We commonly refer to the conscious experience of a duration as time. In physics, time is a fundamental quantity, similar to length, mass, energy in that aspect. The measurement of time involves a time scale whose basic unit is the 'second'. Modern physics regards time as a fourth dimension, on an equal footing with the familiar three dimensions of space. The concept of time has perplexed the best of human minds, and it remains one of the most difficult properties of our universe to understand.