This idea was first explored by mathematician Kurt Gödel in 1949 when he used Albert Einstein’s theory of general relativity to describe a rotating universe. Gödel’s model of a rotating universe was artificial, containing only one ingredient, a negative cosmological constant, to resist the centrifugal force of the rotation and keep the universe static.

Light in a Rotating Universe

Despite the artificial nature of his model, Gödel’s main point was that general relativity allowed for the possibility of a rotating universe at all. And he used his model to show that general relativity allowed for time travel into the past, which should be forbidden.

In a rotating universe, all observers would consider themselves the center of rotation. This means that if you were to stay in one place, you would see the universe wheeling around you. However, if you moved to a new location, you would still see the universe rotating around you. The farther away you go from any one observer, the greater the rate of rotation. This is not just a rotation of matter, but a rotation of space-time itself.

This rotation of space-time means that light, which always follows the curvature of space-time, takes some strange journeys. A beam of light sent out from an observer will curve away as it gets swept up in the rotation of space-time. At some distant point, the rotation will be too much, and the light will turn around and return to the observer.

Other Ways for General Relativity to Allow for Time Travel

This means that there is a limit to how far you can see in a rotating universe, and beyond that, all you will observe is duplicate images of your own past self. The same applies to physical movement; if you were to travel in a rocket through a rotating universe, your movement would double back on itself. When you returned to your starting point, you would find yourself arriving before you had left. In this way, a rotating universe would be capable of rotating your future into your past, allowing you to travel back in time.

Gödel objected to this aspect of general relativity because it allowed for backward time travel, violating our notions of causality and introducing paradoxes. The fact that relativity did not automatically make time travel impossible signaled to Gödel that Einstein’s theory was incomplete.

Thankfully, there is no evidence that we live in a rotating universe. If the cosmos were rotating, light coming from opposite directions of the sky would be redshifted in one direction and have an equivalent amount of blueshifting in the other. Astronomers have tested this by surveying distant galaxies and even the cosmic microwave background, concluding that if the universe is rotating, it’s doing so at a rate of less than 10^-17 degrees per century.

Despite the lack of evidence for a rotating universe, physicists have explored other ways for general relativity to allow for backward time travel, such as wormholes, faster-than-light-speed “warp drive,” and special paths around infinitely long cylinders. However, these contrivances rely on exotic physics that breaks our understanding of how the universe works, such as matter with negative mass.

In contrast, Gödel’s rotating universe is simply a matter of observational test, not a fundamental break with known physics. We could have found ourselves in a rotating universe just as easily as we find ourselves in an expanding one. There is nothing in our knowledge of physics that prevents this kind of universe from existing, so there is nothing in our knowledge of physics that prevents backward time travel.

In conclusion, while there is no evidence that we live in a rotating universe, Gödel’s exploration of the concept highlights the incompleteness of our understanding of general relativity. The fact that backward time travel is possible under some circumstances challenges our notions of causality and paradox