Why does the professor mention driving a car while throwing a rock?

P Well, we’re going to be talking about a special property of light. Actually, it’s a small part of Einstein’s special theory of relativity. Now, don’t worry. I’ll 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, that’s 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, that’s 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 you’re 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, here’s 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 you’ll really have to pay attention. Imagine that you’re 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 he’s in a different frame of reference. Right? Well, the speed of the car and the speed of the rock together equal 70 mph, and that’s the speed he’ll see the rock moving.
S2 What happens if you’re in that same car, still going along at 50 mph, and you turn on the headlights? Doesn’t light always have the same speed? But, wouldn’t 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. Here’s 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 Let’s 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. That’s 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, it’s because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It’s 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, it’s because distance and time, the things we use to measure speed, begin to change once we start to move really, really quickly. It’s totally crazy, right?
Why does the professor say this:
P It’s totally crazy, right?