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.
Get your head around the most mind-bending concepts in science
From black holes to blockchains, from consciousness to the multiverse, we explain it all in this 13-part special
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 …
