at the core of the discussion is the fact that if you travel faster than light , other observer will see you in 2 places at the same time if not in your arrival point BEFORE you even start .
I was thinking long and hard whether to add to this or not, given how videos have already been posted. And I'm not sure that what I'm about to say will help. So, here goes nothing:
The thing about tachyons (FTL particles) and time travel in special relativity, as we understand is as follows, it requires a somewhat specific scenario to really showcase. First off, the premise of special relativity: "all inertial reference frames are equally valid". This means that whatever happens as observed in one inertial frame, actually happens. This principle is yet to be shown as being invalid, or have limits of being applied.
So here's the scenario: we have two rockets, both going at relativistic velocities, let's say 0.5c each. They see each other, and are constantly exchanging signals so as to synchronise time. At one point, call it T1, the first rocket sends out an FTL particle, which will arrive on the other rocket at another point, call it T2'. Both pilots have agreed beforehand that once this happens, they will radically change course so as to be moving in the opposite directions (i.e. they were approaching each other, and will then move away from each other).
Original "timeline", point of view of first rocket: rockets are approaching each other, tachyon is sent out, rocket starts accelerating the other way until it reaches 0.5c again. Point of view of the second rocket: rockets are approaching each other, tachyon is received, rocket starts accelerating the other way until it reaches 0.5c. So far so good, right?
Now here comes the crux of the matter - is our tachyon always moving at superluminal velocities? It has to, to qualify as a tachyon, right? Let's say it does. Let's also say, that much like photons, to any observer it always moves at the same velocity. This is the sticking point right here, repeat it with me: the tachyon moves at a fixed velocity in a vacuum for an observer no matter what reference frame is used.
Ok, so, let us return to the rockets - their frames of reference have now significantly shifted, having changed direction, thus their view of each other will have radically shifted too. But not only that - that same tachyon that was sent from the first rocket to the second? Will now look completely different (as in, a totally different kind of tachyon) to the rocket receiving it. It will have a different velocity for starters, and as such will be perceived as a wholly different kind of tachyon. But what's worse: this new tachyon can now arrive at a point in time that's earlier in the worldline of the first rocket than when it had originally sent out their tachyon.
The underlying physics relates to how one changes from one reference frame to another and how that affects the view on every event around, including observing this tachyon (let's pretend it can be observed). I recommend looking up Minkowski spacetime diagrams, and I'm pretty sure I'd seen an animated version of this explanation somewhere too.
The comparison with soundwaves breaks down on a fundamental level, I'm afraid - the fabric of reality, as we understand it, is tied up with time on a fundamental level through the speed of light. You only get spacetime with specifically the speed of light. The unique thing about the speed of light is that all observers in all reference frames will agree on what it is (and there is exactly one such constant). So, unless our understanding is flawed, being able to send information, or travel, faster than the speed of light is bound to enable timetravel of some form. Every possible experiment to date has confirmed that those changes in reference frames behave exactly as our theories (special and general relativity) predict them to.
... And that is why quantum things happening instantly, as far as we can tell, is "spooky action at a distance" - it is something that violates this fundamental thing in relativity, and nobody's been able to reconcile quantum mechanics with relativity thus far. And both work pretty well within their respective margins.
TL;DR: tachyons violate relativity in a bad way. But relativity can be bent to enable sort-of-ftl (wormholes, warp drive), at least hypothetically. And Stellaris is a game where Relativity isn't even remotely being considered - and almost all sci-fi (and sci-fantasy) features FTL without much concern for what that does to timelines (timetravel plots are another beast from FTL travel, usually). If you want to understand
why exactly tachyons violate relavity in a bad way, learning Special Relativity is sufficient and recommended.
My apologies for rambling, carry on.
. Neither are possible. When a phenomena breaks physics it usually means it's not possible, and in other cases it's only possible in a way we don't understand. The kind of FTL we see in fiction is impossible.
