“Why will we use green lasers for starwisp and light sail propulsion?
“Because stars aren’t green. Can’t be green, in fact, because a black-body spectrum that peaks in the green is broad enough that there’s plenty of other-colored light to make it not sum to green. That makes green the least stealthy color in space.
“So when you’re going to be shining a few hundred terawatts into someone else’s star system, a monochromatic 530 nm green is as good as it gets in letting them know up front that you aren’t trying to sneak something in on them.”
– Argil Medanis-ith-Medanis, Laserider Network
That totally makes sense from a trust perspective
I mean, from a sensor’s perspective, any monochromatic emission is going to jump out of the frequency spectrum of any star anyway, so I’m guessing this is more of a visual reference for people out stargazing? In that case, how would the Eldrae be 100% sure that the civilisation viewing the approach of their linelayer actually has 530nm green in their visual spectrum?
Any civilization which sees something bright enough to be a small star is going to start running spectroscopic tests if they can, if only because it’s a clue on what’s coming to kill them. After they get the results back, it would be very obvious if they’ve done any astronomy at all that it wasn’t a natural phenomenon, which was the point of using that wavelength. It will pop beautifully on pretty much any astronomical instrument, even if the species can’t see that color natively.
The aim’s to demonstrate honesty, and if you use something that fits comfortably into the emission spectrum of your star - well, sure, it’s obvious to spectroscopic analysis, but it does leave open the possibility that you are malign and incompetent. (It may seem unlikely, but people do the damndest things in history.)
But you get the idea. The aim is to be as obvious as you can so that not even the supremely paranoid could possibly argue that you were trying to hide something, and for that, the best option is a color not naturally found in stars or black-bodies in general such that it’s really, really obviously unnaturally generated. Whatever you look at it with.