Einstein's general relativity theory is questioned but still standsChristian Fernsby ▼ | July 29, 2019
More than 100 years after Albert Einstein published his iconic theory of general relativity, it is beginning to fray at the edges, said Andrea Ghez, UCLA professor of physics and astronomy.
World Black hole
Einstein's 1915 theory of general 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 theory of general 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.
Ghez's research team was able to see the co-mingling of space and time near the supermassive black hole.
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.
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.
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. ■