Mysteries of the Black Hole

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Black holes are some of the most fascinating and yet to be fully explored objects in outer space that have kept scientists and researchers on their edge since it was discovered. Simply put, it’s a region in outer space where nothing, not even light, can escape its gravitational pull.

It’s a region in spacetime where the gravity is very strong and doesn’t allow the smallest of particles, electromagnetic radiations, space objects such as comets, planets. Because nothing can get out of these regions, they are not visible to people which is why they are referred to as black holes. Space telescopes with special tools are used to identify these black holes.The theory of general relativity, which was published by Albert Einstein in 1915 and which describes gravitation in modern physics, predicts that a sufficiently compact mass can deform spacetime which then turns into a black hole.

The boundary of the black hole after which nothing can escape its gravitational pull is called the event horizon. According to the general theory of relativity, there are no detectable features of black holes despite the fact that they can have an enormous impact on objects around them. Black holes of stellar mass have temperatures on the order of billionths of a kelvin which makes it impossible to observe.

History

In the 18th century, John Michell and Pierre-Simon Laplace were the first to consider objects whose gravitational pull were strong enough to not allow even light to escape. Later in 1915, Einstein developed the general theory of relativity and showed that gravity does influence light’s motion. Karl Schwarzschild, in 1916, found a solution using relativity theory to characterize a black hole. Later in 1958, David Finkelstein interpreted it further.

The discovery of neutron stars in 1967 sparked further interest in the gravitationally collapsed compact objects.On 11 February 2016, LIGO Scientific Collaboration (LSC) and the Virgo collaboration detected gravitational waves for the first time. It also represented the first observation of a black hole merger. By December 2018, 11 gravitational wave events were observed which involved 10 merging black holes and in April 2019, the first direct image of a black hole and its surrounding was published. The observation was made using Event Horizon Telescope in 2017 of the supermassive black hole in Messier 87’s galactic centre.

What is the definition of a black hole?

Black holes can be defined, according to NASA, as “a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space.”  

As light is unable to escape the black hole’s gravity, it appears completely black – hence the name. Black holes can, however, be “seen” with some special analysis of data collected from a wide range of telescopes (more on this later).

Who first discovered black holes?

While everyone has heard of black holes nowadays, have you ever wondered who first discovered them? 

Technically speaking, we haven’t really “found” a black hole yet, but we can infer their existence through a variety of techniques (more on this later). That being said, scientists have speculated about the existence of something like them for hundreds of years.

In 1783, for example, an English cleric and amateur scientist called John Mitchell managed to demonstrate that Newton’s law of gravity could be used to show a place where gravity was so intense light cannot escape. 

He went even further. Mitchell suggested that although these areas would be invisible, they should reveal their presence by interfering with things like stars that might orbit them. 

His theoretical work would prove to be years ahead of his time, with the later groundbreaking work of the great Albert Einstein. 

Einstein first predicted that such things should exist way back in 1916, in his “General Theory of Relativity”. According to him, big enough stars should be able to collapse under their own gravity and create what we call today black holes. 

For decades after this, black holes remained a purely theoretical concept, and the actual term wasn’t coined until 1967 by the American astronomer John Wheeler.

Mitchell and Einstein’s work was reinforced in 1971 when two British astronomers, Louise Webster and Paul Murdin, independently announced they had discovered one in space using indirect methods. Murdin worked out of the Royal Greenwich Observatory in London and Webster at the University of Toronto.

What they had found was an intense x-ray source, now called Cygnus X-1, orbiting a blue star around 6,000 light-years away. It would be the first of many. 

As amazing as this all is, it wasn’t until very recently that scientists managed to “see” one for the first time. Back in 2019, the Event Horizon Telescope (EHT) collaboration managed to release a computerized image of what is believed to be a black hole. 

The EHT focussed the radio telescopes on the center of the Messier 87 Galaxy (Virgo A) where a black hole was thought to lurk. This galaxy is somewhere in the region of 54 million light-years away from Earth. 

It is thought that the black hole in question has a mass of about 6.5 billion suns. The team was attempting to examine and image the black hole’s event horizon and accretion disk (a large cloud of hot gas and dust trapped in orbit around the black hole).

This they did, and two years later they were able to image the shape of the magnetic fields in the hot gas swirling around the hole. The discovery of this black hole has proved to be groundbreaking, as it is hoped that it will open a whole new area of research into the nature of black holes. In 2021, astronomers took advantage of an ancient gamma-ray burst to detect an intermediate-mass black hole. Information from the Sloan Digital Sky Survey suggests IMBHs may exist in the center of most dwarf galaxies.

How to build a black hole?

The gravitational pull of an object depends on how much stuff it contains. And just as with stars and planets, more stuff — or mass — comes with a greater force of attraction.

Black holes aren’t just massive. They’re dense, too. Density is a measure of how tightly mass is packed into a space. To understand how dense a black hole can be, imagine you could pack your own. Start with a thimble. Fill it with all of your books (you would need to really stuff them in). Add your clothes and any furniture in your room. Next, add everything else in your house. Then throw in your house too. Make sure to squeeze it all down to fit.

Don’t stop there: A black hole with a thimble-sized event horizon contains as much mass as the entire Earth. Stuffing your thimble increases its density, its mass and its gravitational attraction. The same is true with black holes. They pack a huge amount of mass into an incredibly small space. Imagine a black hole the size of New York City. It would have as much mass and gravity as the sun. That means this New York-sized black hole would be able to hold all eight planets (and every other object in our solar system), just as the sun does.

What the black hole wouldn’t be able to do is gobble up the planets. That sort of idea gives black holes a bad rap, says Ryan Chornock. He is an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

One popular misconception you see in science fiction is that black holes are kind of cosmic vacuum cleaners, sucking in things that pass by,” Chornock says. “In reality, black holes just sit there unless something extraordinary happens.”

Sometimes, a star will get too close. In May 2010, a telescope in Hawaii picked up a bright flare from a distant galaxy. That blaze peaked a few months later, in July, and then faded away. A team of astronomers, including Chornock, identified this glow as the last blast from a dying star being ripped apart by a black hole. As the remnants of the star fell toward the black hole, they became so heated that they glowed. So even black holes can create brilliant light shows — by eating stars.

What’s inside a black hole?

By definition, we can’t observe what’s inside a black hole, because no light – no information of any kind – can escape. But astrophysical theories suggest that, at the core of a black hole, all the black hole’s mass is concentrated into a tiny point of infinite density. This point is known as a singularity.

It is this point – this singularity – that generates the black hole’s incredibly strong gravitational field. Consider, however, that the singularity might not exist. That’s because all known physics breaks down under the extreme conditions at the center of a black hole, where quantum effects doubtless play a large part. As we do not yet possess a quantum theory of gravity, it is impossible to describe what actually exists at core of a black hole.

Observing black holes

With no emission from a black hole, scientists can only observe their gravitational effects on nearby objects in space. If there are stars or gas near the black hole, it may be actively “feeding” on them. That is, a black hole may draw in material from these nearby objects. In this case, a black hole will have an accretion disk. This is where material spirals inward before the black hole eats it, like water down a drain.

The accretion disk may rotate at significant percentages of the speed of light: Friction between colliding particles in the disk raises its temperature to million of degrees, radiating huge quantities of X-rays that can be detected with special telescopes.

In April 2019, the Event Horizon Telescope project revealed the first-ever direct image of a black hole, the supermassive black hole at the center of the giant elliptical galaxy M87. A global array of radio telescopes acquired the image. It demonstrated beyond reasonable doubt that black holes exist. Scientists were able to directly test General Relativity’s models of black hole behavior and found that M87’s black hole complied perfectly.

What is the closest black hole to Earth?

The closest black holes yet discovered to Earth are all more than a thousand light-years away from us. At this distance, these black holes will have no discernable effect on our planet or its environment. 

In 2021, astronomers claimed to have found a tiny black hole just 1,500 light-years away, dubbed “the Unicorn”.  The black hole is about three times the mass of our sun and appears to be a companion to a red giant star. The miniature black hole was discovered by analyzing the way that light from the red giant appeared to change in intensity and appearance at various points in its orbit. They surmised that the distortion was caused by a very small black hole.

Bigger and bigger

Again, nothing can escape a black hole — not visible light, X-rays, infrared light, microwaves or any other form of radiation. That makes black holes invisible. So astronomers must “observe” black holes indirectly. They do this by studying how black holes affect their surroundings.

For example, black holes often form powerful, bright jets of gas and radiation visible to telescopes. As telescopes have grown larger and more powerful, they have enhanced our understanding of black holes.

“We seem to be finding bigger and more powerful black holes than we would have expected, and that’s quite interesting,” says Julie Hlavacek-Larrondo. She is an astronomer at Stanford University in Palo Alto, Calif.

Hlavacek-Larrondo and her collaborators recently used data from NASA’s Chandra space telescope to study the jets from 18 extremely large black holes.

“We know big black holes have these incredibly powerful [jets] that can easily extend beyond the size of the galaxy,” says Hlavacek-Larrondo. “How can something so small create an outflow that’s so much bigger?”

The size of the jet can be used to estimate the size of the black hole. That has led to some surprising findings. In December 2012, for instance, Hlavacek-Larrondo and other astronomers reported that some black holes are so big they deserve a new name: ultramassive.

These black holes probably contain anywhere between 10 billion and 40 billion times more mass than does our sun. Even five years ago, astronomers knew of no black holes with a mass above 10 billion times that of our sun, says Jonelle Walsh. She’s an astronomer at the University of Texas at Austin.

With so much mass, the superstrong gravity of an ultramassive black hole can hold together entire clusters, or groups, of galaxies.

Mysteries of the massive

“How do you create these big black holes?” asks Hlavacek-Larrondo. They are so large that they must have slowly gained mass after first forming billions of years ago. Scientists are now starting to explore how black holes have been forming since the Big Bang.

How to build a big black hole isn’t the only mystery. Supermassive black holes are connected, through gravity, to hundreds of billions of stars. Figuring out the link between a black hole and the stars it anchors is a dilemma. Which came first is a bit like the chicken and the egg question.

“We’re still not sure if the supermassive black hole came first — and then gathered galaxies into a linked cluster, admits Hlavacek-Larrondo. Maybe the clustering came first.

Last year brought yet another discovery that deepens the mystery about black holes. Walsh, the Texas astronomer, and her colleagues used the Hubble Space Telescope to study a galaxy called NGC 1277. This galaxy lies more than 200 million light-years away. (A light-year is the distance light travels in one year.)

Even though NGC 1277 is only about one-fourth the size of the Milky Way, Walsh and her colleagues reported in November that the black hole at its center is one of the biggest ever measured. They estimate it is about 4,000 times more massive than our galaxy’s Sagittarius A*.

In other words, “the black hole there is too big for the galaxy it resides in,” Walsh says. Black holes and galaxies are usually believed to grow — and stop growing — together. This new discovery suggests either this black hole just kept growing, by feeding on nearby stars and other black holes, or somehow was oversized from the very start.

Walsh says she wants to know if other galaxies have a similar arrangement — or even the opposite, with a small black hole at the center of a large galaxy. “We can try to infer how the growth of one affects the other,” Walsh says. But how that happens, she notes, “is not fully understood.”

Black holes are some of the most extreme objects in the universe. Astronomers continue to find and observe more of their extreme members, including the biggest, smallest and strangest black holes out there. Explains Walsh: Those observations can help untangle the complicated relationships black holes have with stars, galaxies and clusters of galaxies. That future research, she explains, “will push us toward understanding how everything [in the universe] works together and forms and grows.