Exploring the Physical World

A Black Hole? Where?

• The Jerusalem Post • • TAGS: Astrophysics, Physics, Space

Black Hole

A hole may be an empty space, but a black hole – the heart of darkness – is anything but.

Appearing in a variety of science fiction stories and usually surrounded by an aura of mystery, myths and surprising theories – black holes have engaged the imagination since the idea of their existence arose in the 18th century. Here we take a look at what actually creates a black hole, what effects are created near black holes, what types of black holes exist in the universe, and what other rumors, theories and myths revolve around them.

What is a black hole?

A black hole is an area in space where there is a mass of stars concentrated in a radius of a few kilometers. Since the mass of a black hole is so large and its gravity so strong, anything that comes close enough to it falls into it – even light.

All of the concentrated mass of the black hole falls into its center, called its gravitational singularity; at this point there is infinite density. A few kilometers from the center exists the boundary that marks the “point of no return,” i.e. the point at which the speed needed to escape the gravitational field of the black hole is greater than the speed of light. From this point it is impossible to remove material, light, or any kind of “information.” What falls through is lost forever, and we will never know its fate. From the outside, it appears as a completely dark ball.

How are black holes formed?

What causes a star’s entire mass to be compressed into such a small radius? The answer lies in the gravity of massive stars that manage to continually compress all the gas within the core of the star, until the mass collapses to the size of a dot.

Stars produce a lot of heat and radiation during their lives, and the internal pressure created at the core of the star prevents them from collapsing under their own weight. When the nuclear fuel at the core of a star is depleted, A black hole? SCIENCE Where? A hole may be an empty space, but a black hole – the heart of darkness – is anything but the gas collapses inwards in a violent process releasing matter and energy outwards in vast quantities. If the star is considered sufficiently massive, this exploding process is called a supernova.

Some of the gas from the star remains, yet there is no longer a source of energy to help push against its own gravity, and so the star collapses to a minimal size. If the mass remains large enough, the debris will collapse into a “neutron star,” where all the electrons are united with the protons and a core of neutrons is formed. If the mass of this core is too big even for the repulsion between the neutrons, the star will collapse inwards. At this stage no force known to science can resist the weight of the gas. The mass is concentrated in one point, and a black hole is created.

The history of studying black holes

Using mechanics and Newtonian theory of gravitation, an “escape velocity” can be calculated for each star with a known mass and radius, which is the minimal velocity an object needs to escape the gravity of a planet or star.

At the end of the 18th century, philosopher Reverend John Michell and mathematician Pierre-Simon Laplace, at the same time but separately, proposed the idea of a body so massive and dense that its escape velocity is greater than the speed of light. At that time the notion was dismissed, because in Newton’s Laws there was no explanation for why light would be affected at all by the gravity of a star.

At the beginning of the 20th century, Einstein formulated his general theory of relativity, which comprehensively explained the phenomenon of gravity compared to that given by Newton in the 17th century. The theory of relativity produced the same results as Newton’s theory of gravity referring to a weak gravity, and bodies moving relatively slowly compared to the speed of light. The differences begin to appear only at high speeds and in very strong gravitational fields.

Already in 1917 (two years after Einstein’s publication) physicist and astronomer Karl Schwarzschild published a solution to the equations of relativity near a massive and dense (compact) body. In his equations, there was a clearly visible boundary beyond which light could not escape. According to the theory of relativity, light and matter behave the same way in a gravitational field, which supplied a more natural explanation for the existence of a black hole that was capable of attracting light in the first place.

The black hole was a good theoretical demonstration of the situation where the theory of relativity behaves quite differently from Newtonian gravity due to the strong gravitational fields that exist near it.

The effects of gravity

When you’re far away from the black hole, there is no difference between its gravity and that of a normal star.

So if we would replace our sun with a black hole of the same mass, the Earth would continue in its orbit without interference (if we disregard the lack of sunlight of course). The formidable gravitational force of a black hole stems from its small size. The gravitational pull increases as you get closer to the concentrated mass in the center.

Because of this and due to the fact that the black hole can be equivalent to the mass of a star, but with the radius of only a few kilometers (instead of millions of kilometers), matter is able to get closer to the black hole and be pulled by the very strong gravitational field.

According to the theory of relativity, a massive body distorts the space and time around it. To an external observer, a clock thrown into the black hole will show time slowing as the clock approaches the hole, and in fact from the outside it would look like the clock takes an infinite amount of time to pass the boundary that marks where the hole’s effects begins.

Light that comes from a source behind the black hole can circumvent it, because gravity distorts light rays and diverts them. The black hole can serve as a lens that concentrates the light of background stars. In this sense, it is not only possible to see stars that are from the opposite side of the black hole, it is also possible in certain situations to see the same star reflected several times around the hole, and even a full ring of light that is coming from one source where the light rays surround all sides of the black hole.

In science fiction we often encounter black holes that are the key to a different universe or gateway for traveling back in time. Beyond the strange phenomena already mentioned, there are other ideas that are based on the theory of relativity for the possibility of “wormholes” in such black holes. Of course these are only theories that don’t really have any scientific basis, and anyway there seems to be no way we could actually check whether they are correct.

Observations of black holes

Black holes are exotic bodies that are naturally very difficult to see in space.

They do not emit light, which means the discovery of black holes and spotting them is very difficult, and relies on indirect measurements.

In the 1990s, scientists from the University of California in Los Angeles measured the movement of 90 stars at the center of our galaxy and found they orbited a mass four million times greater than our Sun, with a radius of around a thousandth of a light year. It is hard to imagine such a large amount of mass in an area so small that is not centralized in a supermassive black hole. In that same area in space there is also a strong source of radio waves, known as Sagittarius A.

Today it is assumed that radio waves are emitted from the gas surrounding the black hole that heats up as it falls inwards.

In the past year a Laser Interferometer Gravitational-Wave Observatory experiment detected for the first time gravitational waves that apparently were emitted due to a collision between two black holes, each about 30 times heavier than our sun.

These discoveries have advanced the status of black holes from theoretically interesting phenomena to astrophysical reality. Today, black holes are integral to our understanding of the evolution of galaxies and the stellar life cycle. They provide a platform for new theories of gravity and quantum mechanics and the variety of phenomena surrounding them are promoting our knowledge of physics in a variety of fields.

The writer is a PhD student in the Weizmann Institute of Science and is a writer for the Davidson Institute