Black Holes

Black holes are regions of spacetime where gravity is so strong that nothing, not even light, can escape. They are some of the most fascinating objects in the Universe and continue to be a source of inspiration in the quest for a theory of quantum gravity.

A Brief History of Black Holes

The concept of black holes was first introduced in the 18th century by John Michell and Pierre-Simon Laplace, who discussed the possibility of 'dark stars' with an escape velocity larger than the speed of light. However, the idea didn’t receive much attention until it was revived in 1915, when Karl Schwarzschild found a solution in Einstein’s theory of general relativity describing a region of space where gravity is so intense that nothing can escape, now known as the event horizon. Even then, black holes were believed to be a theoretical curiosity and few believed that they really existed.

In 1939, Oppenheimer and Snyder predicted the gravitational collapse of massive stars into black holes. The theoretical understanding of black holes was further advanced by Penrose who established under which conditions gravitational collapse inevitably leads to the formation of a singularity, a point of infinite density and curvature at the heart of a black hole. At the same time, the first observational evidence for the existence of black holes started to emerge, such as the X-ray observations of the object Cygnus X-1 in 1964.  Today, it is unquestionable that black holes are real.

The image above shows the stunning photograph of the black hole at the center of the galaxy M87. The picture was taken by the Event Horizon Telescope (EHT), a global network of eight radio telescopes. What you see in this image it the light from the hot gas swirling around the black hole. The light is highly bent by the strong gravity near the black hole’s event horizon. The dark central region is the black hole’s shadow.  The inferred mass of the central black hole of M87 is 6.5 billion times the mass of our Sun. At the center of our own galaxy lives a black hole of 4.3 million Solar masses. The existence of this black hole was first inferred indirectly, by observing the orbits of stars around it.

Black holes continue to captivate scientists and the public alike, serving as fascinating cosmic laboratories for testing the limits of our understanding of gravity and the nature of spacetime.

The Information Paradox

In a remarkable paper, Stephen Hawking demonstrated that black holes emit radiation when the effects of quantum mechanics are taken into account, challenging the long-held belief that black holes were completely dark. Detecting this radiation directly from a black hole is extremely challenging, but the theoretical consequences of this radiation are profound. It implies that black holes have an entropy proportional to the area of the black hole’s event horizon. The fact that this entropy depends on the area and not the volume is an example of the holographic principle.

According to classical general relativity, once an object crosses the event horizon of a black hole, it is believed to be lost to the outside Universe, with no information about its specific characteristics retrievable. This challenges the principles of quantum mechanics, which state that information is always conserved, and the evolution of a quantum system is deterministic and unitary. Even in quantum physics, the Hawking radiation coming from the black hole is thermal radiations which doesn’t seem to encode information about what went into the black hole.

Analog Black Holes

The question of whether black hole evaporation results in the loss of information, has remained unresolved since Hawking’s seminal discovery in 1974. To date, the investigations have remained mostly theoretical since it is almost impossible to settle this paradox through direct astrophysical observations. This is mainly because the answer to the information loss paradox relies critically on the end stage of the black hole evaporation. However, the lifetime of a astrophysical black holes is much longer than the current age of the universe, so that these black holes are too young to reveal the secrets of information paradox. Physicists therefore resort to the investigation of 'analog black holes' in the laboratory, where the physics of the analog system mimics the evolution of a real black hole.

In 2017, Prof. Pisin Chen of LeCosPA and Prof. Gerard Mourou of Ecole Polytechnique (2018 Nobel Laureate in Physics) pointed out that state-of-the-art high-intensity lasers can induce a relativistic flying plasma mirror, which would accelerate under a properly tailored plasma target density profile. Such a relativistic accelerating mirror mimics the late-time evolution of black hole evaporation. Critical issues, such as how the information may be preserved, can be addressed through the entanglement between the analog Hawking radiation photons and their partner modes. An international AnaBHEL (Analog Black Hole Evaporation via Lasers) Collaboration was therefor initiated by Prof. Chen that includes experts from Taiwan, France, and Japan.  In recent years, this collaboration has been making sizable progress.

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