听力原文：P Well, were going to be talking about a special property of light. Actually, its

a small part of Einsteins special theory of relativity. Now, dont worry. Ill explain things really, really slowly, and use plenty of examples. OK, first, you have to understand the idea that there are things called frames of reference. Imagine that you are on an elevator. In that box, thats one frame. of reference. And as you go from the first floor to the third, you pass by some guy standing on the second floor. That second floor, thats another frame. of reference. Frame. of reference just means, then, the place where a person is standing. S1 Is there one perfect frame. of reference？ P Good question. Nope. There are no stable frames. All of them move in some way. Now listen to this. Imagine that youre in your living room and you have a block of wood. If you measure the wood sitting on your couch, or if you measure it while riding on a bus, the block of wood will have the same measurements. Let me add here that we have to assume that the bus is going down a nice straight, smooth road and at a constant speed. OK, heres the first part of what Einstein was talking about. The laws of physics are true in all frames of reference. S1 So, that block of wood is going to be the same size in your living room or on the bus？ P Right. Now youll really have to pay attention. Imagine that youre driving in a car and the car is going 50 mph. You throw a rock out the front at 20 mph. In your frame. of reference, the rock is going 20 mph. What about some guy standing by the side of the road？ Remember that hes in a different frame. of reference. Right？ Well, the speed of the car and the speed of the rock together equal 70 mph, and thats the speed hell see the rock moving. S2 What happens if youre in that same car, still going along at 50 mph, and you turn on the headlights？ Doesnt light always have the same speed？ But, wouldnt it be like the rock, the speed of light plus the speed of the car？ P Well, think about speed. Speed is distance divided by time. OK. Heres another crazy part. Distance and time change when things start getting close to the speed of light. How about distance？ Well the faster an object moves, the shorter it gets. Say you buy a really nice limousine, and you want to drive it around and show it off. Well, drive it slowly, because the faster you drive, the shorter the car will appear to the people on the street. You see the car, in your frame. of reference, as staying the same length as when you bought it. The people on the street, standing there in a different frame. of reference, see it as shorter than you do. S2 What about time？ P Lets say you have two frames of reference. One is a guy standing on the ground and the other is a guy flying in a super fast airplane. The guy on the ground sees the guy in the super fast plane as moving slower in time. Thats crazy, right？ Listen one more time. From the point of reference of the guy on the ground, the guy in the plane goes through time slower. So, how does light maintain the same speed no matter what the frame. of reference？ Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car？ Well, its because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. Its totally crazy, right？ OK. Any questions？ P Why is the light from the headlights not moving at the speed of light plus the 50 mph from the car？ Well, its because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. Its totally crazy, right？ Why does the professor say this: P Its totally crazy, right？

What is the lecture mainly about？

A．Special theory of relativity

B．A property of light

C．Definitions of speed

D．Frames of reference