Loop quantum gravity can be visualised, as its name suggests, as a mesh of loops.

ECONOMIST: A new ��theory of everything�� is gaining ground

Loop quantum gravity, as this rival is known, was dreamed up in 1986 by Abhay Ashtekar, of Pennsylvania State University.

ECONOMIST: A new ��theory of everything�� is gaining ground

Indeed, theorists working on loop quantum gravity think that matter itself is merely the result of twisting and braiding ribbons of space-time.

ECONOMIST: A new ��theory of everything�� is gaining ground

But both it and loop quantum gravity harbour unresolved problems.

ECONOMIST: A new ��theory of everything�� is gaining ground

Loop quantum gravity is, in the jargon, background independent.

ECONOMIST: A new ��theory of everything�� is gaining ground

This could be achieved by finding which one is right and which one is wrong, by finding that both string theory and loop quantum gravity are wrong and a third theory is right, or by finding that the two theories can be unified.

ECONOMIST: A new ��theory of everything�� is gaining ground

Experiments currently being undertaken at Stanford, and also by John Price, a physicist at the University of Colorado, will measure the force of gravity over a distance of less than a millimetre (the size of loop expected if there are only two extra gravity-swallowing dimensions).

ECONOMIST: A matter of gravity

The gravity-based system responded as it was designed to, circulating water and steam in a continuous loop to keep the scale-model reactor at a safe temperature automatically.

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