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. 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.
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 on the blackboard.”
One thing that unifies all of these conceptions of space-time is that it is a “continuum”, something that varies smoothly with no abrupt knobs, bumps or tears. But if we want to combine general relativity with quantum mechanics to create a unified theory of quantum gravity, that notion must change. In quantum gravity, space-time is made up of tiny discrete quanta just like everything else – making it a fabric with a discernible warp and weft.
“Fabric as opposed to a rubber sheet means that we are focusing more on what possible microstructures space-time may have,” says Bojowald. of the University of Marseilles, France, visualises this woven microstructure as being made of “tiny fuzzy blobs”, the starting point of his theoretical investigations of quantum gravity. It’s still just a device, though: something that helps him work with the intangible. “If I do not have an image in my head, I cannot even start thinking,” he says.
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This article appeared in print under the headline “How to think about space-time”
Article amended on 22 December 2014
When this article was first published, it mistook the Penn State University campus where Martin Bojowald is based.
