In 1957, Dr. Hugh Everett III came up with the Many Worlds Interpretation of quantum mechanics, as a way to resolve the paradoxes of the Privileged Observer and such thought experiments as Schrödinger's Cat. The Privileged Observer (according to the rival Copenhagen Interpretation) is the observer who causes the quantum wave function to "collapse" into particles with location and speed and other characteristics while under observation. Schrödinger's Cat is inside a box with a vial of deadly poison that will break when a given atom decays and releases an alpha particle, killing the cat. Since quantum-mechanically, the atom might or might not release a particle at the instant of decay, the cat is famously half-alive and half-dead in the Copenhagen Interpretation--until the Privileged Observer opens the box and "collapses" the cat into either the alive or dead state.
According to Everett's interpretation, every quantum "choice" creates two worlds, two timelines. In one, the observer sees a live cat. In the other, the observer sees a dead cat. There is no Privileged Observer, because both outcomes occurred--just not in the same universe. Every single decision, every single atomic flow or switch, creates another worldline. Initially, the Many Worlds Interpretation held that information (and therefore energy, and therefore, matter) could never pass between worlds. However, string theory in the 1960's, and strange experimental results showing seemingly instantaneous "dipole communication" in the next decades, made it seem theoretically possible.
Van Zandt's discoveries did not actually solve the question. Although he discovered many worlds, they weren't of the infinite "one photon displaced" variety the orthodox Many Worlds Interpretation predicts. Instead, they were like "knots" in the superstring--places where the potential fog of alternatives "collapses" into actual worlds. Further, these "knots" are bunched up into different "energy levels" that Van Zandt dubbed the quantum levels, or quanta.
Thus, parachronics actually managed to combine Everett's theory with an empirical Privileged Observer--the parachronic traveler. And that, with minor mathematical fillips, is where the theory remains even now, with nobody satisfied, and with some radical theorists proposing that a kind of Observer Effect holds for parallel worlds, or that the expectations of the traveler have some effect on his destination.
Precisely how the infinite worlds vary and interrelate remains a tangle. Most of the fundamental physical constants remain the same, but some differ wildly while somehow not reducing the universe to quantum fog. It's difficult to make valid crosstime theories work, in large part because physical laws don't seem to hold constant across the continuum. Not only are there worlds where magic works, or where the luminiferous ether exists, there are innumerable subtle variations down to the level of individual electrons. Other changes don't seem to make any real sense, or exist in isolation when they should affect the entire system of physical constants. For example, there are two worlds in which the speed of light is measurably greater by some half of a percent, and one in which it is less by almost a full percent.
In Earth 0, the speed of light is actually decreasing by a very small amount constantly.
The only universal difference is that the second is nanofractionally slower on all known inhabited world-lines except four: Homeline, Centrum, Shikaku-Mon, and Imperium 0 (unknown to the first three).