Black hole photo bolsters Einstein's theory of general relativity

Einstein's theory just got a whole lot harder to beat.

First ever image of a black hole (photo credit: HANDOUT/REUTERS)
First ever image of a black hole
(photo credit: HANDOUT/REUTERS)
Physicists are a step closer to cracking one of the most puzzling questions in science: how to reconcile the theories of general relativity with quantum physics. The answer may lay within supermassive black holes.
The conflict at the heart of modern physics has been raging for over a century, sparked by a pair of papers penned in 1905 by Albert Einstein. One described his theory of general relativity, which accounts for the universe on a grand scale by explaining gravity and its interactions with spacetime, causing planets to orbit suns, galaxies to collide, and us to stick to earth rather than floating off into space.
The other introduced the idea of quanta, which has since been developed to describe the universe at the subatomic level, including how atoms and particles interact with one another.
The problem is, the two are incompatible. General relativity requires the fabric of the universe to be smooth and continuous. At this level events are deterministic, meaning that every effect can be traced back to a cause. Quantum mechanics, on the other hand, describes a universe split into packets of energy, or 'quanta'. At this level, events produced by the interaction of particles happen in jumps, known as quantum leaps, with probabilistic rather than certain outcomes.
While the difference may sound minor, like the difference between high resolution images showing more detail than low resolution, in practice it means that events are possible in the quantum world which are not in the world of spacetime, as particles can be 'linked' even when not in physical proximity to one another. In 2014 Dutch researchers demonstrated that electrons can influence one another instantly even though they were a mile apart.
And when you try to blow up the micro to the macro, or vice versa, things go dreadfully wrong. Relativity gives nonsensical answers when you scale it down to quantum size, eventually producing infinite values when describing gravity. Conversely, quantum fields carry energy, even in seemingly empty space; according to Einstein's theory, energy and mass are equivalent (E=mc2), so increasing energy by expanding a quantum field is the equivalent of building up mass. Go large enough and the amount of energy in a field becomes so dense that it creates a black hole which causes the universe to fold in on itself. Not ideal.
Now scientists at the university of Arizona have found a new way to test the theories and found that general relativity not only stands up to scrutiny, but just got about 500 times harder to beat.
"We expect a complete theory of gravity [combining the two opposing theories] to be different from general relativity, but there are many ways one can modify 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 down the space of possible modifications," said UArizona astronomy professor Dimitrios Psaltis.
Psaltis, who until recently was the project scientist of the Event Horizon Telescope (EHT) collaboration, is the lead author of a new paper, published in Physical Review Letters, detailing the findings.
To test general relativity, the team used the first ever photograph of a supermassive black hole and its shadow, taken with the EHT. The black hole, located in the midst of the Messier 87 galaxy in the Virgo galaxy cluster, 55 million light-years from earth, has a mass 6.5 billion times that of our sun. The size of a black hole is proportional to its mass: the more massive a black hole, the larger its shadow.
"At that time, we were not able to ask the opposite question: How different can a gravity theory be from general relativity and still be consistent with the shadow size?" said UArizona Steward Theory Fellow Pierre Christian. "We wondered if there was anything we could do with these observations in order to cull some of the alternatives."
Their answer was to undertake a broad analysis of many modifications to the theory, to weed out those which gave results that did not match with the observed shadow.
"In this way, we can now pinpoint whether any alternative to general relativity is in agreement with the Event Horizon Telescope observations, without worrying about any other details," said Lia Medeiros, a postdoctoral fellow at the Institute for Advanced Study who has been part of the EHT collaboration since her time as a UArizona graduate student.
"Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein's theory of general relativity by almost a factor of 500, compared to previous tests in the solar system," said UArizona astronomy professor Feryal Özel, a senior member of the EHT collaboration. "Many ways to modify general relativity fail at this new and tighter black hole shadow test."
Özel acknowledged that general relativity stands up well to testing by observable phenomena, but noted that this new test tightens the boundaries of how well it holds up.
"We always say general relativity passed all tests with flying colors – if I had a dime for every time I heard that," Özel said. "But it is true, when you do certain tests, you don't see that the results deviate from what general relativity predicts. What we're saying is that while all of that is correct, for the first time we have a different gauge by which we can do a test that's 500 times better, and that gauge is the shadow size of a black hole."
"When we obtain an image of the black hole at the center of our own galaxy, then we can constrain deviations from general relativity even further," she said.
Next on the team's list is to capture higher fidelity images with other telescopes, including the Greenland Telescope, the 12-meter Telescope on Kitt Peak near Tucson and the Northern Extended Millimeter Array Observatory in France.
"Together with gravitational wave observations, this marks the beginning of a new era in black hole astrophysics," Psaltis said.