STRING theory, which claims that the universe is made of tiny vibrating strings, has long been considered virtually untestable. But that may soon change. If the theory is correct, then the universe could also have giant strings capable of generating bursts of gravitational waves intense enough to be picked up by detectors on Earth 鈥� giving us the first test of string theory.
General relativity predicts that violent events such as the merging of neutron stars should produce gravitational waves, which are ripples in the fabric of space-time. LIGO, a network of gravity wave detectors, and other instruments are looking for these waves, but none has yet been spotted.
These same detectors could be a testbed for string theory which views the building blocks of matter not as particles but as strings. Different modes of vibrations of these strings correspond to different particles such as electrons and quarks, with the proviso that the strings vibrate in 10 space-time dimensions, six of which are too small to be noticed. The beauty of string theory is that it appears to unify the otherwise incompatible theories of general relativity and quantum mechanics.
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But critics argue that all this cannot be tested, because the strings are about 10-33 centimetres long, and impossible to detect. However, string theory says larger strings could have formed in the split second after the big bang, at the end a period of ultra-fast expansion of the universe known as inflation. Theorists have suggested that the energy for this inflation may have come from the annihilation of a 鈥渂rane鈥� 鈥� a higher-dimensional object allowed by theory 鈥� with a kind of antimatter equivalent called the anti-brane. One result of such a process would be the creation of giant strings, some of which would span the universe.
Until recently, physicists had thought that such strings would have been unstable. Now, Edmund Copeland of the University of Sussex in the UK and his colleagues have discovered that these giants would be relatively stable (Journal of High Energy Physics, DOI: 10.1088/1126-6708/2004/06/013). The universe today could be criss-crossed by these vibrating strings, and like all massive, vibrating objects, they would generate gravitational waves.
Ordinarily these would be too faint to detect. But occasionally a giant string might crack like a whip, with a point along the string travelling at virtually the speed of light. 鈥淚t would generate super-intense gravitational waves, broadcast in one direction like a lighthouse beam,鈥� Copeland says.
These distinctive signals could be stronger than those generated by other violent astronomical events, and if some travelled towards Earth, detectors such as LIGO could pick them up. 鈥淭he feeling now in the field is that this is a serious possibility,鈥� says Jim Hough, director of the University of Glasgow鈥檚 Institute for Gravitational Research in the UK. 鈥淎nd we should know relatively soon.鈥�
