The Hawking Radiation: Passport to Escape From a Black Hole

by Stefan Iliescu / 2 July


“My goal is simple. It is a complete understanding of the universe, why it is as it is, and why it exists at all”, said Stephen Hawking, the famous theoretical physicist and cosmologist of the 20th century. The quote emphasizes that he was not one to settle for an easy challenge, a trait that we hope is the basis of every individual in our team. The task he set for himself was too large for an individual to complete in a lifetime, but, even so, the renowned British physicist accomplished substantial parts of it by leading the world to understand the bits of the universe.


Stephen Hawking devoted all his resources to the study of black holes, individually and in collaboration with other acclaimed researchers. His debut took place in 1970, when, together with Sir Roger Penrose, established the theoretical basis (the Penrose – Hawking singularity theorems) for the formation of black holes. Their prediction was proven by recent observational experiments (2015-2019) at the Laser Interferometer Gravitational-Wave Observatory (LIGO) that detected gravitational waves emitted by colliding black holes (or emerging ones).


The same theoretical basis was the expansion of the black hole (this translates into an increase in the area of ​​a black hole’s event horizon) with the absorption of matter and energy from its vicinity. According to the second law of thermodynamics, the entropy of the black hole can only increase, and, as the entropy is an energy-dependent function that possesses the temperature, the scientists wanted to know how high the temperature of a black hole can go. Here comes perhaps the most significant contribution so far in the field, namely the Hawking radiation, which may be responsible for keeping the temperature bellow a „certain limit”. He uncovered that black holes, once thought to be static, unchanging, and defined only by their mass, charge, and spin, are actually ever-evolving engines that emit radiation and evaporate over time. Although this contribution has not yet been proven by any experiment, which is why Hawking did not win the Nobel Prize in his lifetime, it is seen as the only widely recognized result by physicists in the field as support for a unifying theory of quantum mechanics and gravity.


The next question for the scientific world was, logically, whether the radiation emitted by the black hole preserves the information that came with the ingestion of matter, even in a scrambled form. For many years Hawking did not believe so, and proposed in 1997, characteristically for him, a bet (Thorne – Hawking – Preskill bet). In 2004 Hawking updates his own theory stating that the black hole event’s horizon is not really a “firewall” but rather an “apparent horizon” that enables energy and information to escape (from the quantum theory standpoint), thus declaring himself the loser of the bet. Moreover, he considers that he has thus corrected the biggest mistake of his life in the field. Neither Kip Thorne, who was with him in the bet against John Preskill nor half of the scientific world, is considered convinced of this update, today, two years after Hawking’s death. In the absence of solid experimental evidence (which, among other things, will support a quantum theory of gravity), the question of whether and how information leaks from a black hole (through Hawking radiation) remain open.