Knowing A Black Hole

Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106

2018-03-20 07:36:43



Passing away of Prof. Stephan Hawking has once again drawn people’s attention towards Black Holes as the topic was not only very dear to this scientist but he has given many theories on these astronomical mysterious objects. Black holes are among the strangest things in the universe. They are massive objects –collections of mass – with gravity so strong that nothing can escape, not even light. Because of the relationship between mass and gravity, this means they have an extremely powerful gravitational force. Black holes are the only objects in the Universe that can trap light by sheer gravitational force. Scientists believe they are formed when the corpse of a massive star collapses in on itself, becoming so dense that it warps the fabric of space and time. And any matter that crosses their event horizons, also known as the point of no return, spirals helplessly towards an unknown fate. 

The massive gravitational influence of a black hole distorts space and time in the near neighborhood. The closer we get to a black hole, the slower time runs. Material that gets too close to a black hole gets sucked in and can never escape. Material spirals in to a black hole through an accretion disk — a disk of gas, dust, stars and planets that fall into orbit the black hole. The “point of no return” around a black hole is called the “event horizon”. This is the region where the gravity of the black hole overcomes the momentum of material spinning around it in the accretion disk. Once something crosses the event horizon, it is lost to the pull of the black hole. Black holes were first proposed to exist in the 18th century, but remained a mathematical curiosity until the first candidate black hole was found in 1964 and was called Cygnus X-1, an x-ray source in the constellation Cygnus. Despite decades of research, these monstrous cosmological phenomena remain shrouded in mystery. Albert Einstein first predicted black holes in 1916 with his general theory of relativity. The term "black hole" was coined in 1967 by American astronomer John Wheeler, and the first one was discovered in 1971. There are three types: stellar black holes, supermassive black holes and intermediate black holes. Some fascinating facts about black holes are as:

Formation of a black hole: Scientists think the smallest black holes formed when the universe began. Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. Scientists think supermassive black holes were made at the same time as the galaxy they are in. A black hole is formed when a large star starts running out of fuel and begins to collapse under its own gravity. Such a star may become a white dwarf or a neutron star, but if the star is sufficiently massive then it may continue shrinking eventually to the size of a tiny atom, known as a gravitational singularity. A black hole refers to the region in space in which the singularity’s gravitational force is so strong that not even light can escape its pull. The only difference between a black hole and our Sun is that the centre of a black hole is made of extremely dense material, which gives the black hole a strong gravitational field. It's that gravitational field that can trap everything, including light, which is why we can't see black holes. We could theoretically turn anything into a black hole. If we shrunk our Sun down to a size of only 3.7 miles (6 km) across, for example, then we would have compressed all of the mass in our sun down to an incredibly small space, making it extremely dense and also making a black hole. We could apply the same theory to Earth or to our own body. But in reality, we only know of one way that can produce a black hole: the gravitational collapse of an extremely massive star that's 20 to 30 times more massive than our Sun.

Characteristics: A black hole is 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. This can happen when a star is dying. Black hole is said to be formed when a large star caved in and black hole pulls matter from blue star beside it.

Types of black holes: Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object. Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy “the Milky Way”. The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

Life cycle of black holes: As long as there is something nearby for a black hole to absorb—interstellar dust, gas, nearby stars or other black holes—it will continue growing and thriving. Stephen Hawking believes that black holes that are rotating, like most, should always be creating and shooting out particles. If the black hole has nothing to feed on, this process of shooting out particles would cause the black hole to shrink and eventually disappear. This surprising discovery was first predicted by Stephen Hawking in 1974. The phenomenon is called Hawking radiation, after the famous physicist. Hawking radiation disperses a black hole's mass into space and over time, and will actually do this until there is nothing left, essentially killing the black hole. This is why Hawking radiation is also known as black hole evaporation. There is a telescope currently looking for evidence of this, and scientists are also curious to see if they can observe the process using the Large Hadron Collider, which is the largest machine in the world. Scientists use it to cause particles to smash together at very high speeds. Some people got worried that the Large Hadron Collider would create black holes powerful enough to cause trouble on Earth, but physicists have reviewed all of the research very carefully and believe that if they do create any, they will evaporate so fast that it won’t be a problem.

Einstein and black holes: Einstein didn't discover the existence of black holes – though his theory of relativity does predict their formation. Instead, Karl Schwarzschild was the first to use Einstein's revolutionary equations and show that black holes could indeed form. He accomplished this the same year that Einstein released his theory of general relativity in 1915. From Schwarzschild's work came a term called the Schwarzschild radius, a measurement of how small we have to compress any object to create a black hole. Long before this, British polymath John Michell predicted the existence of 'dark stars' so massive or so compressed that they could possess gravitational pulls so strong not even light could escape; black holes didn't get their universal name until 1967.

To see a black hole: Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars. When a black hole and a star are close together, high-energy light is made. This kind of light cannot be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Black holes do not suck: Black holes look like they're sucking in matter from all around, but that's a common misconception. Some think that black holes are like cosmic vacuums that suck in the space around them when, in fact, black holes are like any other object in space, albeit with a very strong gravitational field. If we replace the Sun with a black hole of equal mass, Earth would not get sucked in – it would continue orbiting the black hole as it orbits the Sun, today. Companion stars shed some of their mass in the form of stellar wind, and the material in that wind then falls into the grip of a black hole.

Spaghettification with black holes: Black holes have this incredible ability to literally stretch us into a long spaghetti-like strand and the phenomenon is called 'spaghettification'. The way it works has to do with how gravity behaves over distance. Right now, our feet are closer to the centre of Earth and are therefore more strongly attracted than our head. Under extreme gravity, say, near a black hole, that difference in attraction will actually start working against us. As our feet begin to get stretched by gravity's pull, they will become increasingly more attracted as they inch closer to the centre of the black hole. The closer they get, the faster they move. But the top half of our body is farther away and so is not moving toward the centre as fast.

New universes: It might sound crazy that black holes could spawn new universes – especially since we're not sure other universes exist – but the theory behind this is an active field of research today. A very simplified version of how this works is that our Universe today, when we look at the numbers, has some extremely convenient conditions that came together to create life. If you tweaked these conditions by even a miniscule amount, then we wouldn't be here. The singularity at the centre of black holes breaks down our standard laws of physics and could, in theory, change these conditions and spawn a new, slightly altered universe.

Black holes as energy sources: Black holes can generate energy more efficiently than our Sun. The way this works has to do with the disk of material that orbits around a black hole. The material that is nearest to the fringe of the event horizon on the inner edge of the disk will orbit much more quickly than material at the very outer edge of the disk. This is because the gravitational pull is stronger near the event horizon. Because the material is orbiting and moving so rapidly, it heats up to billions of degrees Fahrenheit, which has the ability to transform mass from the material into energy in a form called blackbody radiation. To compare, nuclear fusion converts about 0.7 percent of mass into energy. The condition around a black hole converts 10 percent of mass into energy. Scientists have even proposed that this kind of energy could be used to power black hole starships of the future.

The first black hole discovered: Cygnus X-1 was first found during balloon flights in the 1960s, but wasn’t identified as a black hole for about another decade. According to NASA, the black hole is 10 times more massive to the Sun. Nearby is a blue supergiant star that is about 20 times more massive than the Sun, which is bleeding due to the black hole and creating X-ray emissions.

The nearest black hole: An erroneous measurement of V4641 Sagitarii led to a slew of news reports a few years back saying that the nearest black hole to Earth is astoundingly close, just 1,600 light-years away. Not close enough to be considered dangerous, but way closer than thought. Further research, however, shows that the black hole is likely further away than that. Looking at the rotation of its companion star, among other factors, yielded a 2014 result of more than 20,000 light years.

Existance of wormholes: What happens if somebody falls into a black hole? Some people believe these objects are a sort of wormhole to other parts of the Universe, making faster-than-light travel possible. Anything is possible since we still have a lot to figure out about physics. Since we do not yet have a theory that reliably unifies general relativity with quantum mechanics, we do not know of the entire zoo of possible space-time structures that could accommodate wormholes.

The Ergosphere: This region of twisted spacetime is called the ergosphere. It is impossible to stand still in this region.A spinning black hole is more like a whirlpool than a pothole. The swirling water in this analogy is spacetime itself. It’s pulled around as the black hole rotates.  A spinning black hole has kinetic energy bound up in its spin, in the same way that a spinning top is more energetic than a top lying down. That energy can be tapped into and transferred to other things in the black hole’s environment.

Spinning of a black hole: When astronomers measure a black hole’s spin, they report the value as a fraction of the maximum allowed spin. For example, the bigger member of the black hole binary in the quasar OJ 287 has a spin, labeled a, of 0.313, or 31.3% of its max. This number is related to the black hole’s angular momentum; it’s not a fraction of the speed of light. But we can turn it into a fraction of the speed of light. 

Validity of physical laws: The singularity at the core of a black hole may shrink to a size smaller than an atom, and eventually become an infinitely small point in space containing infinite mass. Here the gravitational force is so strong that the spacetime surrounding the singularity is bent to infinite curvature, and scientists are left searching for a good quantum theory of gravity to explain what is truly going on inside these incredibly dense objects.  Singularity is said to be a point where all laws of physics break down.


Freeze of an object near a black hole: To an outside observer with a telescope, an object passing the event horizon will appear to slow down then “freeze” in time without ever seeming to pass through the event horizon. This is because the light takes longer to escape the black hole’s gravitational pull and light signals won’t reach the viewer for an infinitely long time. As time elapses, the light subsequently becomes red shifted and dimmer as its wavelength becomes longer, eventually disappearing from the sight of the observer as it becomes infrared radiation, then radio waves.


Dangers of black hole: Black hole is the point of no return, when we’re too close for any hope of rescue. We can safely observe the black hole from outside of this arena. By extension, this means it’s likely impossible for a black hole to swallow up everything in the Universe (barring some sort of major revision to physics or understanding of our Cosmos, of course.). Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now. The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

Future research: It’s a very exciting time for black hole research. A special project called the Event Horizon Telescope, which is made of many radio observatories around the world combining their power to create something like a telescope as wide as Earth, is aimed at studying our galaxy’s supermassive black hole. The first picture of it may be able to be produced in near future, and that will give us tons of new information to study, confirm theories, or correct them.