
Carnot Efficient Dyson Spheres Are Undetectable by Infrared Surveys (2016) - mrfusion
https://www.tillett.info/2016/08/03/carnot-efficient-dyson-spheres-are-undetectable-by-infrared-surveys/
======
falcolas
So, for that additional 4% efficiency, the radius must be increased by 25x;
making the total surface area somewhere around 650x bigger?

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.

~~~
danieltillett
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?

~~~
falcolas
> 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?

~~~
tboughen
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.

~~~
SomeStupidPoint
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.

~~~
sfifs
That is similar to the plot point of one of those Star Wars prequels if I
recall right

------
schiffern
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](https://en.wikipedia.org/wiki/Landauer%27s_principle)

This provides another thermodynamic reason to make a Dyson sphere as large as
possible.

~~~
O5vYtytb
Doesn't this also assume a computer similar to our own? How would this
principle apply to a biological computer for example?

~~~
SAI_Peregrinus
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.

------
_Codemonkeyism
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".

~~~
danieltillett
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.

~~~
_Codemonkeyism
Why does the 1 AU Dyson sphere needs to be that much thicker (more than 625x
thicker to be precise)?

~~~
danieltillett
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.

~~~
_Codemonkeyism
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.

~~~
danieltillett
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.

------
xenophonf
_...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.

------
woofyman
A civilization capable of building a Dyson Sphere wouldn't need to.

~~~
taneq
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.

~~~
korussian
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.

------
outsidetheparty
"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

~~~
danieltillett
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.

------
zeroer
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.

------
mrfusion
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.

------
amelius
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)?

~~~
pilsetnieks
There's this thing: [http://www.wired.co.uk/article/dyson-megastructure-
mystery-d...](http://www.wired.co.uk/article/dyson-megastructure-mystery-
deepens)

Though who knows what it actually is.

------
dynofuz
maybe all the black holes are dyson spheres?!

------
neals
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?

~~~
danieltillett
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 :)

~~~
neals
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?

~~~
jonlawlor
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.

