s L2 point. That’s pretty cool right there.)

    Doug M.

  • Michael February 8, 2014, 12:28

    It may not be the photon emissions that we pick up first, perhaps the nuclear signature from exhaust engines. Like the incomplete fusion by-products deuterium/tritium which are quite rare in space or even the by-products of a fission drive, U235/Pu239/(decay products), which are even rarer. Perhaps we should also be sniffing for ‘anomalous’ chemicals as well.

  • Alex Tolley February 8, 2014, 13:37

    Of course, had he used a realistic number his whole project would have been pretty certain to detect nothing.

    Thanks for the link. I have to agree. The project only has a small chance of success if there are enough suitable planets within the estimated 100 ly radius. Change the likelihood of suitable planets to be lower, and the proposal for this search would collapse.

    @Doug. While the telescope might be sold for the sexier SETI, I would be quite happy if biosignatures could be assessed.

  • Jason Wright February 8, 2014, 14:48

    Thanks for the comments, everyone.

    Bob: If laws we consider fundamental really aren’t, then we can’t really have a physically-motivated search for aliens because you have nothing to go on, no starting point. As you say, we can only look for “anomalies.” In some sense, that is what we are doing (looking for anomalously high amounts of infrared radiation), but it practice it is hard to look for “anything out of the ordinary” both because you have to start somewhere, and because nature creates lots of classes of rare, “odd” things. Lucianne Walkowicz of the Adler Planetarium is attempting something along these lines with a fancy non-parametric search for anomalies in Kepler light curve data: http://science.time.com/2012/11/28/flickering-stars-could-aliens-be-sending-us-signals/

    As for the Second Law of Thermodynamics being violable, I am skeptical, because Einstein was. He wrote that thermodynamics “is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown.” Lots of people have lost money betting against Einstein; I won’t join them.

    Michael: We are looking for very large energy supplies. Up to the total amount of starlight, there is no physical limitation to how large such supplies could be, and we want to know if in the 10 billion years they’ve had to grow, if any alien civilizations have gotten that big. You are right that planet-bound civilizations produce too little infrared radiation for us to detect today.

    Doug and Enzo: I agree that GAIA will do the yeoman’s work that will lead to a quiet revolution in stellar and Galactic astronomy. I also agree it’s a hard sell to the media!

    ljk: If the power that black hole civilizations produce is emitted in the thermal infrared (and I’ve argued it probably would be), then we will be sensitive to those with G-HAT, if their power supplies are large enough.

    Enzo and David: The Colossus telescope idea is just that: an idea, not a real project. For comparison, consider that the world’s astronomy community and biggest donors have struggled for years to produce a 30-meter telescope that will cost over $1B and revolutionize astronomy; it is still over a decade away, at least. A 70-meter telescope could easily be an order of magnitude more difficult to fund, build, equip, and operate.

    But the idea of looking for hot spots on exoplanets is a nifty one, and I like it. I agree that false positives from volcanism and similar phenomena would be a problem. But keep in mind it is just one thing that you could do with a 70-meter telescope (maybe — I’m not actually convinced it would work).

    Michael: I’m not sure what you mean: how would we look for those things without photons? But I agree that seeing spectral lines from obviously artificial isotopes and chemicals would be an interesting approach to SETI; I suspect people have tried.

  • Michael February 8, 2014, 16:04

    @Jason Wright

    ‘Michael: We are looking for very large energy supplies. Up to the total amount of starlight, there is no physical limitation to how large such supplies could be, and we want to know if in the 10 billion years they’ve had to grow, if any alien civilizations have gotten that big. You are right that planet-bound civilizations produce too little infrared radiation for us to detect today.’

    If we do see that much energy usage I would want to avoid contact with that civilisation! they would see ‘us’ as nothings really. Only good for the local zoo.

    ‘Michael: I’m not sure what you mean: how would we look for those things without photons? But I agree that seeing spectral lines from obviously artificial isotopes and chemicals would be an interesting approach to SETI; I suspect people have tried.’

    I was thinking more along the lines off detecting the atoms of the exhaust, they would have certain chemical/mass signatures. A mass spectrometer analyser could look for these anomalies by capturing them. Fission reactions leave quite a dirty trail which could possibly be detected. Perhaps our Earth polar regions could be checked for past signatures as the magnetic field would have funnelled some of them inwards, a slim chance mind you.

  • David February 8, 2014, 22:21


    There was a search for tritium around 53 nearby stars and other objects
    for much of the reason you stated. See

  • Glaas February 9, 2014, 12:06

    @Doug M. February 8, 2014 at 11:46
    First GAIA measurements will be released in 2016, which is as soon as they can be made, for astronomical reasons. Remember, it takes 6 months to scan the entire sky once, and since it measures parallaxes it must make two measurements of the same object to get its apparent movement. And then a 3rd measurement to separate its distance from its proper movement. That’s 18 months. Add 3 months to give the own researchers a head start, and that’s when they will release the data after commissioning now in May. Precision will then improve with further orbits around the Sun.

    But GAIA of course cannot measure parallaxes to any galaxies, and galaxies are what’s interesting to Jason Wright. Maybe they could be convinced to release the first all sky data a year earlier? One single set of all sky data in itself is of no use for GAIA’s primary parallax mission anyway, so “first rights to data” should not be an issue.

  • Michael February 9, 2014, 14:58



    There was a search for tritium around 53 nearby stars and other objects
    for much of the reason you stated…’

    The tritium hyperfine line would be a powerful indicator of technologically advanced life, as would by-products of nuclear fission which could show up in the spectrum of the starlight such as Iodine and Xenon.

    My concern with the Dyson sphere is that if all the light is intercepted and used to perform work it must all degrade to heat eventually and be radiated or stored. Degraded light ‘heat’ requires large radiators as the emission rate is highly dependant on the temperature. How will the heat be got rid of, if not the star and everything inside the sphere will overheat eventually? Does the sphere expand to accommodate this heat-up by speeding up and moving away, i.e. increasing the spheres orbital momentum?

  • Eric February 9, 2014, 16:51

    As I’ve commented before, I suspect that alien “heat signatures” may be more likely to be evident in Oort clouds.

    Icy bodies seem to be the dominant habitat / environment of space. Over time, aliens should colonize this environment. We may want to look for some excess heat waste from their activities as they exploit the trillions or so of cometary bodies that should make up Oort clouds around stars.

    Would such waste heat be observable if large numbers of icy bodies were being exploited? Would it look like a slightly too warm diffuse glow?

  • Dave Moore February 9, 2014, 21:17

    I did a paper published in JBIS:
    Moore, “Lost in Time and Lost in Space: The Consequences of Temporal Dispersion for Exosolar Technological Civilisations,” JBIS Vol. 63, No. 8 (August 2010), pp. 294-301.
    You can read a short version at https://centauri-dreams.org/?p=19246.
    In part of it, I covered the search for exogalactic Kardashev III civilizations by looking for their infrared excess.
    Not having a background in infrared astronomy, I could do no more than make some generalized speculation about exogalactic IR excess, so I’m pleased and excited to see there is some professional work in the area.
    However, I think the measurements are going to have to become a lot more sensitive (in the range of an IR excess of 10 to the 7 to 10 to the 8 of galactic output for the following reasons:
    i) The Dyson Sphere was intended as a thought experiment, not a physical construction even by Dyson. He more envisaged a cloud of orbiting objects, which would probably leave a fair percentage of the starlight through. So the idea of capturing all the starlight is not a realistic scenario.
    ii) A civilization that is scooping up all the starlight in a galaxy is straining it’s resources. From our knowledge of Earth’s history, we know the civilizations that strain their resources do not last long. They collapse. If such a exogalactic civilization did exist, it’s short life span would mean we would have a very poor chance of catching it.
    iii) In my paper, I postulated a Kardashev III civilization that came about through the colonization of cometary clouds, using Hydrogen fusion as a power source. Growth in this manner is a lot more organic than having to break up planets. I calculated that such a cometary civilization could reach a power output of 10 to the 30 watt in our cometary belt, that of a red-dwarf star. To get my galactic figure, I then assumed every 10th star in a galaxy had a cometary belt civilization.
    So a null result would not be particularly significant. I do, however, wish Jason Wright all the best in his endeavors; although, how he is going to tease a faint signal that is masked by our own galactic dust and variations in dust in the target galaxies I don’t know; but amazing things have been done in astronomy over these last few years, so I’m hoping he will find an ingenious method of distinguishing between artificial and natural signals.

    Let me finish by adding what I see as the advantages searching in the IR over looking for radio signals:
    i) The entire civilization’s power goes into producing the signal;
    ii) it’s omnidirectional;
    iii) no assumptions need be made regarding the civilization’s proclivity to signal or their behavior in general.

  • coacervate February 10, 2014, 1:01

    I think some introspection is in order here. WE are intelligent beings and WE know energy is valuable to US in this era. So it is not all that surprising that some would float the idea that REALLY intelligent life would value energy so much they would consume their own star.

    As if a stone-age tribe on an isolated island somehow found a telescope and used it to detect the presence of super-intelligent life on a neighboring island by scanning the shoreline for immense piles of flint shards.

    For example, we are mortal. Think about how your daily life would change if you were to become immortal. We are looking for beings who’ve transcended the material realm by ascribing material aspirations to them. Eeek!

    We value gold, oil, life, status. Of those four, which will always be precious? Look for monuments … If they have ego they will trumpet loudly. If they have Elon Musks, or artists like Andrew Rogers (who does those huge displays) or even if they just have hams we will hear/see them….if we just do a decent job listening. ~Stop, look and listen Baby, that’s my philosophy~

    we must set priorities. SETI, yes of course, but Dyson spheres? Think it through, spend wisely.

  • torque_xtr February 10, 2014, 6:50

    Thanks for the article, it clarifies why the WISE data are not enough and additional data. But are there any significant consraints based on the WISE data? The Dyson swarm with full or significant coverage would radiate in mid and far IR as much as a total output of a main sequence star (assuming energy conservation, of course), and thus would be detectable from very far away, but there weren’t many news on the IR SETI since the WISE release…

  • Brett Bellmore February 10, 2014, 7:12

    “How will the heat be got rid of, if not the star and everything inside the sphere will overheat eventually? ”

    The “Dyson Sphere” is used only for energy collection; You let the inside heat up to the point where you get good thermodynamic efficiency given the temperature of the outside, which is dictated by diameter, stellar output, and emissivity. The whole thing would be very thin, maintained supported by the radiation pressure inside. Which would be enhanced by making the inside of the sphere a mirror, so that radiation would accumulate. This is necessary to get net radiation force, which you don’t have if you just pass through all the stellar radiation without reflecting any. If you reflect 90% of the incident light, you still absorb 100% of the star’s radiation, but get to use 9 times the star’s radiation for support.

    Then you beam the power to habitats and other constructs, far enough away and apart that they don’t heat each other’s radiators.

    So, you’ve got the thermal emission from the sphere around the star, power beam leakage, and habitat thermal emission. All three are good candidates for a search.

  • Brett Bellmore February 10, 2014, 7:16

    Oh, and if they’re sentimental about their planets, they might be detectable, due to having “spotlights” aimed at them from the Dyson sphere, instead of being left in the dark. Potentially each could be given the right amount to be in the habitability zone.

    Such a Dyson sphere, a statite array really, doesn’t require more than the mass to be found in an asteroid belt or modest sized moon. I expect we could start construction of our own within a century or two.

  • Jason Wright February 10, 2014, 10:03

    @Dave Moore:

    Thanks for the link to that paper. I’ll take a look at it; I agree with your analysis in the Centauri Dreams writeup.

    As for your three points:

    i) I agree that the Dyson sphere concept was a thought experiment by Dyson, not a prediction of the form of alien civilizations; this is why I avoided the term in my writeup. Dyson spheres are a useful concept as an idealized upper limit to starlight collection and re-radiation, but invoking them often makes a discussion degenerate into their plausibility and engineering, which is beside the point.

    ii) It is entirely possible that it is a universal phenomenon that resource-limited civilizations are inherently short lived, and that would explain the lack of Kardashev Type III civilizations with energy supplies rivaling the stellar output.

    (And @coacervate: It’s also possible that all other species in the universe that grow transcend their need for energy in some way.)

    Be careful about what I call the “monocultural fallacy”: just because we can imagine an efficient or plausible way for alien civilizations to thrive, that does not preclude other parts of the super-civilization from using less elegant, more detectable methods to grow and do work. If just one silly civilization in the Milky Way builds a 300K Dyson swarm, that’s enough for us to spot it.

    Since it’s cheap to look, it’s worth the effort to see if they’re there and rule out large energy supplies.

    iii) (And to @Eric): I think detecting a waste heat signal at the 10^-7 level in a galaxy and distinguishing it from dust is essentially impossible. We would have to spot the 10^23 Watts of FIR excess around an individual M dwarf nearby (it should stand out — it would be 10–100% of the star’s luminosity).

    And I agree with your three points regarding the benefits of waste-heat SETI.

  • Michael February 10, 2014, 14:44

    @Brett Bellmore February 10, 2014 at 7:12

    ‘The “Dyson Sphere” is used only for energy collection; You let the inside heat up to the point where you get good thermodynamic efficiency given the temperature of the outside…’

    If the aliens were smart they would take the useful wavelengths and let the less useful ones through with a semi-transparent membrane. In which case we should also look for capped spectrum Dyson spheres.

  • ljk February 10, 2014, 15:24

    Michael said on February 10, 2014 at 14:44:

    “If the aliens were smart they would take the useful wavelengths and let the less useful ones through with a semi-transparent membrane. In which case we should also look for capped spectrum Dyson spheres.”

    If you read Robert Bradbury’s idea, the Dyson Shells *are* the aliens:


    Having Dyson Shells, Swarms, whathaveyou built for the express purpose of having organic creatures run around on the inside in a strictly Earthlike environment is so passe, to say nothing of a waste compared to the potential for such systems.

  • Wojciech J February 10, 2014, 17:58

    Jason Wright
    “If just one silly civilization in the Milky Way builds a 300K Dyson swarm, that’s enough for us to spot it.”

    In previous searches for Dyson Spheres, there have been several candidates detected. The problem is with confirming these observations.

  • Jason Wright February 10, 2014, 21:24

    @Wojciech J:

    Yes, Richard Carrigan’s search using IRAS turned up several interesting candidates. His follow-up observations with the spectrograph on IRAS revealed them all to be dusty giant stars and the like.

    As I wrote, GAIA will be a powerful discriminant because these sorts of false positives will be distinguished by their very high luminosities.

  • Tom February 11, 2014, 2:08

    SETI isn’t looking for intelligent life in the Universe. Its looking for technology in the cosmos. The super sophisticated tool building civilizations that are doing pretty cool things.
    Personally, Dysonspheres would be very useful to power ‘wormholes’ and ‘timegates’. This is more my science fiction speculations than anything attainable with actual engineering.
    I there was a low-technology version of myself that was prospering in the dark ages… I’d imagined super cities where all the inhabitants had pegasi and flying carpets, perfected alchemy where everything is made of gold.
    And genies and ‘elixirs’ for youth and health are as common as clouds and blades of grass… enlightened folk who live in Utopia.

    I think we are doing the 21st Century equivalent of making maps for the world with the locations of dragons and cyclopes live?

    But the flip side of the coin is… Intelligence with tool building is a sustainable activity in our Universe.
    I think it’s going to be hands down; that a few million years before Andromeda and our Milky Way collide… we will be meeting the neighbors?
    We have a few gigayears to work with? We are collective intelligent enough to make all this work. Or we will go extinct for lesser pursuits?
    I vote for the former on the ground of very selfish self-interest.

  • ljk February 11, 2014, 11:49

    Current SETI may be too provincial. How about SIPI:


  • Dave Moore February 11, 2014, 20:25

    @Jason Wright
    I was thinking, the the 10^-7 level of energy output to the host galaxy’s is not quite as bad as it seems.

    This was the ratio of the Kardashev III civilization’s energy to the total galactic energy output, but (we assume) the Kardashev III civilization’s output peaks at 10 microns whereas a galaxy’s energy is spread across the spectrum from gamma rays to radio waves. Therefor the ratio at 10 microns would be at lot less. (I would be interested to know what it is.)

    It would be a bit like trying to image exoplanets. You do it in the IR as the ratio of brilliance between the star and the planet is many orders of magnitude less than at optical wavelengths.

  • Jason Wright February 12, 2014, 10:02

    @David Moore:

    In the infrared, the contrast will scale as the fraction of waste heat times (T*/Twaste)^3, where T* is the typical stellar temperature of the stars (in a galaxy, that’s K giants) and Twaste is the ambient or waste heat temperature of the civilization. For T*=4000K and Twaste = 300K that works out to about 2000 x 10^-7 = 2 x 10^-4 = 0.02%. That’s impossible to distinguish from dust (which usually contributes MIR emission equivalent to roughly 1-20% of the galaxy (sometimes higher)).

    That is, if you did see 1.02% excess MIR emission you wouldn’t know if it was 1.02% dust or 1% dust plus 0.02% aliens, and you would have essentially no way to find out.

    Around an individual star, things are much better, since you’re talking about at least 10% of an M dwarf’s luminosity. In that case you have (3000 K / 300 K)^3 * 0.1 = 100 times the normal infrared output of an M dwarf. That will stand out like a beacon.

  • ljk February 12, 2014, 21:50


    Emerging Technology From the arXiv

    February 12, 2014

    How to Design an Interstellar Communications System

    If we want to communicate with other civilisations, it turns out that the laws of physics, the nature of interstellar space and a little common sense place surprisingly strict bounds on how this communication can take place.

    Here’s an interesting question. If we ever want to communicate with civilisations around other stars, what will be the best way to send a message, given that we will know nothing about how they intend to receive it?

    That’s the question considered today by David Messerschmitt at the University of California, Berkeley. It turns out that the laws of physics, the nature of interstellar space and a little common sense place surprisingly strict bounds on how communication can take place. So if extraterrestrials think in a way that is anything like us, communication of one kind or another is distinctly possible.

    Messerschmitt begins by listing the way in which any form of communication is likely to be limited. To begin with, he says that the power of any signal falls with the square of the distance travelled. Assuming that energy is likely to be a limiting factor for a civilisation, an important property of any interstellar transmitter will be to minimise the energy per bit in any signal–while still allowing the reliable extraction of the information it contains, of course.

    It’s possible that this civilisation could have found a way to generate energy very cheaply. But even if energy is more plentiful, there are many ways to consume more energy other than deliberate inefficiency, says Messerschmitt. “They could increase message length, reduce the message transmission time, transmit in more directions simultaneously, or transmit a signal that can be received at greater distances.”

    Another important design requirement will be to overcome any problems associated with transmitting signals through the interstellar medium. Specifically, certain wavelengths tend to be absorbed while others travel without hindrance. For example, the interstellar medium is essentially transparent to large parts of the microwave spectrum.

    In addition to this, says Messerschmitt, a good idea is to exploit the laws of physics and in particular Shannon’s mathematical theory of communication, which determines how much information can be sent to a noisy channel the certain power level. stop

    Finally, common sense dictates that we should keep things simple. The more complex the mode of transmission, the less likely it is that it will match the receiver. Keeping things simple seems the best way to ensure the greatest likelihood of success.

    This version of Occam’s razor also suggests that there is no reason to send narrow bandwidth signals since these require more energy and greater complexity to transmit. Instead the simplest approach is to send broadband signals, preferably in the microwave part of the spectrum. “It is unlikely that a civilization would use more energy than necessary unless for some reason they consider a reduction in bandwidth to be a higher priority,” says Messerschmitt.

    So that narrows down the huge number of potential signals that we could transmit or that we should look out for, to just a few. Provided, of course, that this other civilisation will be thinking in the way that seems obvious to us.

    One potential shortcoming is that the science and technologies that Messerschmitt invokes were all developed on Earth within the last century. That’s a blink of an eye in cosmological terms.

    Back in the 1970s, the American astronomer Carl Sagan pointed out that any alien civilisation is likely to be at a very different stage in its evolution. Should it be less mature than us, however, this civilisation will not have developed radio technology in the first place.

    That means that our potential contacts are likely to be much more advanced, probably centuries or millennia ahead of us. Sagan asked whether it is possible that these civilisations will have stumbled across a better form of interstellar communication technology, one that seems like magic to us.

    That’s a hard question to answer. But if so, then our attempts to contact them with microwaves may fall on deaf ears, regardless of how well designed our transmitters and receivers are.

    Ref: http://arxiv.org/abs/1402.1215 : Design For Minimum Energy In Starship And Interstellar Communication

  • Wojciech J February 18, 2014, 15:52

    “If we want to communicate with other civilisations, it turns out that the laws of physics, the nature of interstellar space and a little common sense place surprisingly strict bounds on how this communication can take place.”
    What would you like to communicate to civilization that likely is 50 or 100 million years ahead of you?

  • ljk February 19, 2014, 0:48

    Wojciech J said on February 18, 2014 at 15:52:

    “What would you like to communicate to civilization that likely is 50 or 100 million years ahead of you?”

    “We are here! We are here! We are here!” – The diminutive Whos in Horton Hears a Who

  • ljk April 15, 2014, 12:08

    Creating “believable” aliens: