Modern science has scrutinized this arrow of time for nearly 300 years. The scientific concept of time is intimately connected with the development of physics. Following Galileo in the 17th century, Newton developed the first great laws of physics: the equations of motion for mechanics and gravitation. These laws were strictly time symmetric—that is, the world works the same going either forward or backward in time. No distinction exists in these equations to point the arrow of time in one direction.

The next great advance in classical physics came with 19th-century Scotsman James Clerk Maxwell and his theory of electromagnetism. Again, these laws are time symmetric. Great physicists are left to wonder: Why does "shaking" an electron (for example, changing a current's flow) in an antenna cause electrons to move (that is, a current to flow) in a distant antenna a few moments later? Why not earlier? A strict mathematical solution of the equations of electromagnetism allows both possibiliites. The principle of cause and effect and Newton's closely related third law ("Every action requires an equal and opposite reaction") require both solutions.

If the laws of classical physics are time symmetric, where can we find some indication of the time asymmetry that permeates our world? How about nonclassical quantum mechanics or relativity? These have shown that time is more complicated than the absolute, smoothly flowing time of Newton. Still, their equations remain time symmetric, though in a more sophisticated and less obvious manner.

For example, how is it that one can see a set of movie frames of a building being demolished and easily put the frames in the correct time order? Classical thermodynamics and statistical mechanics tell us that there is an arrow of time that is set by the increase of disorder in a system, a property known as entropy. Because of entropy, there is an overwhelming probability that a system will degrade and tend toward a more highly disordered state.

Yet while the laws of physics say systems will tend toward entropy, they do not pick out a direction in time. One recent discovery indicates that the weak interaction (one of the four forces) may violate time-reversal symmetry at the level of one part per billion. This result caused a great deal of interest and follow-up experiments, but in no other instance have scientists observed the violation of time-reversal symmetry. In my opinion, the uniqueness and great weakness of the observed effect serve to underline the actual time-reversal symmetry of the laws of physics.

Perhaps Newton's greatest discovery was to divide the formal description of the world into the laws of nature and initial conditions. If the laws of nature are time symmetric but the world is not, then it must be because of the initial conditions. What are those initial conditions? They have to be how the universe started. Scientists debate the initial conditions that were necessary for its formation. This controversy focuses on what it means for the universe to have been created out of nothing. That is, can the universe create itself?

If the universe has such a singular beginning, then the past of everyone and everything begins at a common point (quantum-fuzzed point). All backward- and forward-going processes must match at that point so only those going forward could emerge.

If there is a singular end to the universe—the "big crunch"— isn't that the same as what happened at the beginning? On one level, yes, but there is a big difference between the beginning and the ending of the universe. Gravity during the big crunch would act as a cosmic meat grinder, pulling in and forcing everything together in a big, complicated mishmash. The beginning of the universe, especially according to the "creation from nothing" scenario, was a very smooth and orderly extrusion process. This ties back to the thermodynamic idea of entropy, as well as to our own experience of time (e.g., knowing which way the film of the collapsing building runs). The initial boundary conditions for the creation of the universe, therefore, give us a direction for time, both in terms of physical motion, which acts according to the Newtonian laws of cause and effect, and of entropy, the process of increasing thermodynamical disorder.

Thanks to science and history, disciplines that require the formulation of precise time lines, we know more about what "time it is" than ever before. We know that the sun is middle-aged, about 5 billion years old. It is about halfway through its life cycle as a shining star. The sun has been shining steadily for most of its life and will for another billion years before it grows hotter and begins to expand, leading to serious global warming. Eventually, the human race will have to make an adjustment to this climate change, although it is a long way off—nearly 1,000 times longer than man has existed.

Our galaxy, the Milky Way, has been around nearly twice as long as the sun, or about 10 billion years. But our galaxy is effectively younger, having used up only about 10% of its original material to produce such stars as the sun. Thus if humans have to move to other star systems because of the sun's heat, our galaxy can extend enough light and energy to provide us a haven for at least 100 billion years more.

We know how old our sun and our galaxy are. How about the universe? It's roughly 15 billion years old, not that much older than our galaxy. We have a good idea how the universe will develop, within the range of parameters we can measure. But we are not sure how long the universe will last, although we do know that the universe will last for quite some time, longer by a large factor than it has existed to the present.

As the 20th century comes to a close, we know a lot about the conditions at the beginning of the universe and the laws of physics that govern its development. We have a broad outline of the history of the universe and of the earth, and we have also been able to trace the evolution of life. We know, too, how the galaxies, stars, and planets formed and developed. With all of this information, we now have a deeper and more comprehensive context for the simple question "What time is it?" And we can answer that question in greater detail and with more precision than ever before.

But it is a sure bet that our human curiosity and our fascination with timewill always impel us to learn even more.