Introduced in 1915, theoretical physicist Albert Einstein’s revolutionary theory of general relativity explains the laws of gravity and their relation to other natural forces. It says that matter warps or curves the geometry of space-time, and we experience that distortion as gravity. The existence of extremely massive black holes was one of the first predictions of Einstein’s theory, and even Einstein wasn’t sure that they actually existed.
Using a massive telescope network, scientists now have data in hand that could vastly broaden our understanding of gravity. Black holes are the most densely filled objects in the universe, giving them enormous gravitational pull. Stellar black holes, formed from the collapse of giant stars, can compact the mass of ten suns to the size of New York City. Supermassive black holes at the center of galaxies can have the mass of billions of suns. Their origin remains a mystery.
“Even if the first images are still crappy and washed out, we can already test for the first time some basic predictions of Einstein's theory of gravity in the extreme environment of a black hole,” says radio astronomer Heino Falcke of Radboud University in Nijmegen in the Netherlands. Astronomers have only circumstantial evidence that black holes lie hidden at the heart of every large galaxy in the universe. “They are the ultimate endpoint of space and time, and may represent the ultimate limit of our knowledge,” says Mr. Falcke.
The first-ever photo of a black hole is a milestone in astrophysics and an achievement that validated the pillar of science put forward by Albert Einstein more than a century ago. The somewhat fuzzy photo of the black hole at the center of Messier 87, or M87, a massive galaxy residing in the center of the relatively nearby Virgo galaxy cluster, shows a glowing ring of red, yellow, and white surrounding a dark center.
The research was conducted by the Event Horizon Telescope (EHT) project, an international collaboration involving about 200 scientists begun in 2012 to try to directly observe the immediate environment of a black hole. Capturing the photos took years of planning and cooperation between international partners stretching from the tallest mountain in Hawaii to the frozen terrain of the South Pole to create an electronically linked network of eight observatories and a virtual telescope dish as wide as the planet. Known as the Event Horizon Telescope, the radio-dish network opened its eye on the heavens during a 10-day window that started on April 4.
Black holes, phenomenally dense and coming in various sizes, are extraordinarily difficult to observe by their very nature. A black hole's event horizon is the point of no return beyond which anything – stars, planets, gas, dust and all forms of electromagnetic radiation – gets swallowed into oblivion. The telescope zeroed in on two supermassive black holes: a beast as massive as four million suns called Sagittarius A, which lies at the heart of our Milky Way galaxy, and a black hole about 1,500 times heavier at the core of the nearby galaxy M87. The Event Horizon Telescope has probed the neighborhood of each of these behemoths before, but this is the first time the network has included the South Pole telescope and the Atacama Large Millimeter/submillimeter Array (ALMA), a group of 66 radio dishes in Chile. ALMA sharpens the Event Horizon Telescope’s acuity 10-fold, enabling it to spot objects as small as a golf ball on the moon—and thus image the surprisingly small event horizons of the two black holes.