* Then we will use the speed of light to measure distance as a much more accurate method.
* a pulse of light/electrical current/sound ...的脉冲
pulse waves
e.g. The switch works by passing a pulse of current between the tip and the surface.
* In a diagram (actually we can only produce a three-dimensional one by putting the spatial dimension horizontally) a cone can well depict the future and the past of a pulse of light based on space-time.
* The deduction of the event P
The other events in the universe can be divided into 3 classes: the future of P/the past of P/elsewhere
For example:
If the sun were to cease to shine at this very moment, then it would not affect anything presently on the earth as they would be in elsewhere when the sun went out
(well, it sounds incredible just because it has gone out of my commonsense view....but I think I will manage to go through it. hhh my outlook to see the world is tottering...)
Only after 8 minutes, the time it takes for the light of the sun to reach the earth, will everything on the earth lie in the future light cone of the event when the sunlight went out.
In this sense, when we look into the universe we're simply looking into its past.
* warp 使变形
fig. His judgment was warped by prejudice.
* geodesic （连接曲面两点的）最短线的；测地线的
* Space-time is not flat. Instead, it is curved by the mass and energy inside the Sun. The earth would spontaneously follow the nearest thing to a straight path in the curved space which is called a geodesic (line). In general relativity it actually moves along a straight line but in three-dimensional space it follows a curved path.
If this is hard to imagine, just think about the aeroplane. Though it might follow a straight line as the shortest path, it always looks like moving along a curved one if we observe it from the two-dimensional ground.
* elongated 细长的，拉长的
* ellipse 椭圆
* 这边复习一下apparent。这篇文章就用到了上次整理的seeming之意的apparent。
They therefore change their apparent position relative to other stars.
* The light is actually deflected by the sun.
* ionic 离子的
* advent 出现，到来
e.g. the advent of new technologies
* There is a relation between the energy and the frequency of the light. The greater the energy, the higher the frequency.
As the light travels to the gravitational field of the earth, it loses energy, so its frequency goes down.
So to someone high up, things in below may appear to happen slower.
In this sense, here comes the twins paradox:
if one of the twins went on a trip in a spaceship at the speed of the light, then he would be much younger than the other one when he came back.
* curvature 弯曲，曲度
