Dyson Sphere

[jbjr]

I've got a question for all the astronomers out there (professional or amateur).

It seem like recently our discovery of extra solar planets has been increasing at an ever increasing rate. At our current level of technology and with our current understanding of techniques used to locate extra solar planets would it be possible for us to see a Dyson sphere/swarm around another star?

I know there are a lot of unknowns to this question so i guess i'm looking for an educated guess. Would we be able to notice the dimming of a distant star that would result from this kind of technology (assuming it exists at all) or would the machines involved be too small for us to notice even in great quantity.

 

[MG1962]

You are going to have to wait for the Webb telescope to be launched before we can answer any of your questions. A Dyson Sphere closed or otherwise would only radiate energy in the infra-red.

Dimming stars - thats easier to answer. There is an ongoing intensive program stretching back over a 100 years looking for stars that change their brightness. So stars to a reasonable distance would be noticed pretty quickly if they began to pulsate or dim to any great measure.

How we would see that from Earth would depend on the construction process the alien race would use - and that I can only guess at

 

[Hercules Rockefeller]

If the point of a DS is to capture nearly all of the energy from a star, I would say no.

 

[jbjr]

Hm. I hadn't even considered the energy it would radiate. That would theoretically allow them to be detected even if they were capturing most (or all) of the the star's energy wouldn't it? I suppose it would depend on the method or transmission of the energy that was being collected.

 

[MG1962]

Hm. I hadn't even considered the energy it would radiate. That would theoretically allow them to be detected even if they were capturing most (or all) of the the star's energy wouldn't it? I suppose it would depend on the method or transmission of the energy that was being collected.

Infrared

 

[catsmate]

Hm. I hadn't even considered the energy it would radiate. That would theoretically allow them to be detected even if they were capturing most (or all) of the the star's energy wouldn't it? I suppose it would depend on the method or transmission of the energy that was being collected.

I can't see any technological civilisation not producing a lot of waste heat, this may be better detectable with the Webb telescope.
Fermilab has been scanning the IRAS data for some years for possible candidates for Dysonian megastructures, with seventeen possible candidates detected; however their data is weak, due to the limited ability to filter out other objects with spectra similar to that postulated. There are other searches
And a nitpick, the idea of a solid spherical shell around a star, commonly referred to as a Dyson Sphere, is technically a Dyson Shell; a classic Dyson Sphere is a network of solar collectors encompassing a star.

 

[MG1962]

I can't see any technological civilisation not producing a lot of waste heat, this may be better detectable with the Webb telescope.
Fermilab has been scanning the IRAS data for some years for possible candidates for Dysonian megastructures, with seventeen possible candidates detected; however their data is weak, due to the limited ability to filter out other objects with spectra similar to that postulated. There are other searches
And a nitpick, the idea of a solid spherical shell around a star, commonly referred to as a Dyson Sphere, is technically a Dyson Shell; a classic Dyson Sphere is a network of solar collectors encompassing a star.

Which would raise a red flag for astronomers - once they realize the spectral class of the star is not matching the output in magnitude observed

 

[rjh01]

The problem with a Dyson Sphere is that they are unstable.If one part of the sphere came closer to the star than the opposite end then that it would keep getting closer until either the sphere tore itself apart or smashed into the star.

 

[MG1962]

The problem with a Dyson Sphere is that they are unstable.If one part of the sphere came closer to the star than the opposite end then that it would keep getting closer until either the sphere tore itself apart or smashed into the star.

Exactly the same problem ringworlds have

 

[nathan]

The problem with a Dyson Sphere is that they are unstable.If one part of the sphere came closer to the star than the opposite end then that it would keep getting closer until either the sphere tore itself apart or smashed into the star.

Exactly the same problem ringworlds have

Aren't they different problems? A Dyson sphere is balanced no matter where the star is, whereas a ringworld is unstable unless the star is exactly at the centre.

Consider a small movement of the star from the geometric center.

For a spherical shell the additional gravitational pull of a patch of the now nearer surface is exactly compensated by the increased area of the patch subtending the same solid angle on the opposite side. (Newton used this to prove that spherical objects can be considered as points from the POV of gravitational attraction, btw.)

For a ring, the patch on the opposite side only grows linearly, but the attraction of the nearer side grows quadratically. Hence it'll crash and burn.

<insert obligatory ringworld engineers reference>

 

[TheGoldcountry]

The problem with a Dyson Sphere is that they are unstable.If one part of the sphere came closer to the star than the opposite end then that it would keep getting closer until either the sphere tore itself apart or smashed into the star.

I think know I might be out of my depth here, but wouldn't the speed of rotation around the star balance? Any orbit will decay eventually, but the velocity of the orbiting body should theoretically keep it stable for a few thousand years. (Like I said, there are smarter people than me here, I'm just throwing out my two cents)

 

[nathan]

I think know I might be out of my depth here, but wouldn't the speed of rotation around the star balance? Any orbit will decay eventually, but the velocity of the orbiting body should theoretically keep it stable for a few thousand years. (Like I said, there are smarter people than me here, I'm just throwing out my two cents)

You can't treat a point on the sphere as-if it was a separate object in free-fall around the star. The Sphere's a rigid body -- if it wasn't the poles would immediately fall into the star.

Regular orbits are ellipses -- although a circular orbit is theoretically stable, I doubt there are any perfectly circular orbits in reality, because even minor perturbation would give some eccentricity. Points on the Sphere are, by definition, moving in circular paths around an axis. The stability issue is what happens if that axis drifts away from the central star.

 

[catsmate]

Which would raise a red flag for astronomers - once they realize the spectral class of the star is not matching the output in magnitude observed

Yep. Plus if the shell of collectors wasn't all encompassing, i.e. when it was being constructed, there'd be very noticeable changes.

The problem with a Dyson Sphere is that they are unstable.If one part of the sphere came closer to the star than the opposite end then that it would keep getting closer until either the sphere tore itself apart or smashed into the star.

Exactly the same problem ringworlds have

If you can build a Dyson Sphere (or a ringworld) you can find a means of correcting for this; big plasma engines, a la Niven.

 

[Crossbow]

I've got a question for all the astronomers out there (professional or amateur).

It seem like recently our discovery of extra solar planets has been increasing at an ever increasing rate. At our current level of technology and with our current understanding of techniques used to locate extra solar planets would it be possible for us to see a Dyson sphere/swarm around another star?

I know there are a lot of unknowns to this question so i guess i'm looking for an educated guess. Would we be able to notice the dimming of a distant star that would result from this kind of technology (assuming it exists at all) or would the machines involved be too small for us to notice even in great quantity.

I would think that it would depend upon the people/beings/what-have-you that built the said Dyson Sphere.

After all, if there is a culture that is advanced enough to build such a thing, then I am sure that this culture could build it in such a way to hide it from us Earthers who have a far, far, far lower level of technological development.

Or, if there is a culture that is advanced enough to build such a thing, then they might want to brag about the fact, and thereby make it easier for beings such as us Earthers to detect their Dyson Sphere.

In either case, I would have to say it is up to the Dyson Sphere builders if they want out-worlders to know about their Dyson Sphere or not.

 

[kalen]

...

Consider a small movement of the star from the geometric center.

For a spherical shell the additional gravitational pull of a patch of the now nearer surface is exactly compensated by the increased area of the patch subtending the same solid angle on the opposite side. (Newton used this to prove that spherical objects can be considered as points from the POV of gravitational attraction, btw.)

I'm with you on this. For a spherically symmetric star and Dyson sphere, the net force between the star and sphere would be zero no matter where the star was located inside the sphere.

Ignoring radiation pressure and solar wind, this is still an unstable arrangement as a small perturbation would set the sphere drifting relative to the star at constant velocity until a collision happened.

 

[catsmate]

I'm with you on this. For a spherically symmetric star and Dyson sphere, the net force between the star and sphere would be zero no matter where the star was located inside the sphere.

Ignoring radiation pressure and solar wind, this is still an unstable arrangement as a small perturbation would set the sphere drifting relative to the star at constant velocity until a collision happened.

Given that a truly spherical star, or a completely symmetric mass distribution is unlikely I'd say some form of correction for the orbit of the sphere would be essential. Assuming you could build a sphere in the first place I doubt a correction mechanism would be difficult.

 

[Solitaire]

Light on the inside, gravity pulls you into the sun.
(The gravitational attraction of the sphere does hold you to it.)
Dark on the outside, but gravity holds you on the sphere.
(Of course you've got to keep moving so you don't wind up in the sun again.)

 

[nathan]

Light on the inside, gravity pulls you into the sun.
(The gravitational attraction of the sphere does hold you to it.)
Dark on the outside, but gravity holds you on the sphere.
(Of course you've got to keep moving so you don't wind up in the sun again.)

I'm not sure what you're trying to say in your parenthesized sentances. The first one is wrong and the second makes no sense.

An object inside the sphere feels no gravitational attraction to the sphere. See my above post about the neutral stability of the sphere itself -- it's the same argument. If you lived on the inside, you'd need something else to stop you falling into the star. Centrifugal force would fall off as you moved away from the equator, and would be perpendicular to the axis of rotation, not perpendicular to the sphere's surface.

 

[CapelDodger]

Ignoring radiation pressure and solar wind ...

My instinct is that we can ignore those, since they depend on an inverse-square law - the same kind of law which leads to "no gravitational attraction to the sphere", as nathan points out.

 

[CapelDodger]

For a spherical shell the additional gravitational pull of a patch of the now nearer surface is exactly compensated by the increased area of the patch subtending the same solid angle on the opposite side. (Newton used this to prove that spherical objects can be considered as points from the POV of gravitational attraction, btw.)

That was the first mathematical result I ever recognised as being beautiful (although the word I'd have used at the time was "neat" ).