The Laws of Physics
The various laws of physics are attempts by physicists to explain the behavior of nature in a simple and general way. Even the most accepted laws of physics, however, are subject to change. Nature's behavior does not change, but techniques for determining its behavior do change and become more accurate. In addition, observations are made under new conditions, such as from orbiting satellites. Physicists, therefore, must subject the laws of physics to new tests to see if, under new conditions, they still hold true. If they do not hold true, changes must be made in the laws or entirely new theories must be proposed. New theories must explain not only all phenomena that the old laws explained but also results of the new tests.
At the beginning of the 20th century, the laws of physics were tested extensively and were found to be too narrow to explain many of the new discoveries. A new body of theories was started. The older body of laws is called classical physics; the new is called modern physics.