# What is Time?

#### Written by Dr. Kenneth G. Hayes

People have pondered the nature of time since we became aware of time. And with good reason! One of the most common ways to measure our lives is how much time we have lived and how much time we may have left to live. Birth and death dates are listed on tombstones, and most of us quickly calculate from those dates the duration of the deceased’s life.

Physics equations often contain time as a variable, so one might assume that physicists have a good understanding of the nature of time. Isaac Newton wrote this about the nature of time in his masterpiece, Principia: “I will not define time, space, place and motion, as being well known to all.” That was Newton’s way of avoiding the question, “What is time?” A few sentences later, Newton describes a few properties of time. He believed there was one universal, absolute time that was the same everywhere. This means accurate clocks would measure the same time no matter where they were or how they might be moving relative to other clocks.

In 1905 the young Albert Einstein deduced new properties of space and time from the postulate that all observers would measure the same value for the speed of light (his special theory of relativity). These were the first significant clues that physicists discovered about the nature of time. One property Einstein discovered was that accurate clocks moving relative to each other would tick at different rates. This means, though we think of ourselves as observers carrying accurate clocks, other observers moving relative to us experience different time than we do! Thus every observer carries with him his own time frame. These differences in the rate of time become large when the relative speed between observers approaches the speed of light (186,000 miles per second, 300 million meters per second). Observers moving relative to each other would measure different time intervals between two events. How large a difference they would measure depends not only on their relative speed but how far apart the two events are along the relative direction of motion between the two observers. Thus three-dimensional space and one-dimensional time get unified into a four-dimensional spacetime. Although this doesn’t answer the question, “What is time?” it gives us fascinating and unexpected new pieces of the puzzle.

In 1915, Einstein gave us more pieces of the puzzle when he published his general theory of relativity, which added acceleration and gravity to special relativity. Now Einstein calculated that two clocks that are stationary relative to each other tick at different rates if they are in different gravitational potentials (imagine different strengths of gravity). Time will travel faster for a friend who is at rest relative to you but at a higher altitude (i.e., sitting in class on a higher floor). Both of these effects (the relative speed effect and the different gravitational potential effect) have been exhaustively tested, and the tests agree with Einstein’s theory. The GPS system that allows GPS receivers (like the one in your cell phone) to precisely determine their position on Earth’s surface would not function if these two effects were not included in the receiver’s programming.

One concept about time that has evaded physics is the concept of “now.” We perceive that we live in the present moment, “the now.” But at present, no symbol in any equation in physics refers to “now.” The time variable in all physics equations just orders events in time the same way that a spatial variable orders events along an axis in space. Some future Einstein may give us new clues about time that will provide some insight about the physical meaning of “now,” but we’ll just have to wait until that moment.

Within the constraints provided by physics as discussed above, we are free to speculate on the question, “What is time?” I love what the captain of the Enterprise, Jean-Luc Picard, says in the movie Generations: “Someone once told me that time was a predator that stalked us all our lives. But I rather believe that time is a companion who goes with us on the journey and reminds us to cherish every moment, because they’ll never come again.”

Dr. Kenneth G. Hayes is Chairman and Professor of Physics at Hillsdale College.