Free space

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In classical physics, free space is a concept of electromagnetic theory, corresponding to a theoretically "perfect" vacuum, and sometimes referred to as the vacuum of free space.

Definition

Today, quantum theory has changed the vacuum from the classical concept: quantum vacuum or the vacuum state is no longer empty. An approximate meaning is as follows:

Free space describes a region devoid of real particles in its lowest energy state.

A true, physical, quantum vacuum is an abstraction from nature that has its own properties and a vacuum energy. The quantum vacuum is "by no means a simple empty space"., and again: "it is a mistake to think of any physical vacuum as some absolutely empty void." According to quantum mechanics, empty space (the "vacuum") is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. One measurable result of these ephemeral occurrences is the Casimir effect. Other examples are spontaneous emission and the Lamb shift. The determined reader can explore various nuances of the vacuum in Saunders. A more recent treatment is Genz.

Physicists often use the term "vacuum" to discuss ideal test results that would occur in a perfect vacuum, which they simply call vacuum or free space in this context, and use the term partial vacuum to refer to the imperfect vacuo realized in practice. Thus, free space is considered a baseline or reference state, unattainable like the absolute zero of temperature. In this reference state, for example, radiant energy propagates through free space in the form of electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) at the defined speed of light, c₀ with electric and magnetic fields related by the defined value of the characteristic impedance of vacuum Z₀. 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.

However, the theoretical meaning of the vacuum state is not settled. See, for example, Higgs mechanism and QCD vacuum. To date, there is no suggestion that these uncertainties impinge on the role of the vacuum state in the definitions of the SI units, which are more closely related to the undisputed predictions of quantum electrodynamics.

Realization of free space in a laboratory

By "realization" is meant the reduction to practice, or experimental embodiment, of the term "free space", for example, a partial vacuum. Although in principle free space is unattainable, like the absolute zero of temperature, the SI units are referred to free space, and so an estimate of the necessary correction to a real measurement is needed. An example might be a correction for non-zero pressure of a partial vacuum. Regarding measurements taken in a real environment (for example, partial vacuum) that are to be related to "free space", the CIPM cautions:

♦ that in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum.

In practice, a partial vacuum can be produced in the laboratory that is a very good realization of free space. Some of the issues involved in obtaining a high vacuum are described in the article on ultra high vacuum. The lowest measurable pressure today is about 10⁻¹¹ Pa. (The abbreviation Pa stands for the unit pascal, 1 pascal = 1 N/m².)

Realization of free space in outer space

While only a partial vacuum, outer space contains such sparse matter that the pressure of interstellar space is on the order of 10 pPa (1×10^-11 Pa). For comparison, the pressure at sea level (as defined in the unit of atmospheric pressure) is about 101 kPa (1×10⁵ Pa). The gases in outer space are not uniformly distributed, of course. The density of hydrogen in our galaxy is estimated at 1 hydrogen atom/cm³. In the partial vacuum of outer space, there are small quantities of matter (mostly hydrogen), cosmic dust and cosmic noise. See intergalactic space. In addition, there is a cosmic microwave background with a temperature of 2.725 K, which implies a photon density of about 400 /cm³.

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".

US Patent Office interpretation of free space

The United States Patent Office defines "free space" in a number of ways, at least some of which bear little if any relation to the technical definition of free space outlined above. For example, for radio and radar applications the definition is "space where the movement of energy in any direction is substantially unimpeded, such as the atmosphere, the ocean, or the earth" (Glossary in US Patent Class 342, Class Notes). Another US Patent Office interpretation is Subclass 310: Communication over free space, where the definition is "a medium which is not a wire or a waveguide".

Free space

Definition

Realization of free space in a laboratory

Realization of free space in outer space

US Patent Office interpretation of free space

References and notes

Outside links

See also