Monday, August 9, 2010

Einstein and the end of space-time

Physicists struggling to reconcile gravity with quantum mechanics have hailed a theory – inspired by pencil lead – that could make it all very simple

IT WAS a speech that changed the way we think of space and time. The year was 1908, and the German mathematician Hermann Minkowski had been trying to make sense of Albert Einstein's hot new idea - what we now know as special relativity - describing how things shrink as they move faster and time becomes distorted.

"Henceforth space by itself and time by itself are doomed to fade into the mere shadows," Minkowski proclaimed, "and only a union of the two will preserve an independent reality."

And so space-time - the malleable fabric whose geometry can be changed by the gravity of stars, planets and matter - was born. It is a concept that has served us well, but if physicist Petr Horava is right, it may be no more than a mirage.

Horava, who is at the University of California, Berkeley, wants to rip this fabric apart and set time and space free from one another in order to come up with a unified theory that reconciles the disparate worlds of quantum mechanics and gravity - one the most pressing challenges to modern physics.

Since Horava published his work in January 2009, it has received an astonishing amount of attention. Already, more than 250 papers have been written about it. Some researchers have started using it to explain away the twin cosmological mysteries of dark matter and dark energy.

Others are finding that black holes might not behave as we thought. If Horava's idea is right, it could forever change our conception of space and time and lead us to a "theory of everything", applicable to all matter and the forces that act on it.

For decades now, physicists have been stymied in their efforts to reconcile Einstein's general theory of relativity, which describes gravity, and quantum mechanics, which describes particles and forces (except gravity) on the smallest scales.

The stumbling block lies with their conflicting views of space and time. As seen by quantum theory, space and time are a static backdrop against which particles move. In Einstein's theories, by contrast, not only are space and time inextricably linked, but the resulting space-time is moulded by the bodies within it.

Part of the motivation behind the quest to marry relativity and quantum theory - to produce a theory of quantum gravity - is an aesthetic desire to unite all the forces of nature. But there is much more to it than that.

We also need such a theory to understand what happened immediately after the big bang or what's going on near black holes, where the gravitational fields are immense.

One area where the conflict between quantum theory and relativity comes to the fore is in the gravitational constant, G, the quantity that describes the strength of gravity. On large scales - at the scale of the solar system or of the universe itself - the equations of general relativity yield a value of G that tallies with observed behaviour.

But when you zoom in to very small distances, general relativity cannot ignore quantum fluctuations of space-time. Take them into account and any calculation of G gives ridiculous answers, making predictions impossible.

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