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Einstein’s description of gravity has become much harder to beat

Einstein's description of gravity has become much harder to beat

Visualization of the new gauge developed to test the predictions of the modified gravity theories against the measurement of the size of the shadow M87. Credit: D. Psaltis, UArizona; EHT collaboration


7;s theory of general relativity – the idea that gravity is spacetime that deforms matter – has withstood over 100 years of analysis and testing, including the latest test of collaboration with the Event Horizon telescope, published today in last issue of Physical Review Letters.

According to the results, Einstein’s theory just got 500 times harder to beat.

Despite its successes, Einstein’s robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing for general relativity is important because the ultimate theory of the universe must include both gravity and quantum mechanics.

“We expect a complete theory of gravity to be different from general relativity, but there are many ways to change it. We found that whatever the correct theory is, it can’t be significantly different from general relativity when it comes to black holes. We really squeezed it out. space for possible changes, ”said Arizona astrophysics professor Dimitrios Psaltis, who until recently was the project scientist of the Event Horizon Telescope collaboration. Psaltis is the lead author of a new article detailing the researchers’ findings.

“This is a brand new way to test general relativity using supermassive black holes,” said Keiichi Asada, a member of the EHT scientific council and an expert in radio observations of black holes for the Sinica Institute of Astronomy and Astrophysics.

To perform the test, the team used the first-ever image of the supermassive black hole at the center of the nearby galaxy M87 obtained with EHT last year. Early results showed that the size of the black hole’s shadow was consistent with the size predicted by general relativity.

Einstein's description of gravity just got a lot harder to beat

Simulation of black hole M87 showing the movement of plasma as it swirls around the black hole. The thin, bright ring that can be seen in blue is the edge of what we call the shadow of the black hole. Credit: L. Medeiros; C. Chan; D. Psaltis; F. Özel; UArizona; IAS.

“At that time, we weren’t able to ask the opposite question: how different can a theory of gravity be from general relativity and still be consistent with the size of the shadow?” said Pierre Christian, a member of Arizona Administrator Theory. “We wondered if there was anything we could have done with these observations in order to select some of the alternatives.”

The team carried out a very broad analysis of many modifications to the theory of general relativity to identify the unique feature of a theory of gravity that determines the size of a black hole’s shadow.

“In this way, we can now identify if any alternatives to general relativity are in agreement with the observations of the Event Horizon Telescope, without worrying about other details,” said Lia Medeiros, a postdoctoral fellow at the Institute for Advanced Study who she has participated in the EHT collaboration since she was a graduate student at Arizona.

The team focused on the range of alternatives that had passed all previous tests in the solar system.

“Using the meter we developed, we showed that the measured size of the shadow of the black hole in M87 narrows the leeway for changes to Einstein’s general theory of relativity by almost a factor of 500, compared to previous tests in the system. Solar, “said Arizona astrophysics professor Feryal Özel, a senior member of the EHT collaboration. “Many ways to modify general relativity fail in this new, more rigorous black hole shadow test.”

“The images of black holes provide a whole new perspective for testing Einstein’s theory of general relativity,” said Michael Kramer, director of the Max Planck Institute for Radio Astronomy and a member of the EHT collaboration.

Einstein's description of gravity just got a lot harder to beat

Illustration of the different intensities of the gravitational fields detected by cosmological tests, of the solar system and of black holes. Credit: D. Psaltis, UArizona; NASA / WMAP; ESA / Cassini; EHT collaboration

“Together with the gravitational wave observations, this marks the beginning of a new era in black hole astrophysics,” Psaltis said.

Testing the theory of gravity is ongoing research: Are general relativity predictions for various astrophysical objects good enough for astrophysicists not to worry about any differences or changes to general relativity?

“We always say that general relativity passed all tests with flying colors, if I had a penny for every time I heard it,” said Özel. “But it is true, when certain tests are performed, you do not see that the results deviate from what is predicted by general relativity. What we are saying is that while this is correct, for the first time we have a different indicator thanks to which we can do a test that’s 500 times better and that gauge is the size of a black hole’s shadow. “

Next, the EHT team expects high-fidelity images that will be captured by the wide range of telescopes, which includes the Greenland Telescope, the 12-meter telescope on Kitt Peak near Tucson, and the Northern Extended Millimeter Array Observatory in France.

“When we get an image of the black hole at the center of our galaxy, then we can further limit the deviations from general relativity,” Özel said.

Will Einstein still be right then?

A new analysis of the black hole reveals a wobbly shadow

More information:
Physical Review Letters (2020). dx.doi.org/10.1103/PhysRevLett.125.141104

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