Laser-Induced Quantum Field Research Project

Project Overview　

Is the interior of a black hole truly pitch-black and impossible to look into?

In our Laser-Induced Quantum Field Research Project, we aim to literally illuminate the quantum vacuum, which is deeply connected to the properties of black holes, using high-intensity lasers, and explore its contents.

Recently, with the advancement of quantum information technology, a new perspective on ‘information’ has emerged in physics, leading to a new phase in our understanding of black holes, which were previously thought to be pitch-black and impenetrable. By bringing ideas from a wide range of fields, including particle physics, quantum information, quantum optics, nonlinear optics, and plasma physics, we will experimentally approach and verify unsolved and incomplete problems such as the information paradox and quantum gravity, as well as new frontiers in quantum information technology.​

This project is supported by MEXT KAKENHI Grant-in-Aid for Transformative Research Areas A “Extreme Universe” and JST FOREST.

Members​

Research Background and Content

​​・Black Holes and the Information Paradox

More than 100 years have passed since Einstein's general theory of relativity was used to predict the existence of black holes, a high-density compact object with event horizons from which even light cannot escape. Recently, astronomical observations have even succeeded in capturing the outline of an event horizon.

Black holes are thought to absorb not only matter but also light, appearing as completely dark objects that cannot be seen or emit any light. In other words, the information absorbed into them appears to be lost forever.

On the other hand, Hawking incorporated quantum fluctuations and predicted that black holes emit light near the event horizon through thermal radiation (Hawking radiation). Figure 1 (left) shows the concept on which Hawking radiation is based.

This prediction differs from the currently observed phenomena such as gas around a black hole being heated by its gravity and emitting light as a high-temperature plasma, and/or light from behind the black hole being bent by gravity. Instead, Hawking radiation suggests that the black hole itself, including the region near the event horizon, emits light—a remarkable prediction. However, these two predictions seem to contradict each other, as one claims that black holes are completely dark, while the other claims that they emit light.

This led to the emergence of the unsolved paradox known as the ‘information paradox,’ which posits that although information appears to be lost when absorbed by a black hole, it should be preserved based on quantum theory. This paradox is believed to be deeply connected to the development of the theory of quantum gravity, which is still incomplete, and to the further advancement of quantum information technology, which is currently attracting significant attention.

・The Unruh effect linked to Hawking radiation and the equivalence principle

Hawking radiation, which is predicted to originate near the event horizon of a black hole, provides an important clue to the information paradox. However, its radiation temperature is inversely proportional to the mass of the black hole, making it generally much smaller than the temperature of the cosmic microwave background radiation and thus difficult to observe.

Among these, the Unruh effect, which is linked to Hawking radiation through the equivalence principle (the existence of an accelerated coordinate system which is locally indistinguishable from a gravitational field), one of the pillars of general relativity, is of particular interest. The Unruh effect is a phenomenon where the vacuum, which appears to be empty, appears to be emitting thermal radiation from the perspective of an observer in uniform acceleration. Experimental verification of this effect is considered crucial for resolving the information paradox. A conceptual diagram is shown in Figure 1 (right). However, like Hawking radiation, the Unruh effect is difficult to verify because it generally requires large acceleration.

・Experimental Verification of the Unruh Effect Using High-Intensity Lasers

Lasers composed of photons, which are Bosons, excel in energy concentration and can impart extremely high acceleration to charged particles. Among these lasers, J-KAREN-P at the Kansai Institute for Photon Science (KPSI), as shown in Figure 2, has the highest intensity laser in Japan and is one of the top-class high-intensity lasers in the world. However, in recent years, laser intensity has continued to increase, due to lasers capable of outputting powers of 10 petawatts now operational in Europe and China. To compete with these, we are advancing the further intensification of J-KAREN-P at KPSI while utilizing these high-intensity lasers, including J-KAREN-P, to achieve high-acceleration fields previously unattainable. This will enable us to verify the Unruh effect and ultimately address the ‘information paradox.’

Figure 1: Simplified schematic of Hawking radiation and the Unruh effect linked by the equivalence principle

​The left diagram shows how, during the formation of an event horizon via gravitational collapse, one particle (red) of the vacuum fluctuation caused by quantum effects spanning the horizon emerges as Hawking radiation, while its quantum-entangled partner particle (blue) remains confined within the horizon. The right diagram shows the situation in an accelerated frame linked by the equivalence principle. In this accelerated frame, a non-causal (Rindler) horizon exists. From the accelerating particle, only one particle (red) of the vacuum fluctuation is visible, leading to the prediction of the Unruh effect, where the vacuum appears to glow with thermal radiation.

Figure 2: J-KAREN-P laser at KPSI, QST. This gives us the highest intensity in Japan.