In February 2016, physicists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) reported the first observation of gravitational waves, emanating from two black holes, each about 30 times more massive than the Sun and located 1.3 billion light-years away. Physicists can therefore now study black holes as concrete objects. But are these the same black holes as those predicted by general relativity? Are these real black holes, or objects approaching the description given by Einstein's theory? While many cosmologists have no doubts about the answer, others take more measured positions. Without rejecting the identity of indirectly observed black holes outright, several astrophysicists argue that more observational tests regarding the direct properties of general relativity black holes need to be performed to definitively rule on the matter.
Gravitational wave detectors have spotted four dozen black hole mergers since LIGO's breakthrough detection. In April 2019, an international collaboration called Event Horizon Telescope (EHT) produced the first image of a black hole. By directing radio telescopes from around the world to the supermassive black hole at the heart of neighboring galaxy Messier 87 (M87), the EHT has imaged the structure of the massive object.
Astronomers are also tracking stars approaching the black hole at the center of our own galaxy, following paths that may hold clues to the nature of the black hole itself. The observations are already challenging astrophysicists' assumptions about how black holes form and influence their surroundings. The smaller black holes detected by LIGO and now Europe's Virgo gravitational-wave detector in Italy have turned out to be heavier and more varied than expected, putting astrophysicists' knowledge of massive progenitor stars to the test. .
And the environment around our galaxy's supermassive black hole appears surprisingly fertile, teeming with young stars that shouldn't be forming in such a maelstrom. But some physicists feel the shadow of a more fundamental question:do they really see the black holes predicted by Einstein's theory?
Some theorists say yes. "I don't think we'll learn more about general relativity or black hole theory says Robert Wald, a gravitational theorist at the University of Chicago. Others aren't so sure:"Are black holes strictly the same as you'd expect to see from general relativity, or are they different?" This will be a major focus of future observations says Clifford Will, a gravitational theorist at the University of Florida.
Any anomaly would require a rethink of Einstein's theory, which physicists suspect may not be the most comprehensive theory of gravity, given its current lack of compatibility with quantum mechanics. By using multiple techniques, researchers are already gaining different and complementary points of view on these strange objects.
This is notably the work of Andrea Ghez, an astrophysicist at the University of California, who shared the 2020 Nobel Prize in Physics for inferring the existence of the supermassive black hole at the heart of our galaxy. “We are still a long way from having the complete picture. But we're definitely putting more pieces of the puzzle in place .
Made up of gravitational energy, a black hole is a sum of contradictions. It contains no matter, but has mass and can rotate. It has no surface, but has size. It behaves like a large, massive object, but is really just a particular region of space.
So says general relativity, published by Einstein in 1915. Two centuries earlier, Isaac Newton postulated that gravity is a force that somehow travels through space to attract massive objects towards each other. Einstein went further and argued that gravity arises because massive objects such as stars and planets distort spacetime, causing the trajectories of free-falling objects to curve.
The first predictions of general relativity differed only slightly from those of Newton's theory. While Newton predicted that a planet should orbit its star in an ellipse, general relativity predicts that the orientation of the ellipse should advance slightly, or show precession, with each orbit. In the theory's first triumph, Einstein showed that it explained the previously unexplained precession of the planet Mercury's orbit. It wasn't until years later that physicists realized that the theory also involved something far more radical.
In 1939, theorist J. Robert Oppenheimer and his colleagues calculated that when a sufficiently massive star burned up, no known force could prevent its core from collapsing to an infinitesimal point, leaving behind its gravitational field as a permanent sink. in space-time. At some distance from the point, gravity would be so strong that not even light could escape. Anything close would be cut off from the rest of the universe, according to Caltech theorist David Finkelstein in 1958.
This “event horizon” is not a physical surface. An astronaut falling through would not notice anything special. Nevertheless, according to Finkelstein, who died just days before LIGO was announced in 2016, the horizon would act as a one-way membrane, letting objects fall, but preventing them from coming out.
According to general relativity, these objects—eventually named black holes by famed theorist John Archibald Wheeler—should also bear a startling similarity. In 1963, Roy Kerr, a New Zealand mathematician, discovered how a rotating black hole of a given mass would warp spacetime. Others quickly proved that in general relativity, mass and spin are the only characteristics a black hole can have, implying that Kerr's mathematical formula, known as Kerr's metric, describes each existing black hole.
Wheeler dubbed the result "the baldness theorem," to point out that two black holes of the same mass and rotation are as indistinguishable as bald heads. Some physicists suspected that black holes might not exist outside of theorists' imaginations, says Caltech theorist Sean Carroll. Skeptics have argued that black holes could be an artifact of the subtle mathematics of general relativity, or that they could only form under unrealistic conditions, such as the collapse of a perfectly spherical star.
>However, in the late 1960s Roger Penrose, a theorist at Oxford University, dispelled these doubts with rigorous mathematics, for which he shared the 2020 Nobel Prize in Physics. that, no, no, even if you have a lumpy thing, as long as the density got high enough, it was going to collapse into a black hole says Carroll.
