Carnot Efficient Dyson Spheres Are Undetectable by Infrared Surveys (2016)(tillett.info)
tillett.info
Carnot Efficient Dyson Spheres Are Undetectable by Infrared Surveys (2016)
https://www.tillett.info/2016/08/03/carnot-efficient-dyson-spheres-are-undetectable-by-infrared-surveys/
61 comments
> I'm not a member of an alien civilization
Entirely off topic, but I love how IANAL style disclaimers found their way to astrophysics discussions. IANAMOAAC anyone? I like it.
(Note: I really like it, this is not snarky sarcasm)
Entirely off topic, but I love how IANAL style disclaimers found their way to astrophysics discussions. IANAMOAAC anyone? I like it.
(Note: I really like it, this is not snarky sarcasm)
Heh - well, when we're discussing hypothetical super-advanced aliens with technology and capability to create Dyson spheres, it seems sufficiently outside the scope of my knowledge to justify the disclaimer. ;)
I'm not sure whether to be relieved or disappointed that a member of a super-advanced alien civilization is not actually posting on HN. I guess neither, since if you actually are an alien, you would probably make the same denial :)
Speaking as an alien, we don't have a concept of 'cost' the way humans do.
We long ago overcame biological limitations on our lifespans. Since then, the two things they matter to us are not dying by accident, and not getting bored. As you can imagine these can come into conflict on occasion. But building oversized Dyson spheres is fun and not particularly dangerous m.
We long ago overcame biological limitations on our lifespans. Since then, the two things they matter to us are not dying by accident, and not getting bored. As you can imagine these can come into conflict on occasion. But building oversized Dyson spheres is fun and not particularly dangerous m.
Aren't there long stretches of time where the nanoreplicators just do their thing and all you can do is watch and/or get another cup of space coffee? (somewhat like the computer program compiling processes on earth)
Or are you guys capable of slowing your perception of time so it seems to go really fast?
Or are you guys capable of slowing your perception of time so it seems to go really fast?
The reproductive cycle for quasi-immortal beings is necessarily quite time consuming.
We have created sub-universes with different laws of physics etc. When we get bored, we inject ourselves into the viewpoints of entities in those universes, after temporarily wiping memories of our true selves so we can re-exist as lesser, mortal beings.
I believe the closest equivalent human term is VR MMORPG.
I believe the closest equivalent human term is VR MMORPG.
You don't need to make a single sphere (swarm more likely). If you arrange the shells in layers the outer layers can efficiently capture the long wavelength radiation using relatively little matter.
The question is if you could capture the energy and you had the material resources would you let it escape into the universe at 290K?
The question is if you could capture the energy and you had the material resources would you let it escape into the universe at 290K?
> The question is if you could capture the energy and you had the material resources would you let it escape into the universe at 290K?
If the cost of acquiring and constructing the material into a collector were greater than the benefits of collecting a mere 4% more energy, absolutely!
Plus, for an advanced civilization, if given the choice between 99% efficiency around one star and 95% efficiency around a mere 100 stars - which is more likely to be done?
To make the analogy more relevant to humans: Why - when we're technologically advanced enough and have sufficient materials - do we let so much energy escape Earth?
If the cost of acquiring and constructing the material into a collector were greater than the benefits of collecting a mere 4% more energy, absolutely!
Plus, for an advanced civilization, if given the choice between 99% efficiency around one star and 95% efficiency around a mere 100 stars - which is more likely to be done?
To make the analogy more relevant to humans: Why - when we're technologically advanced enough and have sufficient materials - do we let so much energy escape Earth?
I think it depends entirely on the incentives of the civilization in question. If the primary goal was hiding your world from the Borg/Dalek/Alien bioweapon your species discovered was ravaging the galaxy, then a 99% Carnot efficient Dyson sphere seems like the ideal defensive structure.
No it doesn't.
Anything with sufficiently advanced technology to threaten a dyson sphere wielding species would probably detect the gravitational anomaly of the missing star and go investigate there first.
Taking a tiny portion of the energy from a bunch of stars seems wildly more efficient at surviving from a threat than putting all your resources into creating a system they're sure to investigate.
Anything with sufficiently advanced technology to threaten a dyson sphere wielding species would probably detect the gravitational anomaly of the missing star and go investigate there first.
Taking a tiny portion of the energy from a bunch of stars seems wildly more efficient at surviving from a threat than putting all your resources into creating a system they're sure to investigate.
That is similar to the plot point of one of those Star Wars prequels if I recall right
Or do something else with the 625x more resources you need? I'd probably let the 4% escape and do something nice with the 625x more resources.
You don't need 625x the material resources to capture the last 4%. Think multiple shells with the outermost shell being 1 atom thick.
What atoms - when arranged in a single-atom-thickness structure - could collect, transform, and transfer radiant energy back towards the inner shells, while also remaining rigid enough to not deform under the pressure of that radiant energy (not to mention the energy emitted by other stellar phenomena)?
For better or worse, we have to assume the alien civilizations have the same physics that we do, otherwise the civilization simply has to build their 1au sphere out of artificially created unobtainium to achieve 99% efficiency.
For better or worse, we have to assume the alien civilizations have the same physics that we do, otherwise the civilization simply has to build their 1au sphere out of artificially created unobtainium to achieve 99% efficiency.
Carbon.
The outer shell does not need to be a single structure. A swam of micro satellites orbiting at 25 AU with 1 atom thick carbon antenna collectors (think black snowflake) would on average be not much more than 1 atom thick and would collect 99% of the energy emitted by the star or inner shell(s).
This is all rather beyond what my post was about which is that only inefficient Dyson spheres can be detected in the infrared and that any civilisation capable of building a 95% efficient sphere is able to build one that is 99%.
The outer shell does not need to be a single structure. A swam of micro satellites orbiting at 25 AU with 1 atom thick carbon antenna collectors (think black snowflake) would on average be not much more than 1 atom thick and would collect 99% of the energy emitted by the star or inner shell(s).
This is all rather beyond what my post was about which is that only inefficient Dyson spheres can be detected in the infrared and that any civilisation capable of building a 95% efficient sphere is able to build one that is 99%.
The Three Body Problem (sci-fi book) had a similar puzzle in it--i.e. how can something as small as an atom have such a profound effect on our civilization? I won't spoil it, but it's well worth reading.
Thank you, I was going to make the same point.
Another point worth considering is that if the biology of the aliens were Earthlike, it would be nice to have the "native" intensity of sunlight (or close to it) available everywhere on the inside of the sphere.
Even if the biology were oriented to 4x less insolation, that would only be a 2x increase in sphere radius.
I think the "efficiency" concern is a bit silly also, since advanced alien civilizations will have advanced, high-density power sources available as needed. The only rationale I can see for very high efficiencies would be the desire to hide...and that might be a valid concern.
Another point worth considering is that if the biology of the aliens were Earthlike, it would be nice to have the "native" intensity of sunlight (or close to it) available everywhere on the inside of the sphere.
Even if the biology were oriented to 4x less insolation, that would only be a 2x increase in sphere radius.
I think the "efficiency" concern is a bit silly also, since advanced alien civilizations will have advanced, high-density power sources available as needed. The only rationale I can see for very high efficiencies would be the desire to hide...and that might be a valid concern.
> the costs don't seem worth the benefits, especially without some other form of energy to supplement the 650x increase in energy needs.
That's assuming the larger surface area is undesired, and the extra space won't be utilized for something else, like habitation, or storage.
That's assuming the larger surface area is undesired, and the extra space won't be utilized for something else, like habitation, or storage.
> I'm not a member of an alien civilization with the technology to create a Dyson sphere, but as a lowly human engineer, the costs don't seem worth the benefits
Unless they're hiding from something ... something terrible. Coming to a silver screen near you.
Unless they're hiding from something ... something terrible. Coming to a silver screen near you.
I've seen mention before of the Carnot efficiency of Dyson spheres, but what about Landauer efficiency? Assuming the DS is powering a bunch of computers, and assuming sufficiently advanced technology that these computers approach the limits of physics (reasonable for a Type 2 civilization imo), then their computers will use power proportional to kT. The colder the computer, the less power it uses. https://en.wikipedia.org/wiki/Landauer%27s_principle
This provides another thermodynamic reason to make a Dyson sphere as large as possible.
This provides another thermodynamic reason to make a Dyson sphere as large as possible.
The Wikipedia article you also has this to say about Landauer's principle:
"The principle is widely accepted as physical law; but in recent years it has been challenged^[how?], notably in Earman and Norton (1998), and subsequently in Shenker (2000)^[10] and Norton (2004,^[11] 2011^[12]), and defended by Bennett (2003)^[1] and Ladyman et al. (2007).^[13]"
So even now, a mere 56 years after its proposal, this principle is coming in to question. How much more questionable would it be after millions or even billions of years of physics research that alien civilizations may have under their belt by the time they start building these sorts of Dyson spheres? How many revolutions in physics knowledge might they have had by that point? They may be able to perceive reality in completely different ways than we do, and have unimaginable intelligence at their disposal.
It is certainly fun to speculate, but I personally find it very difficult to put limits on what beings like that might be capable of, especially when those purported limits are the results of mere human thought.
"The principle is widely accepted as physical law; but in recent years it has been challenged^[how?], notably in Earman and Norton (1998), and subsequently in Shenker (2000)^[10] and Norton (2004,^[11] 2011^[12]), and defended by Bennett (2003)^[1] and Ladyman et al. (2007).^[13]"
So even now, a mere 56 years after its proposal, this principle is coming in to question. How much more questionable would it be after millions or even billions of years of physics research that alien civilizations may have under their belt by the time they start building these sorts of Dyson spheres? How many revolutions in physics knowledge might they have had by that point? They may be able to perceive reality in completely different ways than we do, and have unimaginable intelligence at their disposal.
It is certainly fun to speculate, but I personally find it very difficult to put limits on what beings like that might be capable of, especially when those purported limits are the results of mere human thought.
Doesn't this also assume a computer similar to our own? How would this principle apply to a biological computer for example?
The Landauer principle applies to any computer made of matter utilizing energy (in a non-thermodynamically reversible fashion). So it applies just fine to biological computers, quantum computers, and even mechanical computers! It's a result in quantum information theory, and doesn't have anything to do with the overall type of computer used. It just determines how much energy it takes to change the state of a quantum (particle), at a minimum. Things like changing the polarization of a photon or the spin of an electron.
The Landauer limit applies to all non-thermodynamically reversible changes of state in a system. I don't know how your biological computer works, so I can't say 100% that it applies - but it probably does.
> Doesn't this also assume a computer similar to our own?
No. It is about abstract computation, which is based on (our best understanding of) theoretical physics.
No. It is about abstract computation, which is based on (our best understanding of) theoretical physics.
Not sure my mathematics is up to the task but
"using a relatively small increase in resources and time."
A sphere with 99% efficiency would need 25 larger radius according to the article, which is 625x more material assuming the thickness can be the same and doesn't need to be more be be structurally working.
625x to me doesn't sound like a "relatively small increase in resources".
But it looks like some people consider an increase of 62400% in resources a "small increase".
"using a relatively small increase in resources and time."
A sphere with 99% efficiency would need 25 larger radius according to the article, which is 625x more material assuming the thickness can be the same and doesn't need to be more be be structurally working.
625x to me doesn't sound like a "relatively small increase in resources".
But it looks like some people consider an increase of 62400% in resources a "small increase".
Where you are going wrong is assuming the outer layer has to be the same thickness as the inner layers. The outer layer only needs to capture the long infrared radiation emitted by the inner shells. In theory it only needs to be 1 atom thick and can contain gaps half the wavelength to be captured.
Why does the 1 AU Dyson sphere needs to be that much thicker (more than 625x thicker to be precise)?
The inner shells need to be thicker for three reasons - photon pressure, they are at a higher temperature, and the wavelengths they are capturing are shorter. Having said this they would not need to be much thicker than the outer layers.
So
a.) The inner layer is thicker than the outer layer, but not much
b.) The outer layer has a surface area 625x larger than the inner layer
c.) The outer layer needs only a "small increase in resources"
I still don't get it.
a.) The inner layer is thicker than the outer layer, but not much
b.) The outer layer has a surface area 625x larger than the inner layer
c.) The outer layer needs only a "small increase in resources"
I still don't get it.
The only thing we get to observe is the outer layer. We really have no way of knowing what is going on inside or what structures are present.
The relative small increase is not in reference to a smaller sphere, but to the material resources of the stellar system. If you only need 0.00000001% of the material resources of the system to build an inefficient sphere then using 625x more is not a significant increase.
The relative small increase is not in reference to a smaller sphere, but to the material resources of the stellar system. If you only need 0.00000001% of the material resources of the system to build an inefficient sphere then using 625x more is not a significant increase.
We tend to assume that one would want to be able to walk and live inside a 1AU Dyson sphere, so structurally it would have to be sufficiently strong to support such uses.
...put simply, the emission signal from efficient Dyson spheres will be swamped by infrared noise in any wide-field infrared surveys.
I am also not a member of an alien civilization with the technology to create a Dyson sphere, but this strikes me as being very good from the standpoint of such a civilization's operational security.
I am also not a member of an alien civilization with the technology to create a Dyson sphere, but this strikes me as being very good from the standpoint of such a civilization's operational security.
A civilization capable of building a Dyson Sphere wouldn't need to.
Possibly, but stars are a huge waste of energy.
A civilisation capable of building a Dyson sphere should be able to come up with a better use for a star than just lighting up the universe.
A civilisation capable of building a Dyson sphere should be able to come up with a better use for a star than just lighting up the universe.
My guess is that by the time any civilisation gets advanced enough to build a Dyson sphere, ennui has set in to the point where they don't bother.
Any civilization advanced enough to build a Dyson Sphere would by definition be one capable of extreme, long term focus. Since ennui is the enemy of long term focus, they'd probably have conquered it.
yeah. antimatter, dude.
"The good news there is a different approach for finding efficient Dyson sphere, but that is another post." Tease!
Did he ever write that other post? If so it must have a high Carnot efficiency because I can't find it /rimshot
Did he ever write that other post? If so it must have a high Carnot efficiency because I can't find it /rimshot
Sorry I haven’t got around to writing it yet as I have been too busy with work - actually I have not written an article on anything this year.
I'd guess: survey for sources at stellar brightness and low temperature. Dunno if it's been done.
The article posits that maybe advanced alien civilizations may be motivated to build larger (and hence harder to to detect) Dyson spheres for the efficiency gains. While that's a possibility, it's also possible that being harder to detect is an end unto itself for safety reasons.
I'm thinking what's remarkable about this is it could be a potential explanation of dark matter.
It had always been ruled out previously because astronomers said they'd be easily detectable.
It had always been ruled out previously because astronomers said they'd be easily detectable.
So any candidate locations where such Dyson Spheres may currently exist? Can we detect them based on their gravity (i.e. objects orbiting it without the centripetal force accounted for)?
There's this thing: http://www.wired.co.uk/article/dyson-megastructure-mystery-d...
Though who knows what it actually is.
Though who knows what it actually is.
Yes - distant galaxies. This was to be a follow up post that I have just not got around to writing as I have been so busy.
maybe all the black holes are dyson spheres?!
So, just musing here, no actual knowledge on any of this: what if a Dyson sphere was 100% efficient and feed its energy back into it's power-source. Could this sphere exists for all of eternity? Trapping all energy inside and recycling it?
The second law of thermodynamics makes 100% efficiency impossible. The Carnot efficiency is just a measure of how close you can get to 100%.
It must be a slow day on HN for this post of mine to make the front page :)
It must be a slow day on HN for this post of mine to make the front page :)
Is there any way you could explain to the uninformed, like myself, where the energy would leak? Is it impossible to make a 100% isolated sphere? Would radiation or other kind of waves always pass through?
If it was 100% isolated, the heat from the sun would continually increase the internal temperature, which would probably be bad. They're looking for a steady state dyson sphere.
The energy is converted from short wavelength (visble light) to longer wavelengths (infrared). A Dyson sphere would act as a black body [1] and radiates energy at the temperature of the sphere outer surface.
1. https://en.m.wikipedia.org/wiki/Black-body_radiation
1. https://en.m.wikipedia.org/wiki/Black-body_radiation
The problem with a 100% isolated sphere that you can't dump entropy either.
Energy can't be recycled in that sense. There is a maximum amount of useful work which can be extracted from a given source of energy - this is called the "exergy." Exergy is not conserved. Once all the exergy is gone, it doesn't matter how much energy there is; it can't be moved around or manipulated in any way.
https://en.wikipedia.org/wiki/Exergy
https://en.wikipedia.org/wiki/Exergy
That would be equivalent to a perpetual motion machine, so I don't think it's possible for thermodynamical reasons.
No, because it cannot be 100% efficient. If it were you would have a perpetual motion machine.
An exponential increase in size is not "a relatively small increase in resources and time." It goes from "something that could be built with all the mass in the solar system" to "something that requires the mass from an additional couple of hundred solar systems to construct".
I'm also curious about whether a Dyson sphere which would roughly equal Neptune's orbit would have sufficient sunlight at any given point to sustain life (at least, life as we know it).
I'm not a member of an alien civilization with the technology to create a Dyson sphere, but as a lowly human engineer, the costs don't seem worth the benefits, especially without some other form of energy to supplement the 650x increase in energy needs.