There are many popular myths concerning black holes, many of them perpetuated by Hollywood. Television and movies have portrayed them as time-traveling tunnels to another dimension, cosmic vacuum cleaners sucking up everything in sight, and so on. It can be said that black holes are really just the evolutionary end point of massive stars. But somehow, this simple explanation makes them no easier to understand or less mysterious.
NOTE: This section is about what are called "stellar-mass black holes". For information about black holes with the mass of billions of Suns, see Active Galaxies & Quasars .
Black Holes: What Are They?
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the only thing that would change would be the Earth's temperature. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the only thing that would change would be the Earth's temperature. To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
The Schwarzschild radius can be calculated using the equation for escape speed.
vesc = (2GM/R)1/2
For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be
R = 2GM/c2
If the sun was replaced with a black hole that had the same mass as the sun, the Schwarzschild radius would be 3 km (compared to the sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our solar system.
If We Can't See Them, How Do We Know They're There?
Since black holes are small (only a few to a few tens of kilometers
in size), and light that would allow us to see them cannot escape, a
black hole floating alone in space would be hard, if not impossible, to
see. For instance, the photograph above shows the optical companion star to the (invisible) black hole candidate Cyg X-1.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of infalling material for a black hole. A binary system also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star or a black hole, since neutron stars always have masses of about 1.5 times the mass of the sun. Another sign of the presence of a black hole is random variation of emitted X-rays. The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be possible black hole candidates.
Cygnus X-1 is the longest known of the black hole candidates. It is a highly variable and irregular source with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3000 kilometers. This is one fourth of Earth's diameter! So the region emitting the x-rays around Cygnus X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses is too large to be a white dwarf or neutron star, it must be a black hole.
However, there are arguments against Cyg X-1 being a black hole. HDE 2268686 might be undermassive for its spectral type, which would make Cyg X-1 less massive than previously calculated. In addition, uncertainties in the distance to the binary system would also influence mass calculations. All of these uncertainties can make a case for Cyg X-1 having only 3 solar masses, thus allowing for the possibility that it is a neutron star.
Nonetheless, there are now about 10 binaries for which the evidence for a black hole is much stronger than in Cygnus X-1. The first of these, an X-ray transient called A0620-00, was discovered in 1975, and the mass of the compact object was determined in the mid-1980's to be greater than 3.5 solar masses. This very clearly excludes a neutron star, which has a mass near 1.5 solar masses, even allowing for all known theoretical uncertainties. The best case for a black hole is probably V404 Cygni, whose compact star is at least 10 solar masses. With improved instrumentation, the pace of discovery has accelerated over the last five years or so, and the list of dynamically confirmed black hole binaries is growing rapidly.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of infalling material for a black hole. A binary system also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star or a black hole, since neutron stars always have masses of about 1.5 times the mass of the sun. Another sign of the presence of a black hole is random variation of emitted X-rays. The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be possible black hole candidates.
Cygnus X-1 is the longest known of the black hole candidates. It is a highly variable and irregular source with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3000 kilometers. This is one fourth of Earth's diameter! So the region emitting the x-rays around Cygnus X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses is too large to be a white dwarf or neutron star, it must be a black hole.
However, there are arguments against Cyg X-1 being a black hole. HDE 2268686 might be undermassive for its spectral type, which would make Cyg X-1 less massive than previously calculated. In addition, uncertainties in the distance to the binary system would also influence mass calculations. All of these uncertainties can make a case for Cyg X-1 having only 3 solar masses, thus allowing for the possibility that it is a neutron star.
Nonetheless, there are now about 10 binaries for which the evidence for a black hole is much stronger than in Cygnus X-1. The first of these, an X-ray transient called A0620-00, was discovered in 1975, and the mass of the compact object was determined in the mid-1980's to be greater than 3.5 solar masses. This very clearly excludes a neutron star, which has a mass near 1.5 solar masses, even allowing for all known theoretical uncertainties. The best case for a black hole is probably V404 Cygni, whose compact star is at least 10 solar masses. With improved instrumentation, the pace of discovery has accelerated over the last five years or so, and the list of dynamically confirmed black hole binaries is growing rapidly.
What about all the Wormhole Stuff?
Unfortunately, worm holes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat - the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create.
Unfortunately, worm holes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat - the throat, or passageway, is called an Einstein-Rosen bridge. It has never been proved that worm holes exist and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create.
Can You Give Me Some More References?
There is quite a bit of black hole theory out there. For more information on it, try these books:
1. Black Holes and Warped Spacetime - William J. Kaufmann, III
2. Lonely Hearts of the Cosmos - Dennis Overbye
3. Black Holes and Time Warps, Einstein's Outrageous Legacy - Kip S. Thorne
4. The Mathematical Theory of Black Holes - S. Chandrasekhar
5. Black Holes and Baby Universes and other Essays - Stephen Hawking
6. Universe - William J. Kaufmann, III
7. Black Holes and the Universe - Igor Novikov
There is quite a bit of black hole theory out there. For more information on it, try these books:
1. Black Holes and Warped Spacetime - William J. Kaufmann, III
2. Lonely Hearts of the Cosmos - Dennis Overbye
3. Black Holes and Time Warps, Einstein's Outrageous Legacy - Kip S. Thorne
4. The Mathematical Theory of Black Holes - S. Chandrasekhar
5. Black Holes and Baby Universes and other Essays - Stephen Hawking
6. Universe - William J. Kaufmann, III
7. Black Holes and the Universe - Igor Novikov
QUANTUM PHYSICS
Quantum physics is about the characteristics of subatomic particles and it says that energie is not continuously, but in form of quanta (packages). Before I begin with quantum physics I advise the readers who does not know much about nuclear physics to read the homepage of nuclear physics, because otherwise quantum physics might be difficult to understand. But if you known the structure of an atom and particles then it is not nessecary to read the homepage of nuclear physics. Another thing is very important, during reading about quantum physics you must forget about all logical physical laws, because the laws in the subatomic world are inconceivably for us. After reading this website you might have different conception of the world.
Quantum physics is about the characteristics of subatomic particles and it says that energie is not continuously, but in form of quanta (packages). Before I begin with quantum physics I advise the readers who does not know much about nuclear physics to read the homepage of nuclear physics, because otherwise quantum physics might be difficult to understand. But if you known the structure of an atom and particles then it is not nessecary to read the homepage of nuclear physics. Another thing is very important, during reading about quantum physics you must forget about all logical physical laws, because the laws in the subatomic world are inconceivably for us. After reading this website you might have different conception of the world.
Chapter 1: The Particle wave duality
In former times scientists thought that light consists of waves and that electrons, neutrons and protons are particles. But Scientists have discovered that sometimes light has got a wave character and sometimes light has got a particle character but not only light also the other particles which I mentioned sometimes have got a wave character. There is an experiment which shows that light can have a particle character. For this experiment we need a metal plate. When we irradiate this metal plate with light it can happen that some of the electrons of some atoms will leave their atomic shell. But when no electron leaves the atomic shell a classical physicist would say that the intensity is to low and what we need a stronger light source or that we must give the light nearer to the metal plate. But this would not help, because light consists of photons and when we have got a higher intensity there are more photons which bombard the electrons, but one electron can only absorb one photon. This means that the energie of the photon is responsible, wheather an electron leaves his atomic shell or not. The electrons are holded by the positive charged atomic nucleus and so they need a certain energy to break out. So we need radiation with a shorter wavelength to give the electrons enough energy. If the wavelength is shorter the energy and the frequency are higher. Which wavelength do we need depends on the atoms. Simple light is to little so that we need ultraviolet light for example. All this is called photoelectric effect. The best possibility to make this experiment is with an electroscope. There is also an experiment which show us that electrons can have a wave character. It is the double gap experiment which I will describe later, because he is the most important experiment for quantum physics and the consequences of him might change your conception of the world.
Chapter 2: Heisenberg's uncertainty relation
To measure the position and the speed of a certain particle we need light or another radiation. When we use radiation with a long wavelength the position is inexact, but the speed is quite exact. When we use radiation with a short wavelength the position is quite exact, but the speed is inexact. This means that when we want to measure one of these things exact, we cannot measure the other thing exact, too. Some things which are explained in nuclear physics with a simple pattern cannot be explained in quantum physics so easy, too. We have this problem with the Bohr atom model. In real there are not any electrons which fly around the atomic nucleus, but you imagine that they are on certains energie levels. In this situation it is also impossible to say exact where an electron is. So here we find Heisenberg's uncertainty relation, too. But there are so called orbitals where it is very probable that there is an electron, but it is never sure. In quantum physics we have got only probabilities.
Chapter 3: Nothing is real
Perhaps you are fascicled from the particle wave duality or from Heisenberg's uncertainty relation, but this what you will learn here will make the other things less important for you. The experiment begins very simple. You need a light source, a wall with two holes and a screen. On side of the wall there is the light source and on the other side there is the screen. When light passes the wall we can see an interference sample on the screen. The maxima are not behind the holes on the screen, but there is one maximum between the two holes on the screen, otherwise it would not be an interference sample. On the right and on the left of this maximum there are dark areas and then again bright areas, but these bright areas are not as bright as the maximum in the middle. Then we have got two dark areas again and so on. This result should not wonder us, because this are waves and because some waves have got a longer way from the light source to the screen than other waves some waves strengthen each other and other waves extinguish each other. When two wave combs clash then they strengthen each other and when a wave comb and a wave valley clash then they extinguish each other. When one hole is closed the maximum is behind the opened hole. Now we will replace the light source through an electron source and we will make the experiment again. This time we get the same interference sample when both holes are opened. This proofs the wave character of the electrons. But it is important that light or electrons cannot be a wave and a particle at the same time. Now it becomes interesting, we do not let many electrons throught the wall, but only one after the other. When one electron passes the wall it cannot handycap himself and because it can only go throught one of the holes it would be logical that the maxima are behind the two holes. But when we wait until many electrons have passed the wall we saw an interference sample again. When we repeat this experiment and we close one hole the maximum is behind the open hole. It seams that electron knows wheather both holes are opened or only one. When we try to measure throught which hole an electron goes we get two maxima behind the two holes. So it is wrong to say that the electron goes throught one of these two holes, because we can say that it goes throught both holes or we can also say that it goes throught not hole, both answers are correct. The consequence is that nothing is real until an observer saw it. We do not know a reciprocal action between the electron, the observer and the instruments, but there must exist a reciprocal action. An electron has got many possibilities and because of our oberservation the electron must choose one of the possible ways. So when it goes throught one of the holes it is logical that the maxima are behind the holes. It is called collapse of the wave function and every particle has got a probability wave. This means that never can be sure where a particle is, we can only say where the most probable place is. A human being has got also a probability wave which we can find in the whole universe, but her strongest point is there where we are. But there is everytime a very little probability that you can find yourself on Mars for example or somewhere else, but this probability is so little that you need not be afraid. When we know throught measurements where this person is then his wave function collapses, because we know his exact position. As long as we observe something it is real and when we do not observe it it is not real any longer. There is another illustration which is called Schrodinger's cat. It is a thought experiment. We give a cat into a box with a radioactive material and a bottle of poison. Because we never know when an atom decays a radioactive material is very good for this experiment. The probability is very improtant for quantum physics. The box must be closed. When an atom of the radioactive material decays the bottle will be broken and the cat will die. But as long as we do not look after the cat if she is alive or dead, then we can say that she is alive and dead or not alive and not dead, both answers are correct. But in this situation we could not never say that she is alive or dead. I hope that these both experiments could tell you something about nature. So when you believe all this, which is not total sure until today, your conception of the world has changed I think.
Chapter 4: The space-time and time travels
Quantum physics is full of other phenomens. A very interesting possibility are time travels, because we see time travels in many movies, but only little prople know, that there are physical theories which make time travels prossible. All what we know, it is space and time, we call it space-time. We always talk about three dimensions and a fourth dimension which is the time. There are so called space-time-diagrams. On the y-axle we have got the time and on the x-axle, the horizontal axle, we have got the space. In a diagramm like that we can draw lines. When we stand for example the line is parallel to the y-axle, because only the time smears. When we travel faster the line comes closer to the x-axle. The line of an object which has got an infinite speed would be parallel to the x-axle. All known movements can be illustrated in these space-time-diagrams. These lines go from the bottom to the top, from bottom left to top right or from bottom right to top left. Lines which go from the top to the bottom, top right to bottom left or top left to bottom right would be movements back into the past. We cannot imagine movements like that, but for photons which fly with the speed of light it do not matter if they go into the past or into the future. The consequence might be for example that something can be formed of nothing, because Einstein's formula (E=mc2) allows to transform matter into energy and energy into matter. An electron-positron-pair for example can be formed by a no existing photon, which collide to form this photon. This is possible, because photon do not know the differenc between past and future. When we will crook the space-time so strong that the time will be one of the space dimensions and this space dimension will be the time then time travels could be possible, because in the space dimensions we can go forward and backward. In real it is very difficult to crook the space-time so strong, because we need a very strong gravity field. The possibility of time travels is fascinating.
Chapter 5: The Many worlds theory
There also exist another theory which is called many worlds theory. She discribes the nature without saying that something can be unreal. This theory says that there are many realities. So we can say that the electron in the double gap experiment goes in one reality throught one hole and in another reality throught the other hole. So we split one reality into two different realities. It grows like a tree with all his branchs. We can also say that in our world Schrodinger's cat is dead or alive and another reality it is the opposite. When we will travel in the time scientists think that if somebody kills his father before he was born then he disappears. But when there are many realities it can be possible to kill his father in one reality and then the person would not disappear. It is not sure if this theory is correct.
Chapter 6: The unified field theory
The unified field theory is for modern physics the most important problem. This theory says that after the Big Bang only one unified power has existed. This GUT-Power (Grand Unified Theory) was divided into four fundamental powers.
The four fundamental powers
power
range
strength
appearance
strong power
10-15 m
1
between quarks
electromagnetic power
infinite
10-2
between charged particles
weak power
10-15 m
10-13
between leptons (neutrinos, elecrons)
gravity
infinite
10-38
between all particles
power
range
strength
appearance
strong power
10-15 m
1
between quarks
electromagnetic power
infinite
10-2
between charged particles
weak power
10-15 m
10-13
between leptons (neutrinos, elecrons)
gravity
infinite
10-38
between all particles
Today we try to bring the four powers together. It has been discovered that a symmetry has existed between the electromagnetic power and the weak power which has broken. It is important to know that the W+-, the W-- and the Z0-particles are the exchangeparticles of the weak power. In our universe there is the higgs field, which unifieds with the field of the weak power. With high energy it is possible to destroy the higgs field and the exchangeparticles of the weak power are then free, they behave like photons and do not differ from them. For this discovery S. Glashow, S. Weinberg and A. Salam got the noble price. In an experiment in the CERN (Conseil Européenne pour la Recherche Nucléaire), in the proton-antiproton-collider this particles were found and so the electricalweak power was proofed. Now phycisists work to find a connection between the electroweak power and the strong power. Author and Webmaster: Lukas Czarnecki If you have got questions about quantum physics you can send me an e-mail under the following adress: webmaster@hpwt.de
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