A reader signing himself "Waggs" raised some issues in response to the theory articles on parallel and divergent dimension theories, which required a longer response than could be addressed in the comments sections.  Thus this article will attempt to respond to those.

He begins,

Before you start running around saying "because there is an infinite number, it must be like this" consider that although the number is really really really big, it is not infinite.

From this, he observes correctly,

Conservation of Energy is not as absolute as we [...] thought....the lost energy is always taken from somewhere else.

There is a confusion here, in which two distinct concepts, parallel dimensions and divergent dimensions, are being conflated.

That "the lost energy is always taken from somewhere else" is conservation of energy:  the same amount always exists, but we move it from place to place.  In parallel universe theory, the total matter/energy of the entire multiverse has always existed.  When we suggest that matter and energy moved from one universe to another, there is no need to compensate:  the total still exists, just in a different location.

In divergent theory, though, the time travel event creates a new universe which takes the history of the old universe to the point of the time travel arrival but then diverges from it.  A new universe is created at that instant, of the same size and in the same state as the old universe, which also still exists.  Thus there is a matter/energy creation problem here, because what is needed to create the new universe must come from somewhere, and must be equal to the total matter/energy of the original universe.  That is where the conservation problem arises.

It is true that the number of divergent universes would never be infinite; but under certain forms of parallel dimension theory time travel only works if the number of identical parallel universes is infinite, because for any given universe there must be at least one other unique universe to which a traveler to the past could go from which no traveler from the future could come.

Waggs, however, cites a different concept of the multiverse, in which there are not many discrete universes but a single universe in which all possibilities occur but each individual experiences only one set.  Citing the famous thought experiment Schrödinger's cat, he asserts,

Each entangled operation that makes up our world, then, may or may not exist in the others....Future and past, then, would be illusions fixated on cause and effect.  Altering your own past would only produce a new configuration and would not alter your existence.

That Schrödinger created his thought experiment specifically to demonstrate that this conception of the multiverse was absurd does not matter to those who cite him for the explanation of this view; this is what is believed, that all possible states are actual.  Waggs takes the notion one step further, asserting that all are actual within a single cosmos, coexisting but known to individuals only in coherent causal chains.  The causal chains themselves are thus illusory; the fire both lights and fails to light, the forest burns and does not burn, and the events are not causally connected but in our perceptions.  It is a conception of reality which denies reality, in which time travel cannot really change the past because all possible versions of the past already exist within the one universe; it only changes which version of the past the time traveler knows, without eliminating the version from which he came, such that he now knows two distinct realities, both of them real and co-existing.  It is fixed time overlaying parallel dimensions in a single universe that has no specific history.

As interesting as that is as a thought experiment, few could embrace such a subjective conception of reality.  It means that those who claim the Holocaust never happened are as correct as those who assert that it did; that Obama and McCain and many other men each won the 2008 American presidential election and are each currently serving as President of the United States.  Reality ultimately is whatever you believe it to be, while being entirely different for someone else.  Rewriting history is not inventing a different world, but recounting a different true version.  Nothing is false; everything that might have happened did.

This runs afoul of objections previously raised in discussing a divergent dimension theory multiverse, the version of parallel dimension theory in which every possible universe exists.  Additionally, in invalidating the necessity of causality it also invalidates the conservation of matter and energy--the point from which this began.  It is, finally, the scientific declaration that nothing is true, a claim which Socrates observed must apply even to itself.