Change: The only thing that doesn’t

Chang-li Yiu

Professor Emeritus, Mathematics and Physics

Students in beginning physics courses study the motion of projectiles. When a pebble is tossed into the air, its position, speed and direction vary all the time in complicated ways. Students learn to trace these variations by using Newton’s laws of motion.

Soon afterwards, they learn a shortcut based on the principle of energy conservation. Regardless of how complicated a motion is a quantity called “energy” remains constant throughout the movement of the projectile. Using this principle, we can relate the speeds of the projectile at any two points right away, without having to solve Newton’s equation.

Energy is but one of the many constants, or conserved quantities, in physics. These conserved quantities are like beacons in the ever-changing physical world. They can cut through tangled ideas and help us arrive at simple and direct answers to complicated questions.

Take the conservation of mass principle in classical chemistry, for example. It says that the total mass must be the same before and after any chemical reaction. When chemists try to find out the details of a particular reaction, this principle immediately rules out any potential chemical process that does not preserve mass.

There are similar ideas in human affairs.

Once when I was waiting near the office of Dr. George Hauser in Computer Science, I read a note posted on his door: “We trained hard, but it seemed that every time we were beginning to form into teams, we would be reorganized. I was to learn later in life that we tend to meet any new situation by reorganizing. And what a wonderful method it can be for creating the illusion of progress while producing confusion, inefficiency and demoralization.” Everyone knows that, I thought. Why post this note? Then I read the origin of the quotation: “Petronious Arbiter, 66 A.D.”

Some things remain unchanged in this ever-changing world.

We all have our favorite adages from the ancients. These adages remain fresh through the ages and serve as moral beacons in our lives. They are constants of motion in human affairs. Confucius told us to “love others as you love yourself.” That remains good advice after more than two thousand years. But the fact that we still need that advice means that we still don’t love others as much as we love ourselves. Jesus advised his followers to use only the Lord’s Prayer, saying, “Your Father has knowledge of your needs even before you make your requests to Him.” But guess what? Even those who insist on interpreting the Bible literally virtually always say their own prayer first, and then tag on the Lord’s Prayer. There is something stubbornly constant in human beings that even Jesus could not change.

Nonetheless, we still cling to ancient teachings, even when we don’t exactly follow them. We have not given up trying to do better yet; that is a very good constant in human dynamics.

Although we use constants to make sense of our world, even constants change. Long before the global positioning system was invented, the North Star guided travelers for thousands of years. The star can do the job because its position in the sky is fixed in the north, while other stars circle around it during the night. Can this go on forever? Apparently not. Modern astronomy teaches us that in some 10,000 years the North Star will wander substantially far from its current position. Then, just like all other stars, the “North Star” will circle around the point it used to occupy and will no longer guide travelers.

Sometimes we find that our understanding of a constant needs adjustment. Physicists have found that the conservation of mass principle is not strictly correct. To be more accurate, we must consider the conservation of mass and energy together. Similarly, for more than 200 years Newton’s laws of mechanics were considered a permanent mandate of nature. At the beginning of 20th century its limitations were discovered, and Newton’s laws were replaced by more all-encompassing laws. We expect that in the future these new concepts will undergo further changes as we learn more about nature.

So all the golden rules of human behavior, which have been with us for millennia—will they also change? If the lessons in science are to be taken seriously, they might. Don’t get too upset if someone challenges them. You might be witnessing history in the making.

The only unchanging aspect of the universe, observed some ancient Chinese and Greek philosophers, is change. The consolation is that some changes are so slow that even for true conservatives they might as well be constants.