Time travel is one of the great tropes of science fiction and fantasy, going back at least as far as H.G. Wells (though you could argue fables involving time travel have been around a lot longer). Technically, we are all traveling through time, forward at a fixed rate of one second per second. But that’s not what we’re talking about here. We’re talking about the ride in the TARDIS, rev up the flux capacitor to 1.21 gigawatts kind of travel. The wibbly-wobbly timey-wimey, maybe make the universe explode kind of time travel.
First, what exactly is time? In the most literal sense, it’s just a way of measuring celestial movement. But we all know it’s a bit stranger than that. We do not experience time in such a clear-cut, linear fashion. Time is not perceived the same way on a roller coaster as it is when sleeping, or watching golf (which is eternal). But the mere change in perception does not mean that what is being perceived is necessarily fluid itself.
We may know that atoms decay at certain rates, or planets move in predictably decaying orbits, and therefore think we understand time itself. But even the most sciencey parts of science are themselves built upon areas in which actual, empirical knowledge remains uncertain. We leap over numerous paradoxes to try to apprehend the world as we see it. As such, what time measures is one thing. What time is, is another. It may seem like magical thinking to believe that the essential nature of time maybe mutable in such a way that would allow us to travel in it, but nothing we know disproves the possibility.
Oh, and if your argument against time travel is that we have no time machines, consider this. If time travel will ever be possible, then it already is. So the fact that hasn’t been invented yet is irrelevant because “yet” is irrelevant.
Theoretical physicist Lee Smolin posits that time is not relative (as Einstein thought). He says it is a real force, rather than an illusion relative to other factors. “Any truth about the world is a truth about the world within time—there are no timeless truths. And most importantly, there are no laws of nature that are outside of time,” he suggests. This argues against the possibility of time travel in that if no laws of nature are outside of time, no reality can exist outside of time, either, and working around this fundamental truth is not possible.
In order for time, forward or back, to be traversable, it would need to be malleable in a way that Smollin’s perspective (or observable nature) does not allow for. Time is. And what it is, you cannot truly alter.