Black Hole Paradox Could Finally Be Solved

Stephen Hawking’s work has influenced many fields, including those of cosmology: more specifically quantum gravity and black holes . He was the first to point out that black holes behave in a way that puts two fundamental theories at odds with each other, relativity and quantum mechanics. This paradox has baffled scientists for half a century and led some to question the fundamental laws of physics. Recently, scientists say they may have solved it, relying in particular on the fact that black holes have a property they have named “quantum hair of gravity”. This would be a huge advance in theoretical physics.

Black holes are cosmic objects that we do not yet fully understand. Thus, in astrophysics, a black hole is defined as being an object so compact that the intensity of its gravitational field prevents any form of matter or radiation from escaping from it. In other words, their gravity bends spacetime so much that nothing can reach the speed necessary to escape. Such objects can neither emit nor scatter light, and are therefore black, which in astrophysics means that they are “optically invisible”.

In the same way, information cannot escape from it, and Einstein’s theory of general relativity suggests that information about what goes into a black hole therefore cannot leave it, i.e. say that we cannot determine what, a posteriori , entered the black hole. But quantum mechanics says it’s impossible. This is the information paradox highlighted by Stephen Hawking in 1976.

An international quartet of physicists, including a professor and research student from the University of Sussex, have co-authored two papers that could dramatically affect our understanding of black holes and claim to be the solution to the problem that has baffled scientists for nearly 10 years. a half-century. The studies are published in the journals Physical Review Letters and Physics Letters B , respectively .

First, let’s go back to the information paradox. Hawking realized that black holes radiatein a unique way. Their warping of spacetime would alter the wave nature of the surrounding quantum fields so that a form of thermal radiation would be produced. This means that a black hole should slowly evaporate, emitting its energy, photon after photon, out into the Universe. As it radiates, the black hole loses energy and therefore mass. In fact, general relativity implies that information could basically disappear in a black hole, following the evaporation of this one. Conversely, the laws of quantum physics state that information is preserved in black holes. This is where the information paradox lies.

There have been myriad solutions proposed, including the ” wall of fire theory “, in which information was supposed to burn before entering a black hole, the ” fuzzy ball theory of darkinos ” in which black holes are assumed to have fuzzy boundaries. But most of these proposals required rewriting the laws of quantum mechanics or Einstein’s theory of gravity, the two pillars of modern physics.

All of the theories that assume the persistence of information actually describe these remaining connections to the Universe as “hairs”. This is why Xavier Calmet and his collaborators suggest that when matter collapses into a black hole, it leaves a faint imprint in its gravitational field. The authors named it the “quantum hair of gravity” because their theory replaces an earlier idea called the “no hair theorem “, developed in the 1960s. This “bald black hole theory”, based on the classical physics, states that the latter can be considered as surprisingly simple objects, defined only by their mass, their electric charge and their angular momentum, related to their speed of rotation.

Instead of simple objects, the authors claim that black holes are far more complex. They believe that their quantum hair theory provides the mechanism by which information is preserved during the collapse of a black hole. This new solution applies quantum thinking to gravity in the form of theoretical particles called gravitons. These hypothetical elementary particles would convey gravity in most quantum gravity systems, similar to how the photon is associated with the electromagnetic force. Through a series of logical steps showing how gravitons could potentially behave under certain energy conditions, the team demonstrated their model of how information inside a black hole

Specifically, the researchers compared the gravitational fields of two stars with the same total mass and radius, but different compositions. In classical physics, the two stars have the same gravitational potential, but at the quantum level, the potential depends on the composition of the star. When stars collapse into black holes, their gravitational fields preserve the memory of star composition and lead to the conclusion that black holes have hair. Information about the material that fell into the black hole would leave a trace of its passage, giving us access, theoretically, to the composition of the black hole.

Nevertheless, there is no obvious way to test the theory with astronomical observations, the gravitational fluctuations would be too small to measure. As a theory it is interesting, based on a solid framework. But it needs careful scrutiny from the scientific community.

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