This Comprehensive Guide Will Answer the Questions You Have About Black Holes—and Spark Some New Ones

In 1915, physicist Albert Einstein put forth his theory of general relativity, which explained gravity as a product of the curvature of space and time. The theory paved the way for the concept of the black hole—a place in space where matter has been tightly compressed and become dense enough that it produces extreme gravity with an inward pull so powerful that not even light can escape. A black hole with the mass of the entire Earth would be so dense that it would be only about the size of a marble.

Following the circulation of his theory, Einstein himself dismissed black holes as unrealistic in 1939.

But over 100 years after Einstein issued his theory, humanity got its first glimpse of a black hole. In 2019, the Event Horizon Telescope revealed an image of a supermassive black hole, specifically its silhouette, surrounded by glowing gas, at the center of a faraway galaxy.

In their new book Supermassive: Black Holes at the Beginning and End of the Universe, physicists James Trefil and Shobita Satyapal take an approachable look at black holes, covering both the theoretical and the known, including the supermassives that reside in most galaxies. The authors shed light on the ways black holes are studied today and their profound impacts on space-time, and ponder their origins, their evolution, and their roles at the beginning and the end of the universe.

Trefil and Satyapal spoke with Smithsonian about their book, the wider world of black hole science and the mysteries scientists most want to solve but don’t yet understand.

Where do black holes come from?

Stellar black holes are born from the death of massive stars, more than ten times the size of our sun. While those stars burn, nuclear fusion exhorts an outward push to counter the gravitational pull of the star’s own mass. With the star’s fuel is exhausted and fusion burns out, the star explodes in a supernova, ejecting chemical elements out into space and leaving behind a very dense core. Its own gravity causes this core to collapse upon itself and create a black hole.

The origins of other types of black holes are very mysterious, none more so than supermassive black holes, which can reach mind-boggling masses.

What’s a supermassive black hole?

As their name suggests, supermassive black holes are, well, very massive—most are more than one million times the mass of our sun. They also give rise to big questions.

“We still don’t know how they form, or how they got so big,” Satyapal says. Because of their size, scientists suggest that they could not have formed through the collapse of a single star.

“There is one in the center of most of the galaxies but we don’t know what they are doing,” Satyapal adds. That includes our own Milky Way, home to our local supermassive black hole called Sagittarius A*. It has a staggering mass equal to about four million suns, packed into a diameter only about half that of Mercury’s orbit in our own solar system.

Since these black holes sit at the center of so many galaxies, they seem to be tied to the formation and evolution of galaxies and the universe itself. We don’t understand this story yet, but new technologies and telescopes are giving scientists a look at its earliest chapters.

What’s inside a black hole?

Scientists have a name for the center of a black hole, the “singularity,” but they have no concrete explanation for what happens in this peculiar place. Theories of gravity predict that everything that has entered the black hole has all its matter compressed there until it reaches a point of infinite density and zero volume so that it has no size at all.

“It is a point when the familiar rules of physics break down and all hell breaks loose,” Trefil and Satyapal write in the book. “The existence of the singularity was one of the main reasons physicists refused to accept the reality of black holes in the early and mid-20th century.”

“These things may or may not exist, in reality,” Trefil adds. “It’s a signal to us that you have a theory that works for gravity, like Earth going around the sun, but when you push that to the limit you reach a point where the theory doesn’t work anymore. That’s the way I think of the singularity.”

What is a black hole’s event horizon?

Each black hole features an event horizon, and it’s past this point that gravity becomes too strong for anything to escape. “You can think of the event horizon like the brink of Niagara Falls. Once you hit the falls, you’re gone,” says Trefil.

While this sounds ominous, it’s a misconception that black holes are like giant vacuums in space that suck anything and everything into the void. Only objects that approach closely and directly are lost to the black hole.

In fact, objects in space can orbit a black hole just as they would orbit a star. “We’re not going to get sucked into the sun because we’re in orbit around it,” Trefil explains. If our sun were replaced with a black hole of equal mass, Earth would orbit it just as we do now and never be pulled into the black hole.

How can we see black holes?

Astronomers estimate that there could be 100 million black holes in the Milky Way alone. Yet, because light cannot escape them, they are invisible.

Satyapal says we can see and study black holes in two main ways. The first is by understanding their impacts on orbiting stars or other objects. “We infer their presence by observing the motion of matter around them,” she says, “so you can detect the black hole’s gravitational influence on surrounding objects.” Trefil notes that this type of observation is like tracking the moon by its influence on the tides, which remains constant even if the moon is not in sight.

The second way is by observing a source of light that’s actually produced by the black hole. As gas, dust and other matter gets close to the black hole it is compressed and heats up, by friction, to billions of degrees, as it swirls at hundreds of miles per second in a structure called the accretion disk. “It radiates, so you can detect the light from this matter right before it spirals into the central black hole,” Satyapal says. Measuring the speed of this matter allows scientists to calculate a black hole’s mass.

These critical observations are possible because of another astonishing fact about black holes: Although they are invisible, the high-energy radiation of the superheated material surrounding them can be one of the brightest things in the universe.

How do black holes distort space-time?

A black hole’s powerful gravity has major impacts on the objects near it in space. This phenomenon is often illustrated with the idea of a bedsheet, stretched tight and marked with lines to form a square coordinate system. If someone places a bowling ball in its center, that massive object subsequently distorts and warps the surrounding space, so that any objects around it will now move along the grid’s newly distorted lines.

But black holes don’t operate in two dimensions like the sheet. Instead, they bend and warp all three spatial dimensions and a fourth one—time itself. A massive black hole alters the fabric of space-time and causes time to move more slowly as an object approaches. While this may sound like a bit of science fiction, it’s actually just an amplification of the same time dilation phenomenon by which gravity causes time to move a tiny bit more slowly closer to Earth’s surface so that even a person’s head and feet experience time differently.

What are the latest black hole discoveries and questions?

The James Webb Space Telescope is a time machine that enables scientists to see deeper into space, and further back in time toward the birth of the universe, than ever before. The deepest views go back more than 13 billion years, which is only about 400 million years after the Big Bang and incredibly early in cosmic history. Black holes are part of this picture.

“Webb is discovering that every early in the universe there are many, really massive black holes, and they seem to be forming right at the same time that stars are forming, which is not what astronomers thought would happen,” Satyapal says. “If you have a singularity 40 billion times the mass of the sun, you have to have a lot of matter and you need to get it really close together so gravity takes over and squishes it together. But how on Earth do you form that?”

Webb is also spotting black holes in ancient tiny galaxies, far smaller than our own, known as little red dots. Strangely, these black holes seem to be as massive as the entire baby galaxies in which they reside, whereas Sagittarius A* is only 1/1,000th the mass of the Milky Way.

Some people think that means the black hole forms before the galaxy, and the galaxy forms around the black hole, Satyapal notes.

But, she adds, when it comes to black holes, questions are piling up more quickly than answers can even be suggested: “Really, James Webb is finding more and more mysteries that are making all the theorists’ heads spin.”

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.