Last year, in the inaugural Flame Challenge, Alan Alda and the Center for Communicating Science at Stony Brook University challenged scientists to explain what a flame is to an 11-year-old. This year, the subject was time. In particular, we were instructed to "Answer the question — 'What is time?' — in a way an 11-year-old will find instructive, interesting, and maybe even fun." Written entries had a 300-word limit. This is my submission.
In some ways, time is a direction. Certain processes only go in one direction. You can stir chocolate syrup into your milk, but no amount of stirring will separate the syrup from the milk once they're combined.
One fundamental law of nature is that entropy—the amount of disorder in a system—increases with time. A glass of chocolate milk, with milk and chocolate particles all jumbled together, is more disordered than a glass of milk sitting next to a spoonful of chocolate syrup. In fancy language, stirring the syrup into the milk increases the entropy of the system. The future is the direction in which entropy increases, and the past is the direction in which it decreases.
This notion of time has a big limitation: it can say that I ate my broccoli before I made my chocolate milk, but it can't say whether it was five minutes or three years before.
From a physics point of view, determining the amount of time that passes between two events is not just a matter of finding a good stopwatch. Einstein's theory of relativity shows that many of our instinctive notions of time are wrong. Time is not a universal measurement. Instead, the speed of an observer relative to an event affects the amount of time that seems to pass.
We humans usually aren't traveling quickly compared to events we're observing, so these effects are undetectable to us. But if you flew by my window at close to the speed of light (about 670 million miles per hour), you would see me make chocolate milk in slow motion.
Relativity means we can create clocks that measure time between events that occur on and near Earth, but those clocks are based on us, not on fundamental properties of the universe.
I realized after writing this post that I had scheduled it for the day most Americans switch to Daylight Saving Time. The theory of relativity fails to address the discontinuity of spacetime we experience in the one minute between 1:59 am and 3:00 am on the 2nd Sunday in March.