“According to Einstein’s Theory of General Relativity, nothing can escape from the gravity of a black hole once it has passed a point of no return, known as the event horizon,” explained Niayesh Afshordi, a physics and astronomy professor at Waterloo in 2020 about echoes in gravitational wave signals that hint that the event horizon of a black hole may be more complicated than scientists currently think. Afshordi’s conjecture is based on research reporting the first tentative detection of these echoes, caused by a microscopic quantum “fuzz” that surrounds newly formed black holes.
“This was scientists’ understanding for a long time until Stephen Hawking used quantum mechanics to predict that quantum particles will slowly leak out of black holes, which we now call Hawking radiation,” wrote Afshordi. According to Hawking’s 1974 conjecture, if one takes quantum theory into account, black holes should glow slightly with “Hawking radiation”.
According to quantum mechanics, pairs of virtual particles and antiparticles are constantly created and annihilated in normal space. But if a pair of virtual particles or photons is created just outside of the event horizon, one may fall into the black hole while the other escapes. To conserve mass and energy, the escape of a newly created particle or photon must be counteracted by a corresponding decrease in the mass of the black hole. Hence, black holes slowly evaporate due to Hawking radiation. The problem is, no astronomer has ever observed Hawking’s mysterious radiation.
Hawking theorized that the universe’s gravitational behemoths, black holes, were not the dark stars astronomers imagined, but they spontaneously emitted light — The problem is, no astronomer has ever observed Hawking’s mysterious radiation.
“Scientists have been unable to experimentally determine if any matter is escaping black holes until the very recent detection of gravitational waves,” continued Afshordi. “If the quantum fuzz responsible for Hawking radiation does exist around black holes, gravitational waves could bounce off of it, which would create smaller gravitational wave signals following the main gravitational collision event, similar to repeating echoes.”
Afshordi and his coauthor Jahed Abedi from Max-Planck-Institut für Gravitationsphysik in Germany, reported the first tentative findings of these repeating echoes, providing experimental evidence that black holes may lack truly inescapable event horizons, radically different from what Einstein’s theory of relativity predicts.
They used gravitational wave data from the first observation of a neutron star collision, recorded by the LIGO/Virgo gravitational wave detectors.
Echoes Confirm Effects of Quantum Physics and Hawking Radiation
The echoes observed by Afshordi and Abedi match the simulated echoes predicted by models of black holes that account for the effects of quantum mechanics and Hawking radiation.
“Our results are still tentative because there is a very small chance that what we see is due to random noise in the detectors, but this chance becomes less likely as we find more examples,” said Afshordi. “Now that scientists know what we’re looking for, we can look for more examples, and have a much more robust confirmation of these signals. Such a confirmation would be the first direct probe of the quantum structure of space-time.”
In an email to The Daily Galaxy Afshordi wrote: “If echoes are real, they certainly come in different flavors for different black hole merger events. It appears that some mergers show clear evidence for them, while most don’t. We have a hypothesis that the more unequal mergers have louder echoes, which is roughly consistent with the handful of mergers that show evidence for loud echoes.
“What is clear,” Afshordi concluded his email, “is that the search for black hole echoes remains one of the rare windows that we have to realistically probe Planck scale physics. That is why many more theoretical and observational studies of this phenomenon are ongoing.”
The study, “Echoes from the Abyss: A highly spinning black hole remnant for the binary neutron star merger GW170817,” was published in the Journal of Cosmology and Astroparticle Physics in November, and was awarded the first place Buchalter Cosmology Prize this month.
Source: Jahed Abedi et al. Echoes from the abyss: a highly spinning black hole remnant for the binary neutron star merger GW170817, Journal of Cosmology and Astroparticle Physics (2019). DOI: 10.1088/1475-7516/2019/11/010
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