Everything about Free Space totally explained
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. The definitions of the
ampere and
meter SI units are based upon measurements corrected to refer to free space.
Properties of free space
The concept of
free space is an abstraction from nature, a baseline or reference state, that's unattainable in practice, like the
absolute zero of temperature. It is characterized by the
defined value of the parameter μ
0 known as the
permeability of free space or the
constant
, and the
defined value of the parameter ε
0 called the
permittivity of free space or the
constant. These parameters appear in
Maxwell's equations for the electromagnetic fields. Parameter ε
0 also enters the expression for the
fine-structure constant usually denoted by
α, which characterizes the strength of the
electromagnetic interaction.
In the reference state of free space, according to Maxwell's equations,
electromagnetic waves, such as
radio waves and
visible light (among other
electromagnetic spectrum frequencies) propagate at the
defined speed of light,
c0, and according to the
theory of relativity, this speed is independent of the speed of the observer or of the source of the waves. The electric and magnetic fields in these waves are related by the
defined value of the
characteristic impedance of vacuum Z0. In addition, in this reference state the principle of
linear superposition of potentials and fields holds: for example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation.
The ideal
vacuum of free space isn't the same as a physically obtainable
vacuum.
What is the vacuum?
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. The term
partial vacuum is used to refer to the imperfect vacuo realizable in practice. The physicist's term "partial vacuum" does suggest one major source of departure of a realizable vacuum from free space, namely non-zero pressure. Today, however, the classical concept of vacuum as a simple void is replaced by the quantum vacuum, separating "free space" still further from the real vacuum – quantum vacuum or the
vacuum state isn't empty. An approximate meaning is as follows:
Vacuum describes a region devoid of real particles in its lowest energy state. |
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") isn't 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. Related to these differences, vacuum differs from free space in exhibiting nonlinearity in the presence of strong electric or magnetic fields (violation of linear superposition). Even in classical physics it was realized that the vacuum must have a field-dependent permittivity in the strong fields found near point charges. These field-dependent properties of the vacuum continue to be an active area of research. The determined reader can explore various nuances of the vacuum in Saunders. A more recent treatment is Genz.
At present, even the meaning of the vacuum state isn't settled. For example, what constitutes a "particle" depends on the gravitational state of the observer. See the discussion of vacuum in
Unruh effect. Speculation abounds on the role of vacuum in the expanding universe. See
vacuum in cosmology. In addition, the vacuum may exhibit spontaneous
symmetry breaking. See Woit and the articles:
Higgs mechanism and
QCD vacuum. To date, there's no suggestion that these uncertainties affect the use of
SI units, whose implementation is predicated upon the undisputed predictions of
quantum electrodynamics.
In short, realization of the ideal of "free space" isn't entirely a matter of achieving low pressure, as the term
partial vacuum suggests.
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. What is the
operational definition of free space? 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's 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/m
2.)
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 (1×10
5 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/cm
3.
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's a
cosmic microwave background with a temperature of 2.725 K, which implies a photon density of about 400 /cm
3.
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. 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 isn't a wire or a waveguide". This definition bears little if any relation to other technical definitions of free space outlined above.
References and notes
Further Information
Get more info on 'Free Space'.
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