Physicists have struggled for decades to find a grand unified theory. Could superstrings provide an answer?by Ian Stewart / September 20, 2003 / Leave a comment
Published in September 2003 issue of Prospect Magazine
Light from distant galaxies tells us that the universe is expanding-one of the main pieces of evidence that space, time and everything came into existence a little over 12bn years ago in the big bang. In 1998 astronomers, trying to find out whether the expansion will continue forever, or grind to a halt and reverse itself in a big crunch, discovered something much more puzzling. The expansion is speeding up. To explain this baffling acceleration, the cosmologists invented dark energy, a mysterious force that pushes the universe apart.
Does dark energy exist? No one knows. At present nothing known to physics can explain it, so something unknown to physics must be the cause. It’s like something out of Star Wars.
In February this year, American cosmologists Gia Dvali and Michael S Turner put forward a different theory, one in which dark energy does not exist. Instead, gravity is leaking out of our universe into an extra dimension. With less gravity to hold the universe together, it is coming apart faster than expected. It also sounds like something out of Star Wars.
Hidden dimensions? Only in the late 20th and early 21st centuries could physicists say this kind of thing with a straight face. It is a concept associated with Victorian spiritualists, who invented the fourth dimension as a convenient place to hide everything that didn’t make sense in the familiar three.
Why we need the superstring hypothesis
We spent the first half of the 20th century learning that the universe is far stranger than we imagined. Albert Einstein taught us that not only do space and time together make up a four-dimensional continuum; they also get mixed up with each other if we move fast enough-this is relativity. And Werner Heisenberg, Erwin Schr?ger and Paul Dirac discovered that on the tiniest of scales, the universe is plain weird: the quantum world, in which matter is made of waves and cats can be alive and dead at the same time.
We spent the last half of the 20th century puzzling over one gigantic discrepancy: relativity and quantum theory contradict each other. Each works well within its own domain-the very large for relativity, the very small for quantum theory. But when those domains overlap, as they do when we want to understand the early history of the universe, the combination doesn’t work. And so science set off on a quest for a single theory that would unify the whole of physics into a single mathematical law. And out of that quest came a strong suspicion that the familiar three dimensions of space and a fourth of time are mere scratches on the surface of something far bigger. Could the universe be made from ten-dimensional “superstrings,” maybe, with six tightly curled dimensions that are so small we never notice them? Or is the universe just a four-dimensional “brane” floating in a many-dimensional metaverse, like a skin of congealed milk on a cup of coffee?