Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex, written by Sophocles over 2,500 years ago, is the first time travel story.
But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.
The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics(opens in new tab) or relativity. There are also technical challenges: it might be possible but would involve vast amounts of energy.
Apparently we can travel forward in time by accelerating ourselves to high speed or going to a region of higher gravitational force. But, we can't then get back to the timeline of those who did not spend time at high speed or in the high gravity region.
S, where the article states "In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday," it seems to have overstated the situation.
Clearly, we can always get to "next Thursday" at either the normal rate in our inertial frame of reference, or at a faster rate by accelerating to a high speed, staying there for less time than it takes to get to next Thursday, and then decelerating. How much less time it takes depends on how high the speed is.
Alternatively, we could go down into a deep mine or go out into space and orbit a large mass at a distance that has a much higher gravitational acceleration than here on Earth's surface. All of those are opportunities to travel forward in time at a faster rate than if we just stay put on Earth's surface, and presumably would extend our lifetimes to go farther into the future before we die.
It is just going backward in time that seems to be a lot harder to do, if not actually impossible.