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A black hole made from pure light is impossible, thanks to quantum physics 

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Black holes can’t be formed from pure light. Quantum physics would curb their creation under any foreseeable conditions, a new study suggests.

Typically, matter is responsible for black holes. They’re often formed when a star’s core implodes at the end of its life. But matter isn’t necessarily required to form a black hole. According to Einstein’s general theory of relativity, black holes could form from concentrated energy alone.

A black hole formed from electromagnetic energy — that is, light — is called a kugelblitz. That concept has been jangling around in physicists’ brains for decades. But actually producing a kugelblitz seems to be a no-go, theoretical physicist Eduardo Martín-Martínez and colleagues report in a paper accepted to Physical Review Letters. “No known source in the current universe would be able to produce it, neither artificial or natural,” says Martín-Martínez, of the University of Waterloo in Canada.

In recent years, science fiction writers have picked up the kugelblitz mystique and run with it. Fans of the Netflix show Umbrella Academy may be familiar with the term, which is German for “ball lightning.” In season 3, a kugelblitz obliterates large swaths of existence.

In general relativity, gravity results from matter curving spacetime. If enough mass is packed into one region, the spacetime can curve so dramatically that it forms a region within which it’s impossible to escape — a black hole. But in general relativity, energy and mass are equivalent. That means energy can curve spacetime just as matter can, suggesting the wild idea that a black hole could form with no matter at all.

That concept is “a very interesting thought,” says theoretical physicist Juan García-Bellido of Universidad Autónoma de Madrid, who was not involved in the new study, “especially if we want to produce something like this in the laboratory.” Scientists have previously considered whether futuristic lasers might one day form a black hole in a lab, and even proposed using a kugelblitz to power a spacecraft.

Alas, calculations suggest that any attempt at a kugelblitz would result in failure, Martín-Martínez says. “You are not going to get even close. You’re not going to get even something that starts attracting you like Earth would.”

That’s because of a quantum effect that occurs when electromagnetic energy is highly concentrated. According to the well-verified theory of quantum electrodynamics, when light reaches those extremes, pairs of particles and antiparticles begin to form. Those particles — electrons and their positively charged antimatter partners, positrons — would escape the region, taking energy with them. That prevents the energy from reaching the levels needed to form a black hole.

Forming a kugelblitz in a laboratory would require light intensities more than 1050 times that of the state-of-the-art laser pulses, the team calculated. (That’s a mind-bogglingly large factor — a 1 with 50 zeroes after it.) And in nature, the brightest quasars — brilliantly luminous centers of active galaxies — are likewise vastly too dim. 

The kibosh on kugelblitzes applies across a huge range of scales. It rules out itty-bitty kugelblitzes with a radius as small as a hundredth of a quintillionth of a nanometer all the way up to 100 million meters. Even outside that range, Martín-Martínez says, a kugelblitz would still be very unlikely.

García-Bellido, however, notes a possible loophole: “It’s much more likely that things like this might have happened in the early universe.” 

Just after the Big Bang, the universe is thought to have expanded extremely rapidly, a process known as inflation. That inflation may have imprinted fluctuations in the curvature of spacetime that could cause light to collapse into what’s known as a primordial black hole (SN: 8/7/16). So while light won’t form black holes under its own gravity, that preexisting curvature, García-Bellido says, could have allowed something akin to a kugelblitz. 



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