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How to think about… Space-time

It has often been described as a rubber sheet, but Einstein's twisted space-time is more intangible than that

By Anil Ananthaswamy

10 December 2014

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Space-time. Often described as the fabric of reality, this four-dimensional amalgamation of space and time was set at the heart of physics by Einstein (see “How to think about… Relativity”). But what is it?

A popular way of envisaging space-time is as a stretchy rubber sheet that deforms when a mass is placed on it, with the varying curvature analogous to the warping of space-time by gravity.

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It’s a picture that might lead us to believe space-time is itself something physical or tangible. But the physical manifestation of the dimensions we move through is, if anything, the fields they contain (see “How to think about… Fields“). For most physicists, space-time itself is a lot more abstract – a purely mathematical backdrop for the unfolding drama of the cosmos. Martin Bojowald of Penn State University in University Park sees it as a mathematical entity called a manifold. The equations of general relativity allow us to calculate the evolution of this manifold, and so of the universe itself, over time. “The rubber sheet is a picture for such a manifold, so in an abstract way I am indeed using the analogy,” he says.

Don Marolf of the University of California, Santa Barbara, goes even further. “Visualising the ‘shape’ of space-time is very useful,” he says. “But most of us don’t visualise it as something particularly physical. To the extent that we draw pictures, they are just chalk lines …

Article amended on 22 December 2014

When this article was first published, it mistook the Penn State University campus where Martin Bojowald is based.

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