Time travel is a staple of science fiction, and not without reason. Who wouldn’t want to go back in time to explore history, or save the world from catastrophe. Time travel has also been deeply studied within the context of theoretical physics because it tests the limits of our scientific theories. If time travel is possible, it has implications for everything from the origin of the universe to the existence of free will. One of the central problems of time travel theory is that it gives rise to logical paradoxes. But a couple of researchers think they have solved the pesky paradox problem.
Before getting into the details of their work, let’s talk about what time travel means in the context of theoretical physics. In general relativity, there is no great cosmic clock representing the time or age of the cosmos. Time is not absolute but relative. The rate at which time flows for an object depends on how it is moving relative to another object. Each of us experiences time in a unique way. This means the position of an object is defined by its position both in space and time. This is why time is sometimes referred to as the fourth dimension, and while you can’t take the idea too literally, a four-dimensional space decent way to visualize spacetime.
The path of a CTC compared to a normal timelike curve. Credit: Ben Tippett and David TsangAs you move through space, you also move through time. In physics, this means we can draw a line through spacetime that represents your path or motion through space and time. This is known as your timelike curve. Your entire life can be traced as a single line from your glorious birth to your inevitable demise. This is pretty simple in special relativity, but in general relativity, things get complicated. In relativity, gravity is caused by the warping of space and time. As John Wheeler once put it:
Matter tells spacetime how to curve, and spacetime tells matter how to move. Because of gravity, your timelike curve can be bent and twisted.
So what if you could bend your timelike curve so that it looped back on itself. This would be a closed timelike curve (CTC). It would mean that no matter what, your path and your life choices would inevitably return you to the place and
time of your birth. Congratulations, you have somehow traveled back in time. In theoretical physics, if time travel is possible, it must be possible to create a CTC.
It turns out that general relativity allows for closed timelike curves to exist.