Black Holes

Black Holes are the mysterious objects in our universe which no one can see. This puts the black hole into a category all its own. We can never absolutely prove the existence of black holes because they are unable to be seen. This paper will discuss the creation, physical characteristics and optical anomalies of a black hole. In the process of discussing the black holes I will discuss properties of gravity and light, and discuss where the laws of physics no longer apply. Characteristics such as the photon sphere, singularity, Einstein Rings, binary systems and Schwarzschild Radius will also be discussed.

rs=2Gm
c2

Towards the end of a stars life the star will undergo changes that will lead to its permanent state. This state is primarily based on the mass of each star. The less massive stars, like our Sun, will balloon into Red Giants and collapse in on themselves, becoming a white dwarf. Larger stars will explode into supernovae then collapse into a neutron star or a black hole. The smallest black holes science has found are about 4 times more massive than our Sun. The Schwarzschild radius can calculate the threshold a star needs to achieve to become a black hole. If we were to compress the Earth to this threshold, the earth would have a diameter of 16mm before it would become a black hole. The equation to figure the radius of a mass is as follows. Where G is the gravitational constant, c is the speed of light, and m is the mass of the object.

The Black Hole itself is theorized to be a single point in space where matter is compressed to infinite density and zero volume. This point is called the singularity. At this point, it is believed that the properties of physics break down. Space and time are no longer relevant and new rules apply called quantum gravity. It is impossible to know for sure what happens at the singularity because a black hole’s gravity is so strong that not even light can escape. It can only be theorized through mathematics as to what happens at and near this point.

When a black hole is formed, light is emitted that will never enter the black hole, nor will it escape the gravitational pull. This forms a photon sphere or the black holes surface. A more common name for this surface or photon sphere is called the event horizon. The event horizon is the point in which no light can escape. As a result, we will never be able to see what lies behind this event horizon. Anything that crosses the event horizon will be sucked into the black hole. Light and other energy emitted away from the black hole before it crosses will eventually make it into space.

If we were to visit a black hole, what would we see? If we sent a space craft towards a black hole and observed from Earth, there wouldn’t be much to watch. The spacecraft would appear to slow down as it approached the black hole. As the spacecraft got closer and closer, it would appear to start turning red, then slowly fade from view. This happens because as the spacecraft would get closer to the black hole the light being reflected off the craft would be slowed by the gravity of the black hole. This slowing would stretch the light wave into the infrared spectrum. This would make the spacecraft appear to slow down and turn red even though it is accelerating towards the speed of light itself.

If we were to go to a black hole ourselves, it wouldn’t be a very pleasant ride. As we neared the black hole, the immense gravity would pull on our head and feet, pushing inwards on our sides, stretching us into spaghetti. As what is left of our bodies accelerated to almost the speed of light, our molecules would break down and emit radiation. We can only speculate what could happen after we cross the event horizon and head towards the singularity. Beyond the event horizon all our known laws of physics break down and are no longer applicable.

The very nature of black holes makes it very difficult to view and observe them. However, there are several ways we can detect black holes and theorize their existence. One way we can detect a black hole is by the Accretion disk surrounding the black hole. This disk is matter that is rotating around the black hole as it gets sucked past the event horizon. Before the matter is sucked into the black hole it is stretched out into a disk shape around the black hole.

When a star gets too close to a black hole, or a black hole is part of a binary system, we can observe the star rotating around a point in space, or even being pulled apart and entering the Accretion disk of the black hole. As matter stretches towards the event horizon it increases in speed to near the speed of light and releases large amounts of radiation. This radiation that is emitted before the matter crosses the event horizon is another way to detect the presence of a black hole. The radiation will escape the gravity of the black hole and some will make their way to Earth where sophisticated telescopes can pick up and measure the X-rays.

Gravitational Lensing is a phenomenon that occurs around very strong gravitational fields. Strong gravitational fields are able to bend light. This can have some strange effects to the observer. There are instances where we can observe the same star in two different points in the sky, at two different points in the stars life. This is because some light from the star shines directly toward Earth. Some light from that star shines away from earth, but at some point is bent back towards earth by a strong gravitational field. The light that is bent back has traveled further and is dimmer. But that light came from the same star even though it appears to have come from a different source.

A peculiar type of gravitational lensing can occur if the source, lens and observer are aligned properly. Einstein Rings occur around black holes and neutron stars. An Einstein Ring occurs when light from a star shines towards a strong gravitational field and is bent around that field back towards an observer. This makes the star to appear as a ring of light around a black hole or neutron star. There are infinite points at which an Einstein Ring can occur based on the location of the observer. What we do know is that between each Einstein Ring is an entire picture of the sky. This is due to the bending of the light.

There is a particular Einstein Ring called the “Self” Einstein Ring. This ring is directly at the event horizon. If an observer were to look across the event horizon, that observer would be able to see the back of their head. This is because the gravity is so strong at the event horizon that the light energy would bend completely around the black hole.

There are other types of black holes that are thought to be at the center of each galaxy. A Supermassive black hole in the Milky Way Galaxy is thought to be 4.3 million times the mass of our sun. Recent observing techniques have improved the measuring of black holes and the M87 black hole is thought to be up to 7 billion solar masses. These enormous black holes are thought to be formed early in the creation of the universe in immense nebula clouds, not from a dying star.

Stephen Hawking also theorized the existence of tiny black holes in the beginning of the universe after the big bang. These black holes didn’t have a mass much larger than an asteroid and were created by the immense pressure of the big bang. These black holes however, most likely disappeared due to what is called Hawking Radiation. These black holes would emit this radiation until no mass was left to emit, killing the black hole.

The Large Hadron Collider once it is fully operational may produce many tiny black holes. These black holes would only last for nanoseconds and only have a mass of a couple photons. This new device may be able to give us a look into the few nanoseconds after the big bang to better understand the existence and destruction of matter.

There is much speculation and new theories as to what is possible if we could harness the power of a black hole. One popular theory is a wormhole. A wormhole is essentially two connected black holes through space. Since space and time break down inside a black hole, these two connected black holes could be on the opposite side of the universe, yet be connected at the same time. It is theoretically possible (if we could withstand the gravity) to travel into a wormhole and out the other side almost instantaneously. It is also believed that wormholes can unlock the key to time travel.

A new emerging theory is the existence of what is called white holes. White holes are polar opposites of black holes. A white hole ejects matter from its singularity. It is the time reversal of a black hole. This would mean that a white hole and a black hole are the same thing at different points in time, at the same time. This theory also suggests that when a black hole is formed, a new dimension is created and a big bang happens in that dimension. All the matter sucked into a black hole in our universe, in this dimension is ejected through a white hole in a new dimension, creating a new universe.

I have discussed how a black hole may form from a large dying star, the properties of a black hole and individual characteristics of the singularity and photon sphere. I have described how a black hole may look to us from Earth and what it may look like if we were to travel there ourselves. The optical anomalies surrounding a black hole help us detect its existence even though its existence cannot be proven. The appearance of multiple stars and Einstein Rings lead to conclusions of very strong gravitational fields of a neutron star or black hole. The work of Stephen Hawking on Hawking radiation and the speculation and new theories surrounding black holes will be sure to intrigue generations to come. Black holes are fascinating objects in science and many more things than what I have presented can be learned about them.


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