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In 1970, the US "freedom" satellite discovered Cygnus X-1, which is different from other ray sources. Located on Cygnus X-1 is a huge blue planet more than 30 times heavier than the sun, which is towed by an invisible object weighing about 10 suns. Astronomers agree that this object is a black hole. It is the first black hole discovered by human beings< In 1928, salamanian chandraseka went to Cambridge to study with Sir Arthur Eddington, a British astronomer. Chandraseka realized that there is a limit to the repulsive force that the incompatibility principle can provide. The maximum velocity difference of particles in a star is limited by relativity to the speed of light. This means that when the star becomes compact enough, the repulsive force caused by the incompatibility principle will be less than the effect of gravity. Chandraseka calculated; A cold star about one and a half times the mass of the sun cannot support itself against its own gravity This mass is called the chandraseka limit). The former Soviet scientist Lev Davidovich Landau found a similar conclusion almost at the same time
If a star's mass is less than the chandraseka limit, it will eventually stop shrinking and become a "white dwarf" with a radius of several thousand miles and a density of several hundred tons per cubic inch. White dwarfs are supported by the repulsive force of the principle of incompatibility between electrons in their matter. The first one to be observed is the one orbiting Sirius, the brightest star in the night sky
Landau points out that there is another possible final state for stars. Its ultimate mass is about one or two times the mass of the sun, but its size is even smaller than that of a white dwarf. These stars are supported by the repulsive force of the principle of incompatibility between neutrons and protons, not electrons. So they're called neutron stars. They have a radius of only about 10 miles and a density of hundreds of millions of tons per cubic inch. When neutron stars were first predicted, there was no way to observe them. They were not observed until a long time later
on the other hand, when stars whose mass is larger than the chandraseka limit run out of fuel, there will be a big problem: in some cases, they will explode or throw out enough material to rece their mass below the limit, so as to avoid catastrophic gravitational collapse. No matter how big the stars are, this will always happen. Eddington refused to believe chandraseka's results. Eddington believed that a star could not collapse to a point. This is the view of most scientists: Einstein himself wrote a paper declaring that the volume of a star will not shrink to zero. The hostility of other scientists, especially Eddington, his former teacher and the main authority on stellar structure, led chandraseka to abandon his work in this field and turn to other astronomical problems such as stellar cluster motion. However, he won the 1983 Nobel Prize, at least in part because of his early work on the mass limits of cold stars
chandraseka points out that the incompatibility principle cannot prevent stars with mass greater than chandraseka limit from collapsing. But, according to general relativity, what happens to such a star. This problem was first solved by a young American, Robert Oppenheimer, in 1939. However, the results he got showed that there would be no more results with the telescope at that time. Later, e to the interference of the Second World War, Oppenheimer was involved in the atomic bomb program. After the war, most scientists were attracted to physics at the atomic and nuclear scale, so the problem of gravitational collapse was forgotten by most people
in 1967, joseline bell, a graate student in Cambridge, discovered objects that emit regular pulses of radio waves from the sky, which further inspired the prediction of the existence of black holes. At first bell and her mentor, Anthony hevish, thought they might have made contact with alien civilizations in our galaxy. At the seminar announcing their discovery, they called the four earliest sources lgm1-4, which means "little green man". Finally, they and all others came to the conclusion that these objects, which are called pulsars, are actually rotating neutron stars. When the concept of black hole was first put forward, there were two kinds of light theories for these neutron stars: one was Newton's particle theory of light; the other was Newton's particle theory of light; The other is the wave theory of light. Due to the wave particle ality of quantum mechanics, light can be considered as both wave and particle. In the wave theory of light, it is not clear how light responds to gravity. But if light is made up of particles, one can expect that they are just as affected by gravity as shells, rockets and planets. At first, people thought that light particles move infinitely fast, so gravity can't slow them down. But Romer's discovery about the limited speed of light shows that gravity can have an important effect on it< Based on this assumption, John Michel, the supervisor of Cambridge, published an article in the Journal of philosophy of the Royal Society of London in 1783. He pointed out that a star with enough mass and compactness will have such a strong gravitational field that even light can't escape - any light emitted from the star's surface will be attracted back by the star's gravity before it reaches far away. Michelle suggested that there may be a large number of such stars, although we can not see them because the light from them will not reach us, but we can still feel their gravitational attraction. This is what we call a black hole
in fact, because the speed of light is fixed, it is not rigorous to treat light like a shell in Newton's theory of gravity The projectile launched from the ground decelerates e to gravity, and finally stops rising and turns back to the ground; However, a photon must continue to move upward at a constant speed. How does Newton's gravity affect light Before Einstein put forward the general theory of relativity in 1915, there was no theory about how gravity affected the coordination of light, and then the meaning of this theory for massive stars was understood
when observing a star collapse and form a black hole, because there is no absolute time in the theory of relativity, each observer has his own time measurement. Because of the gravitational field of the star, the time of someone on the star will be different from that of someone far away. Suppose an intrepid astronaut on the surface of the collapsing star collapses inward with the star. According to his watch, he sends a signal every second to a spaceship orbiting the star. At a certain time in his watch, such as 11 o'clock, the star just shrinks to its critical radius. At this time, the gravitational field is so strong that nothing can escape, and his signal can no longer be transmitted to the spaceship. When he arrived at 11 o'clock, his partner in the spaceship found that the time interval between a series of signals sent by the astronauts became longer and longer. But the effect is very small before 10:59:59. They only need to wait a little longer than a second between receiving the two signals sent out at 10:59:58 and 10:59:59, but they have to wait infinitely long for the signal sent out at 11:00. According to the astronaut's watch, light waves are emitted from the stellar surface between 10:59:59 and 11:00; Seen from the spaceship, the light wave is scattered into an infinite time interval. The time interval between receiving this series of light waves on the spaceship becomes longer and longer, so the light from the star becomes more and more red and lighter. Finally, the star becomes so hazy that it can no longer be seen from the spaceship, and all that remains is a black hole in space. However, the star continues to act on the spaceship with the same gravity, making the spaceship continue to revolve around the black hole formed< However, e to the following problems, the above situation is not completely realistic. The farther away from the star, the weaker the gravity, so the gravity acting on the intrepid astronaut's feet is always greater than that on his head. Before the star shrinks to the critical radius and forms the event horizon, the difference in force has already pulled the astronaut into spaghetti, or even torn him apart! However, there are many massive objects in the universe, such as the central region of galaxies, which suffer from gravitational collapse and proce black holes; An astronaut on such an object won't be torn apart until the black hole forms. In fact, when he reaches the critical radius, he doesn't feel any different, even when he passes the point of never returning. But as the region continues to collapse, within a few hours, the difference between the force of gravity acting on his head and his feet will become so great that it will tear it apart again
Roger Penrose pointed out in his research between 1965 and 1970 that according to general relativity, there must be a singularity of infinite density and space-time curvature in black holes. This is quite similar to the big bang at the beginning of time, except that it is the end of time for a collapsing object and astronauts. At this singularity, the laws of science and the ability to predict the future have failed. However, any observer remaining outside the black hole will not be affected by the failure of predictability, because neither light nor any other signal from the singularity can arrive. This amazing fact led Roger Penrose to put forward the cosmic supervision conjecture, which can be paraphrased as: "God hates naked singularity." In other words, the singularity proced by gravitational collapse can only occur in places like black holes, where it is gracefully obscured by the event horizon and not seen by the outside world. Strictly speaking, this is the so-called weak cosmic supervision conjecture: it prevents the observer outside the black hole from being affected by the predictable failure at the singularity, but it can't help the poor astronaut who unfortunately fell into the black hole
General Relativity related
there are some solutions to the general relativity equation, which make our astronauts may see the naked singularity. He may be able to avoid hitting the singularity and go through a wormhole to another part of the universe. It seems that this offers great possibilities for travel in space - time. Unfortunately, all these solutions seem to be very unstable; The smallest interference, such as the existence of an astronaut, will change it, so that he can't see the singularity, so he bumps into it and ends his time. In other words, singularity always happens in his future, never in the past. The strong cosmic supervision conjecture is that in a realistic solution, the singularity always exists in the future (such as the singularity of gravitational collapse) or in the past (such as the big bang). Because it is possible to travel to the past near the naked singularity, there is great hope that some form of cosmic surveillance conjecture will hold
the event horizon, that is, the boundary of the inescapable region in space-time, is just like the unidirectional membrane surrounding the black hole: objects, such as careless astronauts, can fall into the black hole through the event horizon, but nothing can escape from the black hole through the event horizon Remember that the event horizon is the space-time orbit of light trying to escape from the black hole
first of all, several concepts are clarified. According to the current science, time cannot be reversed. That is to say, you can only go to the future, but not to the past
then there is the rumor that superluminal speed can go back to the past. Beyond the speed of light, you can only see the "image" of the past. You can't change anything< There are only two ways to obtain universal gravitation: 1. Objects attract each other; 2. Physics is moving at a high speed. We know that the central gravity of a black hole is infinite, and even light cannot escape
the greater the gravitational force on an object, the slower the time will start
when approaching the black hole, its gravity will begin to increase until infinity
black holes can only lead you to the future, not to the past
your relative time will start to slow down when you are subjected to gravity or the speed of motion increases. When you reach the speed of light, your relative time will stop
Why do you say that the more gravity you get, the slower your time will be< For example,
light travels in a straight line. When it propagates from one object to another, it moves forward
the greater the gravitation of an object to other objects, the collapse will appear on the secondary plane of its three-dimensional space, that is, it is like putting a heavy object in the center of an overhead cloth, and the middle of the cloth will sink e to gravity
when the light passes through here, the distance increases compared with the distance without passing through here, because there is space bending in the passing place, and the path becomes longer< When s = VT
V is constant, s increases and t increases
the time will increase, and the propagation time of light not passing through here will increase, so the relative time will slow down
when the gravity is large enough to reach enough traction light, when the light passes through the collapse, it can never go out and reach the destination, so the relative time stops
a black hole is an object that can collapse to a certain extent. Therefore, when it passes through its surroundings, your relative time slows down, so that you can go to the future, that is, it distorts time
I hope I can help you
according to general relativity, the gravitational field will bend the space-time. When a star is large, its gravitational field has little effect on space-time, and the light from a certain point on the star's surface can be emitted in any direction along a straight line. The smaller the radius of a star, the more it bends the space-time around it, and the light emitted at certain angles will return to the surface of the star along the curved space
when the radius of a star is small enough to a certain value (astronomically called "Schwarzschild radius"), even the light emitted from the vertical surface is captured. At this point, the star becomes a black hole. To say that it is "black" means that it is like a bottomless hole in the universe. Once any matter falls in, "it seems" can no longer escape. In fact, black holes are really invisible. We'll talk about that later
so, how do black holes form? In fact, like white dwarfs and neutron stars, black holes are likely evolved from stars
we have introced the formation process of white dwarfs and neutron stars in detail. When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and the energy proced by the center is not much. In this way, it no longer has enough strength to bear the huge weight of the shell. So under the pressure of the outer shell, the core begins to collapse until it finally forms a small, dense star, which is able to balance with the pressure again
stars with smaller mass mainly evolve into white dwarfs, while stars with larger mass may form neutron stars. According to scientists' calculations, the total mass of a neutron star cannot be more than three times the mass of the sun. If this value is exceeded, there will be no more force to contend with its own gravity, leading to another large collapse
this time, according to scientists' conjecture, matter will march towards the central point irresistibly until it becomes a "point" with zero volume and infinite density. Once its radius shrinks to a certain extent (Schwarzschild radius), as we have described above, the huge gravity makes even light can not be emitted, thus cutting off all the connections between the star and the outside world - "black hole" is born
compared with other celestial bodies, black holes are too special. For example, the black hole has "invisibility", people can't observe it directly, even scientists can only make various conjectures about its internal structure. So, how does a black hole hide itself? The answer is - curved space. We all know that light travels in a straight line. This is the most basic common sense. But according to general relativity, space will bend under the action of gravitational field. At this time, although the light still propagates along the shortest distance between any two points, it is no longer a straight line, but a curve. Figuratively speaking, it seems that light is going to walk in a straight line, but the strong gravity pulls it away from the original direction
on the earth, e to the small effect of the gravitational field, the bending is negligible. And around the black hole, the deformation of space is very large. In this way, even though some of the light emitted by the stars blocked by the black hole will fall into the black hole and disappear, another part of the light will pass through the curved space to bypass the black hole and reach the earth. Therefore, we can easily observe the stars behind the black hole, just like the black hole does not exist, this is the black hole's invisibility
What's more interesting is that some stars not only emit light towards the earth directly to the earth, but also emit light in other directions, which may be refracted by the strong gravity of nearby black holes to reach the earth. In this way, we can see not only the "face" of the star, but also its side and even its back
"black hole" is undoubtedly one of the most challenging and exciting astronomical theories in this century. Many scientists are working hard to uncover its mystery, and new theories are constantly put forward. However, the latest achievements of modern astrophysics can not be explained in a few words. Interested friends can refer to special works
black holes are one of the most famous predictions of Einstein's general relativity. It suggests that the gravitational field will bend space-time. When a star is large, its gravitational field has little effect on space-time, and the light from a certain point on the star's surface can be emitted in any direction along a straight line. The smaller the radius of a star, the more it bends the space-time around it, and the light emitted at certain angles will return to the surface of the star along the curved space
when the radius of a star is small enough to a certain value (astronomically called "Schwarzschild radius"), even the light emitted from the vertical surface is captured. At this point, the star becomes a black hole. To say that it is "black" means that it is like a bottomless hole in the universe. Once any matter falls in, "it seems" can no longer escape
special report
when a star is aging, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and the energy proced by the center is not much. In this way, it no longer has enough strength to bear the huge weight of the shell. So under the pressure of the outer shell, the core begins to collapse until it finally forms a small, dense star, which is able to balance with the pressure again
stars with smaller mass mainly evolve into white dwarfs, while stars with larger mass may form neutron stars. According to scientists' calculations, the total mass of a neutron star cannot be more than three times the mass of the sun. If this value is exceeded, there will be no more force to contend with its own gravity, leading to another large collapse
this time, according to scientists' conjecture, matter will march towards the central point irresistibly until it becomes a "point" with zero volume and infinite density. Once its radius shrinks to a certain extent (Schwarzschild radius), as we have described above, the huge gravity makes even light can not be emitted, thus cutting off all the connections between the star and the outside world - "black hole" is born.
EOA is a tool to provide bridging connection between DSL modem and ISP. In bridging connection, data is shared between ISP and their users' networks as if the two networks are in the same physical LAN. Bridging connection does not use IP protocol, it is used as a layer-2 device. However, with the setting of ISP, ADSL modem can also be configured as a connection that can provide routing function. In this way, it uses IP protocol to connect to ISP to exchange data. At this time, ADSL modem becomes a three-layer device, and the protocol is changed to IPOA
when creating or modifying the default settings of EOA interface, please ask your ISP which type of protocol you use< (1) view the EOA settings
click the "EOA" option in the "WAN port" tab, and the "EOA" page will appear (as shown in Figure 26). The EOA table in the figure contains a description of each EOA interface being used in ADSL. If your ISP does not use the EOA protocol, the table may be blank. The following is a detailed description of the table on this page:
① interface: the software name used to identify the EOA interface<
② IPF type (IPF, interface firewall protection): firewall protection interface type, there are three options to choose from, public (public), private (private) or DMZ (middle zone):
> public, which is connected to the Internet (IPOA interface is a typical public type), The packets received on the public interface are restricted by the most strict firewall protection rules set in the system
> private is a private interface in your LAN, such as Ethernet interface. Receiving packets on the private interface is less protected because they are exchanged within the network< DMZ (de militarized zone) means that when a computer accesses the Internet, it can access the information of the public network and the intranet (such as a public server in a local area network), and input the interface of the intermediate zone. Whether the information packets are sent inside or outside the local area network, they are limited by the firewall protection settings, which is between the restrictions of the public and private interfaces
③ low level interface: when the EOA interface is set in the system software, it will be connected to a low-level software and hardware structure (at the bottom, they are connected to a physical port, i.e. WAN port). When the EOA is used, an interface name will be reflected in the next level software, which will be an ATM VC interface, such as aal5-0. For relevant introction, see "configuring ATM VC"< (4) configure IP address and subnet mask: that is, the IP address and subnet mask you want to assign to this interface. If you don't use ADSL as a router in your LAN and use this interface to bridge your ISP, you don't have to specify its IP address. If you turn on the DHCP function for this interface, setting the IP address is only a request for the DHCP server to provide services. If the information in the address bar is not filled in, the actual address assigned by the ISP may be different
⑤ enable DHCP: when set to enable, this setting enables the device to dynamically receive the IP information assigned by the ISP server. If this interface is used to bridge your ISP and you do not use it to exchange data, this box can be left blank
6 default route: use ADSL as the default route in your LAN regardless of whether it uses the assigned IP address on this interface. There are two options: enable and disable. Please refer to the description of default route in "configure IP route" for details< (7) gateway address: the external IP address used by ADSL to access the Internet through the EOA interface, which is the IP address of a typical ISP server
8 status: a green or red ball indicates that the interface is currently in use or disabled status, and we cannot manually open or close the interface. If the interface is closed, there is a problem with the DSL connection< Operation: you can click the icon to edit () and delete () the related EOA interface.