“Just like general relativity is a small correction to Newtonian gravity in our solar system, we may also need such a correction to general relativity in a strong gravitational regime,” says Johannsen. If the researchers find that the shadow requires additional parameters, for example, if it is oval or heart-shaped, then general relativity must be incomplete. This is a direct consequence of the simplicity of black holes in Einstein’s theory. This will be the first time we will actually see a black hole itself.Īccording to general relativity, a black hole such as Sagittarius A* should exhibit a nearly circular shadow based on just two properties – mass and spin. To do this, the scientists will compare their predicted model image with the long-anticipated image of Sagittarius A* from the Event Horizon Telescope. What better place to test the limits of Einstein’s theory of general relativity? Within general relativity, black holes were first predicted by Karl Schwarzschild exactly one hundred years ago as a seemingly impossible solution to Einstein’s field equations: Create an object with zero volume and you create an infinite abyss in the fabric of spacetime. “General relativity is the absolute standard in gravitational theory and it’s passed all the tests in the weak field regime, but it could still totally fail around black holes where gravity is many orders of magnitude stronger than anything we’ve tested before,” says Johannsen.Īn image worth more than a thousand wordsĪ black hole is so compact that neither matter nor light can escape its extreme gravity. Even the Laser Interferometer Gravitational-Wave Observatory’s (LIGO) ground-breaking detection of gravitational waves earlier this year essentially assumed general relativity was valid under the extreme conditions of a binary black hole merger. While reassuring, this means that the application of general relativity to its most extreme environments, where it is not bit player but the star, is fraught with extrapolation. Until now, general relativity has for the most part only been tested where gravity is weak and Einstein makes a small correction to Newton’s theory. “Every theory has to be validated experimentally with an ever increasing level of precision.” “This is how science works,” says Johannsen. Their calculations explore what Sagittarius A* will look like if general relativity holds up, but more importantly what it might look like if the theory’s predictions fail. The researchers’ method, which appeared in a recent issue of the prestigious journal Physical Review Letters, proposes to compare predictions made by general relativity against the first-ever, live image of Sagittarius A*, a black hole four million times the mass of our own sun. Images are captured not in visible light, but in the radio portion of the electromagnetic spectrum. Its primary purpose is to image two supermassive black holes: Sagittarius A* and another located in the centre of the M87 galaxy. The Event Horizon Telescope is a global array of radio telescope sites spanning the Earth, creating a receiver the size of our planet. More complicated shapes like an oval or heart would prove general relativity must be corrected within strong gravitational regimes. A circular shadow would prove general relativity is valid. “It’s unprecedented, the resolution and sensitivity of the data we’ll be working with.”Ī simulated image of the black hole Sagittarius A*. “Based on the observations we expect to make this spring with the Event Horizon Telescope, we’ll be able to make incredibly precise statements about the nature of this black hole,” says Johannsen, also a post-doctoral fellow at the Perimeter Institute for Theoretical Physics. Tim Johannsen and Avery Broderick, along with international colleagues, will use the new method in gravitational conditions that are so extreme, the results will usher in a new era in gravitational research, validating – or disproving – general relativity in its most exotic manifestations. University of Waterloo astrophysicists are leading a team of researchers who have devised a new way to test Einstein’s theory of general relativity within Sagittarius A* - the supermassive black hole at the center of the Milky Way.
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