Famed astrophysicist Stephen Hawking made a remarkable discovery in 1974 about black holes and their boundaries called event horizons. He studied the effects of space-time curvature on quantum matter near the boundary of a black hole. The subject is something that can only be described and talked about using quantum mechanics. Hawking proposed that the vacuum near the horizon splits into positive and negative quantum particles. The latter (negative) falls and can cause the mass of the black hole to decrease (commonly called black hole evaporation), while the equivalent amount of (positive) energy is emitted as radiation. This is now called Hawking radiation. This is a quantum mechanical effect because it is achieved by the quantum division of the vacuum into positive and negative parts.

The event horizon of a black hole could be thought of as consisting of particles of light, called photons. It is well known in astrophysics that when matter accumulates, that is to say falls towards and on a black hole, the horizon stretches and tends to lose its photonic character. However, Hawking radiation can only reach the external observer if it comes from an unextended horizon and has a photon-like character. For Hawking radiation to be emitted, it is therefore essential that the horizon is not stretched by the accretion of matter. Basically, this is necessary for a black hole to always remain “black”, but until now it was not understood how this was possible, with all the oscillations that can be caused by all the matter falling into it.

In a recent research article published in the journal ‘Physical Review D’, Naresh Dadhich of the Inter-University Center for Astronomy and Astrophysics (IUCAA), Pune and Rituparno Goswami of the University of KwaZulu-Natal (UKZN), Durban have addressed the issue. The authors showed that the necessary and sufficient condition for the horizon to remain tense throughout the process of matter accretion is that it radiates the classical “Vaidya radiation”.

In 1941, it was the eminent physicist, relativist and mathematician Professor Prahlad Chunnilal Vaidya, one of the most eminent students of Professor Vishnu Vasudev Narlikar (VVN), who obtained the solution to Einstein’s equation describing the field gravitational force of a radiant star like our Sun. .

The solution provided a simple classical model known as the Vaidya solution, and the radiation is known as Vaidya radiation. It is perhaps not out of place to mention that Dadhich was also a student of VVN at the then Poona University and today at Savitribai Phule Pune University, separated in time by more than 25 years and in space by the distance between Benares and Pune.

The present research shows that Vaidya radiation in case of black hole accretion is generated by heat produced due to tidal deformation of the falling matter. That is, a strong curvature of spacetime (or gravitation) near the horizon heats falling matter, producing Vaidya radiation. It should be noted that this is purely classical radiation produced simply by the heat generated in the accreting matter.

Goswami and Dadhich showed that the necessary and sufficient condition for the horizon to remain unstretched and photon-like, while matter continues to fall, is that heat must be removed outward in the form of Vaidya radiation. It is remarkable that the quantity of energy to be radiated is exactly the same as that necessary for the horizon not to expand. That is, when matter reaches the horizon, it achieves the photon-like character of the horizon: it is perfectly in agreement with the horizon and so there is no need for it to stretch.

Dadhich said: “For a black hole to remain a black hole, the infiltrating fluid must be in harmony with the fluid on the black hole horizon. For this, it must undergo tidal deformations producing a heat flux which manifests itself in the form of classical Vaidya radiation emanating from the edge of the accretion zone. It is amazing that an accumulating black hole radiates not only from the quantum Hawking but also from the classical Vaidya, and the latter paves the way for the former to reach infinity.

The accretion phenomenon of black holes emitting classical Vaidya radiation is a new and novel prediction, which has its own merits. Equally significant, however, is that it paves the way for the famous Hawking radiation to propagate even as matter accumulates, thereby allowing the black hole to “evaporate” quantum mechanically. We can say that an accreting black hole must first radiate classically, to evaporate quantum mechanically!

Goswami said: “It’s quite fascinating how space-time geometry and thermal properties are so closely linked. Fixing the horizon as zero under disturbance manifests itself as a thermal flow over space-time which becomes classical radiation. So when a realistic black hole (which must have some sort of accretion from nearby stars) evaporates, radiation is emitted in both the classical and quantum regimes. »