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can never be escaped.

event horizon

Boundary marking the limits of a black hole.

At the event horizon, the escape velocity is equal to the speed of light. Since general relativity states that nothing can travel faster than the speed of light, nothing inside the event horizon can ever cross the boundary and escape beyond it, including light. Thus, nothing that enters a black hole can get out or can be observed from outside the event horizon. Likewise, any radiation generated inside the horizon can never escape beyond it. For a nonrotating black hole, the Schwarzschild radius delimits a spherical event horizon. Rotating black holes have distorted, nonspherical event horizons. Since the event horizon is not a material surface but rather merely a mathematically defined demarcation boundary, nothing prevents matter or radiation from entering a black hole, only from exiting one. Though black holes themselves may not radiate energy, electromagnetic radiation and matter particles may be radiated from just outside the event horizon via Hawking radiation.

Even though the concept that an object with so much gravity that light cannot escape was proposed in the 1780s by John Mitchell, the notion of black holes has been largely ignored until Einstein's theory of general relativity was published in 1915.

relativity

Concept in physics that measurements change when considered by observers in various states of motion.

In classical physics, it was assumed that all observers anywhere in the universe would obtain identical measurements of space and time intervals. According to relativity theory, this is not so; all measurements depend on the relative motions of the observer and the observed. There are two distinct theories of relativity, both proposed by Albert Einstein. The special theory of relativity (1905) developed from Einstein's acceptance that the speed of light is the same in all reference frames, irrespective of their relative motion. It deals with non-accelerating reference frames, and is concerned primarily with electric and magnetic phenomena and their propagation in space and time. The general theory (1916) was developed primarily to deal with gravitation and involves accelerating reference frames. Both theories are major milestones in the history of modern physics. See also equivalence principle, space-time.

Shortly thereafter, scientist Karl Schwarzschild found solutions to equations that proved that black holes could theoretically exist, though he himself only treated it astheoretical—not something that could appear in nature. Later, in 1939, Robert Oppenheimer and H. Snyder theorized that should sufficiently massive stars collapse, black holes could possibly form. At the time, they were known as "frozen stars", as the collapse that creates them would appear to slow down to an outside observer, because of the slowing down of light as it gets closer to the event horizon. Although considered feasible, frozen stars were generally thought of as nothing more than a mathematical theory until the late 1960s, when pulsars and neutron stars were discovered. In 1967, John Wheeler coined the term "black hole", which attracted far more attention than the previous "frozen star". Since then, black "hole research has grown significantly in popularity and complexity, and more and more evidence is uncovered that black holes are not only possible, but far more common than previously thought.

pulsar

n. (Astronomy) celestial body which radiates electromagnetic pulses at regular intervals (believed to be a rapidly spinning neutron star)

In general relativity, a black hole is known as a "singularity"奇點, a space or event where physical theories break down and no longer work. This is because in general relativity, a black hole is compressed into a point with no volume. As a result, this causes the black hole to have infinite gravity and density. These infinite values create a space that breaks equations and therefore cannot be described by physics. A point of contention rests with theories of quantum mechanics, which state that objects with a size of zero cannot exist. Therefore, to quantum physics, a black hole is just a very

large object compressed into a very small space. However, there are no complete theories that unify quantum theories to general relativity, and general relativity is usually used as it is far more complete than quantum mechanics for explaining the behavior of large objects such as stars.

quantum mechanics

Branch of mathematical physics that deals with atomic and subatomic systems.

It is concerned with phenomena that are so small-scale that they cannot be described in classical terms, and it is formulated entirely in terms of statistical probabilities. Considered one of the great ideas of the 20th century, quantum mechanics was developed mainly by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, and Max Born and led to a drastic reappraisal of the concept of objective reality. It explained the structure of atoms, atomic nuclei (see nucleus), and molecules; the behaviour of subatomic particles; the nature of chemical bonds (see bonding); the properties of crystalline solids (see crystal); nuclear energy; and the forces that stabilize collapsed stars. It also led directly to the development of the laser, the electron microscope, and the transistor.

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quantum

In physics, a discrete natural unit, or packet, of energy, charge, angular momentum, or other physical property.

Light, for example, which appears in some respects as a continuous electromagnetic wave, on the submicroscopic level is emitted and absorbed in discrete amounts, or quanta; for light of a given wavelength, the magnitude of all the quanta emitted or absorbed is the same in both energy and momentum. These particlelike packets of light are called photons, a term also applicable to quanta of other forms of electromagnetic energy such as X rays and gamma rays. Submicroscopic mechanical vibrations in the layers of atoms comprising crystals also give up or take on energy and momentum in quanta called phonons. See also quantum mechanics.

■ Another problem in understanding black holes has been that humans have not been able to re-create them in particle accelerators. ■ While there have been isolated claims 零星宣稱of black-hole genesis, it is generally thought that it would require particle accelerators more powerful than what

discoveries are made though, it is likely that more complete theories will be made.

Although we understand much more about the workings of our universe than we did even a few decades ago, black holes remind us that our knowledge of physics is still incomplete. Future theories of space-time behavior時空行為 will undoubtedly need to be tested against the black hole, which sits on the very edge極限 of physics.