• Physics 17, 119
The formation of a black hole from light alone is allowed by general relativity, but a new study says quantum physics rules it out.
D. Coe, J. Anderson and R. van der Marel/NASA/ESA/STScI
Black holes form from large concentrations of mass, such as extinct stars. But according to general relativity, they can also form from ultra-intense light. Theorists have been speculating on this idea for decades. However, calculations by a team of researchers now suggest that light-induced black holes are not possible after all, because quantum mechanical effects cause too much energy to leak out for collapse to occur (1).
The extreme mass density produced by a collapsed star can bend spacetime so severely that no light entering the region can escape. The formation of a black hole from light is possible according to general relativity because mass and energy are equivalent, so energy in an electromagnetic field can also bend spacetime (2). Putative electromagnetic black holes have become popular as ball lightningGerman term meaning “ball lightning,” following the terminology used by Princeton University physicist John Wheeler in early studies of electromagnetically generated gravitational fields in the 1950s (3).
Ball lightning Researchers have already been asked for speculative theories describing exotic physical phenomena ranging from dark matter to cosmic censorship – the hypothesis that the singularity inside a black hole is never visible. Light-induced black holes have even been proposed as a means of propulsion for spacecraft. But the question remains: are they really possible?
The problem is that the electromagnetic fields thought to be necessary would generate many quantum particles, because highly concentrated photons could spontaneously decay into electron-positron pairs in a process called the Schwinger effect. These particles, accelerated by the intense electromagnetic field, would then escape from the region, taking energy with them.
The question then becomes whether gravitational collapse would still be possible, or whether the energy loss resulting from the Schwinger effect would jeopardize it. Mathematical physicist José Polo-Gómez of the University of Waterloo in Canada and his colleagues have done some calculations to see which way the balance tips.
Jason Bachelor/LLNL
The amount of electromagnetic energy that must be concentrated in a given spherical volume to form a Kugelblitz The theory of general relativity makes it easy to calculate the value of the Schwinger effect, and it depends on the radius, the speed of light, and the gravitational constant. The most difficult part of the problem is to estimate the energy dissipation due to the Schwinger effect, in particular to compare the timescale of particle escape to that of the collapse of the black hole.
One of the main challenges is accounting for how this particle production feeds back on itself, because these particles generate electric fields that influence the creation of more particles. And because the Schwinger effect is more than just a minor correction to black hole formation calculations, “many of the usual tools of quantum field theory are not useful here,” Polo-Gómez and colleagues say in a joint statement. As a result, the researchers had to make simplifying assumptions and approximations, and then demonstrate that these choices did not affect their conclusions.
Polo-Gómez and his colleagues find that the Schwinger effect does indeed dissipate energy from the electromagnetic field before a Kugelblitz can form for all sizes between 10−29 and 108 m. Smaller length scales approach the Planck length, the scale at which quantum field theory breaks down, while at larger scales, no known process in the universe would be energetic enough to create black holes from light. “We think our results put an end to the debate,” Polo-Gómez and colleagues say. “It would be scientifically wonderful to be able to create microscopic black holes using very intense lasers, but our research shows that this is not possible.”
“The results seem convincing to me,” says Silvia Pla García, an expert in quantum field theory and gravity at King’s College London. She adds that the researchers’ approximations all seem reasonable. “This work is exciting from a theoretical point of view because it shows how different things can be when you take quantum effects into account,” she says. She notes that this question has long been debated for classical black holes, which are predicted to slowly evaporate due to pair production near their surfaces. Theoretical physicist Reinhard Alkofer of the University of Graz in Austria agrees that the results are convincing, although he adds that other experts have long suspected that quantum effects could undermine quantum theory. ball lightning.
Polo-Gómez and his colleagues say that even if their calculations cannot be verified experimentally, the predicted creation of particle pairs by the Schwinger effect could potentially be observed. Higher-intensity lasers generate electric fields only 1,000 times weaker than the threshold for creating such particles, they say, so future lasers might be able to generate this effect.
–Philippe Ball
Philip Ball is a freelance science writer based in London. His latest book is How life works (Picador, 2024).
The references
- A. Alvarez-Dominguez et al.“No black holes due to light” Rev. Phys. Lett. 133041401 (2024).
- JMM Senovilla, “Formation of black holes by incoming electromagnetic radiation”, Classical quantum gravity 32017001 (2014).
- JA Wheeler, “Geons”, Rev. Phys. 97511 (1955).
