Continuing the arguments for why Einstein's theory of Special Relativity entails eternalism, I have a four dimensional representation of the train scenario mentioned in my last post to help illustrate this.
The events A and B are equidistant from both observers, and so light travelling from them that's simultaneous must hit them at the same time. For green, this is the case. The lights hit him at the same time and since he knows he's equidistant from the two sources, he knows they must be simultaneous. A and B are on green's "now" slice no matter what perspective we're looking at. Whereas for blue, A happens before B. This must be the case because blue is also equidistant from events A and B, and so if they were objectively simultaneous, they must hit him at the same time.
Neither perspective can be said to be objective, so if you were to try and pick one it would be completely arbitrary. So, this means that no one can objectively say whether A and B happened at the same time or not. Since their now slices each cut the block at a different angle, the past, present and future must exist. Right as they pass by each other, green considers event B to be happening, whereas for blue, B is a future event that hasn't yet happened. That means that blue's future must already exist. Similarly, for green, A is an event that's happening, but for blue it's a past event that already happened. That means that blue's past must still exist. And if we chose two simultaneous events for blue, the same logic would apply to green. This is impossible on presentism and possibilism, as it would lead to a paradox, and only possible and coherent with eternalism by everyone's past, present, and future existing.
For green, since the light from A and B are simultaneous and equidistant, it reaches him at the same time at his event C.
Note: The lines on the block 0,1,2,3,4, are not objective. They are only relevant from a subjective POV.
So there you have it. Special Relativity entails eternalism. It is the most straightforward conclusion one can draw from an understanding of the theory on the nature of time. All other views require messy and, perhaps, even ugly rewrites that include things like the totally undetectable aether.