The idea of a black hole–a body so massive that not even light could escape it–was first theorized back in 1784 by English clergyman John Michell. However, it wasn’t until 1915 when Albert Einstein developed his theory of general relativity that the momentum of black hole research would take shape. Black holes have fascinated not just scientists but also the general public. They have been the source of inspiration for numerous books, songs, movies, etc. But we’ve only been able to see them conceptualized via some form of animation. In fact, the idea of ever seeing one was deemed impossible. That is, until now.
For the first time ever, scientists have captured an image of what had previously been believed to be unseeable. Thanks to an international team of over 200 astronomers, the first direct images of a black hole have been captured. It’s a remarkable feat accomplished by coordinating the combined power of eight major radio observatories on four continents, working together to form a virtual, Earth-sized telescope.
In a series of papers published today in a special issue of Astrophysical Journal Letters, four images of the supermassive black hole at the center of M87–a galaxy within the Virgo galaxy cluster some 55 million light-years from Earth–have been revealed.
The image on the left is an EHT2017 image of M87 from the research. The middle image is a simulated image based on a model. The image to the right is the result of taking the model image and applying a Gaussian beam. Even though the middle image is a model, it represents what the black hole would look like with finer detail. Look at the image to the left and notice the ring-like geometry, the peak brightness temperature, the density and the asymmetry of the ring. This is because of gravitational lensing–a side effect of light traveling along the curvature of space and time where light passing nearby a massive object is deflected slightly toward the mass. Also, the black hole has a spin–a clockwise spin, in fact. The “shadow” or the black hole portion of the image can be seen in contrast to the surrounding emission from the accretion flow or lensed counter-jet. All of what we now see corresponds to what Albert Einstein predicted so many years ago.
You can see how the area towards the bottom of the left image is brighter and the area at the top is dimmer. This is due to what’s called Doppler beaming. As the plasma and photons circle the black hole, the approaching side to the line of sight is Doppler boosted, and the receding side away from sight is Doppler dimmed.
So, what we’re looking at here is a black hole that packs the mass of several billion suns into a very tiny volume. This spinning disc of light, or accretion disk, whirls around the black hole at up to 2 million miles per hour. The material within the disk grinds together as it circles. The innermost regions spin faster than those farther out. This differential rotation causes the magnetic fields to get coiled up, ejecting material falling into the black hole at nearly the speed of light.
What is the Event Horizon Telescope?
First of all, Event Horizon is the name of the boundary of the black hole, the point of no return for any material reaching it. Albert Einstein, in his theory of general relativity, predicted the existence of black holes in the form of infinitely dense, compact regions of space where gravity is so extreme that nothing can escape from within. By definition, this black hole would be invisible. But if a black hole is surrounded by light-emitting material such as plasma, Einstein predicted that the material would create a “shadow,” or an outline of the black hole. This outline is the event horizon and what the telescope is named after.
The EHT is truly an international endeavor. No single telescope on Earth can make an observation like we have now. Researchers have linked up radio telescopes in Arizona, Spain, Mexico, Antarctica, and other places around the world to form a virtual instrument the size of Earth.
The difficulty in imaging a black hole can’t be understated. What has been accomplished with the currently released images is equivalent to reading a text on a phone in New York from a sidewalk cafe in Paris. A telescope’s resolution increases with the size of its receiving dish. So, the EHT uses multiple radio telescopes, separated by very large distances, synchronized together, and focused on a single source in the sky. By doing this they can operate as one very large radio dish.
How big is the black hole in M87?
Sometimes it’s thought that a black hole is a very tiny area of space. Actually, black holes are quite large. For this black hole, theorists and modelers have determined that it is about 6.5 billion times as massive as our sun. It’s thought to be much larger than the orbit of Neptune, which takes 200 years to go around the sun. If this is the case, then any planet orbiting around it would do so within a week and would be traveling close to the speed of light.
What does all this mean?
The EHT project has two main goals: to image an event horizon of a black hole and to determine if Einstein’s theory of general relativity needs any revisions.
So far, Einstein has been right yet again and no revisions to the theory of general relativity are needed.
These results should also help scientists get a better handle on black holes, how gas spirals down into a black holes, the process of accretion disks and how they lead to the generation of powerful jets of radiation. More observatories are scheduled to join the EHT array in an effort to sharpen the image of M87 as well as attempt to see through the dense material that lies between the Earth and the center of our own galaxy. Ultimately, this is the big goal: to get imagery of the supermassive black hole at the center of our own Milky Way Galaxy, known as Sagittarius A.