One of the seminal novels to come out of the Golden Age of science fiction was a book by Hal Clement entitled A Mission of Gravity. It tells of the adventures of some inhabitants of the strange world Mesklin, whose gravity varied from 3g (three times Earth’s gravity) to 700g. The novel proved to be fascinating to many, and has remained in print more or less continuously since its publication in 1953.
Perhaps one reason for the novel’s enduring popularity is that gravity is the member of the four fundamental forces of nature that we are most familiar with, but is arguably the strangest one. Three of these forces, the weak force, electromagnetism, and the strong force have been unified (i.e., shown to have a common mathematical basis), but so far, gravity has stubbornly resisted all efforts at unification.
Classical mechanics, Issac Newton’s theory of physics, assumed that gravity acted at a distance – as an instantaneous force that was not mediated by anything – it was simply a property of the universe. Albert Einstein’s General Theory of Relativity contradicted that view. Einstein described gravity as a warp in spacetime created by massive bodies that acted on the bodies themselves. He envisioned it as propagated by a wave, which traveled at the speed of light, transporting gravitational radiation, in a manner analogous to an electromagnetic wave. However, gravitational waves are incredibly weak and could not be detected by existing instrumentation.
Another difference between gravity and the other three fundamental forces is that a quantum mechanical description of the weak force, electromagnetism, and the strong force has been developed. Quantum mechanics is the branch of physics that describes the behavior of systems over infinitesimally small distances. Each of the three forces has been found to be propagated by a specific messenger particle. Einstein envisioned a messenger particle for gravity dubbed the graviton, but, like gravitational waves, the graviton has never been detected. A quantum theory of gravity likewise remains elusive.
Things changed radically on Monday, March 17, 2014, when scientists involved with the BICEP2 project announced the first direct detection of primordial gravitational waves. BICEP2 is essentially a telescope designed to detect changes in the polarization of light. The light, or more exactly, the electromagnetic radiation that BICEP2 detects, is the Cosmic Microwave Background (CMB) – the radiative signature left by the Big Bang. Using BICEP2, the scientists mapped the pattern of polarization in the CMB, and found that gravitational waves could be the only way to explain this pattern.
The BICEP2 result strongly indicates that gravity must be quantum mechanical, because the polarization patterns observed by BICEP must originate from quantum oscillations in spacetime itself.
BICEP only looked at a small portion of the sky – a sector about 15° by 60°. Scientists expect that even larger gravitational wave patterns exist, but in order to see them, an instrument that can observe the entire sky will be necessary. To do this, a spacecraft rather than an earthbound telescope will be required. Scientists hope that, if these larger patterns are present, they may be able to finally test competing theories of quantum gravity, like string theory and loop quantum gravity.
You can watch the BICEP2 press conference here.
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