Everytime I try to read an explanation of both I don’t really understand the difference. Saying “this happened in the past” and “this happened before that” is just using different words to describe the exact same thing to me. I guess I’m also not understanding the implications of this distinction.
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Firstly, “A Theory” and “B Theory” (or A Series and B Series) are not physics. They are metaphysics; a type of philosophy.
They build a bit on real physics, but it is worth noting that the early work on A Series and B Series ordering of events was done in 1908, in the earliest days of our modern understanding of time from a scientific point of view. They are not science.
Roughly speaking, A Series describes events in relation to a particular present. It uses tenses, and tells you when things happened in relation to a “now.”
B series describes events in relation to each other, with no specified present.
The example Wikipedia gives are the statements “it is raining today” and “it rained on 3 October 2023.” The first statement is A Series – it is based on a particular present (it changes meaning depending on when we say it), whereas the second is B series (the statement works whenever we are).
That then gets us into B Theory. B Theory is an idea that time is tenseless; that the flow of time is a subjective illusion created by our own consciousness, and that past, present a future are all equally real.
From a physics perspective, this is kind of interesting, but not really; we know that the flow of time is relative, but it is not subjective, and there are absolutes.
Years ago I had a talk with a physicist who worked on time research, and he described attending a conference on time with a bunch of physicists and philosophers. He was somewhat dismissive of the philosophers; they were struggling with basic concepts that have been understood by physics for over a century.
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