An artist visualization of the star S0-2 as it passes by the supermassive black hole at the Galactic Center.
Credit: Nicolle R. Fuller, National Science Foundation
Maunakea, Hawaii – More than 100 years after Albert
Einstein published his iconic general theory of relativity, it is
beginning to fray at the edges, said Andrea Ghez, UCLA professor of
physics and astronomy. Now, in the most comprehensive test of general
relativity near the monstrous black hole at the center of our galaxy,
Ghez and her research team report July 25 in the journal Science that Einstein’s theory of general relativity holds up.
“Einstein’s right, at least for now,” said Ghez, a co-lead author of
the research. “We can absolutely rule out Newton’s law of gravity. Our
observations are consistent with Einstein’s general theory of
relativity. However, his theory is definitely showing vulnerability. It
cannot fully explain gravity inside a black hole, and at some point we
will need to move beyond Einstein’s theory to a more comprehensive
theory of gravity that explains what a black hole is.”
Einstein’s 1915 general theory of relativity holds that what we
perceive as the force of gravity arises from the curvature of space and
time. The scientist proposed that objects such as the sun and the Earth
change this geometry. Einstein’s theory is the best description of how
gravity works, said Ghez, whose UCLA-led team of astronomers has made
direct measurements of the phenomenon near a supermassive black hole —
research Ghez describes as “extreme astrophysics.”
The
laws of physics, including gravity, should be valid everywhere in the universe,
said Ghez, who added that her research team is one of only two groups in the
world to watch a star known as S0-2 make a complete orbit in three dimensions
around the supermassive black hole at the center of the Milky Way. The full
orbit takes 16 years, and the black hole’s mass is about four million times
that of the sun.
The researchers say their work is the most detailed study ever
conducted into the supermassive black hole and Einstein’s general theory
of relativity.
The key data in the research were spectra that Ghez’s team analyzed
this April, May, and September as her “favorite star” made its closest
approach to the enormous black hole. Spectra, which Ghez described as
the “rainbow of light” from stars, show the intensity of light and offer
important information about the star from which the light travels.
Spectra also show the composition of the star.
These data were combined
with measurements Ghez and her team have made over the last 24 years.
Spectra — collected at the W. M. Keck Observatory in Hawaii using a
spectrograph built at UCLA by a team led by colleague James Larkin —
provide the third dimension, revealing the star’s motion at a level of
precision not previously attained (images of the star the researchers
took at the Keck Observatory provide the two other dimensions). Larkin’s
instrument takes light from a star and disperses it, similar to the way
raindrops disperse light from the sun to create a rainbow, Ghez said.
“What’s
so special about S0-2 is we have its complete orbit in three dimensions,” said
Ghez, who holds the Lauren B. Leichtman and Arthur E. Levine Chair in
Astrophysics. “That’s what gives us the entry ticket into the tests of general
relativity. We asked how gravity behaves near a supermassive black hole and
whether Einstein’s theory is telling us the full story. Seeing stars go through
their complete orbit provides the first opportunity to test fundamental physics
using the motions of these stars.”
Ghez’s
research team was able to see the co-mingling of space and time near the
supermassive black hole. “In Newton’s version of gravity, space and time are
separate, and do not co-mingle; under Einstein, they get completely co-mingled
near a black hole,” she said.
“Making
a measurement of such fundamental importance has required years of patient
observing, enabled by state-of-the-art technology,” said Richard Green,
director of the National Science Foundation’s division of astronomical
sciences. For more than two decades, the division has supported Ghez,
along with several of the technical elements critical to the research
team’s discovery.
“Through their rigorous efforts, Ghez and her collaborators
have produced a high-significance validation of Einstein’s idea about strong
gravity.”
Keck
Observatory Director Hilton Lewis called Ghez “one of our most passionate and
tenacious Keck users.” “Her latest groundbreaking research,” he said, “is the
culmination of unwavering commitment over the past two decades to unlock the
mysteries of the supermassive black hole at the center of our Milky Way
galaxy.”
The
researchers studied photons — particles of light — as they traveled from S0-2
to Earth. S0-2 moves around the black hole at blistering speeds of more than 16
million miles per hour at its closest approach. Einstein had reported that in
this region close to the black hole, photons have to do extra work. Their
wavelength as they leave the star depends not only on how fast the star is
moving, but also on how much energy the photons expend to escape the black
hole’s powerful gravitational field. Near a black hole, gravity is much
stronger than on Earth.
Ghez
was given the opportunity to present partial data last summer, but chose not to
so that her team could thoroughly analyze the data first. “We’re learning how
gravity works. It’s one of four fundamental forces and the one we have tested
the least,” she said. “There are many regions where we just haven’t asked, how
does gravity work here? It’s easy to be overconfident and there are many ways
to misinterpret the data, many ways that small errors can accumulate into
significant mistakes, which is why we did not rush our analysis.”
An artist visualization of the star S0-2 getting closer to the supermassive black hole at the center of the Milky Way and causing a gravitational redshift that is predicted by Einstein’s General Relativity. By observing this redshift, we can test Einstein’s theory of gravity. Credit: Nicolle R. Fuller, National Science Foundation
Ghez,
a 2008 recipient of the MacArthur “Genius” Fellowship, studies more than 3,000
stars that orbit the supermassive black hole. Hundreds of them are young, she
said, in a region where astronomers did not expect to see them.
It
takes 26,000 years for the photons from S0-2 to reach Earth. “We’re so excited,
and have been preparing for years to make these measurements,” said Ghez, who
directs the UCLA Galactic Center Group. “For us, it’s
visceral, it’s now — but it actually happened 26,000 years ago!”
This
is the first of many tests of general relativity Ghez’s research team will
conduct on stars near the supermassive black hole. Among the stars that most
interest her is S0-102, which has the shortest orbit, taking 11 1/2 years to
complete a full orbit around the black hole. Most of the stars Ghez studies
have orbits of much longer than a human lifespan.
Ghez’s
team took measurements about every four nights during crucial periods in 2018
using the Keck Observatory — which sits atop Hawaii’s dormant Mauna Kea volcano
and houses one of the world’s largest and premier optical and infrared
telescopes. Measurements are also taken with an optical-infrared telescope at
Gemini Observatory and Subaru Telescope, also in Hawaii. She and her team have
used these telescopes both on site in Hawaii and remotely from an observation
room in UCLA’s department of physics and astronomy.
Black
holes have such high density that nothing can escape their gravitational pull,
not even light. (They cannot be seen directly, but their influence on nearby
stars is visible and provides a signature. Once something crosses the “event
horizon” of a black hole, it will not be able to escape. However, the star S0-2
is still rather far from the event horizon, even at its closest approach, so
its photons do not get pulled in.)
Ghez’s
co-authors include Tuan Do, lead author of the Science paper, a UCLA research
scientist and deputy director of the UCLA Galactic Center Group; Aurelien Hees,
a former UCLA postdoctoral scholar, now a researcher at the Paris Observatory;
Mark Morris, UCLA professor of physics and astronomy; Eric Becklin, UCLA
professor emeritus of physics and astronomy; Smadar Naoz, UCLA assistant
professor of physics and astronomy; Jessica Lu, a former UCLA graduate student
who is now a UC Berkeley assistant professor of astronomy; UCLA graduate
student Devin Chu; Greg Martinez, UCLA project scientist; Shoko Sakai, a UCLA
research scientist; Shogo Nishiyama, associate professor with Japan’s
Miyagi University of Education; and Rainer Schoedel, a researcher with Spain’s
Instituto de Astrofısica de Andalucıa.
The National Science Foundation has funded Ghez’s research for the
last 25 years. More recently, her research has also been supported by
the W. M. Keck Foundation, the Gordon and Betty Moore Foundation and the
Heising-Simons Foundation.
In
1998, Ghez answered one of astronomy’s most important questions, helping to
show that a supermassive black hole resides at the center of our Milky Way
galaxy. The question had been a subject of much debate among astronomers for
more than a quarter of a century.
A
powerful technology that Ghez helped to pioneer, called adaptive optics,
corrects the distorting effects of the Earth’s atmosphere in real time. With
adaptive optics at Keck Observatory, Ghez and her colleagues have revealed many
surprises about the environments surrounding supermassive black holes. For
example, they discovered young stars where none was expected to be seen and a
lack of old stars where many were anticipated. It’s unclear whether S0-2 is
young or just masquerading as a young star, Ghez said.
In
2000, she and colleagues reported that for the first time, astronomers had seen
stars accelerate around the supermassive black hole. In 2003, Ghez
reported that the case for the Milky Way’s black hole had been strengthened
substantially and that all of the proposed alternatives could be excluded.
In
2005, Ghez and her colleagues took the first clear picture of the
center of the Milky Way, including the area surrounding the black hole, at Keck
Observatory. And in 2017, Ghez’s research team reported that S0-2 does not have
a companion star, solving another mystery.
Source: W.M. Keck Observatory/News
About Adaptive Optics
W. M. Keck Observatory is a distinguished leader in the field of adaptive optics (AO), a breakthrough technology that removes the distortions caused by the turbulence in the Earth’s atmosphere. Keck Observatory pioneered the astronomical use of both natural guide star (NGS) and laser guide star adaptive optics (LGS AO) on large telescopes and current systems now deliver images three to four times sharper than the Hubble Space Telescope. Keck AO has imaged the four massive planets orbiting the star HR8799, measured the mass of the giant black hole at the center of our Milky Way Galaxy, discovered new supernovae in distant galaxies, and identified the specific stars that were their progenitors. Support for this technology was generously provided by the Bob and Renee Parsons Foundation, Change Happens Foundation, Gordon and Betty Moore Foundation, Mt. Cuba Astronomical Foundation, NASA, NSF, and W. M. Keck Foundation.
About W.M. Keck Observatory
The W. M. Keck Observatory telescopes are the most scientifically productive on Earth. The two, 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
