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. Partial vacuum on Earth may be referred to as terrestrial vacuum or laboratory vacuum.
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 electrodynamic)), and measurements are too inaccurate to verify the theory at extremely low pressures. Consequently, this strategy for defining vacuum under extreme conditions is of unknown accuracy, and cannot be relied upon to check experimentally the behavior of "true" vacuum.
Outer space has very low density and pressure, and is a close physical approximation to perfect vacuum. Nonetheless, no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic centimetre. However, even if every single atom and particle could be removed from a volume, it would still not be "empty"; see Vacuum (quantum electrodynamic).
Stars, planets and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 1 Pa (7.5−3 torr) at 100 km of altitude, the Kármán line, which is a common definition of the boundary with outer space, marking the point where lift cannot support a vehicle, and it must go into orbit. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the sun and the dynamic pressure of the solar wind, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre.
Experiment and theory
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.
The article Vacuum (quantum electrodynamic) describes one of several theoretical approaches to the description of real vacuum. Other theoretical models are considered in quantum chromodynamics and in quantum gravity.
- (1979) “Chapter 3: Partial pressure measurement”, G. L. Weissler, Robert Warner Carlson: Vacuum physics and technology, 2nd ed. Academic Press, pp. 81 ff. ISBN 0124759149.
- Tadokoro, M. (1968). "A Study of the Local Group by Use of the Virial Theorem". Publications of the Astronomical Society of Japan 20 (No. 2). This source theoretically estimates a density of 7 × 10−29 g/cm for the Local Group. An atomic mass unit is 1.66 × 10−24 g, for roughly 40 atoms per cubic meter.
- Michelle M Donegan (2009). “§6: Space basics: getting to and staying in space”, Ann Garrison Darrin, Beth Laura O'Leary, eds: Handbook of space engineering, archaeology, and heritage. CRC Press, p. 84. ISBN 1420084313.
- See, for example, Alexander S Shumovsky (2001). MW Evans & I Prigogine: Modern nonlinear optics, Part 1, 2nd ed. Wiley, pp. 396 ff. ISBN 0471389307.