When we think about time travel and black holes, we usually just imagine circling them and being forced back in time in a similar way to the Tipler Cylinder. However, this next theory draws on the idea that we might actually fly right into the centre of one of the most terror-inducing objects in the universe.
The first realistic concept of rotating black holes was introduced in 1963 by New Zealand mathematician Roy Kerr. He proposed that if dying stars were to collapse into a ring of neutron stars (collapsed stars that are the size of Manhattan but have the density of the Sun), their centrifugal force would prevent them from becoming a singularity. While this sounds complicated, it is relatively simple to understand.
Centrifugal force is the force that draws a rotating body away from its centre of rotation. This is due to the inertia of the body, as the body’s path is constantly changing direction. This is quite easily visualised if you imagine children in on a roundabout – the faster they spin, the more they are pulled away from the centre.
A singularity is slightly more difficult to explain. A simplified version is that a singularity is a point where gravity is mathematically infinite – for example, the centre of a regular black hole. The idea of the Kerr black hole is that it is not a singularity; the centrifugal force of the rotating neutron star will prevent it from becoming one.
As the black hole would not be a singularity, Kerr speculated that it would be safe to enter without fear of its infinite gravity.
It is thought that if these black holes existed, and if we entered one, we would pass through them and exit a white hole. A white hole is essentially the opposite of a black hole; instead of pulling everything into it, it would force everything out – maybe into another time or universe.
While these black holes are entirely theoretical, it could be an interesting way for future generations to explore the past or even the future. After all, there is no telling where (or, when) a Kerr black hole might take you.