measure for measure A unit for nothing The way that we understand free space has varied wildly since our frst conception of the . And how we measure the void has proven just as changeable, as Karl Jousten explains.

ncient Greek philosophers debated of gas density times the product of whether a void space could exist at Boltzmann constant and temperature. The Aall. Among the most famous denying Boltzmann constant is one of the recently the existence of vacuum was , and redefined fixed constants of the SI with he was joined by most philosophers in the zero uncertainty. A priori, quantum- Middle Ages. The prevailing argument was electrodynamical calculations of the that space cannot be defined in a region dynamic polarizability of helium enable where nothing exists. In 1644, however, determination of molecular density from the Italian mathematician Evangelista a measurement of refractive index in a Torricelli proved the sceptics wrong. His fundamental manner. To measure the gas experiment was rather simple: Torricelli density of a vacuum makes sense5, because filled a glass tube of about 1 m in length vacuum pressures do not exert significant with mercury and sealed the open end with forces and most applications of vacuum his fingertip. He then immersed this end need either a long enough mean free in a mercury reservoir before removing his path of the gas molecule or a well-defined fingertip. The outflowing mercury caused gas density: 10–10 Pa corresponds to the column in the tube to sink to a depth 104 molecules per cm3, 105 Pa to 1019 of 76 cm, measured from the liquid surface molecules per cm3. Perhaps, at some point in of the reservoir. In this way, Torricelli the future, molecular density instead of the demonstrated that the space void of mercury pascal will serve as a measure of the nothing. in the tube was in fact a vacuum because it Without any doubt, there are could also be completely filled with water Credit: Universal History Archive / Getty macroscopic areas, for example, small from below and the mercury level was volumes of a few dm3 between galaxies, in independent of the volume above. Torricelli which not a single molecule is present. For had effectively invented the first . such a volume, the term ‘absolute vacuum’ Only four years later, in 1648, the purity and temperature. Second, the unit was introduced. We know today, however, French scientist and philosopher Blaise could not be expressed by the base units that even absolute vacuum is not empty in Pascal (pictured) showed that the mercury of the international system (SI) newly terms of energy. The nature of the energy height varied with air pressure, which introduced between 1948 and 1960. And of a vacuum is yet to be determined, and itself depends on altitude1 and weather third, new ways of vacuum measurement, may be related to the cosmological constant conditions. In the , Otto von Guericke, unrelated to mercury, were developed. introduced by Einstein, but we do know mayor of the German city of , For these reasons, at the 14th Conférence that it permits particles to be generated was the first to generate a large-scale Générale des Poids et Mesures (CGPM) in spontaneously by fluctuating quantum fields vacuum. He ordered members of the city’s 1971, the name pascal (Pa, 133.22 Pa = 1 torr) for short time intervals, even in absolute fire brigade to drain the water from a for the unit of pressure expressed as vacuum. In this sense, there is no space in water-filled vessel with one of their pumps. kg m–1 s–2 was introduced. The mbar, equal the world that is truly empty. ❐ This is why ‘vacuum pumps’ are still named to 100 Pa, was also a widely used unit for pumps, even though — from an vacuum pressures. Now, the most commonly Karl Jousten point of view — these instruments are in used vacuum range spans 15 orders of Physikalisch-Technische Bundesanstalt, Berlin, fact gas compressors. magnitude from 10–10 Pa for extremely high Germany. To measure a vacuum, the original vacuum, which is required in surface e-mail: [email protected] Torricelli tube was formed into a U-shape and elementary particle research, up to 105 Pa, with a connecting tube to the vacuum vessel. corresponding to atmospheric pressure. Published online: 3 June 2019 This laid the groundwork for the definition Several National Metrological Institutes https://doi.org/10.1038/s41567-019-0530-8 of the measure of vacuum with mm Hg, provide fundamental measurement equivalent to one millimetre of a mercury standards to realize this huge scale and References 1. Sparnaay, M. J. Adventures in Vacuum 78 (Elsevier Science, 2 column, identical to 1 torr at 0 °C. The calibrate vacuum gauges . North-Holland, 1992). ‘torr’ and ‘mm Hg’ as a quantity for pressure There is a new trend in the way we 2. Jousten, K. (ed.) Handbook of Vacuum Technology 2nd edn, Ch. suffered from several drawbacks. First, the define the pascal3,4, moving away from 15 (Wiley-VCH, 2016). 3. Jousten, K. et al. Metrologia 54, 146–161 (2017). value depends on the density of mercury a mechanical measurement as force per 4. Hendricks, J. Nat. Phys. 14, 100 (2018). and hence on its isotopic composition, area towards an optical measurement 5. Szwemin, P. Vacuum 82, 174–177 (2008).

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