Vacuum (partial)

Partial vacuum refers to a realizable but non-ideal, or imperfect, vacuum. The partial pressure of a gas in a mixture of gases is the portion of the total gas pressure contributed by that gas. Laboratory vacuum or terrestrial vacuum historically was achieved by pumping down a vacuum chamber, and success was measured by the partial pressures of the residual gases. Because the gases cannot be completely removed, the result of pumping down is a partial vacuum. One instrument important in monitoring the success of pumping down is the mass spectrometer, which ionizes the gases and then detects the ions as a current.

To determine the properties of the ideal vacuum, 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 an empirical approach to defining vacuum. Unfortunately, the theory calculating the properties of vacuum is rather complicated today (see vacuum (quantum electrodynamics)), and measurements are too inaccurate to verify the theory at extremely low pressures. Consequently, this strategy for defining vacuum is of limited accuracy, and cannot be relied upon to check experimentally the behavior of "true" vacuum.

Some experiments affected by vacuum are examined in quantum electrodynamics, such as spontaneous emission and natural spectral linebreadths, the Lamb shift the Casimir force, and quantum beats between spontaneously emitting systems in vacuum. So far these experiments tell us more about atoms than about the vacuum.