One source of entertainment that I enjoy is science fiction, and so I want to address a popular idea from that genre — time travel. I am no quantum physicist, and so I cannot speak to issues of time on the quantum or micro level, but on the macro level of visible objects time travel appears nonsensical. To begin with, characters who time travel in science fiction appear at what seems to be the same space in the different time, but that makes no sense. Everything in space is moving all the time, and so any point in space in the present has no exact corresponding point in the past or future (i.e., a particular place exists only at a particular time). The science fiction writers often use some point on Earth as the point of origin and the destination for the time travel, ignoring that the Earth is continuously moving relative to the Sun and the rest of the universe and that what keeps a non-time-traveling object in place relative to surrounding objects is the totality of all the forces acting on it, including gravity. For the time travel to occur, the object can no longer be subject to those forces, so the object should not respond to any of them, including gravity, and should not remain in the same location. I would guess that the location of the object at a different time, other than the immediate past or future, would be indeterminable.
Maybe a more fundamental problem for time travel to the past is that the time traveler comes from nothing into something (and a something that is not some simple mass of particles but a very complex life form) at the past time, bootstrapping the time traveler’s own existence in the past time, and then as the time traveler has an effect (think the Butterfly Effect of Chaos Theory), the time traveler contributes to creating the preconditions for its own existence in the present time. Not only does that violate the fundamental rule that the sum total of mass and energy is constant (at least outside some sort of Big Bang singularity), but it requires a completely predetermined universe where the time traveler has always been part of the past and has always contributed to creating the preconditions for the time traveler’s act in the present to travel to the past.
One other problem is that even though our brains have created this simple concept of human beings as discrete, disconnected objects, human bodies like all other matter are interconnected parts of the environment and contribute to the mass and energy of that environment and so instantaneously removing them would create an instantaneous hole in space, which I believe would have significant repercussions for the surrounding environment, just as instantaneously adding them to the new environment would cause an instantaneous compression of mass around the entry point, which would also have serious repercussions. Though I do suppose that the proponents of time travel could argue that the addition and subtraction need not be instantaneous.
A related set of problems to those of time travel occur when the protagonist in science fiction travels to a parallel universe that is virtually identical to the protagonist’s universe, at some time in the past, present, or future. I have several problems with this as well.
First, if the protagonist travels from one to the other, they aren’t completely separate parallel universes, as they obviously can be connected, and are instead probably better viewed as different parts of one mega universe, which leads to the question of how they have managed to remain separate for so long.
Second, as with time travel, the problem I discussed earlier related to instantaneously adding mass or removing mass from the environment are going to be there when one switches universes.
Third, if the parallel universe is identical, how would the protagonist know that they entered the other one, and what happened to the identical copy of the protagonist in the other one, i.e., did that individual just switch places, and, if so, how would anyone know, including the protagonist?
Fourth, if it isn’t identical, then how could space and time correspond in such a precise fashion that the individual would cross the boundary to a virtually identical space or time. If it is the same time, the space would need to be virtually identical, or there would be no way of knowing that the spatial point for entry is appropriate, e.g., not in outer space or in the Earth’s solid mass. If it is not the same time, for the reasons cited earlier regarding time travel, that the corresponding space of a different time is indeterminable, then the spatial point of entry cannot be known or predicted.
And that’s why I’ve never been a big fan of stories with time travel or travel to parallel universes.