Time | Research & Encyclopedia Articles

Time

Time, it has been joked, is what keeps everything from happening all at once. It is the clock on our wall, the watch on our wrist, perhaps even the beat of our heart. Time seems to flow forward like a steady stream. We live in time, and time washes over us whether we like it or not.

And yet, time appears to be very different from that other entity in which we live, space. Space, the air and land and seas around us, the sky overhead and the universe beyond, is a more comfortable concept. We can move forward or backward in space, at our will. (The only requirement is that we must stay in our universe.) We can return to a place visited earlier, we can live or walk or run in more places than another, we know how to navigate through space. We might say, "I'll meet you back home," with home specifying a point in space, but we never say, "I'll meet you back yesterday." Yesterday is gone.

Scientists as well as philosophers have grappled with these types of questions, with time, throughout the ages. In the 5th century A.D. St. Augustine said, "What, then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know." Absolutists like Isaac Newton thought that time's existence and properties are "absolute" and independent of the universe--it would exist even if the universe did not. Time is the universe changing, and time was just another variable in Newton's formulation of natural laws, the thing that other variables like position and momentum changed in. This is much the common view of time we have today, because it is a view that works. With it we can establish systems for timekeeping--clocks and such, which are ultimately based on repetitive events in the universe, like the earth's revolution of its passage around the sun. We can establish units for time--the year, the day, the minute, the second. (By international agreement, the second is now defined as 9,192,631,770 periods of the radiation derived from an energy-level transition in the cesium atom.) If we do not fully understand time, we can at least keep track of where we are in it.

The Absolutists view of time was dominant in science until the twentieth century, when in 1905 Albert Einstein's special theory of relativity forever changed the way we think of time. Einstein founded his theory on the principle that the speed of light must be constant and the same for all who tried to measure it, regardless of whether they were at rest on the Earth's surface or traveling through space at high speed. The consequences of this assumption (which is true as far as best we know) were tremendous. Time could no longer be viewed as independent of space, but was now very much like it, the famous "fourth dimension." Space and time became relative, not absolute.

Einstein's view of the universe has surprising consequences that at first strike us as paradoxical. Two events that are simultaneous to one observer may not be simultaneous to another, especially if the second is moving very fast with respect to the first. Moreover, as any object's speed approaches the speed of light, time for that object will slow down relative to others. This raises the possibility of the twin paradox. If one of two twins leaves Earth in a spaceship and travels at appreciable fraction of the speed of light for, say, five years, then turns around and comes back just as fast, she will only have aged 10 years. But her twin who stayed on Earth may have aged far more, 50 or 100 or more years! The mass of rapidly-moving objects also increases, and their length contracts, as measured by an observer "at rest." (In actuality there is no "at rest"--everything in the universe is moving relative to everything else.)

Einstein's theory has been verified to a high degree of accuracy, mostly through experiments involving fast-moving particles, and is considered as true as Newton's theories once were. (In fact, at low, everyday speeds, the two theories are equivalent.) But the consequences of his theory have changed the way we view time and space, now considered as a four-dimensional spacetime in which we move.

If time is relative, is it also reversible? That is, if time were to run backwards, would our science still satisfactorily explain the world? Newton's and Einstein's theories are time symmetric--the direction of time makes no difference. However, there are very rare, exotic events which are not time reversable, such as the decay of a particle known as a K-zero meson. It is not fully understood how such a rare nonreversibility might show up in the larger, everyday world.

Has time existed forever? Will it last forever? Science does not yet know. It appears that our universe was created in big bang about ten billion years ago--an outward explosion from a single point that has forever since been expanding. We know it is expanding because astronomers have been able to detect a slight shift in the wavelengths of light coming from other stars, a doppler-shift that stretches the wavelength out, just as if the star that emitted it is moving away. Will this expansion continue forever? Einstein's 1911 general theory of relativity describes how spacetime changes in the presence of mass and energy, and with some assumptions and some fundamental astronomical observations it can be used to predict the evolution of our universe. The Einstein equations predict that our universe will eventually expand to a certain point, stop, and then reverse itself if there is enough mass in the universe to "stop" the expansion. The universe would then begin to contract, presumably resulting in a big crunch. If there is not enough mass, the universe will expand forever, presumably until it is an immense space with immense spaces between every atom and electron. Some researchers in astronomy are trying to determine approximately how much mass the universe does in fact have. But finding the answers to such questions will take...time.