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What is einstein's theory of relativity? When this theory was first published, it was said that it could be understood by only a dozen or so scientists in the whole world! So obviously, we cannot even begin to try to explain it in any technical detail in this book. But it should be useful to have a general idea of what Einstein was dealing with, the problem he was concerned with. Everybody knows from experience that all motion is "relative." This means that it can only be measured in relation to something else. For example, you're sitting in a railroad train and you look out the window. As you see things moving by quickly, you know you're in motion. But there's a man sitting opposite you, and relative to him you're not moving at all! So the existence of motion can have meaning only when it is considered relative to something which is fixed. That's the first basic part of Einstein's theory. We may state it as: The motion of a body traveling at uniform speed through space cannot be detected by observation made on that body alone. The second basic part of Einstein's theory said that the only absolute unchanging quantity in the universe was the speed of light. Now, we know this to be about 186,000 miles per second. But it is really a fantastic idea to be able to imagine that this cannot change. Here is why this is so strange: If a car is going at 60 miles an hour, we mean that its speed, measured by a someone standing still, is 60 miles per hour. If it passes a car traveling in the same direction at 40 miles per hour, it passes it at a speed of 20 miles per hour. And if the second car, instead of traveling in the same direction, were coming to meet it, they would pass each other at a speed of 100 miles per hour. Now, according to Einstein, if the speed of a ray of light were measured in the same way (for example, if we were racing in one direction and it was going the opposite way), it wouldn't make any difference! That ray of light would still pass at about 186,000 miles per second. This is only the rough, general idea of what Einstein was dealing with in his theory of relativity. Among other things he dealt with were mass and energy and how they can be transformed into each other. For Details
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