Tuesday, October 03, 2017

Gravitational Waves Catch 2017 Nobel

A long time ago in a galaxy far, far away...two monstrously large black holes, perhaps as old as the universe itself, collided. Nearly a billion years later, in late 2015, the Advanced LIGO detectors in Hanford, WA and Livingston, LA came online—just in time to catch the signal as it went by. The men who masterminded this enormous endeavor, observing a warping of spacetime smaller than a proton over a scale of several miles, were recognized this morning for their efforts with the announcement that they would receive the 2017 Nobel prize in physics.
For those who haven't been keeping up, gravitational waves are created when any suitably massive body moves in an asymmetrical way—a concept best illustrated by Richard Feynman's "sticky bead" thought experiment. Although gravity can be thought of simply as an attractive force in that example, Einstein's general relativity models it as a warping of spacetime, the surface of the universe itself.

An artist's impression of two merging black holes, producing gravitational waves.
Image Credit: National Science Foundation
This paradigm shift underlies LIGO's detection method, laser interferometry, which measures the distance between two mirrors by bouncing a beam of light between them. Since light travels at a very well-defined, constant speed, the distance a photon travels and the time it takes to make that journey are two sides of the same coin. The photon's frequency, the number of times it oscillates per meter that it travels (or per unit of time, if you please) is defined by its energy, but this can change depending on local gravity conditions—observers on Earth receiving a laser signal from the moon will see it blueshifted, or higher in energy than it was at its point of origin, thanks to Earth's deeper gravity well. It won't go any faster, but there'll be more cycles per second—it could, for instance, knock an electron free in a solar panel even if this reaction wouldn't have worked on the moon. However, the crux of Einstein's general theory of relativity is that the photon could be thought of as having the same energy, while time itself ticks faster on the surface of the earth than it does on the moon.

LIGO takes advantage of this fact. When a gravitational wave passes through a point in space, there's a slight warping of any photons currently in-flight, and this lies at the heart of the interferometry technique devised by LIGO founder Rainer Weiss.

You might've heard of superposition—how waves can sit on top of one another and either add together in intensity or cancel out, depending on their relative phases. Photons, electromagnetic waves, do this all the time; the shine of a peacock's feather or the rainbows of a soap bubble are all due to superposition and interference of light waves. But imagine pointing a single-photon laser directly at a mirror, perpendicular to the surface so that the reflection forms a perfect counter-wave to the incident photon and the two waves cancel out at every point. Any slight change to the photon's wavelength or the distance between the mirror and the laser disturbs this perfect cancellation, creating a detectable light signal.

The actual mechanism of LIGO's operation is slightly more complicated, combining lasers from two perpendicular, miles-long vacuum tunnels to produce the cancelled beam, but the general idea is the same, and allows for incredibly precise measurements—one part in 1021.

The L-shaped arms of LIGO's Hanford, WA facility are each 2.5 miles long, but the changes they detect are something like one ten-thousandth the diameter of a proton.
Image Credit: via PBS
It's worth noting that we're detecting this kind of subatomic signal as the result of an event happening in another galaxy. While intuitively friendly comparisons and scaling metaphors are our bread and butter here at PhysicsCentral, there's not much we can do to bring this into the realm of everyday objects; the scales involved, and the differences between them, are already near the limits of human comprehension.

As predicted, the Nobel's three recipients are Kip Thorne, Barry Barish, and Rainer Weiss, selected for the honor from among LIGO's thousands of collaborators for their longstanding involvement in the project; Weiss and Thorne were part of LIGO's original triumvirate leadership (along with Ron Drever, now deceased), with Barish taking over a while after the directorship was consolidated into a single role. Although all three received a portion of the honor and will receive some of the associated prize money, half went to Weiss, with Barish and Thorne splitting the remainder.

In addition to confirming yet another aspect of Einstein's relativity, LIGO and similar collaborations have opened the door to a new kind of astronomy—being able to detect gravitational waves is something like having a whole new sense available to us; it's like hearing for the first time. The largest project ever funded by the US's National Science Froundation, the LIGO collaboration and the scientists that comprise it are a point of pride for American science—and though it wouldn't have been possible without the three people honored today, we shouldn't forget the contributions from thousands across the world—scientists and citizen-scientists alike—that have helped us open our eyes and see the universe in a new way.

Stephen Skolnick

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