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.

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.”