Free space

In physics, free space is a concept of electromagnetic theory, corresponding to a theoretical "perfect vacuum".

Definition

Free space simply means that there is no material or other physical phenomenon present except the phenomenon under consideration. Free space is considered the baseline state of the electromagnetic field. Radiant energy propagates through free space in the form of electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies). The constant value <math>\mu_0 \,</math> is known as the permeability of free space. The permittivity of free space, <math>\varepsilon_0 </math>, is the ratio of the electric displacement field to the electric field in free space. This permittivity is used in the construction of the fine-structure constant. According to relativity, radiant energy in free space propagates at the speed of light, independent of the speed of the observer or of the source of the waves.

Ideal states and real-world applications

Free space conveys that the region is absolutely devoid of matter and has no external fields or forces other than those considered in the problem at hand. Free space, with a gaseous pressure of absolute zero is a mathematical idealization. For example, in the "vacuum" of outer space, there are small quantities of matter (mostly hydrogen), and cosmic noise. The density of the interplanetary medium and interstellar medium though is extremely low, and, for many applications, the interplanetary and interstellar regions are "free space". The cosmic microwave background has a temperature of 2.725 K, which implies a photon density of about 400 /cm³.^[1] Free space is a philosophical concept that is never observed in practice. The United States Patent Office defines "free space" for radio and radar applications as "space where the movement of energy in any direction is substantially unimpeded, such as the atmosphere, the ocean, or the earth". (US Patent Class 342, Class Notes ^[2]) This is considerably different from the theoretical definition of free space as there exists considerable material in all of the USPTO's examples. As per this definition, at various electromagnetic frequencies some density conditions are reasonable approximations to free space.

See also

• Vacuum energy
• Medium (optics)
• Dielectric constant
• Dirac sea

Notes

1. ^ Martin J. Rees (1978), "Origin of pregalactic microwave background", Nature 275: 35-37., <http://www.nature.com/nature/journal/v275/n5675/abs/275035a0.html>
2. ^ U.S. Patent Classification System - Classification Definitions as of June 30, 2000

References

• Davida, "What is the significance of permittivity of free space?". Ask A Scientist, Physics Archive.
• Susan Lea, "Electromagnetic waves in free space". (PDF)
• Eric W. Weisstein, "Permittivity of Free Space".
• Eric W. Weisstein, "Permeability of Free Space".
• "Characteristic impedance of free space". TechTarget, 2005.