Science Weblecture for Unit 56
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Relativity asks the question: "What does it mean for two events to be simultaneous?" It recognizes that different observers use different ways of measuring time and location from some point of reference. Einstein's thought experiments involved observers who where stationary (not moving) relative to one another but at different distances from an event. He realized that because light travels at a finite speed, the two observers would see the event at different times. In normal life, where people are relatively close to one another, the small differences in observations does not affect our calculations or general conclusions, but when we start looking at objects on very large or very small scales, or try to reconcile the experiences of observers moving at very different speeds, classical mechanics can no longer explain what we actually observe.
Any time we discuss velocity or acceleration, we use a frame of reference to measure distance traveled and time elapsed. The frame of reference can be any convenient set of coordinates. For example, when considering a car with an open sunroof moving down the road, we can use a frame of reference that is fixed to the road. We also pick directions -- for example, we might start counting from Main Street, positively in the north direction and negatively in the south direction.
We could also use a frame of reference fixed to the car, with a point of origin in the middle of the sunroof.
Consider now a person standing on the seat of the car with his head and arms through the sun roof as the car rolls down a deserted street at 20 mph, facing south as the car moves north. Our aspiring baseball player tosses a ball at 95mph away from the car. From his point of view, in the frame of reference of the car, the ball is moving at 95mph with respect to the car. However, from the frame of reference of a person on the street, who has to take into account both the motion of the car forward at 20mph and the motion of the ball rearward from the car at 95mph, the ball only appears to be going 75mph.
In classical mechanics, frames of reference for non-accelerating systems are equivalent. If we analyze the forces involved using either frame of reference, we will get equivalent answers once we take into account the difference in velocities of the frames of reference relative to each other.
To see how this works, study the basic concepts on Frames of Reference.
Now that you've got that down, it is time to enter the weird and wonderful world of relativity.
Read about the theory of special relativity at the Nobel Prize site. There are seven sections, one each on
As formulated, special relativity applies only to inertial reference frames. In point of fact, most systems are accelerating, so Einstein proposed a general theory of relativity to cover those situations as well.
Read about general relativity at the USC astronomy course site. [1 long page.]
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