Isn't there two events here: the actual supernova and our observation of the supernova. So the actual supernova event could have occurred but our observation is when it crosses our 'lightcone'?
You are right, there are two points in space-time (the definition of the word "event" in special relativity). But the "space time distance" between them is zero! (another way to say that is that the two points have light-like separation)
All of the weirdness of special relativity stems from that definition of "space time distance", which is just an extension of Pythagoras' theorem:
distance = sqrt(x^2+y^2+z^2-(c*t)^2)
x, y, and z are the differences in spacial coordinates between the two events (in a given coordinate system) and t is the difference in time coordinates (in the same coordinate system). The minus sign is where the interesting effects stem from.
The issue is that "now" is not well-defined since it depends on the reference frame. Therefore what we consider "already happened" another observer may consider "still in the future".
All observers will agree on the order of events if and only if the light from each event traveled to the next before the next happened.
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That is extremely incorrect. If for example we know for sure that the observations we've made so far lead to supernova explosion in 10 years and thus the resulting life-threatening gamma burst reaching Earth in 2028, it would make sense to do something before the actual "observation" of the burst.
In relativistic terms, "the actual supernova" is an event (an x, y, z, t quad). It's a perfectly well understood entity in relativity. The point in relativity is that different coordinate systems put different numbers on x, y, z, and t, but they all are talking about the same event.
When we see it, we will be able to put a perfectly accurate x, y, z, and t on the supernova in our coordinate system, with t being "8000 years ago". All this has nothing to do with relativity, merely with the finite propagation speed of light.
If we call the moment at which the light reaches us t=0, then we will say that the supernova occurred at t=-8000, because we know where it occurred, and we know what the speed of light is.
General relativity provides a toolset for a complete transformation from "our coordinate system" to any other: a diffeomorphism between coordinates, and laws of physics written in generally covariant form and obeying some constraint equations for values on any three-dimensional submanifold.
If the supernova's matter is that of the standard model of particle physics, you get everything after the comma in the previous paragraph.
So any two observers of the supernova have a straightforward mechanism for relating observations against their idiosyncratic choice (or choices, they aren't restricted to just one!) of system of coordinates. The actual observables are independent of the choice of coordinate systems.
You'll find (mathematical) proofs of this in discussions of the initial value problem in General Relativity; (physical) evidence is the same as that from astrophysical tests of General Relativity.
As a practical matter, realistic observers will only measure a tiny subset of the total observables of the supernova, and different observers will measure a different subset, even if they use precisely the same system of coordinates. Obscuration by dust or distance is not caused by the coordinates in which one expresses distance or the size of dust particles.
> There is no "the actual supernova". That's the whole point of relativity.
No, coarsely, there is a 4d-worldtube along which one finds a WR star, a supernova of type Ib or Ic, and a remnant. Or more finely, there are worldtubes for large numbers of particles that are bound together gravitationally and through standard-model interactions into these more macroscopic objects. In some compact region of spacetime a collection of these particles is best described as the start of a Ib or Ic supernova; their worldtubes diverge (possibly spectacularly) elsewhere in spacetime. Or, if you like, we take a roughly 4-cylindrical section through the spacetime-filling fields of the standard model such that somewhere in that 4-cylinder the excitations of the fields are best described as the start of a Ib or Ic supernova. Really, it doesn't matter, because we know mathematically how to relate each of these descriptions, and have a good idea about why we might choose one description over another when asking various questions. That is the whole point of relativity.
