We have entered the era of gravity waves, a window on the universe that will reveal startling new realities much as Galileo's light wave telescope, four centuries ago, revealed a universe unlike anything imagined by medieval culture.
Just as the motion of electrons and other electrically charged objects creates light waves, the motion of massive objects (having "weight") creates gravity waves. Isaac Newton discovered gravity three centuries ago, but gravity waves weren't remotely imagined until Einstein created his monumental General Theory of Relativity in 1915, and even Einstein wasn't sure about them.
Einstein saw space and time -- "spacetime" -- as a fabric or substance that interacts with massive objects. As a two-dimensional analogy to the real (but impossible to visualize) four-dimensional spacetime reality, imagine a perfectly flat trampoline. This is analogous to empty spacetime, with no massive objects. Now imagine a bowling ball rolling across the trampoline. This warps the trampoline, just as spacetime is warped by massive objects.
Now imagine a bowling ball and a marble rolling, side by side, across the trampoline. The large depression made by the bowling ball causes the marble to veer toward the bowling ball. That's how warped spacetime causes massive objects to move toward each other. Newton thought gravity was a pull or "force" acting between any two objects possessing mass, but Einstein saw it differently: Gravity warps spacetime, and massive objects simply follow the warped contours.
Finally, imagine two bowling balls moving past each other in opposite directions. Both create large warps. If the warps are deep, and if the balls are moving slowly, the balls will veer toward each other and begin to move in circular orbits around each other. As the two warps merge into one, the balls spiral inward. This creates outward-spiraling ripples in the trampoline -- analogous to gravity waves in spacetime.
These are waves unlike any other. Water waves, for example, simply cause water to vibrate up and then down as the wave passes any fixed point. But as a gravity wave passes by, spacetime itself vibrates. Imagine a gravity wave moving northward along a highway. As the wave passes a fixed point on the highway, the space in the remaining two dimensions (east-west, and up-down) alternately contract and expand, with one dimension contracting while the other expands.
Such waves were detected last Sept. 14 at Earth's first gravity wave observatory, called "LIGO," in Hanford, Wash., and Livingston, La. The two installations received a short burst of gravity waves that were identical except that the burst reached Livingston 0.007 seconds before Hanford. This duplication confirmed that the burst came indeed from an extraterrestrial source rather than from, say, a passing truck, and that gravity waves travel at light-speed, as expected.
LIGO's two installations are "laser interferometers." Each has two four-kilometer-long pipes, running along the ground and oriented perpendicular to each other, that intersect near one end. A single laser is at one of the short ends and a half-reflecting half-transmitting mirror (like a shop window that reflects your image) is at the intersection. The mirror is angled to send the reflected light beam (from the laser) down one tube and the transmitted beam down the other tube. The two beams reflect from mirrors at the ends of the tubes, pass again through the reflecting/transmitting mirror, and impact a single light detector. Since light is a wave, the two beams "interfere" when they arrive together at the detector: they can cancel each other if crests from one beam impact the detector simultaneously with valleys from the other beam.
The lengths of the two paths are adjusted to produce cancellation under normal conditions. Thus the laser beam can strike the detector only when one of the laser beams contracts slightly, and the other expands slightly -- as would happen if a gravity wave were passing through the two tubes.
The observed gravity wave was tiny, shortening and lengthening the tubes by only one-thousandth of the width of one proton! This wave's source was enormous: Two "black holes" -- stars that have completely collapsed -- circling in on each other at about a billion light-years from Earth. So the black holes collided a billion years ago. One black hole possessed 36 times the sun's mass, the other 29. During the two-tenths of a second burst, the two monsters circled each other every 0.01 seconds, and their energy radiated as gravity waves exceeded the energy of all the visible radiation from all the combined stars of the universe.
With this completely new way of examining astrophysical objects and phenomena," the researchers at LIGO report, "gravitational waves will truly open a new window on the universe, providing astronomers and other scientists with their first glimpses of previously unseen and unseeable wonders, and greatly adding to our understanding of the nature of space and time itself."
We will detect many more gravity waves. Stay tuned.
Commentary on 04/12/2016