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String theory is part of 21st-century physics that fell by chance into the 20th century.
Daniele Amati

The first superstring revolution left us with five consistent theories, where one-dimensional strings moved around in ten-dimensional spacetime. They all seemed to describe different worlds. Which, if any, was the correct one?

In 1995, Edward Witten gave a talk at the yearly “Strings” conference. He proposed that these five theories were actually all part of a single framework. But he could not fully describe his vision. Nevertheless he demonstrated how ordinary superstrings give us tantalising hints about the properties of this ultimate theory. He called it M-Theory.

Witten argued that there were dualities between the different superstring theories. Others had already suggested some of these, but Witten drew them all into a coherent picture. Each theory is an alternative way of looking at the same world. Which one you need depends on the values of certain physical parameters. He also showed that M-theory doesn’t have ten spacetime dimensions, but eleven!

There isn’t a string theory in eleven dimensions, but there is a supersymmetric theory of gravity, called supergravity. During the late 1970s, whilst their colleagues worked on incorporating supersymmetry into the Standard Model, some physicists tried to combine supersymmetry and gravity. The result was supergravity. It was largely ignored by string theorists, who worked in ten dimensions, not eleven! Ignored, that is, until Witten realised that supergravity is also part of the M-theory picture.

With supergravity came a supermembrane theory, describing two-dimensional membranes in an eleven-dimensional spacetime. For M-theory to hold together in eleven dimensions, it must also include surfaces called membranes.

If M-theory is correct, then why did it take physicists so long to spot it? The answer is simple. An eleven-dimensional theory with membranes looks just like a ten-dimensional theory with strings. Realising this requires a little imagination.

Suppose you live in an eleven-dimensional world, where one of your dimensions is a circle. Take a two-dimensional membrane sheet and wrap it around the circular dimension to make a cylinder.

Now imagine you make the circular dimension extremely small. From your perspective the world is now ten-dimensional and your cylinder has become extremely thin. In fact its thickness is now so small that it looks exactly like a one-dimensional string!

Shortly after Witten’s inspiring lecture, Joseph Polchinski realised that membranes with up to nine spatial dimensions had a very simple description in string theory. These so-called D-branes have been central to research ever since.

It quickly became clear that D-branes suggested new symmetries in M-theory. The most famous was introduced by Juan Maldecena in a 1997 paper. His result is known as the AdS-CFT Correspondence. It is essentially a duality between string theory and a type of quantum field theory.

D-branes, AdS-CFT and M-theory open the doors to the study of non-perturbative physics. They also provide a set of tools with applications from black holes to condensed matter. Much research today involves developing of the insights first articulated in the the mid-to-late 1990s.

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