The first image of a black hole may hide a treasure, but physicists disagree on whether it has been found.
A team of scientists claims to have discovered a ring of photons, a thin halo of light around the supermassive black hole in the galaxy M87. If real, the ring of photons would provide further evidence of the black hole’s intense gravity. But other scientists dispute the claim. Despite multiple news headlines suggesting the photon ring has been found, many physicists remain unconvinced.
Revealed in 2019 by scientists with the Event Horizon Telescope, or EHT, the first image of a black hole revealed a donut-shaped glow of hot matter swirling around the black hole’s dark silhouette (Serial number: 04/10/19). But according to Einstein’s general theory of relativity, a thinner ring should overlap that thick doughnut. This ring is produced by photons, or light particles, orbiting close to the black hole, pulled by the giant’s gravity before escaping and zooming toward Earth.
Thanks to this circumnavigation, the photons should provide “a fingerprint of gravity,” revealing the black hole’s properties more clearly, says astrophysicist Avery Broderick of the University of Waterloo and the Perimeter Institute for Theoretical Physics in Canada. He and his colleagues, a subset of scientists from the EHT collaboration, used a new method to get that fingerprintreported on August 10 astrophysical journal.
Create images with EHT it’s not a simple point and shoot affair (Serial number: 04/10/19). The researchers piece together data from EHT’s squadron of observatories scattered around the world, using various computational techniques to reconstruct an image. Broderick and his colleagues created a new image of a black hole assuming it had diffuse emission and a thin ring. On three of the four days of observations, the data matched an image with the thin ring added better than one without the ring.
But that method has received harsh criticism. “The claim of detecting a ring of photons is absurd,” says physicist Sam Gralla of the University of Arizona in Tucson.
One main point of contention: The ring of photons is brighter than expected, emitting about 60 percent of the light in the image. According to predictions, it should be more than 20 percent. “That’s a giant red flag,” says physicist Alex Lupsasca of Vanderbilt University in Nashville. More light should come from the black hole’s main bright donut than from the thin ring of photons.
This unexpected brightening, Broderick and colleagues say, is because some of the light from the main glow bundles up with the ring photons. So the apparent brightness of the ring does not depend only on the light coming from the ring. The researchers note that the same effect appeared when testing the method on simulated data.
But that mixing of the supposed photon ring light with other light isn’t a very convincing detection, critics say. “If you want to claim that you’ve seen a ring of photons, I think you have to do a better job than this,” says astrophysicist Dan Marrone of the University of Arizona, a member of the EHT collaboration who did not co-author the new paper.
The new result only suggests that an added thin ring gives a better match to the data, says Marrone, not whether that shape is associated with the photon ring. Thus, the question arises whether scientists are seeing a ring of photons or simply picking out an unrelated structure in the image.
But Broderick argues that the characteristics of the ring, the fact that its size and location are as expected and consistent from day to day, support the photon ring interpretation.
Meanwhile, in a similar independent analysis, Gralla and physicist Will Lockhart, also of the University of Arizona, find no evidence of a photon ring, they report in a paper posted August 22 on arXiv.org. His analysis differed from that of Broderick and his colleagues in part because it limited how bright the photon ring could be.
To convincingly detect the photon ring, some scientists propose adding telescopes in space to the EHT observatory crew (Serial number: 03/18/20). The further apart the telescopes are in the network, the more fine details they will be able to detect, potentially including the ring of photons.
“If there was ring photon detection,” Lupsasca says, “it would be the best thing in physics this year, if not for many years.”