What in the world gives direction to time?
When a bag of popcorn is heated in a microwave oven, the corn kernels explode into fluffy, edible structures that are just the right snack for a football game. If, however, you then decided to remove thermal energy from the popped kernels by sticking them in a refrigerator, they would never change back to their original unpopped state. Why can’t the process that occurred in the oven be reversed, like a videotape run backwards?
The answer is in this lesson.
What Is Physics?
Time has direction, the direction in which we age. We are accustomed to oneway processes—that is, processes that can occur only in a certain sequence (the right way) and never in the reverse sequence (the wrong way). An egg is dropped onto a floor, a pizza is baked, a car is driven into a lamppost, large waves erode a sandy beach—these one-way processes are irreversible, meaning that they cannot be reversed by means of only small changes in their environment.
One goal of physics is to understand why time has direction and why one-way processes are irreversible. Although this physics might seem disconnected from the practical issues of everyday life, it is in fact at the heart of any engine, such as a car engine, because it determines how well an engine can run.
The key to understanding why one-way processes cannot be reversed involves a quantity known as entropy.
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