On April 11, this newspaper published a photo of an object never before seen by humans: a black hole.
The black disk at the center of that photo is the black hole at the center of galaxy M87. A "galaxy" is a giant pancake-shaped disk of stars and gas. Our own Milky Way galaxy has a similar but far smaller central black hole 26,000 light-years away (a "light-year" is the distance light travels in one year). It's the size of planet Mercury's orbit around the sun. The black hole in M87 is much larger, the size of Neptune's orbit. It contains the mass of 6.5 billion suns and is 55 million light-years away.
Every black hole has an outer edge, called an "event horizon," from which nothing, not even light, can escape. Black holes come in two varieties: "Galactic black holes" such as those at the centers of M87 and the Milky Way, and "stellar black holes" which are remnants of burned-out stars and have masses of "only" tens of suns.
The event horizon telescope is a 25-year effort to detect the galactic black holes in M87 and the Milky Way. To "see" through the dust and gas surrounding every galactic center, the event horizon telescope must use microwaves rather than light. Telescopes detecting these wavelengths are typically dish-shaped antennas, larger than satellite TV dishes.
To detect small objects at great distances, telescopes need a large "aperture" (opening) because all waves spread out in space and waves coming through smaller apertures spread more widely causing poor "resolution" -- a fuzzy image. To view the tiny black hole in M87 at 55 million light-years away, you need an aperture the size of Earth. Amazingly, this is possible by using many individual telescopes (dishes) located all over Earth, and linking their data by recording the precise time of each image at each telescope so images can later be reconstructed to form a single image from all these telescopes. The data-processing requirements for this are unprecedented, requiring two years to bring the mainframe computers from event horizon telescope's eight sites to Germany where the image was then reconstructed.
There were 80 dishes at eight sites: South Pole, Chile, Mexico, California, Arizona, Hawaii (two sites) and Spain. Some sites hosted single large dishes while other hosted linked arrays. Viewing was scheduled for 10 days in April 2017 during which astronomers hoped for five days when all eight sites would be clear. The image of our galactic black hole is still being processed.
The black hole in M87 is actually a relatively thin disk with the small spherical black hole at its center. Its bright halo is a thin "accretion disk" of light and matter orbiting the black hole at up to light speed and spiraling into the black hole. It's astonishing: light itself orbits the black hole, forming a circle. Our view from Earth is slightly off-center, so we see the flat disk of light slightly edge-on. Thus, half the disk rotates partly toward us, while the other half rotates partly away from us. This causes half the accretion disk to appear hotter and brighter, the other half cooler and darker.
One mystery is M87's two enormous "jets" shooting roughly toward us and away from us, perpendicularly to the disk. Some of the material consumed by the black hole is transferred to these jets and shot thousands of light-years into space. The jets are organized by electromagnetic fields near the black hole and are generated by either the visible orbiting gases or by the black hole itself, but the details are not understood.
Black holes are so mind-boggling that Einstein, whose theory predicts them, thought they could not exist "in the real world." According to Einstein's General Theory of Relativity, the bright visible outer halo forms because the black hole's enormous mass (billions of suns squeezed into a volume as small as our solar system!) bends space, and light simply follows this bent shape. The black hole itself should be perfectly circular. Future viewing seasons will check the perfection of this circle to greater and greater accuracy, providing an important check on the accuracy of Einstein's theory.
The results of the April 2018 viewing period are being processed. In the near future you can expect new, improved images annually, additional new sites resulting in better image resolution, an array of space telescopes forming an aperture much larger than Earth, and a Nobel Prize awarded to three leading event horizon telescope scientists.
The event horizon telescope represents a stunning intellectual achievement in an epic tale that extends over millions of years of human fascination with the starry heavens.
Commentary on 04/30/2019