Physics lecturer discusses first-ever images of black hole

Melissa Halford, a lecturer in the Department of Physics in Auburn’s College of Sciences and Mathematics who studies clusters of stars and star formations, was eager to see the first-ever photo of a black hole because of the information astronomers can gain about the universe from the image.

“The Event Horizon Telescope, which is actually an array of several telescopes, is designed to take the first ever picture of the event horizon of a black hole. A black hole’s event horizon is the distance from which nothing can escape a black hole’s gravity. This project is quite challenging because black holes are very far away and their event horizons are relatively small. Astronomers must combine the signals from telescopes located all over the world to achieve the resolution that is needed to see them. This image is exciting because this has never been done before and because it will help astronomers learn about how material falls into black holes,” she said.

Halford offered additional answers below about black holes.

What do you hope to learn from the first direct image of a black hole?
 
With scientific advances like this one, it’s difficult to predict what the most important lessons will be, because often the most exciting results are the ones that are completely unexpected. But there are a lot of things we don’t yet understand about black holes, some of which may begin to be addressed by the images produced by the Event Horizon Telescope. One question is exactly how material falls into a black hole. Black holes themselves don’t emit light — that’s why they’re called black holes. So, this image won’t show light coming from inside the black hole. The reason we can see any light at all from the surroundings of a black hole is that nearby material is heated to high temperatures as it falls in. The heated material glows, and we can study the light it produces to understand physically what is happening near the black hole.

A second possible insight from a close-up image of a black hole comes from the fact that it will allow us to see some of the effects of the extreme physics that happens near black holes. Current physics does a good job of describing very small things in the universe as well as very massive things. However, it doesn’t work well for objects like black holes, which are both very small and very massive. Science is about making predictions and then testing those predictions in new situations. Until now, we haven't been able to test our current understanding of physics this close to a black hole, which is an environment that differs from any other we’ve observed directly.

What have we already discovered about black holes in the past year or two that has changed humanity’s outlook on their existence and their importance in the universe?

One of the more important recent discoveries related to black holes is the detection in the last few years of gravitational waves (‘ripples’ in space and time) produced when two black holes collide and merge. The theory of general relativity predicts that these extremely energetic events should produce gravitational waves, but none had been detected directly until 2015. The confirmation of their existence provides support for the theory of general relativity. As we detect more of these events, we’ll also learn about the binary black hole systems that produce them, which will tell us more about the universe’s population of black holes.

Why is it critical for us to better understand black holes?

In addition to being unique locations to test fundamental physics, black holes provide insights into other areas of astronomy. Relatively small black holes (those that are several times as massive as the sun) are created when massive stars die. By studying these stellar mass black holes, we can learn about the stars that formed them.

Stellar mass black holes aren’t the only kind of black hole in the universe – there are also the supermassive black holes that exist at the centers of galaxies. The black hole at the center of our own galaxy, the Milky Way, is about four million times as massive as the sun. But some other galaxies have black holes that are much more massive, up to billions of times the sun’s mass. There’s an interesting but not completely understood relationship between the mass of a supermassive black hole and the properties of its host galaxy. The existence of this relationship indicates that the growth of a supermassive black hole and the growth of its galaxy are somehow related, but astronomers don’t yet understand how. An explanation of how galaxies and supermassive black holes affect each other as they evolve would help to fill in some of the gaps in our understanding of how galaxies evolve. This, in turn, informs our understanding of the history of the universe as a whole and helps to address some of the basic questions in astronomy, like why the universe is the way it is.

If you could have just one question answered about black holes, what would it be and why?

There are many open questions about black holes, but an interesting one is how supermassive black holes grow to their current sizes. Some black holes are so massive that astronomers don’t understand how they would have had enough time to grow to their observed sizes even if they started accumulating mass early in the universe’s history. The answer to this mystery may provide information about the early universe.