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ondes gravitationnelles déformation espace-temps

The collision of a black hole will change spacetime forever

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According to general relativity, each gravitational wave must leave a permanent imprint on the structure of spacetime; You must use the space continuously. This theoretically predicted “memory effect” has yet to be confirmed, and since the first detection of gravitational waves in 2016, physicists have been trying to find a way to measure them.

By exploring the connections between matter, energy and spacetime, physicists hope to better understand Stephen Hawking’s black hole information paradox, which has been at the center of theoretical research for five decades. ” There is a close connection between the memory effect and space-time symmetry. It is ultimately related to the loss of information in black holes, which is a very deep problem in the structure of space and time. “,” Cape Thorne said, a physicist at the California Institute of Technology, co-winner of the 2017 Nobel Prize in Physics for his work on gravitational waves.

When Stephen Hawking, in the mid-1970s, hypothesized that black holes could evaporate very slowly, until they disappear, the following problem arose: What happens to what is absorbed? This is called the information paradox. General relativity indicates that information can essentially disappear in such a black hole, but this loss implies the irreversibility and non-unitary evolution of quantum states, which is contrary to the assumptions of quantum mechanics.

Hidden symmetries in spacetime

Why does a gravitational wave permanently change the structure of spacetime? This is due to the close relationship that general relativity maintains between spacetime and energy. When a gravitational wave passes in front of a LIGO gravitational wave detector, it disturbs laser beams that travel through the installation’s massive arms (which form the letter L). If we imagine a circle defining the arms (the center of the circle at their intersection), then a gravitational wave will periodically distort this circle, compressing it successively vertically, and then horizontally, until the wave passes. This distortion causes a slight difference in the measured length between the two detector arms, which reflects the passage of the wave.

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But with the effect of memory, the circuit should still be slightly distorted. The objects discovered by LIGO are so far away that their gravity is negligible; But a gravitational wave has a longer range than the gravitational force. The same goes for the property responsible for the memory effect: gravitational potential. The latter corresponds to the amount of energy that an object will gain if it falls from a certain height – the speed of this object on the ground makes it possible to evaluate the potential energy gained during the fall.

In the context of general relativity, where space-time expands and contracts in different directions according to the motions of objects, potential dictates more than potential energy somewhere: it also determines the shape of space. -the time. The energy of the transient gravitational wave creates a change in the gravitational potential and the latter distorts spacetime even after the wave has passed. ” Memory is nothing but a change in the gravitational potential, but it is a relative gravitational potential Explain Thorne.

The possibilities of deformation are endless, but above all, they are equal to each other in terms of energy. Thus, when a gravitational wave is emitted, it transfers space-time from one configuration to another so that it remains in its lowest energy state. This indicates that there are subtle symmetries in the space-time structure.

Possible solution to the information paradox

In the 1960s, as part of research in general relativity, considering a hypothetical region infinitely far from any mass and energy in the universe, physicists demonstrated an infinite set of symmetries in addition to the expected ones (translation symmetries, spin and momentum). These new symmetries called “supertranslation” indicated that individual sections of spacetime could be stretched, compressed, and sheared, and that behavior in this infinitely remote region remained the same.

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In the 1980s, physicist Abhay Ashtekar discovered that the memory effect is the physical manifestation of these “super-symmetries”. In other words, it is over-translation that can lead to a distortion of spacetime. In 2016, Stephen Hawking and other physicists realized that the horizon of a black hole contains such symmetries in supertranslation. Logically, there would thus be an associated memory effect. This means that free-falling particles can change space-time near the black hole, thus changing its informational content. Thus, knowledge of particle properties will not be lost, but will be permanently encoded in the fabric of spacetime.

However, LIGO scientists haven’t noticed any evidence of a memory effect yet. The change in the distance between the detector’s mirrors when the gravitational wave passes is negligible (on the order of a thousandth of the width of a proton!) and the memory effect should be 20 times smaller. Not to mention, the earthquake sounds coming from the ground greatly hamper the mission. In addition, Earth’s gravity tends to return the LEGO mirrors to their original position, erasing any evidence of deformation. So the researchers will need to measure the displacement of the mirrors caused by the memory effect before gravity has time to affect them.

Detecting a memory effect associated with a single gravitational wave is impractical with current technology, but astrophysicists have devised an alternative: What you can do is effectively stack the signal from multiple mergers, to aggregate the clues in a very statistically rigorous way. It would take more than 1,000 gravitational waves to be able to claim to observe the memory effect,” suggests Paul Lasky, of Monash University in Australia. But thanks to ongoing improvements to LIGO, along with contributions from other detectors (notably Virgo and KAGRA), Lasky believes That it would only take a few years to reach those 1,000 discoveries.

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source : Quanta Magazine