Talk:Vacuum (laboratory)

Proposed deletion and subsequent action

The page vacuum (laboratory) is exactly what this low pressure definition describes. On the other hand, vacuum (laboratory) is redundant as vacuum (partial) means the same thing, and may be clearer. The designation vacuum (science) could mean:

• Vacuum (classical) [r]: The term classical vacuum as used in classical electromagnetism and in the definition of the SI units refers to an ideal reference medium devoid of all particles, with ideal properties. These ideal properties include: independence from field strengths, direction, frequency, or polarization, and from temperature. ^[e]
• Vacuum (quantum electrodynamic) [r]: The term quantum electrodynamic vacuum, or QED vacuum, refers to the ground state of the electromagnetic field, which is subject to fluctuations about a dormant zero average-field condition. ^[e]
• Vacuum (partial) [r]: A realizable vacuum with a gaseous pressure that is much less than atmospheric. ^[e]

Therefore, vacuum (science) should be used as a general designation for all of the above, and not restricted to a narrower meaning. John R. Brews 13:36, 5 January 2011 (UTC)

Various vacuums

From an historical perspective, vacuum originally meant "absence of matter". However, that has become too vague with later developments. In the early 1900's we have the development of electromagnetism and the advent of relativity. Here "vacuum" took on some very specific properties like isotropy, linearity in the fields, and so forth, that are related to matter (or no matter), but the relation is more complicated than before. For example, it was realized that fields become infinite near point charges and the ideal vacuum wasn't going to work there. Then comes quantum electrodynamics and vacuum now is no longer linear or isotropic or any of the classical properties. It isn't even "empty" in principle.

The laboratory vacuum historically was achieved by pumping down a vacuum chamber, and success is measured by the partial pressures of residual gases. Originally the idea was that measurement of properties as pumping down took place could be fitted to theoretical expressions and extrapolated to zero pressure to find the behavior of "true" vacuum. That is a sort of empirical approach to vacuum. Unfortunately, theory is rather complicated today, and measurements are rather crude and cannot verify the theory, so this whole methodology is in limbo as a strategy for defining vacuum.

In the metrology arena, the SI units have further removed matters from the obvious by defining vacuum to have the properties ε₀, μ₀, and c₀ (a defined speed of light). By virtue of these defined properties this "vacuum" is like classical vacuum: it is isotropic, linear, nondichroic and dispersionless. It also happens to be imaginary - it doesn't exist in nature, not even at NIST. So real vacuums are all imperfect attempts to achieve this defined reference medium. The metrology practice is very empirical: corrections to measurements are made to refer them to the defined "vacuum" and this list of corrections grows longer day by day as more and more complications are discovered.

This is the mess that has to be straightened out here. John R. Brews 14:23, 5 January 2011 (UTC)