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Into a black hole
Lectures by Stephen Hawking
31/03/2015
DUBOIS Arnaud |

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Contents
1. Introduction 3 2. Main Part 3 2.1. Stephen Hawking 3 2.2. The evolution of a star 4 2.2.1. Small star 4 2.2.2. Large star 5 2.3. The black hole 6 2.3.1. Wormholes 6 2.3.2. Back to reality 7 2.3.3. Supermassive black holes 7 2.4. Critics 8 3. Conclusion 9 4. Personal Opinion 10 4. Annexes 11 5. Bibliography 13

1. Introduction
The question whether black holes exist or not has been at the centre of Stephen Hawking's works. During my last year in high school, I already wrote a thesis about it. But since 2009 (my last year in high school) a lot has been changed about the subject. Stephen Hawking even said in 2014: "Notion of an 'event horizon', from which nothing can escape, is incompatible with quantum theory."
First of all, I'll shortly speak about Stephen Hawking himself. Then the real text about all his work can be started. I'll first introduce the birth of a star followed by its evolution till its death. Then the concept of "black hole" will be initiated. I'll write a chapter about the critics who were made about his work and eventually I'll resume everything in a short last paragraph. 2. Main part 2.1. Stephen Hawking
Stephen Hawking was born on January 8, 1942, in Oxford, England. Since the beginning of his existence he showed a lot of interest in physics. We all know him as an very intelligent person wandering around in an electric wheelchair while he speaks in robotic accents. In fact, he was diagnosed with a progressive disease during his 21st birthday: amyotrophic lateral sclerosis (ALS). The nerves in his muscles were shutting down. The doctors examining him gave him about 2 to 3 years to live. Eventually, his disease paralysed him over the decades, but he lived on. The doctors were wrong. At this point, he isn't even able to speak for himself. It is some kind of computer monitored system which does it for him.
Later, he married a woman named Jane Wilde. He met her in 1963 and married her two years later. That was one thing he wanted to do before he would die (according to the doctors). But time goes by and Stephen Hawking was still alive. From the moment he was diagnosed with ALS, he completely focussed on his research.
Since his young age, Stephen Hawking was awarded with lots of honours for his work at the Cambridge University, among-other. His novel "A brief history of time" (Stephen Hawking, 1988) became a bestseller for more than 4 years. It sold over 10 million copies in 20 years. 2.2. The evolution of a star
Stellar evolution is the process by which a star changes during its lifetime. Depending on their mass, the lifetime of a star is variable (longer for the least massive stars). Stars are born from collapsing gas and dust (called nebulae), becoming protostars. These now-called "protostars" develop till they become normal stars, also known as "main-sequence stars".
First of all, we have to distinguish two kind of developing stars: the small and the large ones. A common large star is about ten times larger than our sun, for comparison. The big difference between both kind of stars is their ending. Our sun, for example is a small star. Given the fact that smaller stars consume less energy than larger stars, we can say that smaller stars have a longer lifetime. Indeed, a larger star needs to produce more light and heat (energy) and will burn out its fuel more rapidly. About our sun, we don't have to worry, it's at about half its stable existence and still has more than 4 million years to pursue in this state.
The evolution of a small or large star will be explained in a very simple and concise way. The real and complete explanation is very complicated. Indeed, lot of scientists have written entire books about it, but none of them is entirely complete. Therefore, the explanation of the creation of a black hole will also be very brief.
2.2.1. Small star
Our sun is an example of a small star. Like said before, it all starts with the collapse of gas (hydrogen) and dust particles, called nebulae. During this process, a whole lot of chemical reaction occur (very complicated ones, really). Finally, the main-sequence star is formed. Its lifespan varies from a million to thousands of trillions of years. In the case of small stars, their lifespan is often longer than the larger ones, because they need to consume less energy.
Once the star is out of energy (out of hydrogen fuel), a red giant is formed. Their diameter becomes larger, the brightness is higher than before, but their temperature is lower than before. These giants have diameters between 10 and 100 times that of the sun. This state is achieved during the last millennia of the lifetime of the star.

Finally, once the last bits of fuel are consumed, a planetary nebula is created. Planetary Nebula are the outer layers of a star that are lost when the star changes from a red giant to a white dwarf. A white dwarf is the last stage of a stellar evolution. They have the same mass as before (during its main-sequence stage), are 100 times smaller and cooler, but brighter.
These white dwarfs are the shrunken remains of normal stars, whose energy have been used up. Their density is very high. At that stage, white dwarfs just "are". They cool and fade over billions of years, till they become black dwarfs; insignificant, not brightening, but extremely hot stars.
2.2.2. Large star
Concerning the larger stars, the creation of it is similar than that of a small one. It is also created from the nebulae. The difference between both stars lays in its lifetime. A larger star needs to consume more hydrogen fuel to assume the brightness and heat of its large surface. Knowing that the quantity of fuel doesn't differ that much between large and small stars, we assume that the larger ones run out of energy more quickly. The biggest star known to mankind has a lifespan of about a million years, which is far less than normal stars (like our sun) which have a lifespan of about 10 million years (in stable form).
Once its fuel is consumed, the star turns into a super giant. These super giants have the same characteristics as the red giants, but they are far more impressive. These stars are 1000 times bigger than the sun and have luminosities often a million times greater than the sun.
Finally, once the entire fuel is consumed, a supernova occurs. This is the explosive death of a star (often showed in movies). These explosions gain the brightness of a hundred suns for a short period before it becomes a neutron star or a black hole (more of it later).
There are two types of supernovas. The type I happens when gas of a star approaches a white dwarf, causing the dwarf to explode. The second type takes place in massive stars. They then suffer from internal nuclear reactions causing it to explode. It is these kind of supernovas who create black holes or neutron stars.
Neutron stars are believed to be mainly composed of neutrons. The largest stars form black holes. Stars with a bit less mass create neutron stars. Neutron stars are extremely dense. They have a mass of three times the sun but only a diameter of about 20 kilometres.
2.3. The black hole
The main idea of a black hole is that nothing can escape from it. How is this kind of sorcery possible? The gravity exercised by the hole is so great that even light can't escape. It sucks literally everything up that comes too close to it.
A black hole is mainly composed of two known items: The event horizon and the singularity. The explanation of an event horizon is situated in the footnote on page 3. The singularity is defined as the centre of the black hole. It is the most dense part of the hole. Nothing much is known about it, because there isn't any known way to actually see it.
The question now arises: What happens when I, for example, would enter a black hole? Different theories about this have come to the front, but I will mainly speak about Stephen Hawking's theory (which is completed by Einstein's theory of quantum physics).
2.3.1. Wormholes
This hypothetical feature has been defined as a shortcut through space and time. It is like a tunnel binding two ends in different spaces and times. You would enter the wormhole from one side in 2015 near the sun, but you would come out in 2046 near the star "Sagitarrius" or even in another universe. This could open the doors to time-travel. So, in theory this all sounds nice and well, but it is a lot more complicated than that in real life. Nowadays, Stephen Hawking argues that such use for time-travel is no more possible.
First of all, wormholes are believed to be microscopic, no man could possibly enter a wormhole whose diameter is about 10-33 centimetres. Another problem comes from stability. A wormhole would collapse too quickly to really get an advantage out of it. And of course, the third and most problematic issue is that it is situated at the centre of a black hole.
As said before, the gravity is so great nothing can escape it. Knowing this, you must imagine your body once you get closer to the singularity. Your body would be completely crushed to pieces leaving nothing behind but microscopic atoms of yours, floating around till it reaches the centre of the hole.
Even though, a lot of science-fiction movies have considered including wormholes to their script to be a key to success, we can't suppose it's possible in the reality.
2.3.2. Back to reality
Putting aside everything that sounded possible in movies, we have to go back to reality. What exactly does a black hole? If a black hole passes through a cloud of interstellar matter, for example, it will draw the matter inward in a process called the "accretion disc". Material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon called the "accretion disc". Although no-one has ever actually seen a black hole or even its event horizon, this accretion disc can be seen. Indeed, the particles spinning around the black hole at a huge speed is noticeable. The fact that their speed is so high makes them produce heat and when you produce heat, you also produce light. This light is what astronomers see when they think they've spotted a black hole. A similar process can occur if a normal star passes close to a black hole. In this case, the black hole tears the star apart as it pulls it toward itself. As the attracted matter accelerates and heats up, it emits x-rays that radiate into space. These are the radiations that the scientists observe when then think they've spotted a black hole.
2.3.3. Supermassive black holes
If you put aside the song made by Muse in 2006, a supermassive black hole (SMBH) is the largest type of black hole. It is found at the centre of almost all massive galaxies. In our galaxy, the milky way, our SMBH is believed to correspond with the location of Sagittarius A. The origin of such hole remains, badly enough, open fielded. Nothing much is known about SMBH, but the facts that their density at the surface can be less than that of water and that their diameter is about two-and-a-half million times that of our sun. 2.4. Critics
A lot of critics have been written about the work of Stephen Hawking about black holes. Since his new study where he argues that "event horizons" don't exist, many scientists remain unconvinced of his new work. They refer to his paper as "short and does not have a lot a detail, so it is not clear what his precise picture is, or what the justification is".
In addition to this recent critics, Stephen Hawking has to endure other critics each day from other brilliant scientist. If you feel like watching the enormous debate between Stephen Hawking and John Oliver, I gladly refer to this amazing youtube video where Hawking completely mocks Oliver: https://www.youtube.com/watch?v=T8y5EXFMD4s
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3. Conclusion
To conclude this paper about my personal favourite scientist, I'll resume every main idea that has been brought up. First of all, we took a closer look to Stephen Hawking's life. If you don't like to feel reading his complete biography, I invite you to watch the film "the theory of everything". It's a beautiful film about the personal life of Stephen Hawking, seen through the eyes of his first wife. I saw it and it's worth your time.
Then, we entered the centre of this subject, in particular stellar evolution. We examined, in a brief way, how a star was created. Once a star shines, we saw how its total mass could influence the way it would die. If the star was small, it would end up as a white dwarf, before turning into a black dwarf. If the star was large, it would end its life in a massive explosion, called a supernova. If the star was large enough, it would implode to a black hole. If a large star hadn't enough mass, it would end up as a neutron star.
Once we introduced the concept of black holes, we took a closer look at the matter. We got informed about the structure of a black hole, what it really does, what would happen if I got stuck in it. We entered the concept of "wormhole" which is like a tunnel joining two universes in different times and spaces. But we also got informed that all this was just very theoretical and only the science-fiction movies really got out of the game.
We were taken back to reality by re-examining everything that could be seen about a black hole. The accretion disc was one of the subjects that caught our attention. If you remember, an accretion disc was the lightning matter around a black hole, making it "visible" to the blind eye. Why "visible"? Because everything from the event horizon to the centre of the black hole (the singularity) can't be seen. So the only thing that really can be viewed is what is around, in particular, the accretion disc.
Finally, I introduced you to the concept of supermassive black holes. These are black holes situated at the centre of massive galaxies. This concept remains very theoretical, because nothing much is known about it.
At the end, we viewed some critics our dear doctor Stephen Hawking endured during his lifetime. I personally advised you to take a quick look to his debate with John Oliver. It is quite funny to see a presenter be mocked by the most intelligent man alive. 4. Personal Opinion
During the writing of my essay in sixth grade, I made a tremendous amount of research. I even read the book "A brief history of time: from the big bang to black holes" from Stephen Hawking. During my research, I got very familiar with the concept of black holes, even though the subject is more than complicated. My teachers had a difficult time understanding my oral presentation, but eventually the main idea got understood.
The reason why I choose to select this subject for this essay is simple: I already had good understanding of the concept and I already had a lot of background information. The difficulty resided in the language. Indeed, my sixth grade essay was written in Dutch and it counted about forty pages. Rewriting a five-to-eight pages essay in English wasn't that easy, especially for such a difficult subject.
Another reason why I choose to write about this, is that the subject is fascinating. Exploring concepts that are connected to uncertainty and infinity is, indeed, gripping. The concept of black holes is terrifying. It is something that sucks up everything there is, something that you can't escape from, something that intrigues most scientists. That is a topic I wanted to explore more than ever.
I've always been attracted to the mysterious things in life. I like the things that don't really have answers to the questions surrounding it. We don't know what's in a black hole and we don't know where it could take us if someone would enter it. It could maybe be a door to time-travel, but today's technology is insufficient to think about truly doing it.
I really like science-fiction movies and in many of them, they approach the matter. It intrigued me. I wanted to know more about it, if it really could be possible in the future. That's also why I choose to introduce my teacher about the subject.
But, the most important reason why I choose to make a new English essay about this subject is because of Stephen Hawking's last lecture. When he said that black holes don't exist, I couldn't believe it. Stephen Hawking argued their existence since always and all of a sudden he doesn't agree with his own ideas anymore? I wanted to know why. I wanted to know for myself what made him change his mind. This is the main reason of my research.

5. Annexes
Annexe 1: Event Horizon

Annexe 2: Stellar Evolution

Annexe 3: Wormholes

Annexe 4: The accretion disc

6. Bibliography
2.1. Stephen Hawking http://www.biography.com/people/stephen-hawking-9331710 http://www.hawking.org.uk/about-stephen.html
Movie: "The theory of everything" by James Marsh. Premier on the 7th of November 2014.
2.2. The evolution of a star http://www.telescope.org/pparc/res8.html http://cosmos.phy.tufts.edu/~zirbel/ast21/handouts/StellarEvolution.PDF http://en.wikipedia.org/wiki/Stellar_evolution http://www.astronomytoday.com/cosmology/evol.html
2.3. The black hole http://www.hawking.org.uk/into-a-black-hole.html http://www.space.com/20881-wormholes.html https://www.youtube.com/watch?v=P1OBL5J_n98 http://science.nasa.gov/astrophysics/focus-areas/black-holes/ https://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=265 http://www.physicsoftheuniverse.com/topics_blackholes_event.html http://www.space.com/18539-milky-way-s-supermassive-black-hole-seen-eating-something-video.html http://csep10.phys.utk.edu/astr162/lect/active/smblack.html http://en.wikipedia.org/wiki/Supermassive_black_hole 2.4. Critics http://www.space.com/24454-stephen-hawking-black-hole-theory.html http://www.aias.us/documents/uft/paper120.pdf http://universe-review.ca/R15-17-relativity07.htm --------------------------------------------
[ 1 ]. The edge of the black hole where the gravity becomes strong enough to hold light. See Annexe 1.
[ 2 ]. http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583
[ 3 ]. See Annexe 2
[ 4 ]. http://abyss.uoregon.edu/~js/ast122/lectures/lec14.html
[ 5 ]. https://www.youtube.com/watch?v=oajSXi4NTB8: "la supernova qui a explosé deux fois"
[ 6 ]. Little reminder: Light is the fastest "thing" that exists in the universe, about 300 million meters per second.
[ 7 ]. See Annexe 3.
[ 8 ]. See Annexe 4.
[ 9 ]. But apparent horizons do. Read more about it on http://www.space.com/24454-stephen-hawking-black-hole-theory.html
[ 10 ]. http://loldamn.com/wp-content/uploads/2014/06/funny-Stephen-Hawking-and-John-Oliver-interview.jpg