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本文摘要:You might think that physicists would be satisfied by now. They have been testing Einstein’s theory of general relativity, which explains what gravity is, ever since he first described it 100 years ago this year. And not once has it been found wanting. But they are still investigating its predictions to the nth decimal place, and this centenary year should see some particularly stringent tests. Perhaps one will uncover the first tiny flaw in this awesome mathematical edifice.你有可能以为,物理学家现在早已失望了。


You might think that physicists would be satisfied by now. They have been testing Einstein’s theory of general relativity, which explains what gravity is, ever since he first described it 100 years ago this year. And not once has it been found wanting. But they are still investigating its predictions to the nth decimal place, and this centenary year should see some particularly stringent tests. Perhaps one will uncover the first tiny flaw in this awesome mathematical edifice.你有可能以为,物理学家现在早已失望了。他们仍然在对爱因斯坦的广义相对论展开检验。爱因斯坦在整整100年前第一次明确提出了广义相对论,它说明了引力是什么。


或许不会有人找到这座非凡数学大厦的第一个微小缺失。Stranger still is that, though general relativity is celebrated and revered among physicists like no other theory in science, they would doubtless react with joy if it is proved to fail. That’s science: You produce a smart idea and then test it to its breaking point.更加奇怪的是,虽然在物理学家中,广义相对论取得的赞美和推崇多达了所有其他科学理论,但如果检验证明它站不住脚,他们毫无疑问不会深感伤心。

这就是科学:你明确提出了一个聪慧的点子,然后检验它至无限大。But this determination to expose flaws isn’t really about skepticism, far less wanton nihilism. Most physicists are already convinced that general relativity is not the final word on gravity. That’s because the theory, which is applied mostly at the scale of stars and galaxies, doesn’t mesh with quantum theory, the other cornerstone of modern physics, which describes the ultra-small world of atoms and subatomic particles. It’s suspected that underlying both theories is a theory of quantum gravity, from which general relativity and conventional quantum theory emerge as excellent approximations just as Isaac Newton’s theory of gravity, posed in the late 17th century, works fine except in some extreme situations.但是说明了该理论缺失的这种决意,只不过无牵涉到猜测主义,和大肆的虚无主义堪称相比之下扯不上关系。大多数物理学家早已相信,广义相对论并不是引力的最后定论。

这是因为该理论主要应用于在恒星和星系的规模,和量子理论没空集。量子理论是现代物理学的另一块基石,针对的是原子和亚原子粒子级别的微观世界。科学家们实在,这两个基本理论的相结合是一个量子引力理论,广义相对论和常规量子理论是它的绝佳近似值,这就像艾萨克·牛顿在17世纪后期明确提出的万有引力理论,除某些极端情况外,应用于一起一般来说都没问题。The hope is, then, that if we can find some dark corner of the universe where general relativity fails, perhaps because the gravitational fields it describes are so enormously strong, we might glimpse what extra ingredient is needed — one that might point the way to a theory of quantum gravity.科学家的期望是,如果能寻找广义相对论站不住脚的一些黑暗角落——这有可能是因为它叙述的引力场如此强劲——那么我们也许不会找到它缺乏了哪些成分,而这可能会说明通向量子引力理论的道路。

General relativity was not just the last of Einstein’s truly magnificent ideas, but arguably the greatest of them. His “annus mirabilis” is usually cited as 1905, when, among other things, he kick-started quantum theory and came up with special relativity, describing the distortion of time and space caused by traveling close to the speed of light. General relativity offered a broader picture, embracing motion that changes speed, such as objects accelerating as they fall in a gravitational field. Einstein explained that gravity can be thought of as curvature induced in the very fabric of time and space by the presence of a mass. This, too, distorts time: Clocks run slower in a strong gravitational field than they do in empty space. That’s one prediction that has now been thoroughly confirmed by the use of extremely accurate clocks on space satellites, and in fact GPS systems have to adjust their clocks to allow for it.广义相对论某种程度是爱因斯坦最后一个宏大点子,而且可以说道是他最最出色的设想。他的“奇迹年”一般来说被指出是1905年,这一年他开始设想量子理论,并明确提出了狭义相对论,叙述了相似光速的运动造成的时空变形。广义相对论则刻画了更为辽阔的画面,探究了变速运动,比如物体在转入引力场时经常出现的加快。


事实上,GPS系统必需考虑到这种影响,来调整自己的时钟。Einstein presented his theory of general relativity to the Prussian Academy of Sciences in 1915, though it wasn’t officially published until the following year. The theory also predicted that light rays will be bent by strong gravitational fields. In 1919 the British astronomer Arthur Eddington confirmed that idea by making careful observations of the positions of stars whose light passes close to the sun during a total solar eclipse. The discovery assured Einstein as an international celebrity. When he met Charlie Chaplin in 1931, Chaplin is said to have told Einstein that the crowds cheered them both because everyone understood him and no one understood Einstein.爱因斯坦1915年向普鲁士科学院(Prussian Academy of Sciences)递交了广义相对论的论文,不过月公开发表是在第二年。

该理论还预测,强劲的引力场不会造成光的倾斜。在1919年,英国天文学家亚瑟·爱丁顿(Arthur Eddington)通过仔细观察一次日全食中一些恒星的方位,证实了这一预测,这些恒星的光线不会通过邻近太阳的区域。爱因斯坦自此沦为国际名人。当他在1931年与亨利·卓别林(Charlie Chaplin)见面时,据传卓别林对他说道,公众为他们两人掌声,是因为每个人都解读自己的电影,但没一个人解读爱因斯坦的理论。

General relativity predicts that some burned-out stars will collapse under their own gravity. They might become incredibly dense objects called neutron stars only a few miles across, from which a teaspoon of matter would weigh 10 billion tons. Or they might collapse without limit into a “singularity” — a black hole from whose immense gravitational field not even light can escape, since the surrounding space is so bent that light just turns back on itself.广义相对论应验,一些燃料消耗的恒星将因自身引力而坍塌。它们被称作中子星,其密度可能会显得十分之大,直径只有几英里,但一小勺就有100亿吨。

或者可能会无限地坍塌下去,变为“奇点”,也就是一个黑洞,其极大引力场甚至连光都无法逃离现场,因为周围的空间过于过倾斜,光不会必要弯道返回原地。Many neutron stars have been seen by astronomers: Some, called pulsars, rotate and send out beams of intense radio waves from their magnetic poles, beams that flash on and off with precise regularity. Black holes can only be seen indirectly from the X-rays and other radiation emitted by the hot gas that surrounds and is sucked into them. But astrophysicists are certain that they exist.自那之后,天文学家找到了很多中子星:有些被称作脉冲星,它们转动运动,从磁极升空出有反感的电波,升空和暂停不存在着精准的规律性。黑洞不能通过X射线和热气体弥漫的其他电磁辐射被间接看见,黑洞被这些热气体围困着,并将它们排出。但是天体物理学家深信黑洞是不存在的。

While Newton’s theory of gravity is mostly good enough to describe the motions of the solar system, it is around very dense objects like pulsars and black holes that general relativity becomes indispensable. That’s also where it might be possible to test the limits of the theory with astronomical investigations. Last year, astronomers at the National Radio Astronomy Observatory in Charlottesville, Virginia, discovered the first pulsar orbited by two other shrunken stars, called white dwarfs. This situation, with two bodies moving in the gravitational field of a third, should allow one of the central pillars of general relativity, called the strong equivalence principle, to be put to the test by making very detailed measurements of the effects of the white dwarfs on the pulsar’s metronome flashes as they circulate. The team hopes to carry out that study this year.虽然牛顿的引力理论基本上不足以叙述太阳系的运动,但对于密度很大的物体,比如脉冲星和黑洞,广义相对论就不可或缺了。这也是用天文研究检验这个理论的局限的地方。去年在弗吉尼亚州夏洛茨维尔,国家射电天文台(National Radio Astronomy Observatory)的天文学家找到了一颗脉冲星,绕着它运动的另外两颗增大的恒星被称作白矮星,而这一现象是前所未见的。

在这种情况下,有两个星体在第三个的引力场中运动,如果在白矮星绕行脉冲星运动的时候,十分精细地测量它们对脉冲星电波升空规律的影响,应当可以检验广义相对论的核心支柱之一“强劲等效原理”。该团队期望今年积极开展这项研究。But the highest-profile test of general relativity is the search for gravitational waves. The theory predicts that some astrophysical processes involving very massive bodies, such as supernovae (exploding stars) or pulsars orbited by another star (binary pulsars), should excite ripples in space-time that radiate outwards as waves. The first binary pulsar was discovered in 1974, and we now know the two bodies are getting slowly closer at just the rate expected if they are losing energy by radiating gravitational waves.但最引人注目的广义相对论检验是对引力波的找寻。该理论预测,一些十分可观的星体,比如超新星(发生爆炸的恒星)或者被另一颗恒星环绕飞过的脉冲星(脉冲双星),和它们有关的天体物理过程应当在时空中唤起涟漪,像波一样向外电磁辐射。

第一个脉冲双星是在1974年找到的,科学家假设两个星体电磁辐射了引力波,因而损耗了能量,计算出来出有了它们投向的速率,我们现在早已告诉,它们显然在以这个速率渐渐投向。The real goal, though, is to see such waves directly from the tiny distortions of space that they induce as they ripple past our planet. Gravitational-wave detectors use lasers bouncing off mirrors in two-kilometer-long arms at right angles, like an L, to measure such minuscule contractions or stretches. Two of the several gravitational-wave detectors currently built — the American LIGO, with two observatories in Louisiana and Washington, and the European VIRGO in Italy — have just been upgraded to boost their sensitivity, and both will start searching in 2015. The European Space Agency is also launching a pilot mission for a space-based detector, called LISA Pathfinder, this September.不过,确实的目标是,当这些波经过我们的星球时,必要从它们造成的微小空间变形中看见它们。

引力波探测器让激光在长两公里、摆成L形的干预臂上往返光线,从而对这种微小膨胀或扩展展开测量。目前世界上许多台引力波探测器,其中两台——美国的LIGO,在路易斯安那州和华盛顿有两个观察站;以及欧洲的VIRGO,坐落于意大利——刚对灵敏性展开了升级,它们都将在2015年开始找寻引力波。去年9月,欧洲航天局还用太空中的LISA Pathfinder探测器积极开展了一个试点任务。

If we’re lucky, then, 2015 could be the year we confirm both the virtues and the limits of general relativity. But neither will do much to alter the esteem with which it is regarded. The Austrian-Swiss physicist Wolfgang Pauli called it “probably the most beautiful of all existing theories.” Many physicists (including Einstein himself) believed it not so much because of the experimental tests but because of what they perceived as its elegance and simplicity. Anyone working on quantum gravity knows that it is a very hard act to follow.幸运地的话,2015年就不会是我们证实广义相对论优势和局限性的一年。但这会对它受到的尊崇产生过于大影响。奥地利-瑞士物理学家沃尔夫冈·泡利(Wolfgang Pauli)称之为广义相对论“有可能是现有理论中美丽的”。



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