# How Does Relativity Work?

Most recent answer: 10/22/2007

Q:
how does the theory of relativity work.
- dave stein (age 18)
u of i
A:
This is a little question with a big answer. Einstein’s special and general theories of relativity are based on the idea that the laws of physics are the same for all observers. For instance, Einstein interpreted the speed of light in a vacuum as a law of physics that should be measured the same no matter what speed you are traveling at. Imagine you are standing on the side of a road, and you see a car go by on the road at a speed of fifty miles per hour. A passenger in the car throws a baseball out the window with a speed of ten miles an hour relative to the car. As you see it, the ball moves with a velocity of sixty miles an hour relative to the ground, right? Now suppose the same passenger shines a flashlight in the direction he is traveling. Shouldn’t the speed of that light be sixty miles an hour plus the speed of light as viewed from rest? No, said Einstein. The constant speed of light is a law of nature that does not change. This is where the relative part comes in. For the speed of light to be the same in all states of motion, your view of space and time must be different from that of the man in the car. If the car started traveling at very fast speeds, the man in the car would see that everything out the window had gotten scrunched along the direction he was moving. You, standing on the road, would not feel or see these effects but see them going on in the car. Other seemingly crazy things happen, such as mass increasing and velocities appearing different. That is what as known as Einstein’s special theory of relativity.

In the general theory he took into account gravity and accelerations. He said that gravity and acceleration were essentially the same thing and that they were produced by mass warping the fabric of space and time like a metal ball on a sheet of rubber. General relativity predicts that in the presence of gravity, clocks should tick at different rates depending on where they are and light rays should be bent due to the curved space and time. It even predicts that gravity can get strong enough some times to trap light rays (and everything else) in a region of space and keep them from escaping! This is known as a black hole. Many have been found. For more info on relativity and black holes, I recommend the book Black Holes and Time Warps: Einstein’s Outrageous Legacy by Kip Thorne. The first two or three chapters go in depth on relativity in a way that is easy to understand, and the rest of the book tells the fascinating story of black holes.

(published on 10/22/2007)

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