There are five different superstring theories, each ten-dimensional, all seemingly incompatible. But in 1995, Edward Witten proposed that the five theories were actually all part of a large, mysterious and uncharted framework that he dubbed M-theory.
We donâ€™t have the full equations for M-theory, but there are many hints as to how it works. Witten showed that the five theories are linked to each other via dualities: one formulation at strong coupling is identical to another at weak coupling. M-theory is the complete skeleton whilst the five superstring models are individual bones.
M-theory doesn’t have ten spacetime dimensions, but eleven – ten space and one time! Now there isnâ€™t a string theory in eleven dimensions, but there is a supersymmetric theory of gravity, called supergravity. Witten showed that there was a continuous path between the ten-dimensional string theories and the eleven-dimensional theory of supergravity; supergravity is part of the M-theory web.
Our understanding of M-theory is by no means complete. It seems to be the single unifying structure into which all string theories fit. Dualities allow us to relate some of the fringes, where interactions are very weak or very strong. But the middle of the web remains impenetrable.
Some of the duality calculations are surprising, and impressive. Nonetheless we can only see the edges of the picture, and we grasp little of its mathematics. We have yet to derive any concrete predictions and experimental evidence of the required extra dimensions remains elusive.Â Like an artistic masterpiece with a hole through the middle, it gives us a tantalising glimpse of what might be the ultimate unifying theory.