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In physics, we encountered many numbers that we believe do not change with time. These numbers are usually called constants or more appropriate, universal constants. The most familiar example is the speed of light, usually denoted by a small letter c, and has a value of about metres per second, or 300,000 kilometres per second. Another familiar example is the gravitational constant G, derived from the proportionality of the force between two masses and the square of their distance apart, i.e., . G is known as the Newton's law of gravitation.

There are about four so-called universal constants of nature: c, the speed of light, G, the Gravitational constant, h, the Planck constant and k, the Boltzmann constant. Some people believe there are more but to keep within the scope of this essay, only these four will be discussed. The values of these four constants appear unchanged over time and valid throughout the entire universe, i.e., it doesn't matter whether you live on Earth or in another galaxy, and they all have the same values.

Speed of light in vacuum

Gravitational constant

Planck constant

Boltzmann constant

Light is an electromagnetic wave. In the absence of matter, the speed of light is a constant. When light travels through a medium, e.g., glass, its speed is reduced according to the density of the glass. In space, however, the density of matter between stars is very low, so that speed of light is essentially the same as in vacuum.

Newton's law of universal gravitation states that any two objects attract each other with a force proportional to the product of their masses divided by the square of their separation. The constant of proportionality, called G, is what is known as the Gravitation constant. The value of G was first measured in 1798 by Cavendish and co-workers. The value obtained was accurate to about 1%. At the turn of century, Einstein's theory of General Relativity reinterprets the meaning of G in terms of the curvature of space-time around the objects, i.e., G is proportional to the curvature of space-time when a unit mass of material is present. In this theory, the gravitational attraction between two objects is a result of the curvature of space-time around them.

Before the advent of modern physics, electromagnetic radiation was thought to be continuous wave. The concept of discrete radiation was first devised by Planck. His theory of quantized energy, which we now called quantum theory, proposed that radiations are emitted or absorbed in discrete units called quanta. The energy of these quanta is proportional to the frequency of the radiation. The constant of proportionality is what is known as the Planck constant. The value of the Planck constant was first measured in 1916 by the American physicist Robert Millikan.

The ideal gas law says that the product of the pressure and volume is proportional to the number of molecules times the temperature. The constant of proportionality is known as the Boltzmann constant k. The constant relates the average kinetic energy of a molecule to its absolute temperature.

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