奇异的黑洞

capricornus_tai

若要解释何为黑洞，就要从万有引力说起。
牛顿的万有引力定律认为，宇宙间的一切都有强大的相互吸引力，他们能牢牢吸住附近一切天体。例子就是地球的引力使之地球的一切不能轻易飞离地球。人们要把卫星送上围绕地球的轨道，那火箭就至少要有每秒8000米的速度，不然地球的引力会把人造卫星拉回地球，这被称为第一宇宙速度。若要把飞船送出地球引力的控制，那么就至少需要11000米的秒速，这速度称为第二宇宙速度。不同的天体，表面脱离速度相对不同，这与其质量有相对的密切关系。月球质量比地球小，因此表面脱离速度就大约是地球的六分之一。

有人曾发问：有没有可能宇宙中有种天体的表面脱离速度为30万千米秒速，就是说比光速还大，因此连他自己发射的光都脱离不了他的引力？

1798年法国天文学家拉普拉斯从牛顿的引力学大胆地设想，认为宇宙最明亮的天体，很可能我们根本看不到他。因为在万有引力的理论下，当光速低于一个天体的表面脱离速度，那么就是说光根本到达我们这里，所以我们就看不到该天体的存在。这就是早期的黑洞理论。
依据牛顿的引力论，当一个球型天体的质量高出太阳的两倍，那么就会有可能引发引力崩溃。也就是说，它可能会向自己的中心引力坍缩，成为一个体积无限小，质量无限大的质点。依据爱因斯坦的广义相对论，德国科学家史瓦西计算出一个可能具备无穷大引力的天体半径。当一个天体半径一旦达到这个长度以上，就有可能有无穷大的引力，任何物质都逃不过他的引力。这个连光也逃不过的引力天体，人们无法看到他的存在，因此被称为黑洞。
这个洞就是一种天体具有一个封闭的边界称为视界。视界的封闭也是相对而言，外界的物质和辐射可以进入视界，但是视界的一切都无法逃逸引力到外面去。简单的说，黑洞不会向外界发出或反射任何光线，因为光完全无法逃出黑洞的引力。任何东西被吸进去，就再也出不来。黑洞就像是永远出饥饿状态，是填不饱的无底洞，这就是科学家口中的星坟。

capricornus_tai

黑洞

星系NGC4261内黑洞周围的尘埃圆盘
　　“黑洞”很容易让人望文生义地想象成一个“大黑窟窿”，其实不然。所谓“黑洞”，就是这样一种天体：它的引力场是如此之强，就连光也不能逃脱出来。

　　根据广义相对论，引力场将使时空弯曲。当恒星的体积很大时，它的引力场对时空几乎没什么影响，从恒星表面上某一点发的光可以朝任何方向沿直线射出。而恒星的半径越小，它对周围的时空弯曲作用就越大，朝某些角度发出的光就将沿弯曲空间返回恒星表面。

　　等恒星的半径小到一特定值（天文学上叫“史瓦西半径”）时，就连垂直表面发射的光都被捕获了。到这时，恒星就变成了黑洞。说它“黑”，是指它就像宇宙中的无底洞，任何物质一旦掉进去，“似乎”就再不能逃出。实际上黑洞真正是“隐形”的，等一会儿我们会讲到。

　　那么，黑洞是怎样形成的呢？其实，跟白矮星和中子星一样，黑洞很可能也是由恒星演化而来的。

　　我们曾经比较详细地介绍了白矮星和中子星形成的过程。当一颗恒星衰老时，它的热核反应已经耗尽了中心的燃料（氢），由中心产生的能量已经不多了。这样，它再也没有足够的力量来承担起外壳巨大的重量。所以在外壳的重压之下，核心开始坍缩，直到最后形成体积小、密度大的星体，重新有能力与压力平衡。

　　质量小一些的恒星主要演化成白矮星，质量比较大的恒星则有可能形成中子星。而根据科学家的计算，中子星的总质量不能大于三倍太阳的质量。如果超过了这个值，那么将再没有什么力能与自身重力相抗衡了，从而引发另一次大坍缩。

　　这次，根据科学家的猜想，物质将不可阻挡地向着中心点进军，直至成为一个体积趋于零、密度趋向无限大的“点”。而当它的半径一旦收缩到一定程度(史瓦西半径)，正象我们上面介绍的那样，巨大的引力就使得即使光也无法向外射出，从而切断了恒星与外界的一切联系——“黑洞”诞生了。
幻想从星系NGC4261核心附近的一颗行星上看到的夜空，这颗行星将注定被黑洞所吞噬

　　与别的天体相比，黑洞是显得太特殊了。例如，黑洞有“隐身术”，人们无法直接观察到它，连科学家都只能对它内部结构提出各种猜想。那么，黑洞是怎么把自己隐藏起来的呢？答案就是——弯曲的空间。我们都知道，光是沿直线传播的。这是一个最基本的常识。可是根据广义相对论，空间会在引力场作用下弯曲。这时候，光虽然仍然沿任意两点间的最短距离传播，但走的已经不是直线，而是曲线。形象地讲，好像光本来是要走直线的，只不过强大的引力把它拉得偏离了原来的方向。

　　在地球上，由于引力场作用很小，这种弯曲是微乎其微的。而在黑洞周围，空间的这种变形非常大。这样，即使是被黑洞挡着的恒星发出的光，虽然有一部分会落入黑洞中消失，可另一部分光线会通过弯曲的空间中绕过黑洞而到达地球。所以，我们可以毫不费力地观察到黑洞背面的星空，就像黑洞不存在一样，这就是黑洞的隐身术。

　　更有趣的是，有些恒星不仅是朝着地球发出的光能直接到达地球，它朝其它方向发射的光也可能被附近的黑洞的强引力折射而能到达地球。这样我们不仅能看见这颗恒星的“脸”，还同时看到它的侧面、甚至后背！

　　“黑洞”无疑是本世纪最具有挑战性、也最让人激动的天文学说之一。许多科学家正在为揭开它的神秘面纱而辛勤工作着，新的理论也不断地提出。

[ 本帖最后由 capricornus_tai 于 24-2-2006 09:48 PM 编辑 ]

capricornus_tai

Black hole


in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping. The term was first used in reference to a star in the last phases of gravitational collapse (the final stage in the life history of certain stars; see stellar evolution) by the American physicist John A. Wheeler.

Gravitational collapse begins when a star has depleted its steady sources of nuclear energy and can no longer produce the expansive force, a result of normal gas pressure, that supports the star against the compressive force of its own gravitation. As the star shrinks in size (and increases in density), it may assume one of several forms depending upon its mass. A less massive star may become a white dwarf, while a more massive one would become a supernova. If the mass is less than three times that of the sun, it will then form a neutron star. However, if the final mass of the remaining stellar core is more than three solar masses, as shown by the American physicists J. Robert Oppenheimer and Hartland S. Snyder in 1939, nothing remains to prevent the star from collapsing without limit to an indefinitely small size and infinitely large density, a point called the "singularity."

At the point of singularity the effects of Einstein's general theory of relativity become paramount. According to this theory, space becomes curved in the vicinity of matter; the greater the concentration of matter, the greater the curvature. When the star (or supernova remnant) shrinks below a certain size determined by its mass, the extreme curvature of space seals off contact with the outside world. The place beyond which no radiation can escape is called the event horizon, and its radius is called the Schwarzschild radius after the German astronomer Karl Schwarzschild, who in 1916 postulated the existence of collapsed celestial objects that emit no radiation. For a star with a mass equal to that of the sun, this limit is a radius of only 0.9 mi (1.5 km). Even light cannot escape the black hole but is turned back by the enormous pull of gravitation.

It is now believed that the origin of some black holes is nonstellar. Some astrophysicists suggest that immense volumes of interstellar matter can collect and collapse into supermassive black holes, such as are found at the center of some galaxies. The British physicist Stephen Hawking has postulated still another kind of nonstellar black hole. Called a primordial, or mini, black hole, it would have been created during the "big bang," in which the universe was created (see cosmology). Unlike stellar black holes, primordial black holes create and emit elementary particles, called Hawking radiation, until they exhaust their energy and expire. It has also been suggested that the formation of black holes may be associated with intense gamma ray bursts. Beginning with a giant star collapsing on itself or the collision of two neutron stars, waves of radiation and subatomic particles are propelled outward from the nascent black hole and collide with one another, releasing the gamma radiation. Also released is longer-lasting electromagnetic radiation in the form of X rays, radio waves, and visible wavelengths that can be used to pinpoint the location of the disturbance.

Because light and other forms of energy and matter are permanently trapped inside a black hole, it can never be observed directly. However, a black hole can be detected by the effect of its gravitational field on nearby objects (e.g., if it is orbited by a visible star), during the collapse while it was forming, or by the X rays and radio frequency signals emitted by rapidly swirling matter being pulled into the black hole. A small number of possible black holes have been detected. The first discovered (1971) was Cygnus X-1, an X-ray source in the constellation Cygnus. In 1994 astronomers employing the Hubble Space Telescope announced that they had found conclusive evidence of a supermassive black hole in the M87 galaxy in the constellation Virgo. The first evidence (2002) of a binary black hole, two supermassive black holes circling one another, was detected in images from the orbiting Chandra X-ray Observatory. Located in the galaxy NGC6240, the pair are 3,000 light years apart, travel around each other at a speed of about 22,000 mph (35,415 km/hr), and have the mass of 100 million suns each. As the distance between them shrinks over 100 million years, the circling speed will increase until it approaches the speed of light, about 671 million mph (1080 million km/hr). The black holes will then collide spectacularly, spewing radiation and gravitational waves across the universe.

问题：我们能否看到黑洞？

~藤原拓海~

我想知道科学家是怎样拍到黑洞的?

capricornus_tai

原帖由 timothylim 于 25-2-2006 01:52 PM 发表
问题：我们能否看到黑洞？

原帖由 ~藤原拓海~ 于 26-2-2006 10:28 AM 发表
我想知道科学家是怎样拍到黑洞的?

事实上已上面的理论来说，黑洞是看不到的，但是科学家却拍到黑洞。
根据我的印象，科学家拍到的是光与星体在一堆黑暗中消失，因此引发黑洞的猜测。也就是说科学家拍到黑洞吞噬的过程，也拍到黑洞，只不过是看不到。
我也不大确定。

[ 本帖最后由 capricornus_tai 于 26-2-2006 04:57 PM 编辑 ]