Similarly, the past of P can be defined as the set of all events from which it is possible to reach the event P traveling at or below the speed of light. It is thus the set of events that can affect what happens at P. The events that do not lie in the future or past of P are said to lie in the elsewhere of P(Fig. 2.5). What happens at such events can neither affect nor be affected by what happens at P. For example, if the sun were to cease to shine at this very moment, it would not affect things on earth at the present time because they would be in the elsewhere of the event when the sun went out (Fig. 2.6).
We would know about it only after eight minutes, the time it takes light to reach us from the sun. Only then would events on earth lie in the future light cone of the event at which the sun went out. Similarly, we do not know what is happening at the moment farther away in the universe: the light that we see from distant galaxies left them millions of years ago, and in the case of the most distant object that we have seen, the light left some eight thousand million years ago. Thus, when we look at the universe, we are seeing it as it was in the past.
If one neglects gravitational effects, as Einstein and Poincare did in 1905, one has what is called the special theory of relativity. For every event in space-time we may construct alight cone (the set of all possible paths of light in space-time emitted at that event), and since the speed of light is the same at every event and in every direction, all the light cones will be identical and will all point in the same direction. The theory also tells us that nothing can travel faster than light. This means that the path of any object through space and time must be represented by a line that lies within the light cone at each event on it (Fig. 2.7). The special theory of relativity was very successful in explaining that the speed of light appears the same to all observers (as shown by the Michelson-Morley experiment) and in describing what happens when things move at speeds close to the speed of light. However, it was inconsistent with the Newtonian theory of gravity, which said that objects attracted each other with a force that depended on the distance between them. This meant that if one moved one of the objects, the force on the other one would change instantaneously. Or in other gravitational effects should travel with infinite velocity, instead of at or below the speed of light, as the special theory of relativity required. Einstein made a number of unsuccessful attempts between 1908 and 1914 to find a theory of gravity that was consistent with special relativity. Finally, in 1915, he proposed what we now call the general theory of relativity.
Emit 發出
Instantaneously 即刻地
類似地，P的過去可被定義為下述的所有事件的集合，從這些事件可以等於或小於光速的速度運動到達事件P。這樣，它就是能影響發生在P的東西的所有事件的集合。不處於P的未來或過去的事件被稱之為處於P的他處（圖2.5）。在這種事件處所發生的東西既不能影響發生在P的東西，也不受發生在P的東西的影響。例如，假定太陽就在此刻停止發光，它不會對此刻的地球發生影響，因為地球的此刻是在太陽熄滅這一事件的光錐之外（圖2.6）。我們只能在8分鐘之後才知道這一事件，這是光從太陽到達我們所花的時間。只有到那時候，地球上的事件才在太陽熄滅這一事件的將來光錐之內。同理，我們也不知道這一時刻發生在宇宙中更遠地方的事：我們看到的從很遠星系來的光是在幾百萬年之前發出的，在我們看到最遠物體的情況下，光是在80億年前發出的。這樣當我們看宇宙時，我們是在看它的過去。
如果人們忽略引力效應，正如1905年愛因斯坦和彭加勒所做的那樣，人們就得到了稱為狹義相對論的理論。對於時空中的每一事件我們都可以做一個光錐（所有從該事件發出的光的可能軌跡的集合），由於在每一事件處在任一方向的光的速度都一樣，所以所有光錐都是全等的，並朝著同一方向。這理論又告訴我們，沒有東西走得比光更快。這意味著，通過空間和時間的任何物體的軌跡必須由一根落在它上面的每一事件的光錐之內的線來表示（圖2.7）。
狹義相對論非常成功地解釋瞭如下事實：對所有觀察者而言，光速都是一樣的（正如麥克爾遜——莫雷實驗所展示的那樣），併成功地描述了當物體以接近於光速運動時的行為。然而，它和牛頓引力理論不相協調。牛頓理論說，物體之間的吸引力依賴於它們之間的距離。這意味著，如果我們移動一個物體，另一物體所受的力就會立即改變。或換言之，引力效應必須以無限速度來傳遞，而不像狹義相對論所要求的那樣，只能以等於或低於光速的速度來傳遞。愛因斯坦在1908年至1914年之間進行了多次不成功的嘗試，企圖去找一個和狹義相對論相協調的引力理論。1915年，他終於提出了今天我們稱之為廣義相對論的理論。
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