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The Kepler Space Mission has discovered over 1,000 planets. These discoveries may increase the confidence of some people that other civilisations exist. That is an opinion issue, and not what I want to ask about here.

If a hypothetical civilisation far from Earth picked up our human created radio signals, which by now have extended to a sphere around 100 light years across, and decided to use lasers to contact us:

  • Assuming a 50 light year distance, how much power would it take to send us a laser signal?

  • Would this laser signal be so spread out, i.e. undetectable, that radio signals would still be a better option for communication?

Normally, I would have assumed that the laser signal would have spread out so much over that distance, that it would be virtually undetectable, (and I would not ask this question), but this Wikipedia article: Interstellar Communication states:

It has also been proposed that higher frequency signals, such as lasers operating at visible light frequencies, may prove to be a fruitful method of interstellar communication; at a given frequency it takes surprisingly small energy output for a laser emitter to outshine its local star from the perspective of its target.

It may well be that sending the signal is not a major issue, but I don't know enough about the divergence of laser light to determine if detecting the signal is the main problem. To hopefully clarify this, would a large, much too large in practice, focusing lens be required?

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The divergence of the signal is closely related to the angular resolution of the "antenna" (lens, reflector, ...) you use. This will in turn be a function of the size of the source in relation to the size of the lens - or in the limit, the ratio of dish diameter and wavelength (the familiar $\frac{1.22\lambda}{d}$ relationship)

Typically this means it is easier to get a narrow beam of laser light than radio waves. But the narrower you make it, the better your aim needs to be. How well can you aim your transmission at the point where Earth will be 50 years from now?

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  • $\begingroup$ Thanks for that, if I really wanted to talk to someone, I would use a large number of "dishes / lenses", spread around to hopefully increase the chances of catching their attention. But I didn't know if even one lens would be a practical impossibility, making whole idea unworkable. I am not gone on speculative questions, but the wikipedia article made it seem a bit more likely than I imagined. $\endgroup$
    – user81619
    Commented Sep 2, 2015 at 15:22
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    $\begingroup$ The more you focus the power, the fewer civilizations you will reach - collimation reduces the available solid angle. The other thing you can do is concentrate the power in time - short bursts have a better chance of being "seen" above the background for the same average power. $\endgroup$
    – Floris
    Commented Sep 2, 2015 at 15:27
  • $\begingroup$ How well can you aim your transmission at the point where Earth will be 50 years from now? The optics that the aliens would use to transmit the signal would be the same or similar to the optics that they use to track us. If they could see our planet through their telescopes, then of course they would be able to predict its trajectory for a mere 50 revolutions, and hit us with a beam transmitted through the same telescopes. $\endgroup$
    – Solomon Slow
    Commented Sep 2, 2015 at 17:40
  • $\begingroup$ @jameslarge - from 50 light years you are likely to have a problem resolving Earth in its orbit around the Sun - the same reasoning means you probably end up creating a cone of light that is quite a bit wider than Earth's orbit... "aim roughly at the Sun and you're good". $\endgroup$
    – Floris
    Commented Sep 2, 2015 at 22:17
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When you say "laser", I'm guessing that you mean visible or near-visible light.

The problem is, where are they going to build their laser? If they build it on their home planet, then it's going to be so close to their sun, that our telescopes won't be able to distinguish the one from the other. We won't even be able to tell that it's there unless it is a lot brighter (as seen from Earth) than their Sun.

If you're going to transmit a signal from the neighborhood of a star, then it makes a lot more sense to use a wavelength that the star does not emit.

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  • $\begingroup$ Thanks very much for your answer. I did orginally mean visible light, the wikipedia snippet in the post above says at a given frequency it takes surprisingly small energy output for a laser emitter to outshine its local star from the perspective of its target. I took that to mean we know where to look, basically. If we don't, then I agree completely with your last line. I presume, because I am no expert, that a higher frequency means faster information transfer, so possibly in the UV area, if their Sun is G class. I am out of my depth on lasers, I admit:) $\endgroup$
    – user81619
    Commented Sep 2, 2015 at 18:13
  • $\begingroup$ @AcidJazz nothing can go faster than light. higher frequency might allow for more information but not faster. $\endgroup$
    – anna v
    Commented Sep 2, 2015 at 18:49
  • $\begingroup$ @anna v Thanks anna, I might have got my terminology mixed up in typing, but what you say above is what I meant. Once the hypothetical aliens hear our old 1920's radio broadcasts, and if they still want to talk to us after that, I guessed they would want to transfer data as fast as possible. $\endgroup$
    – user81619
    Commented Sep 2, 2015 at 19:00
  • $\begingroup$ @AcidJazz, the article also says, 'citation needed.' :-) I did a really rough calculation: If you want to build a laser that would appear as bright as our sun from 50 light years away, and assuming a beam divergance of 0.1 arcsecond (same as the resolution of the James Webb Space Telescope), I figure you would need about five Terawatts of optical power. That might be surprisingly small compared to the total power output of our Sun, but I don't think we will be building one any time soon. $\endgroup$
    – Solomon Slow
    Commented Sep 2, 2015 at 19:35
  • $\begingroup$ @AcidJazz, I figure the laser could two or three orders of magnitude less powerful IF the folks on the receiving end knew what exact wavelength to look for, and IF they knew to look for faint variations in the average brightness of our Sun. In other words, if we could use some other means to send them details of what to look for and where to look for it, then maybe we could transmit using a laser as small as five Gigawatts. That might almost be worth attempting, if only we knew that they were out there and ready and willing to listen. $\endgroup$
    – Solomon Slow
    Commented Sep 2, 2015 at 19:40

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