Monday, March 24, 2014

Gravitational Reverberations from NYT

Dennis Overbye, writing in the NYT, has a nice article on the significance of the BICEP2 measurements. He makes a point that I had not understood (I'm assuming that he is right).

The gravitons themselves, theory says, are produced by the same process by which black holes leak. It is known as Hawking radiation, after Stephen Hawking of Cambridge University, the renowned black hole theorist, best-selling author and avatar of cosmic mystery who discovered it in a prodigious calculation in 1973.

Shortly thereafter, William Unruh, now at the University of British Columbia, showed that you didn’t need black holes to see this radiation, just acceleration in space. In this case, the role of the black hole you can’t get out of is played by the rapidly retreating horizon you can’t reach in the inflating universe.

Hawking radiation has been part of the physics firmament for decades; it’s the best-known prediction of quantum gravity.

“Now it seems that Hawking and Unruh were right!” said Max Tegmark, a cosmologist at M.I.T., noting that some physicists had wondered whether gravity obeyed the dice-playing quantum principles that Einstein had disdained. “Now we know that gravity is indeed quantized, involving graviton particles,” he added.

So Hawking and Unruh might need to be added to the list of those getting Nobels out of this. The fact that the Hawking and Unruh effects come directly out of semi-classical quantum field theory may be significant too. One way of looking at the trick behind these calculation is that part of the quantum field modes get cut off by a horizon - or, alternatively, that one member of a virtual particle pair fall through the horizon and leaves its partner stuck and real on the other side.