An event is something that happens at a particular point in space and at a particular time. So one can specify it by four numbers or coordinates. Again, the choice of coordinates is arbitrary; one can use any three well-defined spatial coordinates and any measure of time. In relativity, there is no real distinction between the space and time coordinates, just as there is no real difference between any two space coordinates.
One could choose a new set of coordinates in which, say, the first space coordinate was a combination of the old first and second space coordinates. For instance, instead of measuring the position of a point on the earth in miles north of Piccadilly and miles west of Piccadilly, one could use miles northeast of Piccadilly, and miles north-west of Piccadilly. Similarly, in relativity, one could use a new time coordinate that was the old time (in seconds) plus the distance (in light-seconds) north of Piccadilly.
It is often helpful to think of the four coordinates of an event as specifying its position in a four-dimensional space called space-time. It is impossible to imagine a four-dimensional space. I personally find it hard enough to visualize three-dimensional space! However, it is easy to draw diagrams of two-dimensional spaces, such as the surface of the earth.
(The surface of the earth is two-dimensional because the position of a point can be specified by two coordinates, latitude and longitude.) I shall generally use diagrams in which time increases upward and one of the spatial dimensions is shown horizontally. The other two spatial dimensions are ignored or, sometimes, one of them is indicated by perspective. (These are called space-time diagrams, like Fig. 2.1.) For example, in Fig.2.2 time is measured upward in years and the distance along the line from the sun to Alpha Centauri is measured horizontally in miles. The paths of the sun and of Alpha Centauri through space-time are shown as the vertical lines on the left and right of the diagram. A ray of light from the sun follows the diagonal line, and takes four years to get from the sun to Alpha Centauri.
Diagram 圖表
Latitude 緯度
Longitude 經度
一個事件是發生於特定時刻和空間中特定的一點的某種東西。這樣，人們可以用四個數或座標來確定它，並且座標系的選擇是任意的；人們可以用任何定義好的空間座標和一個任意的時間測量。在相對論中，時間和空間座標沒有真正的差別，猶如任何兩個空間座標沒有真正的差別一樣。譬如可以選擇一族新的座標，使得第一個空間座標是舊的第一和第二空間座標的組合。例如，測量地球上一點位置不用在倫敦皮卡迪裡圓環以北和以西的裡數，而是用在它的東北和西北的裡數（1英里=1.609公里）。類似地，人們在相對論中可以用新的時間座標，它是舊的時間（以秒作單位）加上往北離開皮卡迪裡的距離（以光秒為單位）。
將一個事件的四座標作為在所謂的時空的四維空間中指定其位置的手段經常是有助的。對我來說，摹想三維空間已經足夠困難！然而很容易畫出二維空間圖，例如地球的表面。（地球的表面是兩維的，因為它上面的點的位置可以用兩個座標，例如緯度和經度來確定。）通常我將使用二維圖，向上增加的方向是時間，水平方向是其中的一個空間座標。不管另外兩個空間座標，或者有時用透視法將其中一個表示出來。（這些被稱為時空圖，如圖2.1所示。）例如，在圖2.2中時間是向上的，並以年作單位，而沿著從太陽到α-半人馬座連線的距離在水平方向上以英哩來測量。太陽和α-半人馬座通過時空的途徑是由圖中的左邊和右邊的垂直線來表示。從太陽發出的光線沿著對角線走，並且要花4年的時間才能從太陽走到α-半人馬座。
閱讀更多 聰哥記單詞 的文章
