Will Earth or Sun ever become a Black Hole?

There are physical reasons why earth will never become a Black Hole or why our Sun won't either. In the 1930's, an Indian scientist named Subrahmanyan Chanrasekhar, showed that a white dwarf with a mass more than 1.4 times that of the Sun would collapse indefinitely and form a Black Hole. Chanrasekhar's ideas were not excepted until the 1960's, when the pulsars were discovered. After that a new interest in Black Holes reinvigorated the scientific community. In the 1970's an X-ray source known as Cygnus-X was shown to rotate around another star known as HDE 226868. By studying the orbits of the two stars scientists calculated that the X-ray source must have a volume smaller then that of earth but with a mass over the Oppenheimer-Volkoff limit. The Oppenheimer-Volkoff Limit states that an remnant left over after a supernovae with a mass more then 3 times that of the Sun would collapse into a Black Hole. Using the information gathered by observation of Cygnus-X it was concluded that it was a Black Hole.

If a black hole existed, would it suck up all the matter in the Universe?

A black hole will not do that becase of it's "horizon," which means a region from which you can't escape.If you cross the horizon, you're doomed to eventually hit the singularity. But as long as you stay outside of the horizon, you can avoid getting sucked in. In fact, to someone well outside of the horizon, the gravitational field surrounding a black hole is no different from the field surrounding any other object of the same mass. In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects.

Event Horizon

Everyone knows the most fundamental characteristic of a Black Hole: Once you go into it you can never get out. But where do you physically go into a Black Hole? That is where the Event Horizon plays its role. The Event Horizon is an area of space that surrounds the singularity of the Black Hole. If you're familiar with Star Trek then you can imagine the Event Horizon as similar to a force-field around a spaceship. You can't actually see it but you know the consciences of crossing it. Let's say you were traveling in a spaceship and you were approaching a Black Hole and didn't know it, you could easily cross the Event Horizon and never realize it. But say you figured out that you just crossed the Event Horizon and you wanted to leave. You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so if you fire your rockets hard enough, you can give yourself enough energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape. So the next time you're cruising around the Universe, remember: You can't see an Event Horizon, You wouldn't know it if you crossed an Event Horizon, and there is no way to escape across the Event Horizon. Happy Travels!

Properties of Event Horizon

The horizon has some very strange geometrical properties. To an observer who is sitting still somewhere far away from the black hole, the horizon seems to be a nice, static, unmoving spherical surface. But once you get close to the horizon, you realize that it has a very large velocity. In fact, it is moving outward at the speed of light! That explains why it is easy to cross the horizon in the inward direction, but impossible to get back out. Since the horizon is moving out at the speed of light, in order to escape back across it, you would have to travel faster than light. You can't go faster than light, and so you can't escape from the black hole. (If all of this sounds very strange, don't worry. It is strange. The horizon is in a certain sense sitting still, but in another sense it is flying out at the speed of light. It's a bit like Alice in "Through the Looking-Glass": she has to run as fast as she can just to stay in one place.) Once you're inside of the horizon, spacetime is distorted so much that the coordinates describing radial distance and time switch roles. That is, "r", the coordinate that describes how far away you are from the center, is a timelike coordinate, and "t" is a spacelike one. One consequence of this is that you can't stop yourself from moving to smaller and smaller values of r, just as under ordinary circumstances you can't avoid moving towards the future (that is, towards larger and larger values of t). Eventually, you're bound to hit the singularity at r = 0. You might try to avoid it by firing your rockets, but it's futile: no matter which direction you run, you can't avoid your future. Trying to avoid the center of a black hole once you've crossed the horizon is just like trying to avoid next Thursday. Incidentally, the name 'black hole' was invented by John Archibald Wheeler, and seems to have stuck because it was much catchier than previous names. Before Wheeler came along, these objects were often referred to as 'frozen stars.' I'll explain why below.

How big is a Black Hole?

There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. Let's talk first about the masses of black holes. There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I'm using scientific notation: 10^{31} means a 1 with 31 zeroes after it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms. The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun.

Evidences of Black Holes

Suppose you have found a region of space where you think there might be a black hole. How can you check whether there is one or not? The first thing you'd like to do is measure how much mass there is in that region. If you've found a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there. There are two kinds of systems in which astronomers have found such compact, massive, dark objects: the centers of galaxies (including perhaps our own Milky Way Galaxy), and X-ray-emitting binary systems in our own Galaxy. According to a recent review by Kormendy and Richstone (to appear in the 1995 edition of "Annual Reviews of Astronomy and Astrophysics"), eight galaxies have been observed to contain such massive dark objects in their centers. The masses of the cores of these galaxies range from one million to several billion times the mass of the Sun. The mass is measured by observing the speed with which stars and gas orbit around the center of the galaxy: the faster the orbital speeds, the stronger the gravitational force required to hold the stars and gas in their orbits. (This is the most common way to measure masses in astronomy. For example, we measure the mass of the Sun by observing how fast the planets orbit it, and we measure the amount of dark matter in galaxies by measuring how fast things orbit at the edge of the galaxy.) These massive dark objects in galactic centers are thought to be black holes for at least two reasons. First, it is hard to think of anything else they could be: they are too dense and dark to be stars or clusters of stars. Second, the only promising theory to explain the enigmatic objects known as quasars and active galaxies postulates that such galaxies have supermassive black holes at their cores. If this theory is correct, then a large fraction of galaxies -- all the ones that are now or used to be active galaxies -- must have supermassive black holes at the center. Taken together, these arguments strongly suggest that the cores of these galaxies contain black holes, but they do not constitute absolute proof. Two very recent discovery has been made that strongly support the hypothesis that these systems do indeed contain black holes. First, a nearby active galaxy was found to have a "water maser" system (a very powerful source of microwave radiation) near its nucleus. Using the technique of very-long-baseline interferometry, a group of researchers was able to map the velocity distribution of the gas with very fine resolution. In fact, they were able to measure the velocity within less than half a light-year of the center of the galaxy. From this measurement they can conclude that the massive object at the center of this galaxy is less than half a light-year in radius. It is hard to imagine anything other than a black hole that could have so much mass concentrated in such a small volume. (This result was reported by Miyoshi et al. in the 12 January 1995 issue of Nature, vol. 373, p. 127.) A second discovery provides even more compelling evidence. X-ray astronomers have detected a spectral line from one galactic nucleus that indicates the presence of atoms near the nucleus that are moving extremely fast (about 1/3 the speed of light). Furthermore, the radiation from these atoms has been redshifted in just the manner one would expect for radiation coming from near the horizon of a black hole. These observations would be very difficult to explain in any other way besides a black hole, and if they are verified, then the hypothesis that some galaxies contain supermassive black holes at their centers would be fairly secure. (This result was reported in the 22 June 1995 issue of Nature, vol. 375, p. 659, by Tanaka et al.) A completely different class of black-hole candidates may be found in our own Galaxy. These are much lighter, stellar-mass black holes, which are thought to form when a massive star ends its life in a supernova explosion. If such a stellar black hole were to be off somewhere by itself, we wouldn't have much hope of finding it. However, many stars come in binary systems -- pairs of stars in orbit around each other. If one of the stars in such a binary system becomes a black hole, we might be able to detect it. In particular, in some binary systems containing a compact object such as a black hole, matter is sucked off of the other object and forms an "accretion disk" of stuff swirling into the black hole. The matter in the accretion disk gets very hot as it falls closer and closer to the black hole, and it emits copious amounts of radiation, mostly in the X-ray part of the spectrum. Many such "X-ray binary systems" are known, and some of them are thought to be likely black-hole candidates. Suppose you've found an X-ray binary system. How can you tell whether the unseen compact object is a black hole? Well, one thing you'd certainly like to do is to estimate its mass. By measuring the orbital speed of visible star (together with a few other things), you can figure out the mass of the invisible companion. (The technique is quite similar to the one we described above for supermassive black holes in galactic centers: the faster the star is moving, the stronger the gravitational force required to keep it in place, and so the more massive the invisible companion.) If the mass of the compact object is found to be very large very large, then there is no kind of object we know about that it could be other than a black hole. (An ordinary star of that mass would be visible. A stellar remnant such as a neutron star would be unable to support itself against gravity, and would collapse to a black hole.) The combination of such mass estimates and detailed studies of the radiation from the accretion disk can supply powerful circumstantial evidence that the object in question is indeed a black hole. Many of these "X-ray binary" systems are known, and in some cases the evidence in support of the black-hole hypothesis is quite strong. In a review article in the 1992 issue of Annual Reviews of Astronomy and Astrophysics, Anne Cowley summarized the situation by saying that there were three such systems known (two in our galaxy and one in the nearby Large Magellanic Cloud) for which very strong evidence exists that the mass of the invisible object is too large to be anything but a black hole. There are many more such objects that are thought to be likely black holes on the basis of slightly less evidence. Furthermore, this field of research has been very active since 1992, and the number of strong candidates by now is larger than three.

What is a white hole?

The equations of general relativity have an interesting mathematical property: they are symmetric in time. That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you'll get another valid solution to the equations. If you apply this rule to the solution that describes black holes, you get an object known as a white hole. Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out. White holes are a perfectly valid mathematical solution to the equations of general relativity, but that doesn't mean that they actually exist in nature. In fact, they almost certainly do not exist, since there's no way to produce one. (Producing a white hole is just as impossible as destroying a black hole, since the two processes are time-reversals of each other.)

Space Travel - a wormhole?

It is possible to travel around the Earth in 80 days, but impossible to travel around the Universe(at least not now) in that short period of time. In fact, it takes us a few light year just to get to a nearby Star(that is to constantly traveling at the speed of light for some years, not possible now, though). But there could be another alternative to allow us travel to other galaxies? So far, we have only considered ordinary "vanilla" black holes. Specifically, we have been talking all along about black holes that are not rotating and have no electric charge. If we consider black holes that rotate and/or have charge, things get more complicated. In particular, it is possible to fall into such a black hole and not hit the singularity. In effect, the interior of a charged or rotating black hole can "join up" with a corresponding white hole in such a way that you can fall into the black hole and pop out of the white hole. This combination of black and white holes is called a wormhole. The white hole may be somewhere very far away from the black hole; indeed, it may even be in a "different Universe" -- that is, a region of spacetime that, aside from the wormhole itself, is completely disconnected from our own region. A conveniently-located wormhole would therefore provide a convenient and rapid way to travel very large distances, or even to travel to another Universe. Maybe the exit to the wormhole would lie in the past, so that you could travel back in time by going through. All in all, they sound pretty cool. But before you apply for that research grant to go search for them, there are a couple of things you should know. First of all, wormholes almost certainly do not exist. As we said above in the section on white holes, just because something is a valid mathematical solution to the equations doesn't mean that it actually exists in nature. In particular, black holes that form from the collapse of ordinary matter (which includes all of the black holes that we think exist) do not form wormholes. If you fall into one of those, you're not going to pop out anywhere. You're going to hit a singularity, and that's all there is to it. Furthermore, even if a wormhole were formed, it is thought that it would not be stable. Even the slightest perturbation (including the perturbation caused by your attempt to travel through it) would cause it to collapse. Finally, even if wormholes exist and are stable, they are quite unpleasant to travel through. Radiation that pours into the wormhole (from nearby stars, the cosmic microwave background, etc.) gets blueshifted to very high frequencies. As you try to pass through the wormhole, you will get fried by these X-rays and gamma rays.

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