Noticing visitors to our galaxy
Prelude
Prelude TLDR; A fleet of ships is passing through our galaxy, stopping off at particular stars and causing small dips in the light from stars.
This is related to a couple of other questions (and a little suspension of belief to make it fit the story I want). The other questions are refueling and detection.
So the idea is that explorers from another galaxy have sent a fleet of their ships into ours - the ships need to refuel regularly by collecting fusion fuels from red giants (I've decided the ships also use the light and heat from the stars so I can 'explain' using stars rather than gas giants for fuel).
The fleet$^{\dagger}$ obscures some of the light from a star as it collects the fuel, making a small dip in the intensity of the light we receive from that star. Then it moves on, using up fuel to power cryo, travel, AI etc and then stopping to survey and refuel at the next red giant.
$^{\dagger}$ One big ship can have a large volume without presenting such a large area to block out light. As explained in this answer we need a large area...so I'm making it a fleet with a main mother ship rather than one big ship.
The situation to reality check
As the fleet pass through our galaxy they obscure very small amounts of light to the point where they're almost lost in the noise of solar flares, sun spots and general brightness changes (I'm going to say they orbit their chosen star fairly quickly so we can fold the data and get a better signal).
A scientist in my story has come across this unusual trend of a small but very regular dip which appears for a specific amount of time, then disappears and turns up in another system...this trend seems unnatural to our scientist but has been overlooked by others. For the sake of argument I'm going to say that, so far, the ship has passed three star systems in the hundred years of data our scientist has but if you think he needs more I can change this.
I want our scientist to decide this is a sign of intelligent life and track the path of their ship (joining up the dots of red giants) and send out a message to them by predicting where they will be when a message can get to them.
(I want this message to include information about the star system he thinks they'll be in and information about ours so they can find us...but that is just background and for another question).
So how realistic is it that he would be able to track their path? Does he have enough information about their movements to predict where they will be?
(I'm aware he will be long dead before the aliens reach us, I want to give humanity generations of expecting the Aliens before they actually arrive).
Edit: I was imagining slower than light travel between stars and the initial group of red giants being close to each other.
This post was sourced from https://worldbuilding.stackexchange.com/q/79775. It is licensed under CC BY-SA 3.0.
1 answer
As mentioned; we already know there are rogue planets and possibly even black holes out there. Is the fleet in question cover an area the size of Jupiter?
Why wouldn't the better explanation be that these red giants have large planets orbiting them, with total orbit duration > 100 years? We already know most stars have planets.
Plus, a straight line to Earth eyes may be a triangle in 3-d space; and a closest-neighbor walk for the aliens would seem like a drunken walk among stars to us; until we map them in 3-D to see a clear path.
That said, I think a solution to your problem may exist: Make the changes to the Red Giants, as made by the aliens, produce a more permanent feature which your scientist can observe, and might plausibly be the first to measure. for example, a particular signature combination of reduced or increased elements in the spectra of the red giant; or something else permanently observable about the star. As if the aliens extract something useful from the star necessary for their propulsion, or manipulate the star in some way.
Then your observations can be millions of years old; covering hundreds of stars, and your scientist's measurement clearly partitions red giants into Mined vs. UnMined, and furthermore he can see the nearest-next-red-giant path in the 3-D map. There is no natural explanation for such a well defined path; it must be due to intentional selection.
Then further than that: He can see, by comparison of current spectra to spectral observations taken less than 100 years ago, that the last two red giants on only one tail of this path were the most recently changed, just a few decades apart, the last point just a few years ago, so the next star on that path is one we could signal in time to communicate with these aliens before they arrive.
Perhaps further plot developments allow him to measure the trip-time between red giants, say he discovers a subtle decay rate in the spectral signatures, and using that he discovers the aliens sometimes travel at light speed, but other times take decades longer than expected before arriving at the next red giant: He concludes they must stop and visit places for some reason, perhaps they discover life, or just something else worth mining besides red giants.
Feel free to use that if it is useful; I am just spit balling here...
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