廣義相對論的基礎
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廣義相對論是愛因斯坦繼狹義相對論之后，深入研究引力理論，于1913年提出的引力場的相對論理論。這一理論完全不同于牛頓的引力論，它把引力場歸結為物體周圍的時空彎曲，把物體受引力作用而運動，歸結為物體在彎曲時空中沿短程線的自由運動。因此，廣義相對論亦稱時空幾何動力學，即把引力歸結為時空的幾何特性。
如何理解廣義相對論的時空彎曲呢？這里我們借用一個模型式的比擬來加以說明。假如有兩個質(zhì)量很大的鋼球，按牛頓的看法，它們因萬有引力相互吸引，將彼此接近。而愛因斯坦的廣義相對論則并不認為這兩個鋼球間存在吸引力。它們之所以相互靠近，是由于沒有鋼球出現(xiàn)時，周圍的時空猶如一張拉平的網(wǎng)，現(xiàn)在兩個鋼球把這張時空網(wǎng)壓彎了，于是兩個鋼球就沿著彎曲的網(wǎng)滾到一起來了。這就相當于因時空彎曲物體沿短程線的運動。所以，愛因斯坦的廣義相對論是不存在“引力”的引力理論。
進一步說，這個理論是建立在等效原理及廣義協(xié)變原理這兩個基本假設之上的。等效原理是從物體的慣性質(zhì)量與引力質(zhì)量相等這個基本事實出發(fā)，認為引力與加速系中的慣性力等效，兩者原則上是無法區(qū)分的；廣義協(xié)變原理，可以認為是等效原理的一種數(shù)學表示，即認為反映物理規(guī)律的一切微分方程應當在所有參考系中保持形式不變，也可以說認為一切參考系是平等的，從而打破了狹義相對論中慣性系的特殊地位，由于參考系選擇的任意性而得名為廣義相對論。
我們知道，牛頓的萬有引力定律認為，一切有質(zhì)量的物體均相互吸引，這是一種靜態(tài)的超距作用。
在廣義相對論中物質(zhì)產(chǎn)生引力場的規(guī)律由愛因斯坦場方程表示，它所反映的引力作用是動態(tài)的，以光速來傳遞的。
廣義相對論是比牛頓引力論更一般的理論，牛頓引力論只是廣義相對論的弱場近似。所謂弱場是指物體在引力場中的引力能遠小于固有能，力場中，才顯示出兩者的差別，這時必須應用廣義相對論才能正確處理引力問題。
廣義相對論在1915年建立后，愛因斯坦就提出了可以從三個方面來檢驗其正確性，即所謂三大實驗驗證。這就是光線在太陽附近的偏折，水星近日點的進動以及光譜線在引力場中的頻移，這些不久即為當時的實驗觀測所證實。以后又有人設計了雷達回波時間延遲實驗，很快在更高精度上證實了廣義相對論。60年代天文學上的一系列新發(fā)現(xiàn)：3K微波背景輻射、脈沖星、類星體、X射電源等新的天體物理觀測都有力地支持了廣義相對論，從而使人們對廣義相對論的興趣由冷轉熱。特別是應用廣義相對論來研究天體物理和宇宙學，已成為物理學中的一個熱門前沿。
愛因斯坦一直把廣義相對論看作是自己一生中最重要的科學成果，他說過，“要是我沒有發(fā)現(xiàn)狹義相對論，也會有別人發(fā)現(xiàn)的，問題已經(jīng)成熟。但是我認為，廣義相對論不一樣。”確實，廣義相對論比狹義相對論包含了更加深刻的思想，這一全新的引力理論至今仍是一個最美好的引力理論。沒有大膽的革新精神和不屈不撓的毅力，沒有敏銳的理論直覺能力和堅實的數(shù)學基礎，是不可能建立起廣義相對論的。偉大的科學家湯姆遜曾經(jīng)把廣義相對論稱作為人類歷史上最偉大的成就之一。
英文： the basis of general relativity
Following the general theory of relativity Einstein's special theory of relativity are after-depth study of the gravitational theory, in 1913 proposed the relativistic gravitational field theory. This theory is completely different from Newton's gravitational theory, it comes down to the gravitational field of space-time bending around objects, put objects by the gravitational effects of exercise, reduced to objects in space and time along the curved lines of free short-term exercise. Thus, also known as space-time geometry of general relativity dynamics, that is attributed to the gravitational properties of space-time geometry.
How to understand general relativity space-time bending it? Here we use a model of analogy to illustrate. If both have a lot of quality ball, according to Newton's view, they attract each other due to gravity will be near each other. Einstein's general theory of relativity and then do not think that exist between the two ball Attraction. They are close to each other because we do not have the ball appears, the space-time around the Net like a draw, and now put two balls this time and space bending Net, so the ball both on the Net roll bending down up to 1. This is equivalent to a result of bending space-time line of objects along the short-range exercise. Therefore, Einstein's general theory of relativity there is no "gravity" theory of gravity.
Furthermore, this theory is built on the equivalence principle and the principle of the generalized covariant basic assumptions on which the two. Objects are from the equivalence principle of inertial mass and gravitational mass equal to the basic facts that the Department of gravity and speed up the equivalent of the inertial force, which in principle can not distinguish; generalized covariant principle of equivalence principle can be considered a math that the physical laws that reflect all the differential equations at all the reference system should be to maintain the same form can also be said that the reference system are all equal, thus breaking the special theory of relativity in the special status of inertial system, because reference system of the arbitrary selection of the above-mentioned known as general relativity.
We know that Newton's law of gravity that all objects have the quality of both the mutual attraction, which is a static role of ultra-distance.
General relativity in the material produced in the gravitational field of the law by the Einstein field equations that reflect the gravity of its role is dynamic, with the speed of light to convey.
General relativity is Newton's gravitational theory than the more general theory, Newton's gravitational theory of general relativity is only a weak field approximation. Refers to the so-called weak-field gravitational objects in the gravitational field can be far less than the intrinsic energy, force field, the only difference between the two shows, when the application of general relativity must be right to deal with the gravitational problem.
Set up in 1915 in general relativity, the Einstein proposed that can be tested from three of its correctness, the so-called three experiments. This is the light deflection near the sun, Mercury precession, as well as spectral lines in the gravitational field of the frequency shift, which at that time is soon confirmed by the experimental observation. It was designed after the time delay of radar echo experiments soon confirmed on a more high-precision general relativity. 60's on a series of astronomical discoveries: 3K microwave background radiation, pulsars, quasars, X-launched a new power supply, such as astrophysical observations have vigorously supported the general theory of relativity, so that the interest of the general theory of relativity from cold to hot . In particular the application of general relativity to study astrophysics and cosmology, physics has become a popular front.
Einstein's general theory of relativity has put his own life as the most important scientific achievements, he said, "If I do not see the special theory of relativity, would have found someone else, the question is ripe. But in my opinion, not the same as general relativity . "Indeed, the general theory of relativity special theory of relativity contains more than deep thought, the new theory of gravity is still a best theory of gravity. NOT bold innovation and indomitable perseverance, there is no sharp instincts and a solid theory of basic math, it is impossible to establish a general theory of relativity. Thomson great scientist once said that as a general theory of relativity put up the history of mankind's greatest achievements.
