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
正如我们已经看到的，麦克斯韦方程预言，不管光源的速度如何，光速应该是一样的，这已被精密的测量所证实。这样，如果有一个光脉冲从一特定的空间的点在一特定的时刻发出，在时间的进程中，它就会以光球面的形式发散开来，而光球面的形状和大小与源的速度无关。在100万分之1秒后，光就散开成一个半径为300米的球面；100万分之2秒后，半径变成600米；等等。这正如同将一块石头扔到池塘里，水表面的涟漪向四周散开一样，涟漪以圆周的形式散开并越变越大。如果将三维模型设想为包括二维的池塘水面和一维时间，这些扩大的水波的圆圈就画出一个圆锥，其顶点即为石头击到水面的地方和时间（图2.3）。类似地，从一个事件散开的光在四维的时空里形成了一个三维的圆锥，这个圆锥称为事件的未来光锥。
以同样的方法可以画出另一个称之为过去光锥的圆锥，它表示所有可以用一光脉冲传播到该事件的事件的集合（图2.4）。
对于给定的事件P，人们可以将宇宙中的其他事件分成三类。从事件P出发由一个粒子或者波以等于或小于光速的速度运动能到达的那些事件称为属于P的未来。它们处于从事件P发射的膨胀的光球面之内或之上。这样，在时空图中它们处于P的未来光锥的里面或上面。因为没有任何东西比光走得更快，所以在P所发生的东西只能影响P的未来的事件。
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