ETI and dark matter

A very wild though came to me, so please to kill me if I'm talking nonsense.

What if ETIs found a way to use the energy of a star for their own benefit,
and that is why we don't see a good portion of the matter in the Galaxy ?

For example, they could have formed Dyson spheres (solved the instability
problem) around stars and thus absorb all the star light. The only evidence
of the star would be in deep infrared (radiation of the Dyson sphere's
temperature).

If done on truely gallactic scale, then intelligent life could be
responsible for the missing 40% of matter in the Galaxy, which we currently
address to (so far theoretical) "dark matter"....

OK. Shoot me.

Rob

I just noticed that Pete Lynn had the exact same though and posted it in a
side-thread half an hour before me.

If two people have the same thought in such a short time span, then it is
likely that this idea came up before.

So did we already discuss this at some point in time, and is there any
observational data in favor or against this idea (ETI responsible for dark
matter), as far as anyone knows ?

Rob

Wasn't it Rob Dekker who wrote:
A very wild though came to me, so please to kill me if I'm talking nonsense.

What if ETIs found a way to use the energy of a star for their own benefit,
and that is why we don't see a good portion of the matter in the Galaxy ?

For example, they could have formed Dyson spheres (solved the instability
problem) around stars and thus absorb all the star light. The only evidence
of the star would be in deep infrared (radiation of the Dyson sphere's
temperature).

If done on truely gallactic scale, then intelligent life could be
responsible for the missing 40% of matter in the Galaxy, which we currently
address to (so far theoretical) "dark matter"....

OK. Shoot me.

That scenario would be detectable by the observations that were used to
test for the MACHO theory of dark matter. If there were a significant
number of Dyson spheres in the galactic halo, then those observations
would have detected large numbers of gravitational lensing effects as
they passed in front of distant stars.

--
Mike Williams
Gentleman of Leisure

In message , Mike Williams
writes
Wasn't it Rob Dekker who wrote:
A very wild though came to me, so please to kill me if I'm talking nonsense.

What if ETIs found a way to use the energy of a star for their own benefit,
and that is why we don't see a good portion of the matter in the Galaxy ?

For example, they could have formed Dyson spheres (solved the instability
problem) around stars and thus absorb all the star light. The only evidence
of the star would be in deep infrared (radiation of the Dyson sphere's
temperature).

If done on truely gallactic scale, then intelligent life could be
responsible for the missing 40% of matter in the Galaxy, which we currently
address to (so far theoretical) "dark matter"....

OK. Shoot me.

That scenario would be detectable by the observations that were used to
test for the MACHO theory of dark matter. If there were a significant
number of Dyson spheres in the galactic halo, then those observations
would have detected large numbers of gravitational lensing effects as
they passed in front of distant stars.

FWIW, my favourite fictional version of this idea is in Fritz Leiber's
"The Wanderer", where
'the planets are so thick around each sun they shroud its light... It
is the boast of our engineers "Wherever a sunbeam escapes, we place a
planet"... You haven't heard this news, simply because of the snaily
slowness with which light travels. If you could wait a billion years,
you'd see the galaxies grow dim, not by the death of stars, but by the
masking and miserly hoarding of their light by the stars' owners".
--
What have they got to hide? Release the ESA Beagle 2 report.
Remove spam and invalid from address to reply.

Jonathan Silverlight wrote:
If you could wait a billion years,
you'd see the galaxies grow dim, not by the death of stars, but by
the
masking and miserly hoarding of their light by the stars' owners".

Yeah, and Larry Niven's "ringworld" or Dyson spheres can also blot out
light in similar way.

"Mike Williams" wrote in message ...
[....]
That scenario would be detectable by the observations that were used to
test for the MACHO theory of dark matter. If there were a significant
number of Dyson spheres in the galactic halo, then those observations
would have detected large numbers of gravitational lensing effects as
they passed in front of distant stars.


Interesting.
However, the MACHOs they are looking for are considered small (smaller than the stars behind them) and rather dense.
A Dyson shere will be large (range of 1AU radius), and thus much larger than the stars behind it. They are not so dense, since all
it's mass is concentrated in the center (the star that it encircles).
I don't know much about the physics of microlensing, but common sense tells me that a Dyson sphere might actually block the
starlight behind it more than 'microlense' it.
If that is so, stars behind it will fluctuate in strength, but will go down in intensity rather than get a microlense boost...
Stars behind a Dyson sphere would then show up more as 'variable' stars... And a lot of 'variable' stars have been detected.
Does this make sense ?

--
Mike Williams
Gentleman of Leisure

Wasn't it Rob Dekker who wrote:

"Mike Williams" wrote in message news:MUxgKBA5NDwBFw
...
[....]
That scenario would be detectable by the observations that were used to
test for the MACHO theory of dark matter. If there were a significant
number of Dyson spheres in the galactic halo, then those observations
would have detected large numbers of gravitational lensing effects as
they passed in front of distant stars.


Interesting.
However, the MACHOs they are looking for are considered small (smaller than the
stars behind them) and rather dense.
A Dyson shere will be large (range of 1AU radius), and thus much larger than the
stars behind it. They are not so dense, since all
it's mass is concentrated in the center (the star that it encircles).
I don't know much about the physics of microlensing, but common sense tells me
that a Dyson sphere might actually block the
starlight behind it more than 'microlense' it.
If that is so, stars behind it will fluctuate in strength, but will go down in
intensity rather than get a microlense boost...
Stars behind a Dyson sphere would then show up more as 'variable' stars... And a
lot of 'variable' stars have been detected.
Does this make sense ?

Good point.

I've now looked into this and it seems like you'd get a fairly normal
looking microlensing effect as long as the radius of the Dyson sphere is
significantly smaller than its Einstein Ring Radius. I think you get a
fairly normal looking occultation if the radius of the Dyson sphere is
significantly larger than its Einstein Ring Radius. I suppose that if
the radii are fairly similar in size then you get some sort of hybrid
effect.

The Einstein Ring Radius depends on the mass of the lensing object and
the distances to the lens and to the lensed star. For example, if the
lensing object has the same mass as our Sun at a distance of 2
kiloparsecs from here and the light source is very distant, then the
Einstein Ring Radius is about 4 AU.

E = 4.03 * sqrt(M * D/2) AU

Where "E" is the Einstein Ring Radius
"M" is the mass of the lens in Solar Masses
"D" is the "reduced distance" the lens in kiloparsecs

The "reduced distance" is given by
D = Dlens * (1 - Dlens/Dsource)
If the source is very remote (as it typically was in the MACHO searches)
then the reduced distance becomes close to the actual distance.


Note also that the MACHO observations didn't throw away the data that
came from "variable stars". They passed information about thousands of
such observations on to astronomers who happen to be interested in
studying variable stars. If there were lots of Dyson Sphere occultations
in that data then someone should have spotted an interesting pattern.

--
Mike Williams
Gentleman of Leisure

Mike
Thanks for checking the physics of micro lensing on this !
Where did you find the formula's ?

Any way, I guess you are right in that if half the Galaxy is filled with
Dyson spheres, that someone would have found some consistent pattern. Not
just in the MACHO search, but also in infrared I guess.

I also realized that if Dyson speres (or other ETI influence) is responsible
for missing matter, than at least we should see wild variations in missing
matter in other Galaxies...

So, it seems that intelligent life never makes it to the ultimate stage of
absorbing all energy in a Galaxy for its own benefit (unless we have
overlooked some 'dark' galaxies that only radiate in balmy infrared).

In our Galaxy, life did not make it to using (==transforming) a substantial
part of energy.

Finally one more 'quick' question : it seems that MACHO search is
concentrated on the 'halo' of the Galaxy. Why is that ? Can the 'missing'
matter in the Galaxy only be explained in the halo ? What if it is uniformly
distributed among the stars (thus, in a disk) ?
Or are we looking mainly in the halo because it is so nice and dark there ?

Rob

Wasn't it Rob Dekker who wrote:
Mike
Thanks for checking the physics of micro lensing on this !
Where did you find the formula's ?

Google found lots of sites. These are the two that I fond most useful:

http://spiff.rit.edu/classes/phys240...microlens.html
http://www.journals.uchicago.edu/ApJ...729/34729.html


Any way, I guess you are right in that if half the Galaxy is filled with
Dyson spheres, that someone would have found some consistent pattern. Not
just in the MACHO search, but also in infrared I guess.

I also realized that if Dyson speres (or other ETI influence) is responsible
for missing matter, than at least we should see wild variations in missing
matter in other Galaxies...

There may well be wild variations in the dark matter ratios of different
galaxies.

This month's Sky At Night TV program included a short section with a guy who has
been using the VLT to investigate dark matter in the dwarf galaxies of the local
group. He mentioned that the dwarf galaxies have a vastly higher dark matter
ratio than our own galaxy. The Internet copy of the program hasn't gone online
yet at:
http://www.bbc.co.uk/science/space/s...proginfo.shtml

I've tried several Google searches to try to find any information on how
variable the dark matter ratios of different galaxies are, but have failed to
find anything.

So, it seems that intelligent life never makes it to the ultimate stage of
absorbing all energy in a Galaxy for its own benefit (unless we have
overlooked some 'dark' galaxies that only radiate in balmy infrared).

Google finds lots of information about theoretical dark matter galaxies, and a
few observations that might possibly indicate a lensing event caused by a dark
matter galaxy.

In our Galaxy, life did not make it to using (==transforming) a substantial
part of energy.

Finally one more 'quick' question : it seems that MACHO search is
concentrated on the 'halo' of the Galaxy. Why is that ? Can the 'missing'
matter in the Galaxy only be explained in the halo ? What if it is uniformly
distributed among the stars (thus, in a disk) ?
Or are we looking mainly in the halo because it is so nice and dark there ?

What we do is to plot the observed speeds at which the stars orbit various
galaxies against their distance from the centre. We then model the galaxy as if
it were built from four simple components:-
Bulge modelled as a uniform sphere
Disc modelled as a uniform thin disc
Halo modelled as a large uniform sphere
Nucleus modelled as a small uniform sphere
We then adjust the values for the mass and diameter of the components of this
model until we get something that comes close to matching the observed curve. It
happens to turn out that this always requires most of the mass to be in the Halo
component.

There's a Java applet here that shows how this works for one component of such a
model. http://www.astro.queensu.ca/~dursi/dm-tutorial/rot-vel.html

I've got a Java applet from Manchester University that allows me to do the same
sort of thing but with all four components. MU don't seem to have ever made
their applet available to the public.

--
Mike Williams
Gentleman of Leisure

"RD" == Rob Dekker writes:

RD Any way, I guess you are right in that if half the Galaxy is
RD filled with Dyson spheres, that someone would have found some
RD consistent pattern. Not just in the MACHO search, but also in
RD infrared I guess.

Yes. There have been some targetted searches for IR signatures of
Dyson spheres, all of them negative. Also, I think that if the Galaxy
were filled with lots of 300 K, 1-AU "spheres," they would have been
detected by IRAS, 2MASS, DENIS, ....

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