My son (15) invented what seems me to be a new form of inter-stellar communications, that we've dubbed "Dark Star". The idea is to deploy a swarm of small (perhaps a bit larger than nano, for reasons you'll see in a moment) satellites in the space between the asteroid belt and (let's say) the orbit of Pluto. These are the so-called "Dark Stars." The job of the Dark Star swarm is to send an inter-stellar signal by dynamically BLOCKING the narrow rays of light transiting through our solar system. Looking at, say, the Alpha Centuri system with even a moderately strong camera for only a few minutes of exposure one can see many stars that appear to be within that system that are actually far behind it -- that is, we're seeing narrow beams of light from those distant stars transiting the Alpha Centauri system. Someone there looking at our system would see something similar (the two systems being similar in size, although obviously the pattern of background stars would be different from different points of view). The Dark Star swarm would be programmed to dynamically block those transiting beams of light that would reach a distant system at specific times in the future. One could create a temporal pattern on a single ray, or block multiple rays to form a very complex pattern -- even an inter-stellar video of sorts, and a moderately large Dark Star satellite would not need the precise time-space targeting that a very tiny one would. Obviously there are many complex calculations involved in such a communications system, but it seemed novel. Has anyone previously suggested this sort of thing? We've written a short paper on it. Is there a forum for such speculative ideas that we could submit it to?
-
$\begingroup$ Is the intent to communicate across an interstellar civilization or to see if anyone is out there? $\endgroup$– Darth PseudonymCommented Jun 18 at 15:07
-
3$\begingroup$ [+1] cool question and kudos for creatively engaging youthful minds! About "narrow beams of light", if I look at a start the beam is 6 mm in diameter near my eye (or a few meters if I have a telescope) but it's really many "beams" from different areas of that star's surface, so it's stellar-sized at the other end. Something half way in the middle would have to be half the diameter of the star to fully block it, and 1/1000 the diameter of the start ot modulate the light intensity by even 1/1,000,000 (based on relative areas) I don't see how anything short of a SciFi megastructure can work here. $\endgroup$– uhohCommented Jun 19 at 1:05
-
$\begingroup$ @DarthPseudonym Well, both, I suppose. Because the process does not require either high power or interstellar flight, initially we would be sending, but we could also be watching now for others who are sending, and then later for those sending back messages they have received from us. In fact, it occurs to me that we could be looking for this now by trying to find weird patterns of occlusion, not of any star, but of stars whose rays transit exo-systems. Probably many, perhaps all do so, and perhaps many times before reachng us. (It less likely that multiple such would be using this method.) $\endgroup$– jackisquizzicalCommented Jun 19 at 15:01
-
1$\begingroup$ @jackisquizzical when you write “the Alpha Centauri system”, do you mean AB, ABC, or even AB+beta? If you see thousands of stars with a “moderately strong camera” I fear you are considering ABC or AB+beta. See the first picture (and associated caption) in en.wikipedia.org/wiki/Alpha_Centauri for reference of who is who in there. If you mean ABC then the distance between AB and C is currently 13000 AU, so the “rays” of most of those stars are actually passing AB or C at distances far far greater than the size of our solar system. $\endgroup$– jcaronCommented Jun 20 at 22:36
-
1$\begingroup$ @jackisquizzical - no, the background stars are tiny, even if we look at them with tekescopes :) The problem is, at the distance of 4 light years the light blockers are even smaller than the background stars. The things should be very big to completely occult a background star in your scheme, a planet-size. $\endgroup$– HeoppsCommented Jun 21 at 9:44
2 Answers
I don't know if this counts, but a more static, repeating version has been proposed by David Kipping in his Intertemporal Communication video, where megastructures with gaps in the structure corresponding to number sequences are used to create intensity curves that could be read by something like Kepler.
The answer here talks about the prior art, but lets throw some maths at this. Warning, terrible rounding ahead!
If we want to fully block out the sun to a specific point 5 light years away, working in Astronomical units.
Five light years is 316205.5 AU The sun is 0.0093 AU across Asteroid belt is ~3AU out
So our shadow array needs to be 0.99999 the size of the sun, which is quite large. If we do not know the area to be shadowed, then we need to make it bigger, thankfully not that much, about 1.01 times the size of the sun.
The shadow system runs the full diameter of that 3AU belt orbit, making it 18.84 AU long, or 2827433388km.
Some solar sail numbers get 0.7 grams per square meter, or 700kg per square km. For full occlusion our total area is 2827433388km * 1400000km for 3.98 * 10^15 square km, for a system mass of 2.77 * 10^15 tonnes. This is a big number, but there is enough stuff in the asteroid belt to make at least one of these if fully re-purposed.
Note that:
This assumes random asteroids can be refined into ultra high performance solar sail material at high efficiency
that nobody has other uses for most of the easily accessible mass in the solar system
That full occlusion is required - halving the width gets half the mass/cost/effort but more than half occlusion (since sun is circular and we are cutting across the middle)
That our default message is 'dark' - half light/half dark further halves the weight.
That all messages are being sent in the baseline belt orbit - doing plane changes by solar sail is very slow, taking decades and most of the candidate places we might send to are not helpfully aligned with our solar system.
per above this does not seem to make a very good 'life was here' beacon as from most directions it will be dark, and light that is present will have same spectra as the sun.
The data rates of this system will be slow. The baseline transition time for this system is 'darkness' to cross the full sun width. At 18kms crossing the sun for an outside observer will take most of a day. Having the shadows able to act as shutters allows things to be much faster, but means shadow sails must be much heavier to change geometry in minutes or seconds. Even with mechanical shuttering video data speeds would appear impossible.
In general it would seem easier to build a ~1 sun area mirror and use it to reflect additional light to a target than try to send data in shadow. For similar levels of effort solar panels and laser based transmission would get higher data rates.
Certainly it would not seem plausible for a current or near future earth civilization to build, though one that is already building dyson swarms for solar energy collection might choose to use them to send messages, if only for novelty value (maybe selling advertising spots?).
-
$\begingroup$ Thank you for this detailed analysis. I may be mis-understanding you a bit here, or else I didn't make my question clear enough (probably a bit of both! :-) We're not proposing to block out Sol (our sun), but rather the light from suns that are far behind our sun that pass through our solar system on their way to another. $\endgroup$ Commented Jun 19 at 14:57