Physics Primer

物理入门

Simultaneity

同时性

Two events are simultaneous if they happenat the same time. However, in Einstein’s theory of relativity things get morecomplicated, so it is important to distinguish between these scenarios:

两个事件同时发生是同时发生的。然而，在爱因斯坦的相对论中，事物变得更加复杂，因此区分这些场景是很重要的：

1. Local Simultaneity: Two eventshappen at the same time, in the same place. Local Simultaneity is the only typeof simultaneity which is directly observable.

1. 地方同时性：两个事件同时发生在同一地点。局部同时性是直接观察到的唯一同时发生的类型。

2. Simultaneity at a distance: Twoevents happen at the same time, but in different places. This type ofsimultaneity cannot be directly observed because an observer cannot be in twoplaces at the same time. How can we infer that the two events weresimultaneous? Imagine that two local observes write down the time when twoevents A and B take place. Because each of the observers is at the place whereone of these events happens, they can simply write down the time. However, theycannot simply compare their time measurements because their clocks might notagree with each other. They first need to coordinate their clocks to make surethe clocks show the same thing at the same time.

2. 同时发生：两个事件同时发生，但发生在不同的地方。这种类型的同时性不能直接观察到，因为观察者不能同时在两个地方。我们怎么能推断这两件事同时发生呢？想象一下，两个本地观察者写下A和B两个事件发生的时间。因为每一个观察者都在某个事件发生的地方，他们可以简单地记录下时间。然而，他们不能简单地比较他们的时间测量，因为他们的时钟可能不一致。他们首先需要协调他们的时钟，以确保时钟显示相同的东西在同一时间内。

3. Simultaneity for moving frames:Would two events, that are simultaneous for an observer at rest, still besimultaneous for a moving observer? In classical mechanics the answer is yes,but things change radically in the case of Einstein's theory of relativity.We'll see why in the next lesson.

3. 移动帧的同时性：对于静止的观测者来说，两个同时发生的事件是否同时是一个移动观察者的同时？在经典力学中，答案是肯定的，但在爱因斯坦相对论的情况下，事物发生了根本性的变化。我们将在下一课中看到为什么。

Events

事件

We call events localized phenomena thattake place at one point in space and time, e.g. an explosion. One, most often,cannot see the event itself but only its effects. If an observer only hears thesound of an explosion that took place far away, he is not directly observingthe event of the explosion. Of course, the observer hearing the sound of theexplosion is also an event but should not be mistaken with the initial event ofthe explosion.

我们称事件为局部化现象，发生在空间和时间的某一点，例如爆炸。一个，最常见的，看不到事件本身，但只有它的效果。如果观察者只听到远处发生的爆炸声，他不会直接观察爆炸事件。当然，观察员听到爆炸声也是一个事件，但不应误以为爆炸的最初事件。

Events are independent of any observer, buttheir position in space and time will depend on the reference frame of theobserver. Think about a dot on a page. The existence of the dot is independentof any coordinate system, but if we want to describe its position we need tochoose a system of coordinates. However, the distance between two dots isindependent of any system of coordinates. Does that mean that the distancebetween two events in space-time is also independent of any coordinate system?The lessons that follow will allow you to answer that question.

事件独立于任何观察者，但它们在空间和时间上的位置将取决于观测者的参考帧。思考一页上的一个点。点的存在与任何坐标系无关，但如果我们要描述它的位置，就需要选择一个坐标系。然而，两个点之间的距离与任何坐标系无关。这是否意味着时空中两个事件之间的距离也与任何坐标系统无关？接下来的课程将会为你回答这个问题。

思考：

An observer hears the noise from twoexplosions at the same time. Can he conclude that the explosions took place at the same, i.e. that they were simultaneous?

观察员同时听到两起爆炸声。他能断定爆炸是在同一时间发生的，也就是说它们的发生具有同时性？