What is Time (1) ?
What is time? Scientists often think of time as a direction you can travel in. Just as we can move up, down, left, or right in space, we can move in time. Something is wrong with this comparison, though. When you walk forward two steps, you can turn around and walk backwards two steps, too, and get back where you started. Time is different. If you wait twenty-four hours to travel to tomorrow, there’s no way to turn around and travel back a day to where you started. This irreversibility is called the arrow of time. So, what makes time different from space?
The Law of Legos Say you really like Legos. (Or imagine one of your friends who does. You probably have one—Legos are pretty cool!) So, you build a really nice Lego castle. It’s so nice that you decide to keep it on display on the kitchen table. Over time, your friends come by and admire it. Your mom admires it, too, but she’s a little clumsy and she breaks a tower off. So, you dig around in your collection, find some pieces to help repair your castle, and you fix it. Your castle stays on display for a while, and sometimes people accidentally bump into it or push on it too hard. Each time it breaks, you dig through your collection to find the remaining pieces to fix it. But since you’re very eager to fix your castle, each time you go through your collection, you leave it a little bit more disorganized. No matter what happens, your Lego collection will always become less organized. To prevent your castle from decaying, you have to make the rest of your collection messier. Scientists have a fancy name for messiness: entropy. Over time, the total entropy of the universe never decreases. In other words, things can only get messier. You can trade entropy from one place to another, like when you fix your castle (decreasing entropy) by going through the rest of your collection (increasing entropy), but the total entropy of the whole collection will still increase. Scientists call this the Second Law of Thermodynamics.
Legos and Time Earlier we asked what makes time different from space. Why can you go into the future but not the past? Since the universe, like your Lego collection, obeys the Second Law, it’s always getting messier and never getting neater. Because both neatness and time can’t go in reverse—you can’t get neater and you can’t go to yesterday—scientists think that the Second Law is actually what makes time irreversible. If you went back in time, it would fix your Lego castle without messing up the rest of your collection, so that your collection as a whole would get less messy. Since this would break the Second Law, you can’t go back in time!
Telling Time Even after you know why you can’t travel into the past, time is still pretty confusing. We measure space as a distance, with rulers and meter-sticks (or yardsticks). If time is a direction, just like forward or up, we should be able to measure a distance in time. But we measure time with clocks. What’s going on? Fortunately, we have a very nice way of measuring time with a meter-stick. The secret is light. Light is a funny thing—it’s very shy. If you’re chasing a beam of light and you speed up to try to catch it, the light will speed up the exact same amount! No matter how fast you go, the light speeds up to stay ahead of you, so that the difference between your speeds is always the same. This is why we say the speed of light is constant: it is always going faster than you by a constant amount. Because light always goes this same speed more than you, you always know its speed (300,000,000 meters per second, in fact—whew!), so you can use its speed to figure out how to measure time with a meter-stick. A distance in time is the amount of time it takes light to travel that distance in space. For example, we can talk about a “second” as the time it takes for light to travel 300,000,000 meters, instead of going around in circles saying, “Well, a second is just…a second, you know?”
Deceptively Simple So, what did we discover? We looked at a clock, which only goes forward and never backward, and we asked, “Why does it do that? What is time?” By thinking about Legos and light, we discovered that time is a direction, just like forward, backward, left, and right. But it’s not quite like a direction in space. There’s something else. We can’t go back in time because it would make the universe a neater, tidier place, and this is impossible—the universe always gets messier. Such a simple question and such a complicated answer! This is how physics always works. Scientists ask simple questions, but we often find that the simpler a question is, the harder it is to answer. But when you find a really nice answer to a really hard, simple question, it’s very fun and very cool.
collected from the web.
