[ExI] fermi question alive and well

Stuart LaForge avant at sollegro.com
Sat Mar 30 21:09:01 UTC 2019

Quoting John Clark:

> On Thu, Mar 28, 2019 at 11:56 AM Stuart LaForge <avant at sollegro.com> wrote:
>> *So how precisely do we distinguish between brown dwarfs and Dyson
>> swarms? *
> A star needs at least 75 Jupiter masses (or 8% of the sun's mass) for it to
> undergo nuclear fusion, a Brown Dwarf is less massive than that and thus
> would be a lousy power source because it is not a star.  The smallest true
> stars are M class stars but they can't be Dyson Spheres. If you plot the
> total energy output of a star against its color (principle wavelength
> outputted) you get a Hertzsprung Russell diagram.

Yes. I don't think you quite understood what I was saying. The  
spectrum of a Dyson swarm that is far away could be confused for a  
brown dwarf that is much closer. Its because of how apparent versus  
absolute magnitudes depend on the distances involved.

> This plot has been made
> for many millions of stars and the space on the diagram where Dyson Spheres
> should be is a big blank, no star has the needed combination of luminosity
> and color. For example a star that had the same luminosity as our sun but
> radiated most of its energy in the infrared would be a very strong
> candidate for being a Dyson Sphere, but nobody has ever seen such a thing;
> there are infrared stars but all of them are either thousands of times less
> luminous than the sun or thousands of times more luminous. In ever star so
> far discovered the relationship between luminosity and color can be
> explained with the standard theory of stellar evolution with no need to
> resort to ET.
> Hertzsprung Russell Diagram
> <https://cdn-images-1.medium.com/max/1600/0*k1vaMquGoiOVBiOE.jpg>

Wow. Ok yeah, Gaia is a wonderful instrument. It can measure distances  
to stars using parallax and get absolute magnitudes from that. That  
means it is a good detector for waste heat within the accuracy of its  
parallax measurements which are on the order of 10^4 light-years range.

Yet still the Gaia dataset only contains around a billion stars so  
that is less than one percent of the stars in the Milky Way.

Incidentally all this data from Gaia allows us to update our Bayesian  
priors regarding the existence of Dyson technology. Assuming the  
existence or non-existence of a Dyson sphere around given stars are  
measurements independent of one another, one gets that P(DS) the  
probability that a given star has a Dyson sphere in our galaxy has an  
upper bound of

P(DS) <= 1-[(n+1)/(n+2)] where n is the number of stars that Gaia has  

Assuming she has screened exactly 1 billion stars to date, the  
probability of the existence of one or more Dyson spheres in our  
galaxy is at most 1/1,000,000,001.

This could mean ET doesn't exist but I think it more likely means  
Dyson spheres are not practical to build.

This makes biological sense in a way. Senses, heads, and ultimately  
brains evolved to deal with the evolution of motility. Plants, fungi,  
sea anemones, and other stationary creatures never developed brains  
because their inactive lives did not need them. Fish on the other hand  
needed senses and brains to cope with avoiding obstacles and predators  
while swimming around at high speed.

Maybe jupiter brains that stay put around star systems vegetate and  
become stupid due to lack of stimulation.

>> *When it comes to ET, I don't think we know precisely what we are  looking
>> for or how to go about looking for it.*
> We are looking for a civilization that makes use of high frequency light
> and outputs low frequency light as a waste product, and we know exactly how
> to look for it, with infrared and microwave telescopes, but we've never
> seen even a hint of it.

Admittedly Gaia is a game changer, before her it was very easy for an  
astronomer to overlook a far away Dyson sphere by mistaking it for a  
nearby brown dwarf. Especially since the farther away it was the more  
likely light from it would pass through clouds of gas and pick up  
absorption lines.

Stuart LaForge

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