• 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 are known to form from large concentrations of mass, such as burned-out stars. But according to general relativity, they can also be formed by ultra-intense light. Theorists have speculated 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 the collapse to continue [1].
The extreme mass density produced by a collapsed star can warp space-time so strongly 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 the energy in the electromagnetic field can also bend spacetime [2]. The supposed electromagnetic black holes have become popular as KugelbliceGerman for “ball lightning”, following terminology used by Princeton University physicist John Wheeler in early studies of electromagnetically generated gravitational fields in the 1950s [3].
Kugelblitz have previously been involved in speculative theories describing exotic physical phenomena ranging from dark matter to cosmic censorship – the hypothesis that a singularity in a black hole is never visible. Light-induced black holes have even been proposed as a means of propelling starships. But the question remains: are they really possible?
The problem is that the electromagnetic field strengths thought to be necessary would generate a lot of quantum particles because highly concentrated photons can spontaneously decay into electron-positron pairs in a process called the Schwinger effect. These particles, accelerated by the intense electromagnetic field, will then exit the region, carrying away energy.
The question then is whether gravitational collapse would still be possible, or whether the energy loss resulting from the Schwinger effect would undermine it. Mathematical physicist José Polo-Gómez of the University of Waterloo in Canada and his colleagues have performed calculations to see where the balance tips.
Jason Laurea/LLNL
The amount of electromagnetic energy that must be concentrated in a given spherical volume to form a kugelblitz is easily calculated from general relativity and depends on the radius, the speed of light, and the gravitational constant. The hard part of the problem is to estimate the energy dissipation from the Schwinger effect—in particular, how the particle escape time scale compares to the black hole collapse time.
One of the main challenges is accounting for how this particle production feeds back on itself, as such particles generate electric fields that influence the creation of more particles. And because the Schwinger effect is little more than a small adjustment to calculations of black hole formation, “many of the usual tools of quantum field theory are not useful here,” Polo-Gómez and colleagues said in a joint statement. As a result, the researchers had to make some simplifying assumptions and approximations and then had to demonstrate that these choices did not affect their conclusions.
Polo-Gómez and colleagues found that the Schwinger effect does dissipate electromagnetic field energy before kugelblitz can be formed 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 make black holes from light. “We believe our results settle the debate,” said Polo-Gomez and colleagues. “Although scientifically it would be wonderful if we could create microscopic black holes using very intense lasers, 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 seem reasonable. “The work is exciting from a theoretical point of view because it’s an example of how different things can be when quantum effects are taken into account,” she says. She notes that this question has long been debated for conventional black holes, which are predicted to evaporate slowly due to the production of pairs near their surfaces. Theoretical physicist Reinhard Alkofer of the University of Graz in Austria agreed that the results were compelling, although he added that other experts had long suspected that quantum effects would undermine Kugelblice.
Polo-Gómez and colleagues say that even if their calculations cannot be experimentally confirmed, the predicted creation of particle pairs by the Schwinger effect could potentially be observed. The highest-intensity lasers generate an electric field strength only 1,000 times weaker than the threshold for such particle creation, they say, so future lasers may be able to generate the effect.
– Philip Ball
Philip Ball is a freelance science writer in London. It’s his last book How life works (Picador, 2024).
References
- A. Alvarez-Dominguez et al.“There are no black holes from light,” Phys. Rev. Lt. 133041401 (2024).
- JMM Senovilla, “Black hole formation from incoming electromagnetic radiation,” Classical quantum gravity 32017001 (2014).
- JA Wheeler, “Geons”, Phys. Rev. 97511 (1955).
