We live the effects of gravitational force on a daily basis, but the scientific explanation for the force has been much more complicated than our clear-cut observations of the force. A cornerstone of Einstein’s work revolved around general relativity, which, in part, described the nature of gravity: a warping of space time, in which giant objects create a deformation in a similar way to which a cue ball would if suspended on a sheet. Other small objects like marbles would follow the furrows of the sheet toward the fold created by the cue ball.
Peering into the abyss
When massive stars collapse, they form a black hole, a region of unfathomably dense matter that sucks in everything in its near vicinity, including light waves. This latter point is what has created such difficulty in ever directly observing black holes. Our visualization techniques revolve around reflected beams of light, so the very nature of a black hole was thought to make it impossible to observe.
Rather than observing the black holes themselves, scientists would observe the behavior of matter surrounding the collapsed star. The hole is dense and small, which limits the amount of matter that can pass into it at a given time. Because of this, matter surrounding the black hole begins to bottle neck, similar to water flowing down a drain.
The event horizon
However, in the space surrounding a black hole, the bottle neck appears as a flat spinning disk, glowing red and white, with a linear cross-section. This formation is known as an accretion disk. Deep in the center of the accretion disk is the event horizon, a single well-defined point beyond which matter and light has reached its terminal velocity into the center mass and cannot escape.
While accretion disks have been previously observed, the event horizon has remained elusive. This is due to the nature of its light behavior and also its tiny size. The extreme distance and precision that need to be covered in order to observe an event horizon is analogous to viewing a grapefruit on the surface of the moon with a radio telescope. For a single telescope to achieve this feat, it would have to be the size of earth itself.
Whereas the prospect of building an Earth-sized telescope was out of reach of any organization, a creative alternative was devised. Astronomers determined that a series of interconnected telescopes could work in tandem through the process of interferometry to capture an image of a supermassive black hole. This process combines signals from telescope pairs to observe the way that they interfere with one another.
Seeing a black hole
For the purpose of detecting the event horizon of black hole M87—a stunning scientific feat that occurred just recently in 2019—an international team of astronomers and scientists assembled eight giant telescopes for the task, forming what became known as the Event Horizon Telescope. While their approach was ingenious, their work was far from finished.
The collective observations of eight telescopes stationed across Chile, Spain, Mexico, Arizona, Hawaii, and Antarctica produced a gargantuan amount of data. Dan Marrone of the EHT described the amount of data as equivalent to an “entire selfie collection over a lifetime for 40,000 people.” In order to make sense of all the data, Katie Bouman devised an algorithm now known as the “Continuous High-resolution Image Reconstruction using Patch priors” (CHIRP) algorithm to accurately parse the data and come up with the world’s first image of an event horizon.
There are two major breakthroughs that come from the visualization of M87. First of all, it confirms the existence of black holes to begin with. Though we have observed accretion disks in the past, they are formations that can also form around any massive object, such as a star. Because of this, without direct observation, it was impossible to confirm that these images were of black holes. The image of M87 confirms previously held theories. Finally, the structure and size of the observations prove not just that Einstein’s early theories were correct but that the equations of general relativity accurately predicted the dimensions of the structures.