Most of us consider time to be something built in to the world, a self-evident truth. We were born in the past, we are alive today, and we will die in the future. Death is one of the greatest certainties of life, and so the reality of time must be no less certain. After all, without time there would be no future, and no death. But what is the nature of this thing called time? Can we really define it or find it in experience? In a famous passage of his Confessions, St. Augustine writes:
For what is time? Who can easily and briefly explain it? Who can even comprehend it in thought or put the answer into words? Yet is it not true that in conversation we refer to nothing more familiarly or knowingly than time? And surely we understand it when we speak of it; we understand it also when we hear another speak of it. 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.
Today, our first inclination might be to look to science for a definition of time. In Newtonian physics, time is conceived as a linear continuum that contains events ordered from past to future. This temporal continuum was assumed to be a universal and objective reality, providing a common universal time for everyone everywhere. This conception of time, however, was discarded when Einstein discovered that there is no absolute or universal rate at which time flows, or order of events from past to future. The rate of time flow and the ordering of events is not objective, but depends on the reference frame one happens to choose. Since Nature does not prefer any reference frame over another, there is no objective time.
The best we can do is pick a convenient reference frame (e.g., the Earth) and use that as our standard for defining time. We should realize, though, that the “time” thus defined is not an objective reality, but a consequence of our free choice of a particular reference frame. If we had chosen the Sun or Moon instead of the Earth, we would have a “time” with a different rate of flow and a different ordering of events. Moreover, choosing the Earth does not uniquely specify a single reference frame. We need to say whether we’re talking about the non-rotating Earth or the rotating Earth, since rotation affects time flow. In addition, we need to specify whether we’re talking about the surface or center of the Earth, since there is a difference in the rate of time flow due to the difference in gravitational strength. Even on the surface, we need to choose a particular altitude (e.g., mean sea level). Again, this is a free choice and the meaning of “time” will depend on the choice. Time on the top of Mt. Everest will be different than time at the bottom of Death Valley.
Even if we choose a specific reference frame in order to define a unique physical time, there still needs to be some way to measure and distinguish different durations of time. If there is no difference between days and years, for example, time has little meaning. So, in addition to specifying a reference frame, we need to specify some convention for measuring time. In short, we need a clock. And not just any clock. We need a clock that provides a reliable and uniform way of measuring time.
The periodic movement of the sun across the sky every day has been used in the past to define time, but this movement is not actually uniform due to irregularities in the Earth’s rotational speed. In other words, days do not all have the same length. Irregularities are also present in the revolution of the Earth around the Sun and in the revolution of the Moon around the Earth, so the year and the month also do not have consistent lengths. Due to these problems with astronomical clocks, scientists now use atomic clocks to define the measurement of time. A second, according to the current convention, is defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two hyperfine levels of the ground state of cesium-133 at rest at a temperature of 0 K. So, the measurement of time is defined by the detailed behavior of a particular type of atom under very specific conditions. Again, this definition is not some objective fact given to us by Nature, but a convention we have adopted. Using another convention, we would have a different definition of what a “second” of time is. It also makes clear that the meaning of time is not some fixed abstraction, but depends on concrete physical phenomena.
It should be clear now that there is no objective reality to time. It is something that arises out of our own definitions, choices, and conventions. When we talk about time, we are implicitly assuming a choice of a particular reference frame and a particular definition of time measurement. Only when we forget that we have played a role in defining time does it appear to have an independent existence.
sumana
31 October 2010
I would rather worry about what aging is than try to define time. If there was no such thing as aging – no birth and no death – then we would think of time like any other physical quantity like mass/ temperature/ gravity etc. Imagine a situation where only a certain number of human beings were created and these people lived forever till ..whatever time God wanted them to inhabit the earth in the same way that planets continue to move around the sun. We would still talk of yesterday/today/tomorrow but since nothing would change in the absence of aging, we probably would not be talking of the reality of time the way we do so now. In other word the word ‘time’ has real significance only because there is something called ‘aging’