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What is a black hole?
A black hole is defined by the escape velocity that would have to be attained to escape from the gravitational pull exerted upon an object. For example, the escape velocity of earth is equal to 11 km/s. Anything that wants to escape earth's gravitational pull must go at least 11 km/s, no matter what the thing is a rocket ship or a baseball. The escape velocity of an object depends on how compact it is; that is, the ratio of its mass to radius. A black hole is an object so compact that, within a certain distance of it, even the speed of light is not fast enough to escape.
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How is a stellar black hole created?
A common type of black hole is the type produced by some dying stars. A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life. In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy to push outward. For most of a star's life, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight. For small stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully. This type of star is called the "white dwarf." When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely collapses under its own weight.
To create a massive core a progenitor (ancestral) star would need to be at least 20 times more massive than our Sun. If the core is very massive (approximately 2.5 times more massive than the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole. Then the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is referred to as a singularity. When this happens, escape would require a velocity greater than the speed of light. No object can reach the speed of light. The distance from the black hole at which the escape velocity is just equal to the speed of light is called the event horizon. Anything, including light, that passes across the event horizon toward the black hole is forever trapped.
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Since light has no mass how can it be trapped by the gravitational pull of a black hole?
Newton thought that only objects with mass could produce a gravitational force on each other. Applying Newton's theory of gravity, one would conclude that since light has no mass, the force of gravity couldn't affect it. Einstein discovered that the situation is a bit more complicated than that. First he discovered that gravity is produced by a curved space-time. Then Einstein theorized that the mass and radius of an object (its compactness) actually curves space-time. Mass is linked to space in a way that physicists today still do not completely understand. However, we know that the stronger the gravitational field of an object, the more the space around the object is curved. In other words, straight lines are no longer straight if exposed to a strong gravitational field; instead, they are curved. Since light ordinarily travels on a straight-line path, light follows a curved path if it passes through a strong gravitational field. This is what is meant by "curved space," and this is why light becomes trapped in a black hole. In the 1920's Sir Arthur Eddington proved Einstein's theory when he observed starlight curve when it traveled close to the Sun. This was the first successful prediction of Einstein's General Theory of Relativity.
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Among the physical quantities used to describe the universe, some are considered fundamental quantities whereas others are derived quantities, comparable to the designation of definitions and undefined terms in a mathematical system. Although it does not really matter which particular quantities are the ones designated as fundamental, the most common are length, mass, and time. In scientific work the two major systems of units for these quantities are the mks (meter-kilogram-second) and the cgs (centimeter-gram-second). Every measurement is a comparison with the standards that are universally accepted as definitions of these fundamental units. In astronomy and space science, where large distances are common, the meter and even the kilometer are too small to be convenient; in Problems 5, 9, and 10 of this chapter, we show how more suitable units for length are defined.
Dimensional analysis (manipulation of units according to the rules of algebra) is the procedure used to ensure consistency in the definition and use of units. For example, since force is, by definition, the product of mass and acceleration, measured respectively in kg and m/(s^2) in the mks system, the unit of force in this system must be equivalent to kg x m/(s^2). A new term, the newton, was created to describe the unit of force: 1 newton = 1 kg x m/(s^2)
SCIENTIFIC LAWS OFTEN FOUND INCONSISTENT WITH SCIENCE FICTION
NASA rocket scientists often perform feasibility studies which examine a spacecraft design based on the laws of science (physics, mechanics, chemistry, etc.). Scientists, also, use spacecraft design conventions known as "rules of thumb" to determine whether a spacecraft concept has merit. A basic engineering education includes an introductory course in college physics. The same material is covered in less depth in high school physics as well as grade school science. A student can learn much about the laws of science by attempting an informal feasibility study on a spacecraft from a science fiction comic book, a sci-fi magazine cover, or a STAR WARS or STAR TREK video. Listed below are some of the laws of science often violated by artists and authors. Included are descriptive examples of errors often revealed by such feasibility studies.
One of the best ways of determining what is scientifically reasonable is to imagine yourself inside the subject vehicle. Next, try to compare the vehicle to driving a car, piloting an airplane, or even riding a bicycle. These vehicles must obey the laws of science. Each requires a means of steering, an engine or motor for transporting the vehicle, and a way of positioning the driver or passengers in order for them to control and ride inside the vehicle. When something does not seem correct about the science fiction art, try to classify which of the items below may be violated by the drawing.
For example, some artists fail to draw a viewing window for the pilot of a spacecraft to see where the craft is going. Others, leave out a hatch for the astronaut to enter or leave the vehicle. Can you imagine a car without doors? Below is a list of some of the most obvious abuses of the laws of science in science fiction art.
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