One notable feature of string theories is that these theories require extra dimensions of spacetime for their mathematical consistency. In bosonic string theory, spacetime is 26-dimensional, while in superstring theory it is 10-dimensional, and in M-theory it is 11-dimensional.

There are many different ways of achieving supersymmetry, all predicting different masses for the selectrons, the stop quarks, the sneutrinos and everybody else. To date, no evidence for supersymmetry has been found, and experiments at the Large Hadron Collider have ruled out the simplest supersymmetric models.

String theory (or, more technically, M-theory) is often described as the leading candidate for the theory of everything in our universe. But there’s no empirical evidence for it, or for any alternative ideas about how gravity might unify with the rest of the fundamental forces.

String theory has so far failed to live up to its promise as a way to unite gravity and quantum mechanics. At the same time, it has blossomed into one of the most useful sets of tools in science.

Although it hasn’t had the same media exposure, loop quantum gravity is so far the only real rival to string theory. The basic idea is that space is not continuous, as we usually think, but is instead broken up into tiny chunks 10-35 metres across. These are then connected by links to make the space we experience.

If string theory is true, our universe has extra dimensions that lie curled up at every point around us. And some physicists now view strings as a failed theory because it doesn’t make useful predictions about the universe.

Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, traversable wormholes, and Alcubierre drives.

Einstein’s theory of general relativity mathematically predicts the existence of wormholes, but none have been discovered to date. However, a naturally occurring black hole, formed by the collapse of a dying star, does not by itself create a wormhole.

Scientists can’t directly observe black holes with telescopes that detect x-rays, light, or other forms of electromagnetic radiation. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby.

Wormholes are tunnels in space-time that can theoretically allow travel anywhere in space and time, or even into another universe. In many ways, wormholes resemble black holes. Both kinds of objects are extremely dense and possess extraordinarily strong gravitational pulls for bodies their size.