Black hole thermodynamics: a history from Penrose to Hawking
New research explores the historical context of Penrose’s theory of black hole energy extraction, and how it inspired collaborations across political boundaries: ultimately leading to Stephen Hawking’s celebrated theory of black hole radiation.
Peer-Reviewed PublicationIn 1969, English physicist Roger Penrose discovered a property which would later allow for a long-awaited link between thermodynamics, and the far stranger mechanics of black holes. Through new analysis published in EPJ H, Carla Rodrigues Almeida, based at the University of São Paulo, Brazil, sheds new light on Penrose’s motivations and methods, and explores their historical influence on the groundbreaking discovery of Hawking radiation.
Prior to the 1950s, many physicists were reluctant to accept the idea that black holes are physical objects, consistent with the well-established laws of thermodynamics. This picture transformed entirely over the next two decades, and in 1969, Penrose showed for the first time how energy can be extracted from a rotating black hole. His theory hinged on a newly-conceived region named the ‘ergosphere.’
Although it lies just outside the boundary of a black hole, spacetime within the ergosphere rotates alongside the body, like the gas in a planet’s atmosphere. If a piece of matter enters the region, Penrose proposed that it may split into two parts: one of which can fall into the black hole; while the other can escape, carrying more energy than the original particle.
Over the next few years, Soviet physicist Yakov Zel’doivh explored Penrose’s discovery through the lens of quantum mechanics. Although his work was held back by political circumstances, Zel’doiv established friendly collaborations with Western physicists. Ultimately, the theories that emerged through these relationships led to Stephen Hawking’s discovery of novel quantum effects, which can cause black holes to radiate mass. Finally, the physics community was convinced that black holes can indeed obey the laws of thermodynamics.
In her study, Almeida investigates Penrose’s proposal within this historical context. By revisiting original papers, analysing technological details, and exploring relationships between Western and Soviet physicists, she aims to uncover the history they hide. The article moves through the chain of reasoning which led from Penrose’s proposal, to an analogy between thermodynamics and black hole physics; and ultimately, to the formulation of Hawking radiation.
Reference
References: C R Almeida, The thermodynamics of black holes: from Penrose process to Hawking radiation. EPJ H 46, 20 (2021). https://doi.org/10.1140/epjh/s13129-021-00022-9
JOURNAL
The European Physical Journal H
ARTICLE TITLE
The thermodynamics of black holes: from Penrose process to Hawking radiation
Examining the accelerating universe
A new collection of papers focuses on the paradigm of the accelerating expansion of the Universe in turn unpacking some of cosmology’s most pressing questions.
Peer-Reviewed PublicationA special edition of EPJST, edited by Balasubramanian Ananthanarayan, Centre for High Energy Physics, Indian Institute of Science, Bangalore, and Subhendra Mohanty, Department of Theoretical Physics, Physical Research Laboratory, Navrangpura, Ahmedabad, brings together a collection of papers focusing improving our understanding of the accelerating expansion of the Universe and the nature of the dark energy that drives it.
“Despite all the advances in theory and observations in particle physics and cosmology we only understand about 5% of the Universe,” says Mohanty. “The remaining matter and energy of the Universe consist of dark matter, which accounts for the rotational speeds of galaxies and formation of cosmic structure, and dark energy which accelerates the expansion of the Universe.”
In addition to the lingering mysteries of the so-called ‘dark universe’ as theories have grown more robust and observations more precise, troubling disparities have presented themselves between our best descriptions of the Universe. For instance, the rate of acceleration delivered by astronomical observations and the standard model of cosmology is much smaller than the value presented by the standard model of particle physics. “If the discrepancy between different observations is not resolved even after more refined observations, then it will mean that the base model of Lambda CDM – the most favoured standard model of cosmology – needs to be changed,” Mohanty explains. “It is possible that there are interactions between different sectors like dark matter and dark energy which we have not yet recognised.”
The researcher points out that failure to resolve this disparity could also mean that the way we currently measure cosmological distance using the spectroscopic red-shift and the use of standard candles like Type-1a supernovae or Cepheid variables – stars whose luminosity varies periodically with time – needs to be revised.
Mohanty continues by explaining that there are two strands of research in this field, the first being the examination and interpretation of the observational data about what it tells us about the existence of dark energy. The second strand is the microscopic understanding of the nature of dark energy – a fluid that has negative pressure. This, as Mohanty points out, makes dark energy unlike any other particle or field observed so far.
“Unravelling the nature of dark energy by study of the accelerated Universe will unlock the deepest level of our understanding of the Universe,” Mohanty concludes. “The best way to proceed in the understanding of dark energy is to closely relate theory with observations which are now possible due to a plethora of new precision experiments in cosmology and particle physics.”
Reference
Ananthanarayan, B., Mohanty, S. The accelerating universe: evidence and theories. Eur. Phys. J. Spec. Top. 230, 2051–2053 (2021). https://doi.org/10.1140/epjs/s11734-021-00259-x
JOURNAL
The European Physical Journal Special Topics
ARTICLE TITLE
The accelerating universe: evidence and theories
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