As we have seen, Maxwell’s equations predicted that the speed of light should be the same whatever the speed of the source, and this has been confirmed by accurate measurements.
It follows from this that if a pulse of light is emitted at a particular time at a particular point in space, then as time goes on it will spread out as a sphere of light whose size and position are independent of the speed of the source. After one millionth of a second the light will have spread out to form a sphere with a radius of 300 meters; after two millionths of a second, the radius will be 600 meters; and so on. It will be like the ripples that spread out on the surface of a pond when a stone is thrown in. The ripples spread out as a circle that gets bigger as time goes on. If one stacks snapshots of the ripples at different times one above the other, the expanding circle of ripples will mark out a cone whose tip is at the place and time at which the stone hit the water (Fig. 2.3). Similarly, the light spreading out from an event forms a(three-dimensional) cone in (the four-dimensional) space-time.
This cone is called the future light cone of the event. In the same way we can draw another cone, called the past light cone, which is the set of events from which a pulse of light is able to reach the given event (Fig. 2.4).
Given an event P, one can divide the other events in the universe into three classes. Those events that can be reached from the event P by a particle or wave traveling at or below the speed of light are said to be in the future of P. They will lie within or on the expanding sphere of light emitted from the event P. Thus they will lie within or on the future light cone of P in the space-time diagram. Only events in the future of P can be affected by what happens at P because nothing can travel faster than light.
Pond 池塘
正如我們已經看到的，麥克斯韋方程預言，不管光源的速度如何，光速應該是一樣的，這已被精密的測量所證實。這樣，如果有一個光脈衝從一特定的空間的點在一特定的時刻發出，在時間的進程中，它就會以光球面的形式發散開來，而光球面的形狀和大小與源的速度無關。在100萬分之1秒後，光就散開成一個半徑為300米的球面；100萬分之2秒後，半徑變成600米；等等。這正如同將一塊石頭扔到池塘裡，水錶面的漣漪向四周散開一樣，漣漪以圓周的形式散開並越變越大。如果將三維模型設想為包括二維的池塘水面和一維時間，這些擴大的水波的圓圈就畫出一個圓錐，其頂點即為石頭擊到水面的地方和時間（圖2.3）。類似地，從一個事件散開的光在四維的時空裡形成了一個三維的圓錐，這個圓錐稱為事件的未來光錐。
以同樣的方法可以畫出另一個稱之為過去光錐的圓錐，它表示所有可以用一光脈衝傳播到該事件的事件的集合（圖2.4）。
對於給定的事件P，人們可以將宇宙中的其他事件分成三類。從事件P出發由一個粒子或者波以等於或小於光速的速度運動能到達的那些事件稱為屬於P的未來。它們處於從事件P發射的膨脹的光球面之內或之上。這樣，在時空圖中它們處於P的未來光錐的裡面或上面。因為沒有任何東西比光走得更快，所以在P所發生的東西只能影響P的未來的事件。
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