In 1796, Simon Pierre LaPlace predicted that there are objects in the universe which are so massive that light cannot escape them. As a result, they would be “invisible” to us. These objects have become known as black holes. More than 100 years later, Einstein published one of the most successful theories of modern science: General Relativity. A year later, Karl Schwartzchild used Einstein’s equation to define a black hole and calculate its radius, which is now known as the Schwarzschild radius. While black holes captured the imagination of many scientists, Einstein himself did not like them. In fact, in 1939, he published a paper that attempted to show they cannot exist.
Over the years, however, several lines of indirect evidence have supported the existence of black holes. For example, astronomers can measure the speed of objects in orbit around other objects. The speed of the orbiting object indicates the mass of the object being orbited. In the center of a nearby galaxy charmingly named “M87,” there is a disc of hot gas that is orbiting so quickly that the mass of what is being orbited must be three billion times the mass of our sun. However, the size of the object is, at most, the size of our solar system. Those measurements are consistent with Schwartzchild’s description of a black hole.
Of course, it’s always possible that the speed measurements are wrong, or that there is a very massive object that is consistent with what we think a black hole might be but isn’t actually a black hole. Thus, we need some other means by which to analyze the object. That’s where the Event Horizon Telescope (EHT) comes in. Despite it’s name, it is not a single telescope. It is a combination of eight different telescopes that are found in different geographic locations. Those telescopes examined the center of M87 for several days, and their data were combined together to produce the images seen at the top of this post. They are exactly what one expects those telescopes to detect if the center of M87 is a black hole. Thus, as the title of this post indicates, they represent the best evidence to date for the existence of a black hole!
Now while these images are excellent evidence for a black hole at the center of M87, it is important to know what they are and what they are not.
Let’s start with what they are not. Despite what the popular press is saying, these are not pictures of a black hole. They are not pictures of the “shadow” of a black hole. In fact, they are not pictures at all! They are computer-generated images that illustrate what radio telescopes detected at the center of M87. Pictures are images that come from visible light. The radio telescopes used in the study do not detect visible light. They detect radio waves. Now, radio waves are a kind of light, but the wavelength of the light is too long for us to see with our eyes. However, they can be detected, which is what a radio does. Radio telescopes are fancy versions of a radio receiver. They receive radio waves from space, and scientist who study the data use that to infer what the radio telescopes were pointed at.
The EHT looked at the center of M87 and studied just one set of radio waves: those that had a wavelength of 1.3 millimeters. That kind of radio wave is actually called a microwave. The microwave oven you have in your home uses microwaves to cook your food, but those microwaves have a longer wavelength than what the telescopes were detecting. Since the telescopes were looking at only one wavelength of light, and since that wavelength is not anywhere close to visible, what you see above are not pictures. In addition, when you think of a picture, you think of something taken with a single lens, like the lens of a camera. In this case, the image was constructed from the combination of eight different instruments in far-flung locations including North America, Central America, South America, and Europe.
Despite the fact that these images are not pictures, they are very impressive. They represent data that many thought could never be collected. After all, while M87 is “close” as galaxies go, it is still more than 53 million light years away! That means from our perspective here on earth, the center of the galaxy is incredibly tiny. In order to see tiny objects, telescopes have to collect a lot of data from them, so generally speaking, the tinier the object, the larger the telescope needed to study the object. To study something as small as what is shown in the images above, a single telescope would have to be so big that it would probably collapse on its own weight! That’s why it took eight telescopes in different parts of the world to get the job done.
More importantly, what the telescopes detected is exactly what one would predict for a black hole that is surrounded by hot gas. Because of a black hole’s unique properties, it severely warps space and time in its vicinity. As a result, the microwaves emitted by the hot gases do not travel in straight lines. They travel in curved lines as shown in the animation below:
Not only do the images show what is expected from a black hole (a ring of microwaves with a disc-like void in the middle), they show that there are more microwaves in certain areas of the ring (the brighter areas in the images) as compared to other areas. Once again, this is what the black-hole model predicted the telescopes should detect. Even the way the bright spots change over the four different days was predicted by the model.
Now while I do consider these images to be excellent evidence for the existence of a black hole at the center of M87, I will make one caveat. The data from eight different radio telescopes had to be combined in order to produce the images, and that’s not an easy task. After all, each telescope is at a different part of the earth, which means they each view the center of M87 a bit differently. As a result, the data must be added together with a mathematical model. That model necessarily includes assumptions, so it is possible that what we are seeing is an artifact of the model. However, as the scientists state in their peer-reviewed paper, multiple independently-produced models were used, and the differences in the results are negligible. That provides a lot of confidence that the images are real.
These results open up a entirely new realm of astronomical observations, and as astronomers get better at this technique, it will provide further tests of the validity of black-hole physics as well as General Relativity. It is definitely the most exciting astronomical discovery since the detection of gravitational waves!