So I've been debating someone on whether Special Relativity entails eternalism and he is absolutely clueless as to what he's talking about. At the beginning of the debate he barely even had a basic conceptual comprehension of Special Relativity, and he kept making mistake after mistake, forcing us to spend weeks outlining his mistakes so that we could actually be on the same page. And this, despite the fact that he kept proclaiming he has "sufficient" knowledge in the subject matter.
His latest gaffe is below. He created this gif that tries to claim that presentism is compatible with Special Relativity, even though we cannot ever verify an objective reference frame, and furthermore that eternalism is not logically inferred from Special Relativity, which, of course, it is.
There are so many problems with this gif I don't even know where to begin.
The image in the picture comes from a popular image showing a spacetime diagram of the train scenario that Einstein thought of early on when he was formulating the theory. A video of that train scenario is here.
Here's a short synopsis of the scenario:
In the train scenario, the man (green) and the woman (blue) are both an equal distance from the two lights sources at the front and back of the train (which could be labelled A and B respectively). Green is on the platform, and blue is on the train. Green receives the two light signals from events A and B at the same time, while Blue receives the light from event A first and event B second.
Here are the problems with the gif:
First, we’re seeing the spacetime diagram drawn from green’s POV both on the left and on the right. In other words, green is drawing the diagram, not blue. If blue’s POV were drawing the diagram on the right, we would not see both of the lightning strikes on the horizontal line for both diagrams; one would be above the other on the right diagram and both would be horizontal on the left diagram. And that means my friend here would not be able to have done what he did—which is try to claim that either perspective could be the one objective perspective. This is apparent to anyone “sufficiently” knowledgeable in Special Relativity, which my friend proves over and over that he is not.
Second, on his own diagram this is apparent. See above. In his option B, which uses the "now" slice of blue's POV, for green, event A happens first, and thus it’s light is emitted first, and would therefore hit green earlier than B!! For green, event A is happening, while event B hasn’t happened yet!! This means that it is impossible for green—who is on the platform in the train scenario, to receive the lights from A and B at the same time. My friend is totally, completely lost in his knowledge of Special Relativity.
In Special Relativity both perspectives are correct. That’s how we avoid paradoxes like the ladder paradox. If he were correct, he'd get stuck in the numerous paradoxes in relativity because he'd have no way to use the relativity of simultaneity—which is how you get out of those paradoxes. So of course each of them individually are compatible with SR. He's not saying anything brilliant. What is incompatible with Special Relativity is claiming that either green's POV or blue's POV is correct. In order to do that you must deny the two postulates of Special Relativity:
- The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
- The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
You’d have to deny postulate 1 because you will be denying one POV’s legitimacy in terms of what events are physically happening on it's "now" slice according to its reference frame, and to do that you'd be saying the laws of physics are not invariant. You'd literally be picking one POV and saying that could be the objective one, and then saying the same with the other. But you'd be denying that both could be true This is total nonsense and incompatible with Special Relativity. You'd have given no reason to think one’s POV is not physically legitimate.
And finally he tries to attack eternalism from a philosophical framework asking you to entertain "existential problems." For humans, airplanes, and other complex machines to have "always existed" is only bizarre if you come to eternalism with presentist assumptions. Once you conceive of the spacetime block, the "timescape" where all of time is laid out—akin to a landscape where all of land is laid out, there is no problem, certainly no logical one. In one part of the block, humans, airplanes, and other complex machines exist. In another part they don't. To say it's bizarre that they "always existed" implies that they exist at every moment in time. But eternalism doesn't say this. A moment in time is a slice of the block universe. If we could cut the universe up into a 100 quadrillion time slices from the big bang to 100 billion years after, humans would not have "always existed." They would exist on some of those slices and not others. And their evolution from bacteria would all trace paths in the universe from the big bang. This makes perfect sense once you understand eternalism.
If eternalism gives you an existential crisis, so be it. Determinism can do the same thing. So can the many worlds formulation of quantum mechanics. So can evolution for crying out loud. Your personal feelings about the implications of our best scientific theories are not any good reason against the theory or its implication. Only if you can come up with some logical incoherence would that be relevant to the truth of something like eternalism.
So, over all this argument is so bad, so amateur, and such a waste of time for me.