Even on a black hole’s turf, an essential constant of nature holds steady.

According to standard physics, the fine-structure constant, which governs interactions of electrically charged particles, is the same everywhere in the universe. Some alternative theories, however, suggest that the constant might be different in certain locales, such as the extreme gravitational environment around a black hole. But when put to the test near the supermassive black hole at the center of the Milky Way, the number didn’t budge, physicists report in a paper accepted in Physical Review Letters.

The fine-structure constant is one of an assortment of unchanging numbers found in physics formulas, such as the mass of an electron or the speed of light. It determines the strength with which electrically charged particles pull on one another. Scientists don’t know why it has the value it does — about 1/137. But its size seems crucial: If that number were much different, atoms wouldn’t form (SN: 11/2/16).

Using experiments on Earth, scientists have previously shown that the fine-structure constant doesn’t vary over time. “What’s interesting here is to try to search for variation somewhere else in the universe, in a totally different environment,” says physicist Aurélien Hees of SYRTE at l’Observatoire de Paris.

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Using observations of light from five stars that cruise around the supermassive black hole at the center of the galaxy, Hees and colleagues searched for hints of an altered fine-structure constant. When the starlight is separated into different wavelengths, it shows features called absorption lines, which indicate particular wavelengths of light that are absorbed by certain atoms. If the fine-structure constant were altered at the galaxy’s center, the separation between those absorption lines would differ from measurements of those absorption lines made on Earth.

But the absorption lines agreed with expectations. The researchers calculated that the fine-structure constant near the black hole agreed with its earthly value to better than a thousandth of a percent.

It’s the first time scientists have searched for a variation of the fine-structure constant in the general vicinity of a black hole, says Wim Ubachs of Vrije Universiteit Amsterdam, a physicist who previously has searched for changes in various constants of nature.

A 2010 study gave tentative hints that the fine-structure constant might vary as scientists look farther out into space, with the number increasing or decreasing in certain directions, but the evidence for that phenomenon is not conclusive (SN: 9/3/10). So scientists are probing the constant in a variety of ways, including near a black hole.

“The work is very important because it denotes the beginning of a new type of study,” namely, searching for variation of the fine-structure constant at the center of the galaxy, says physicist John Webb of the University of New South Wales in Sydney.

In previous research, Webb and colleagues found no variation while probing the fine-structure constant in an environment that’s even more gravitationally extreme: the surface of dense dead stars called white dwarfs. So if the new research had found any indication of change in the steadfast constant, Webb says, “I would have been very surprised.”