Physics Professor Sidney Perkowitz writes: “Seven years after their discovery, the ripples in spacetime have opened new windows on the universe’s deepest secrets.”
When Galileo Galilei first pointed a small telescope at the heavens in 1609, he began a revolution in astronomy. Today huge telescopes and radio dishes tell us about the universe. But they cannot directly probe those invisible sharply curved regions of spacetime called black holes or find those so far undiscovered invisible tunnels through spacetime called wormholes, theorized to offer instantaneous cosmic transport and possibly time travel.
Now, though, we have discovered gravitational waves, undulations in spacetime that can directly reveal black holes and wormholes by how they ripple the fabric of the universe. Rainer Weiss of MIT, who with Barry Barish and Kip Thorne of Caltech shared the 2017 Nobel Prize for the discovery, tells Nautilus that astronomical research in the wake of the initial observation “has produced so much science it’s unbelievable.”
Einstein predicted gravitational waves in 1916 after developing general relativity, his theory of gravitation that does not treat gravitation as a force, but as arising from the curvature of spacetime. He showed that an accelerating mass would generate gravitational waves moving at the speed of light, analogous to but different from the electromagnetic waves produced by accelerating electric charges. Einstein himself was unsure if they would ever be discovered, but he fully understood their importance as potentially giving direct experimental evidence of his view of spacetime as the underlying cosmic fabric.
Einstein wondered about the discovery of gravitational waves because they are exceedingly weak, too weak to detect unless they come from massive bodies in rapid motion. Fortunately, these criteria are met by black holes when they collide. Gravitational waves were finally detected on Sept. 14, 2015, by two separate LIGO (Laser Interferometer Gravitational-Wave Observatory) installations in the United States.1 Each consists of two legs, both 4 kilometers long, that form an L-shape. Laser beams bouncing between mirrors traverse the legs and meet at the corner of the L, where their interference pattern responds to the tiny spatial changes as a gravitational wave sweeps by.
This first observation came from two black holes, 36 and 29 times as massive as our sun, when their mutual gravitational attraction sent them spiraling into each other, reaching half the speed of light just before collision. LIGO detected the resulting transient gravitational wave as a signal that accurately matched the prediction from general relativity. This direct evidence for gravitational waves also showed for the first time that two black holes can merge; in this case, into a black hole of 62 solar masses.
Note: The missing 3 solar masses after this merger was converted into the energy of a gravitational wave pulse generated in the final fraction of a second as the orbital velocity of the inspiraling black holes reached over half the speed of light. The luminosity of this pulse exceeded that of the entire visible universe by about 50 times. Spreading outward at the speed of light, this resounding tremor in spacetime reached Earth some 1.3 billion years later, just days after the LIGO gravitational wave detector came back online after an upgrade that made it sensitive enough to detect the far-flung ripples of this incredible shaking of the heavens. (E. Hedin)
LIGO displayed remarkable sensitivity in registering this event, responding to distortions in space that changed the 4 kilometer distance between mirrors by less than the diameter of a proton. This feat jump-started an astronomical program at LIGO under the auspices of Caltech and MIT and in collaboration with the Virgo gravitational wave observatory in Italy, with the KAGRA observatory in Japan joining in 2020. By 2021 the consortium had observed 90 gravitational wave events. Centuries of electromagnetic wave astronomy have produced catalogs of stars and galaxies that classify them by their fundamental properties. Now gravitational wave astronomy is doing the same for black holes and their cousins, neutron stars, the dense objects that can form on the way to the formation of a black hole.
As new gravitational instruments are built, the result will be a group of gravitational wave observatories operating over different frequency ranges, much as conventional observatories examine electromagnetic waves of different frequencies from gamma rays to radio waves. That spectral breadth gives electromagnetic wave astronomy its wide window on the universe. Now seven years into the regime of gravitational wave astronomy, a wide gravitational window is opening to give a unique but complementary view of the cosmos. We can hardly imagine what wonders this dual vision will uncover.
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