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.
Secondly, since this shows the relativity of simultaneity is not a mere illusion of perspective, then it's ontic, and if it's ontic, that falsifies presentism and possibilism and entails eternalism. Eternalism is the only view compatible with the effects of Special Relativity. To deny an ontic relativity of simultaneity, you'd have a real dilemma on your hands in this scenario. For one thing, you'd basically be saying that the famous Michelson-Morely experiment is false - because the light on one track would have to travel more distance than the other due to the earth's movement and thus would take more time. But of course that isn't true. The movement of the earth has no effect on the measurement of light. And the more recent tests that try to search for gravity waves that can measure changes in the time it takes light to bounce back on two different tracks to within an accuracy of 1/100th the diameter of a single proton have additionally confirmed this.
So what would this scenario look like on a spacetime diagram? Well, first a spacetime diagram is a 2 dimensional representation of spacetime as it is described in Special Relativity. It was developed by Herman Minkowski in 1908, and is sometimes also referred to as a Minkowski diagram.
On the diagram there is a vertical t axis representing time from past to future in the up direction, and a horizontal d axis which represents distance and events that are considered simultaneous. (Sometimes this is shown as an x axis.) Using what scientists call "natural units" the speed of light travels one unit of t for every unit of d and so it always travels at a 45 degree angle in either direction from every point as seen in the spacetime diagram below from the green dot. The two lines form what is called a light cone. For every observer there is a future light cone, and a past light cone. (For an interactive spacetime diagram, click here.)
In the diagram below the blue line is the platform. It goes along the t axis as it travels thought time. The person on the platform is in the center of it represented by the yellow dot. The train path is along the orange line, passing left to right. The person on the train is in the center of the train, equidistant from the front and the back, represented by the red dot.
When the train passes the person on the platform, he and the person on the train are equal distant from the front and back of the train. Events A and B are the two flashing lights that travel along the yellow lines at the speed of light (represented by the lightening strikes in the video.) They come from the front and back of the train respectively. Since the man on the platform knows he's equidistant from point A and B at time 0 on the x axis, both lights hit him at the same time (event C) and he concludes A and B must be simultaneous. This is because all observers will measure the speed of light at the same speed in their reference frame. So two sources of light that are equidistant from you will always hit you at the same time and they must be simultaneous, regardless of your motion relative to other objects. This is represented by the diagram below.
From the person on the train's reference frame, she is also the same distance from the front and the back of the train. In her reference frame, two equidistant sources of light will hit her at the same time and she will conclude that the sources of light were simultaneous events (just like the person on the platform.) This is, again, because all observers will always measure light at the same speed in their reference frame, regardless of their motion relative to other things. So does the person on the train see the two sources of light at the same time like the person on the platform did? No. For the person on the train, the light from event A hits her first at event C, and the light from event B hits her later at event D. Since in her reference frame she is at rest and the person on the platform is moving, and since she is equidistant from the front and back of the train, she concludes that event A must have happened before event B. There is no way that events A and B could be simultaneous for her. In order to claim that they were, and that the lights hit her at different times, you'd have to violate two of the fundamental principles in Special Relativity: The laws of physics are the same for everyone in their reference frame, and the speed of light will always be measured at the same speed.
So there is no way that this scenario can be explained by a mere trick of perspective about the time it takes light to travel. If you think this is the case, you have to violate the two fundamental principles in Special Relativity. Additionally, when we do the pole and barn paradox, it makes things even more difficult because you'd fall in the paradox without a true ontic relativity of simultaneity.
Without the right tools, it's very difficult to draw a 3 dimensional version of these kinds of scenarios. Here's a simple diagram showing two events, A and B, and according to the green dot, they happen simultaneously at time 2 because green is still relative to them. The blue dot is travelling past green in the direction of the arrow. It's time slice intersects with green's time 0 on the left and 2 on the right. Blue thinks that event B happens first, and then event A because it cuts the block at a different angle due to its motion relative to A and B. Mind you, the picture isn't going to be perfect because I don't have the best tools.
From the person on the train's reference frame, she is also the same distance from the front and the back of the train. In her reference frame, two equidistant sources of light will hit her at the same time and she will conclude that the sources of light were simultaneous events (just like the person on the platform.) This is, again, because all observers will always measure light at the same speed in their reference frame, regardless of their motion relative to other things. So does the person on the train see the two sources of light at the same time like the person on the platform did? No. For the person on the train, the light from event A hits her first at event C, and the light from event B hits her later at event D. Since in her reference frame she is at rest and the person on the platform is moving, and since she is equidistant from the front and back of the train, she concludes that event A must have happened before event B. There is no way that events A and B could be simultaneous for her. In order to claim that they were, and that the lights hit her at different times, you'd have to violate two of the fundamental principles in Special Relativity: The laws of physics are the same for everyone in their reference frame, and the speed of light will always be measured at the same speed.
So there is no way that this scenario can be explained by a mere trick of perspective about the time it takes light to travel. If you think this is the case, you have to violate the two fundamental principles in Special Relativity. Additionally, when we do the pole and barn paradox, it makes things even more difficult because you'd fall in the paradox without a true ontic relativity of simultaneity.
Without the right tools, it's very difficult to draw a 3 dimensional version of these kinds of scenarios. Here's a simple diagram showing two events, A and B, and according to the green dot, they happen simultaneously at time 2 because green is still relative to them. The blue dot is travelling past green in the direction of the arrow. It's time slice intersects with green's time 0 on the left and 2 on the right. Blue thinks that event B happens first, and then event A because it cuts the block at a different angle due to its motion relative to A and B. Mind you, the picture isn't going to be perfect because I don't have the best tools.
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