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Vaidya Radiation may enable Hawking Radiation - New finding |
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Renowned Astrophysict, Stephen Hawking made a remarkable discovery in 1974, about Black holes and their boundaries called Event horizons. He considered the spacetime curvature effects on the quantum matter in the neighbourhood of a black hole's boundary. This matter is something that can only be described and talked about using quantum mechanics. Hawkings proposed that the vacuum near the horizon splits into positive and negative quantum particles. The latter (negative) fall in and can result in a decrease in black hole mass (commonly known as black hole evaporation), while the equivalent amount of (positive) energy is given out as a radiation. This is what has come to be known as the Hawking radiation. This is a quantum mechanical effect because it is realised through quantum splitting of vacuum into positive and negative parts.
A Black Hole's event horizon could be thought of as made of particles of light, called photons. It is well known in Astrophysics that when matter accretes i.e. falls towards and onto a Black Hole, the horizon gets stretched and tends to lose its photon-like character. However, Hawking radiation can reach to the outside observer only if it comes off the unstretched horizon having a photon-like character. For Hawking radiation to come out - it is therefore critical that the horizon must not get stretched by the matter accretion. Basically this is required for a Black Hole to always remain "black", but till now it was not understood how it is possible, with all the wobbles that may be caused by all the matter falling into it.
A recent research paper published in the journal Physical Review D (Letters), Rituparno Goswami of the University of KwaZulu-Natal (UKZN), Durban, and Naresh Dadhich of Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune addresses this issue. The authors have shown that the necessary and sufficient condition for the horizon to remain unstretched all through the process of accreting matter is that it must radiate out classical "Vaidya Radiation".
In 1941, it was the eminent physicist, relativist, mathematician Professor Prahlad Chunnilal Vaidya, one of the most distinguished students of Prof. Vishnu Vasudev Narlikar (VVN), who obtained the solution of Einstein's equation describing the gravitational field of a radiating star like our Sun. The solution provided a simple classical model known as the Vaidya solution, and the radiation now is known as Vaidya radiation. It may not be out of place to mention that Prof. Dadhich was also VVN's student at the then Poona University and now Savitribai Phule Pune University, separated in time by over 25 years and in space the distance between Benares and Poona.
The present research shows that the Vaidya radiation in case of an accreting Black hole is generated by heat produced due to tidal deformation of the in-falling matter. That is, strong spacetime curvature (or gravitation) near the horizon heats up the in falling matter producing Vaidya radiation. It is to be noted that this is purely classical radiation produced simply by heat generated in the accreting matter.
Goswami and Dadhich have shown that the necessary and sufficient condition for the horizon to remain unstretched and photon-like, while the matter keeps falling in - is that the heat must be eliminated outward as Vaidya radiation. It is remarkable that the amount of energy to be radiated out is exactly the same as required for the horizon to remain unstretched. That is, as matter reaches the horizon it attains the photon-like character of the horizon -- it is perfectly in consonance with the horizon, and hence there is no need for it to get stretched.
The phenomenon of accreting black holes giving out classical Vaidya radiation is a new and novel prediction, standing on its own merit. It is however an equally significant fact, that it paves the way for the well-known Hawking radiation to propagate out even when matter is accreting, and thus enabling the black hole to "evaporate" quantum mechanically. We can say that an accreting black hole must first radiate classically, to evaporate quantum mechanically!
A Black Hole's event horizon could be thought of as made of particles of light, called photons. It is well known in Astrophysics that when matter accretes i.e. falls towards and onto a Black Hole, the horizon gets stretched and tends to lose its photon-like character. However, Hawking radiation can reach to the outside observer only if it comes off the unstretched horizon having a photon-like character. For Hawking radiation to come out - it is therefore critical that the horizon must not get stretched by the matter accretion. Basically this is required for a Black Hole to always remain "black", but till now it was not understood how it is possible, with all the wobbles that may be caused by all the matter falling into it.
A recent research paper published in the journal Physical Review D (Letters), Rituparno Goswami of the University of KwaZulu-Natal (UKZN), Durban, and Naresh Dadhich of Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune addresses this issue. The authors have shown that the necessary and sufficient condition for the horizon to remain unstretched all through the process of accreting matter is that it must radiate out classical "Vaidya Radiation".
In 1941, it was the eminent physicist, relativist, mathematician Professor Prahlad Chunnilal Vaidya, one of the most distinguished students of Prof. Vishnu Vasudev Narlikar (VVN), who obtained the solution of Einstein's equation describing the gravitational field of a radiating star like our Sun. The solution provided a simple classical model known as the Vaidya solution, and the radiation now is known as Vaidya radiation. It may not be out of place to mention that Prof. Dadhich was also VVN's student at the then Poona University and now Savitribai Phule Pune University, separated in time by over 25 years and in space the distance between Benares and Poona.
The present research shows that the Vaidya radiation in case of an accreting Black hole is generated by heat produced due to tidal deformation of the in-falling matter. That is, strong spacetime curvature (or gravitation) near the horizon heats up the in falling matter producing Vaidya radiation. It is to be noted that this is purely classical radiation produced simply by heat generated in the accreting matter.
Goswami and Dadhich have shown that the necessary and sufficient condition for the horizon to remain unstretched and photon-like, while the matter keeps falling in - is that the heat must be eliminated outward as Vaidya radiation. It is remarkable that the amount of energy to be radiated out is exactly the same as required for the horizon to remain unstretched. That is, as matter reaches the horizon it attains the photon-like character of the horizon -- it is perfectly in consonance with the horizon, and hence there is no need for it to get stretched.
The phenomenon of accreting black holes giving out classical Vaidya radiation is a new and novel prediction, standing on its own merit. It is however an equally significant fact, that it paves the way for the well-known Hawking radiation to propagate out even when matter is accreting, and thus enabling the black hole to "evaporate" quantum mechanically. We can say that an accreting black hole must first radiate classically, to evaporate quantum mechanically!
Quotes from Authors:
Naresh Dadhich - " For a black hole to remain a black hole, infalling fluid must be in consonance with fluid on black hole horizon. For that it has to undergo tidal deformations giving out heat flux which manifests as classical Vaidya radiation emanating from the boundary of accreting zone.
It is amazing that an accreting black hole not only radiates quantum Hawking but also classical Vaidya, and the latter paves the way for the former to reach out to infinity."
Rituparno Goswami - "It is quite fascinating how the geometry of spacetime and the thermal properties are so closely related. Fixing the horizon to be null under perturbation, manifests as a thermal flux on the spacetime that becomes a classical radiation. Thus when a realistic black hole (which must have some kind of accretion from nearby stars) evaporates, radiation is emitted in both classical and quantum regimes"
It is amazing that an accreting black hole not only radiates quantum Hawking but also classical Vaidya, and the latter paves the way for the former to reach out to infinity."
Rituparno Goswami - "It is quite fascinating how the geometry of spacetime and the thermal properties are so closely related. Fixing the horizon to be null under perturbation, manifests as a thermal flux on the spacetime that becomes a classical radiation. Thus when a realistic black hole (which must have some kind of accretion from nearby stars) evaporates, radiation is emitted in both classical and quantum regimes"
Research contacts:
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Prof. Naresh Dadhich Inter-University Centre for Astronomy and Astrophysics (IUCAA) Savitribai Phule Pune University Campus, Pune 411 007 E-mail: nkd_at_iucaa.in |
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Prof. Rituparno Goswami Astrophysics Research Centre (ARC, UKZN), School of Mathematics Statistics and Computer Science University of KwaZulu-Natal (Westville Campus), Durban 4000 E-mail: vitasta9_at_gmail.com |