It seems it is not possible to form black holes out of light alone. If energy and mass are one and the same, as Albert Einstein’s theories posit, then it should be possible to create a region of light so dense it collapses into a black hole – but a closer look has demonstrated quantum effects prevent that from happening.
The idea of a black hole formed by the collapse of light, called a kugelblitz, has been around for decades, with researchers considering everything from how to make them in a laboratory to whether they could be used to power starships. But when at the University of Waterloo in Canada and his colleagues incorporated quantum effects into our previous understanding of kugelblitze, it became clear these strange objects are probably impossible to form.
“In the current conditions of our universe, it’s not possible to form a kugelblitz naturally or artificially, even with super-powerful lasers that don’t even exist yet,” says MartĂn-MartĂnez.
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This is because of a quantum phenomenon called the Schwinger effect, in which matter is generated by an extreme electromagnetic field. When a powerful field, such as the kind generated by an intense beam of light, is concentrated in a small region of space, pairs of electrons and positrons spontaneously appear. Their formation steals some of the energy from their parent field.
The researchers found the incredible concentration of light required to create a kugelblitz would produce a similarly incredible rate of electrons and positrons from the Schwinger effect. These particles and antiparticles would hurtle away from the light, thus preventing a black hole from condensing. “These effects hinder the formation of the black hole way before the structure that is formed is anywhere near a black hole,” says at the University of Waterloo.
The researchers found this rule holds for black holes with a radius from 100,000 kilometres down to 10-29 metres, trillions of times smaller than a proton. “The range of length scales which is ruled out is enormous, so I think it covers all realistic scenarios, at least according to our current understanding of physics,” says at the University of Sheffield in the UK, who was not involved with this work.
The one possible exception is during the early universe, when the behaviour of space-time as a whole was different. Such dissimilarity could have enabled kugelblitze to form. The research also does not rule out the possibility of forming a black hole from matter and then growing it with light.
“Once a black hole is formed, you can certainly throw light into it,” says MartĂn-MartĂnez. “But once it crosses the event horizon of a black hole, it’s not light anymore – general relativity predicts that there is no way to tell what a black hole is made of.” This also means if kugelblitze formed in the early universe, we would have no way to differentiate them from regular black holes.
“The equations of general relativity have lots of exotic solutions which involve what we might regard as unphysical phenomena,” says Winstanley. “So [this type of analysis] is important for identifying realistic solutions of general relativity and ruling out solutions which cannot arise in the physical universe.”
arXiv
