Standard: A framework for establishing measurement units.

Physical constant: An empirically based value.

Physicists establish standards so they can measure things consistently; how they define a standard can change over time. For example, the length of a meter is now based on how far light travels in a precise interval of time. This replaces a standard based on the wavelength of light emitted by krypton-86. Prior to that, the meter was defined as the distance between enscribed marks on platinum-iridium bars. Advances in technology, and the requirement for increased precision, cause scientists to change the method used to define the standard. Scientists strive for precise standards that can be reproduced as needed and which will not change.

By choosing standards, scientists can achieve consistent results around the globe and compare the results of their experiments. Well-equipped labs can measure time using atomic clocks like the one shown in Concept 1 on the right. These clocks are based on a characteristic frequency of cesium atoms. You can access the official time, as maintained by an atomic clock, by clicking here.

You will encounter two types of constants in this textbook. First, there are mathematical constants like π or the number 2. Second, there are physical constants, such as the gravitational constant, which is represented with a capital G in equations. We show its value in Concept 2 on the right. Devices such as the torsion balance shown are used to gather data to determine the value of G. This is an active area of research, as G is the least precisely known of the major physical constants.

Constants such as G are used in many equations. G is used in Sir Isaac Newton’s law of gravitation, an equation that relates the attractive force between two bodies to their masses and the square of the distance between them. You see this equation on the right.