Time travel has been a staple of science fiction for decades. Everyone has wished at some point or another that they could travel to the past to fix their mistakes, which explains some of the appeal of the subject. Naturally, since the theme is so common, many people have gotten the impression that time travel is realistically possible, and have come up with various methods by which one might travel through time. However, when it comes to real science, time travel stays in the realm of fantasy.
The most obvious and well known issue with time travel is the paradox. A man travels to the past and kills his own grandfather before the birth of his father. Since his father was never born, the man himself never could have been born, and thus never could have gone back in time to kill his own grandfather, creating a paradox. Several explanations have been proposed to solve this problem, but all are unsatisfactory. Some propose that time travel is indeed possible, and that future time travelers are part of their own past. In this case, the gun would perhaps misfire, and the grandfather would go on to tell his grandson about the time traveler who tried to shoot him, prompting the grandson to build his time machine. However, this simply moves the paradox to another place. If the grandson then fails to build the time machine for whatever reason, the past becomes impossible. Another idea is that any paradox would have universe ending consequences. However, since it is almost certain that some other sentient race somewhere in the universe has advanced enough to discover time travel if it were possible, and the universe still exists, one must conclude this to be faulty.
Perhaps the best explanation for paradoxes is the multiple universe hypothesis. In this hypothesis, every potential action splits off into two separate universes so both courses of action are taken. A time traveler would create his own timeline, where his grandfather died and he himself was never born, only existing in his current state. However, this has additional problems. Firstly, every model of physics that predicts alternate universes does not predict a multiverse that would operate by these rules. Some models predict alternate universes floating in dimensions separate from ours, but these universes would be wholly different, not a byproduct of our own. Secondly, and more importantly, the very basis of time travel is that you stop existing in one time and start existing in another, potentially one where you already exist in some form. This constitutes an obvious breach of the Law of Conservation of Mass and Energy, which would completely undermine all known laws of physics, and allows infinities to creep in, since if you can create energy from nothing, there’s no limit to how much you can create.
The simplest argument against time travel is, “Where are the time travelers?” Assuming humanity manages not to destroy itself in the coming centuries, and we were to invent time travel, why is our time not filled to the brim with temporal tourists? Why are aliens who might visit our planet tens of millions of years from now and see the remains of our civilization not here, studying us? Some would argue that it is impossible to travel to a point before the creation of your time machine, but it seems that they are starting from the assumption that time travel is possible, and then finding reasons for the lack of visitors, hardly a scientific perspective. Although it may provide for entertaining science fiction universes, in reality, we must consign time travel to the bin of pseudoscience.