Readers ask about photon rings and more

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Rings and rainbows

A team of scientists claim they’ve uncovered a thin ring of light called a photon ring around the first black hole to have its picture taken, the supermassive black hole in the galaxy M87. But skeptics aren’t convinced, Emily Conover reported in “Physicists dispute photon ring claim” (SN: 9/24/22, p. 8).

Assuming that Earth and the photon ring are on the same plane, reader James P. Rice wondered why we see the light as a circle rather than a straight line.

“The photon ring is a little like a rainbow,” physics writer James R. Riordon says. “It’s an optical effect, not a physical ring.” Rainbows are caused by the way raindrops affect paths of light. A person can see a rainbow in the rain in front of them as long as the sun is behind them. But where the rainbow appears to the observer changes depending on where that person stands relative to the sun.

In a similar way, a photon ring is caused by a black hole’s influence on paths of light, Riordon says. A photon ring will appear as a circular ring around a black hole no matter which direction you look at the black hole from. “You can’t be in the plane of a photon ring any more than you can walk to the end of a rainbow,” he says.

Just wondering

Science News readers often ask questions that are unrelated to our journalism but are fascinating nonetheless. We’re indulging our nerdy impulses to try to answer those questions.

Reader Marc Sapir asked if light from the early solar system could bend and come back to us during space travel so that we might someday see past Earth events.

“Everything bends light a little bit,” says Sam Gralla, a physicist at the University of Arizona in Tucson. But for light to come back all the way to us, it would have to bend a lot.

Our sun, for instance, “bends light by a little less than one one-thousandth of a degree, so it won’t help us see our own past,” Gralla says. If, hypothetically, the sun collapsed to a black hole of the same mass, “then it could bend light all the way around, which would be pretty cool,” he says. “But only a very small fraction of emitted light gets bent so much, so we’d have to have amazing telescopes to see it.” Even if we did have such telescopes, humans and other life on Earth would not survive without the sun’s light, Gralla says. And alas, our sun is destined to become a red giant when it dies, not a black hole.

Some light from our early solar system also could reach Sagittarius A*, the black hole at the Milky Way’s center. “Sgr A* can bend light a lot,” Gralla says. “But unless the light is emitted very near it, only a tiny fraction is actually bent.” For light from our solar system to return to us, it would have to approach Sgr A* at precisely the right direction, he says. “The proverbial needle in a haystack.”

“Still, it’s fun to think that aliens near black holes might be looking at themselves in the ‘black hole mirror’ at this very minute,” Gralla says.

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