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Earth, present time or very near future. Astronomers have just announced that it is believed that almost 9 billion habitable Earth-like planets exist in the Milky Way alone (this is true, but there is much scepticism over the number).

World bodies convene, and decide that we should try and communicate our presence, and think about ways we could catch the attention of another civilisation.

Now, the most obvious part of our solar system is the Sun itself, and my question is this - how would you, using current or soon feasible technology, alter the luminosity of the Sun in a way that is:

  1. Noticeable from at least 100 light years away (think about the fluctuations we can detect in stars a similar distance away).
  2. Doesn't cause permanent change to in the Sun (effects are temporary).
  3. Doesn't negatively impact life on our planet (effect is either not dangerous or too short an interval to damage us).

I'm thinking something that causes to Sun to flare brighter or dimmer for a short period of time, or maybe even changing the colour it emits of that is possible, by introducing foreign objects into the Sun, but I'm open to any ideas! Obviously the more feasible and repeatable over time, the better (I'm hoping this could be a sustainable form of communication), so if your method is dangerous, but not in short bursts, please consider the interval required between repeats in order to maintain safe use.

Note that an answer saying "you can't do this without causing damage to us or the Sun" is perfectly acceptable, provided it's true!

Things I'm not looking to consider:

  1. The method of communication - that's a different problem. I just want to be able to make the Sun look different from very far away for a short but detectable period of time, should a people with similar levels of technology to us be looking.

  2. The wisdom of doing it in the first place - no "just inviting invasion" discussions please.

  3. "The chances that someone would see it are tiny" or "probably no one would see it for hundreds of years". I'm not interested in the probability of life on other planets.

P.S. I know a similar question has been asked about using the Sun as a giant signal lamp, but it specifically referred to the use of a Dyson sphere which is way beyond our level of technology. I'm looking for something that alters what the Sun looks like from far away, not something that physically blocks it at intervals.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ Very much related, certainly not a duplicate: How would lighthouses work in space? $\endgroup$ – a CVn May 17 '16 at 13:30
  • $\begingroup$ Maybe some huge array of venetian blinds, just like you get with signalling lamps aboard ships, could do the trick. It would have to be huge, but a one-atom-thick layer of metal should be able to work, alternatively a clored foil. You would not even have to cover the entire surface of the Sun (in the desired direction) to make the change noticeable. You could use it for morse communication. This system would obviously be re-usable without damaging either the sun or a body in our system. To avoid disturbing life here just leave the blinds open as Earth passes in front. $\endgroup$ – Burki May 17 '16 at 13:42
  • $\begingroup$ Here's the question I believe the OP refers to in his PS: worldbuilding.stackexchange.com/questions/37897/… $\endgroup$ – Kys May 17 '16 at 15:33
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With current or near-future technology, no, it's not possible.

First of all, terminology: the Luminosity of an astronomical object is

[..] The total amount of energy emitted by a star, galaxy, or other astronomical object per unit time.

Changing the Sun's luminosity would require significantly increasing or reducing the rate of fusion the Sun performs. Short of dumping several Jupiters worth of hydrogen into the Sun to significantly increase its weight, fusion rate, and luminosity, I don't see how this could ever be achieved, let alone with near-future technology.

What you are looking for is the perceived brightness of an object from a certain point in the universe, which is a property that depends on the sun's luminosity, distance of observer, and blocking objects in the line of sight from sun to observer. Since the distance is determined by the position of the observer and we can't change the Sun's luminosity, we have to work with line-of-sight-blocking objects.

To use the Sun as a signal lamp, you would do the exact same: You put up and remove shades periodically to create a signal. Then other civilizations can use standard transit method to catch that signal. Depending on the distance over which you want to transfer the signal and assumed or known technical capability of the receivers, you will need shades with moon- to planet-sized diameters and a means to control their orbit around the Sun. A solar sail of sufficient size should do the job.

In order to differentiate a sail from an actual planet and make it clear that it is an artifical signal, you probably want to create some sort of signal that comes to close to actual communication. For example, you could build several sails and arrange them in such a way that their light/shade pattern communicates a mathematical sequence over time, such as the first few prime numbers, or the start of the Fibonacci sequence. You can also make the sail rectangular rather than circular, to produce a sharper drop in brightness on the detecting end.

As for technological feasability: In 2014, the largest solar sail launched to date was a square of only 38m on a side, for an area of 0.0014 km² and a weight of 32 kg. Earth's radius is about 6370 km, so a disk with Earth's radius (or an equal-sized rectangle) would have an area of 127 million km², or 88 billion times larger (and heavier!) than the 2014 solar sail.

In short, no, using the Sun as a signal lamp is not feasible with reasonably expected near-future technology. You are looking at millions of tons of material that has to be manufactured and then launched into a solar orbit.

Note that even with future technology, when it would be possible it would not at all be reasonable. If you want to announce your presence at the speed of light, simply turn on a large radio broadcast at a specific wavelength to penetrate the interstellar medium, or, if you know where the aliens are, shine a bright, focused laser beam at their solar system. Those methods are infinitely cheaper and simpler than changing the brightness of the Sun, and, best of all, they are trivially available with 1950's or earlier technology.

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  • $\begingroup$ Since the line of sight is determined by the observer's position, we would have to at the very least know where a prospective civilization is located. Unless we are looking at building a Dyson sphere which can be opened and closed, which certainly is not feasible with near future tech by any stretch of the words... $\endgroup$ – a CVn May 17 '16 at 13:35
  • $\begingroup$ Yes I glanced over that. If you want to send the signal in all directions, you would need to enclose the sun in shades. If you cover only parts of the sun along its equator, then you will of course only cover that part of the sky. The whole idea is firmly located in megascale astroengineering, which is far future stuff. $\endgroup$ – Hackworth May 17 '16 at 13:38
  • $\begingroup$ Thanks for the good answer! I had a feeling it wasn't feasible but thought it worth the question. $\endgroup$ – Ieuan Stanley May 19 '16 at 12:55
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Perhaps the star shade we use to try an observe the alien worlds will be visible as an extended transit to them! hey, the dipping of the starlight started and then stayed put for a few days before returning to normal! What kind of planet does that?!

So, to a civilization with a program like our Kepler mission, they'll know we're looking at them.

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