New research shows that extreme objects known as “kugelblitzes” – black holes formed only by light – are impossible in our universe, a challenge Einstein’s general theory of relativity. The discovery places significant constraints on cosmological models and demonstrates how quantum mechanics and general relativity can be reconciled to answer complex scientific questions.
Black holes — massive objects with such a strong gravitational pull that even light cannot escape their grip — are among the most intriguing and strange objects in the universe. They are usually formed by the collapse of massive stars at the end of their life cycle, when the pressure from fusion reactions in their cores can no longer counteract the force of gravity.
However, there are more exotic hypotheses regarding the formation of black holes. One such theory involves the creation of “kugelblitz,” German for “ball lightning.” (The plural form is “kugelblitze.”)
“A Kugelblitz is a hypothetical black hole that, instead of forming from the collapse of ‘ordinary matter’ (whose main constituents are protons, neutrons and electrons), is formed from the concentration of vast amounts of electromagnetic radiation, such as light,” study co-author Jose Polo-Gomezphysicist at the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada, told Live Science in an email.
“Even though light has no mass, it carries energy,” Polo-Gomez said, adding that in Einstein’s theory of general relativity, energy is responsible for creating curvatures in space-time that lead to gravitational pulls. “That’s why, in principle, it’s possible for light to form black holes – if we concentrate enough of it in a small enough volume,” he said.
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These principles are valid under classical general relativity, which does not account for quantum phenomena. To investigate the potential impact of quantum effects on kugelblitz formation, Polo-Gómez and colleagues investigated the influence of the Schwinger effect.
“When there is incredibly intense electromagnetic energy – for example, due to huge concentrations of light – some of this energy is transformed into matter in the form of electron-positron pairs,” the study’s lead author Alvaro Alvarez-Dominguez from the Institute of Particle Physics and Space (IPARCOS) at the Universidad Complutense de Madrid, told Live Science in an email. “It’s a quantum effect called the Schwinger effect. It’s also known as vacuum polarization.”
In their studywhich has been accepted for publication in Physical examination letters but not yet published, the team calculates the rate at which electron-positron pairs produced in an electromagnetic field would deplete their energy. If this rate exceeds the energy replenishment rate of the electromagnetic field in a given region, a kugelblitz cannot form.
The team found that even under the most extreme circumstances, pure light can never reach the energy threshold necessary to form a black hole.
“What we demonstrate is that kugelblitzes are impossible to form by concentrating light, either artificially in the laboratory or in natural astrophysical scenarios,” study co-author Louis J. Garay, also from IPARCOS, told Live Science. “For example, even if we used the most intense lasers on Earth we would still be more than 50 orders of magnitude from the intensity needed to create a kugelblitz.”
This discovery has profound theoretical implications, significantly constraining previously considered astrophysical and cosmological models that suggest the existence of kugelblitzes. It also dashes any hopes of experimentally studying black holes in the laboratory by creating them through electromagnetic radiation.
Nevertheless, the positive result of the research shows that quantum effects can be effectively integrated into problems related to gravity, thus providing clear answers to real scientific questions.
“From a theoretical point of view, this work shows how quantum effects can play an important role in understanding the mechanisms of formation and emergence of astrophysical objects,” Polo-Gómez said.
Inspired by their findings, the researchers plan to continue investigating the influence of quantum effects on various gravitational phenomena that have both practical and fundamental importance.
“A number of us are very interested in continuing to investigate the gravitational properties of quantum matter, especially in scenarios where this quantum matter violates traditional energy conditions,” said Eduardo Martin-Martinez, also of the University of Waterloo and the Perimeter Institute. “This type of quantum matter can in principle give rise to exotic spacetimes, leading to effects such as repulsive gravity or the production of exotic solutions such as Alcubierre’s warp drive or traversable wormholes.”
