One of the most fascinating concepts in all of science and philosophy is the idea of eternalism. Eternalism is "a philosophical approach to the ontological nature of time, which takes the view that all points in time are equally "real", as opposed to the presentist idea that only the present is real." This view on time can be traced back to Herman Minkowski who took Einstein's Special Theory of Relativity (STR) to its logical conclusion. STR entailed that there cannot be a universal present—a "now" which all observers can agree upon that's simultaneous. Instead, two events that are simultaneous to you, might not be for me depending on our motion relative to one another. It utterly destroyed Newton's notion absolute simultaneity.
For over a hundred years this has been debated by scientists, philosophers, and theologians alike. One common view against eternalism is the idea that the relativity of simultaneity isn't ontologically real, but is actually just an illusion resulting from the amount of time it takes it light to reach an observer. So is this the case? Is the relativity of simultaneity just a subjective illusion?
Well no, it isn't. Here is a scenario that can show that the relativity of simultaneity must be ontic and not just an illusion resulting from the time it takes light to reach you from different events. That scenrio is show in this video here:
The person on the train is equidistant from the front and back of the train. If the two flashes were objectively simultaneous, for her the light would reach her at the same time. The light in the back wouldn't take longer because the train is moving — the train's movement is relative. That's why it's called relativity. All movement is relative to other things. In the woman on the train's reference frame, she is still and the man on the platform is moving. So it would be incorrect to think that the person on the ground's view is somehow the "correct" one.
Consider this. If you were on a spaceship travelling at 1 million miles an hour and you measure light in any direction, you would always measure it at the same speed. If the ship was 100 feet long and you were sitting at the 50 foot mark, two lights flashing simultaneous in the front and back of ship would hit you at the same time, regardless of the ship's movement or speed relative to other things. The light from the front wouldn't hit you first and light from the back wouldn't hit you latter due to the ship's motion. So the person on the train must conclude that the two lights happened at a different time ontologically in her reference frame, disagreeing with the person on the platform. This is a true ontic relativity of simultaneity.