Amal Kumar Raychaudhuri

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The Raychaudhuri Equation

Subhro Niyogi & Prithvijit Mitra, October 8, 2020: The Times of India

The world may be celebrating British physicist Roger Penrose’s Nobel Prize but only a handful know that his journey into the heart of one of astrophysics’ greatest mysteries — Black Holes — had a strong Kolkata connect, and that he was standing on the shoulder of a city physicist, Amal Kumar Raychaudhuri.

The Raychaudhuri Equation in general relativity, derived by Raychaudhuri, is in the spotlight after the Royal Swedish Academy of Sciences awarded one half of the 2020 Physics Nobel to Penrose for throwing light on Black Holes. The other half of the prize was jointly shared by Reinhard Genzel and Andrea Ghez.

Penrose had collaborated with cosmologist Stephen Hawking and used an equation that Raychauduri had published in the journal ‘Physical Review’ in 1955 for a mathematical description of Black Holes in 1969. Raychaudhuri had worked on the equation in the early 1950s while teaching at Asutosh College. The paper was published when he was at the Indian Association for the Cultivation of Science in 1955.

“Raychaudhuri had used geometry to arrive at a singularity where laws of physics break down and physical quantity becomes infinitely large in the context of gravity and where space time becomes infinitely large,” said IISER physical sciences department professor Narayan Banerjee, who was present at the Jadavpur University workshop in 1987 when Raychaudhuri and Penrose had first met each other. “This equation was the building block on which the Penrose-Hawking singularity theorems published in 1969 were based. Both Penrose and Hawking have acknowledged Raychaudhuri’s contribution on numerous occasions, both in published papers and books,” said Banerjee.