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{{short description|Space that is empty of matter}}
{{hatnote group|
{{About|empty physical space or the absence of matter|the appliance|vacuum cleaner|other uses}}
{{Redirect|Free space}}
}}

[[File:Kolbenluftpumpe hg.jpg|thumb|300x300px|Vacuum pump and [[bell jar]] for vacuum experiments, used in science education during the early 20th century, on display in the Schulhistorische Sammlung ('School Historical Museum'), [[Bremerhaven]], Germany]]

A '''vacuum''' ({{plural form}}: '''vacuums''' or '''vacua''') is  [[space]] devoid of [[matter]]. The word is derived from the Latin adjective {{lang|la|vacuus}} (neuter {{lang|la|vacuum}}) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous [[pressure]] much less than [[atmospheric pressure]].<ref name="chambers">{{cite book |first=Austin |last=Chambers |date=2004 |title=Modern Vacuum Physics |publisher=CRC Press |location=Boca Raton |isbn=978-0-8493-2438-3 |oclc=55000526}}{{page needed|date=May 2013}}</ref>  Physicists often discuss ideal test results that would occur in a ''perfect'' vacuum, which they sometimes simply call "vacuum" or '''free space''', and use the term '''partial vacuum''' to refer to an actual imperfect vacuum as one might have in a [[laboratory]] or in [[outer space|space]]. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure.<ref>{{cite book |last=Harris |first=Nigel S. |date=1989 |title=Modern Vacuum Practice |publisher=McGraw-Hill |page=3 |isbn=978-0-07-707099-1 }}</ref> The Latin term '''''in vacuo''''' is used to describe an object that is surrounded by a vacuum.

The ''quality'' of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas [[pressure]] means higher-quality vacuum. For example, a typical [[vacuum cleaner]] produces enough [[suction]] to reduce air pressure by around 20%.<ref>{{cite book |last=Campbell |first=Jeff |date=2005 |isbn=978-1-59486-274-8 |page=97 |title=Speed cleaning |publisher=Rodale |url=https://books.google.com/books?id=hqegeIz9dyQC&pg=PA97}} Note that 1 inch of water is ≈0.0025 [[Atmosphere (unit)|atm]].</ref> But higher-quality vacuums are possible. [[Ultra-high vacuum]] chambers, common in chemistry, physics, and engineering, operate below one trillionth (10<sup>−12</sup>) of atmospheric pressure (100&nbsp;nPa), and can reach around 100 particles/cm<sup>3</sup>.<ref name=Gabrielse /> [[Outer space]] is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space.<ref name=tadokoro>{{cite journal
 | last=Tadokoro | first=M. | title=A Study of the Local Group by Use of the Virial Theorem
 | journal=Publications of the Astronomical Society of Japan
 | volume=20 | page=230 | date=1968
 | bibcode=1968PASJ...20..230T }} This source estimates a density of {{val|7|e=-29|u=g/cm<sup>3</sup>}} for the [[Local Group]]. An [[atomic mass unit]] is {{val|1.66|e=-24|u=g}}, for roughly 40 atoms per cubic meter.</ref>

Vacuum has been a frequent topic of [[philosophical]] debate since ancient [[Ancient Greece|Greek]] times, but was not studied empirically until the 17th century. [[Clemens Timpler]] (1605) philosophized about the experimental possibility of producing a vacuum in small tubes.<ref>{{Cite book |last=Jörg Hüttner & Martin Walter (Ed.) |title=Clemens Timpler: Physicae seu philosophiae naturalis systema methodicum. Pars prima; complectens physicam generalem. |publisher=Hildesheim / Zürich / New York: Georg Olms Verlag |year=2022 |isbn=978-3-487-16076-4 |pages=28–37}}</ref> [[Evangelista Torricelli]] produced the first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure. A Torricellian vacuum is created by filling with mercury a tall glass container closed at one end, and then inverting it in a bowl to contain the mercury (see below).<ref>[https://books.google.com/books?id=7igDAAAAMBAJ&pg=PT3 ''How to Make an Experimental Geissler Tube''], [[Popular Science]] monthly, February 1919, Unnumbered page. Bonnier Corporation</ref>

Vacuum became a valuable industrial tool in the 20th century with the introduction of [[incandescent light bulb]]s and [[vacuum tube]]s, and a wide array of vacuum technologies has since become available. The development of [[human spaceflight]] has raised interest in the impact of vacuum on human health, and on life forms in general.

== Etymology ==
The word ''vacuum'' comes {{ety|la||an empty space, void}}, noun use of neuter of ''vacuus'', meaning "empty", related to ''vacare'', meaning "to be empty".

''Vacuum'' is one of the few words in the English language that contains two consecutive instances of the vowel ''[[u]]''.<ref name=double_u>{{cite web | title=What words in the English language contain two u's in a row? | website=Oxford Dictionaries Online | url=https://en.oxforddictionaries.com/explore/what-words-in-the-english-language-contain-two-u-s-in-a-row/ | archive-url=https://web.archive.org/web/20180808104345/https://en.oxforddictionaries.com/explore/what-words-in-the-english-language-contain-two-u-s-in-a-row/ | url-status=dead | archive-date=August 8, 2018 | access-date=2011-10-23  }}</ref>

== Historical understanding ==
Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient [[Greek philosophy|Greek philosophers]] debated the existence of a vacuum, or void, in the context of [[atomism]], which posited void and atom as the fundamental explanatory elements of physics. [[Lucretius]] argued for the existence of vacuum in the first century BC and [[Hero of Alexandria]] tried unsuccessfully to create an artificial vacuum in the first century AD.<ref name="genz">{{cite book |last=Genz |first=Henning |title=Nothingness: The Science of Empty Space |date=1994 |publisher=Perseus Book Publishing |isbn=978-0-7382-0610-3 |edition= |place=New York |publication-date=1999 |oclc=48836264}}</ref>

Following [[Plato]], however, even the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, itself, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite literally nothing at all, which cannot rightly be said to exist. [[Aristotle]] believed that no void could occur naturally, because the denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void. In his ''[[Physics (Aristotle)|Physics]]'', book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continue ''ad infinitum'', there being no reason that something would come to rest anywhere in particular. 

In the medieval [[Muslim world]], the physicist and Islamic scholar [[Al-Farabi]] wrote a treatise rejecting the existence of the vacuum in the 10th century.<ref>{{Citation |url=https://seop.illc.uva.nl/entries/al-farabi/|year=2016|last=Druart|first=Therese-Anne|title=al-Farabi|encyclopedia=[[Stanford Encyclopedia of Philosophy]] |editor-last=Zalta |editor-first=Edward N. |edition= Winter 2021 |access-date= 2022-10-25}}</ref> He concluded that air's volume can expand to fill available space, and therefore the concept of a perfect vacuum was incoherent.<ref>{{Citation |last=McGinnis |first=Jon |title=Arabic and Islamic Natural Philosophy and Natural Science |date=2022 |url=https://plato.stanford.edu/archives/spr2022/entries/arabic-islamic-natural/ |encyclopedia=[[Stanford Encyclopedia of Philosophy]] |editor-last=Zalta |editor-first=Edward N. |edition=Spring 2022 |access-date=2022-08-11}}.</ref> According to [[Ahmad Dallal]], [[Abū Rayhān al-Bīrūnī]] states that "there is no observable evidence that rules out the possibility of vacuum".<ref name=Dallal>{{cite web|first=Ahmad|last=Dallal|date=2001–2002|title=The Interplay of Science and Theology in the Fourteenth-century Kalam|publisher=From Medieval to Modern in the Islamic World, Sawyer Seminar at the [[University of Chicago]]|url=http://humanities.uchicago.edu/orgs/institute/sawyer/archive/islam/dallal.html|access-date=2008-02-02|archive-url=https://web.archive.org/web/20120210094416/http://humanities.uchicago.edu/orgs/institute/sawyer/archive/islam/dallal.html|archive-date=2012-02-10|url-status=dead}}</ref> The [[suction]] [[pump]] was described by Arab engineer [[Al-Jazari]] in the 13th century, and later appeared in Europe from the 15th century.<ref name="Hill2">[[Donald Routledge Hill]], "Mechanical Engineering in the Medieval Near East", ''Scientific American'', May 1991, pp. 64–69 ([[cf.]] [[Donald Routledge Hill]], [http://home.swipnet.se/islam/articles/HistoryofSciences.htm Mechanical Engineering] {{webarchive|url=https://web.archive.org/web/20071225091836/http://home.swipnet.se/islam/articles/HistoryofSciences.htm|date=2007-12-25}}).</ref><ref>[[Donald Routledge Hill]] (1996), ''A History of Engineering in Classical and Medieval Times'', [[Routledge]], pp. 143, 150–152.</ref>

European [[scholasticism|scholars]] such as [[Roger Bacon]], [[Blasius of Parma]] and [[Walter Burley]] in the 13th and 14th century focused considerable attention on issues concerning the concept of a vacuum. The commonly held view that nature abhorred a vacuum was called ''[[horror vacui (physics)|horror vacui]]''. There was even speculation that even God could not create a vacuum if he wanted and the 1277 [[Paris condemnations]] of [[Bishop]] [[Étienne Tempier]], which required there to be no restrictions on the powers of God, led to the conclusion that God could create a vacuum if he so wished.<ref name="Barrow">{{cite book |last=Barrow |first=John D. |url=https://archive.org/details/bookofnothingvac0000barr |title=The Book of Nothing: Vacuums, Voids, and the Latest Ideas about the Origins of the Universe |date=2000 |publisher=Pantheon Books |isbn=978-0-09-928845-9 |edition=1st American |location=New York |oclc=46600561 |author-link=John D. Barrow |url-access=registration}}</ref> From the 14th century onward increasingly departed from the Aristotelian perspective, scholars widely acknowledged that a [[supernatural]] void exists beyond the confines of the cosmos itself by the 17th century. This idea, influenced by [[Stoic physics]], helped to segregate natural and theological concerns.<ref name="Barrow2002">{{cite book |first=J.D. |last=Barrow |date=2002 |title=The Book of Nothing: Vacuums, Voids, and the Latest Ideas About the Origins of the Universe |series=Vintage Series |publisher=Vintage |isbn=978-0-375-72609-5 |lccn=00058894 |url=https://books.google.com/books?id=sU_K0wbBeugC&pg=PA77 |pages=71–72, 77}}</ref>

Almost two thousand years after Plato, [[René Descartes]] also proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything [[dichotomy]] of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of his [[Cartesian coordinate system|namesake coordinate system]] and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct.{{citation needed|date=August 2024}}

[[File:Baro 0.png|thumb|100px|left|[[Evangelista Torricelli|Torricelli]]'s [[mercury (element)|mercury]] [[barometer]] produced one of the first sustained vacuums in a laboratory.]]

Medieval [[thought experiment]]s into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated.<ref name=grant>{{cite book
 | title = Much ado about nothing: theories of space and vacuum from the Middle Ages to the scientific revolution
 | author = Grant, Edward
 | publisher = Cambridge University Press
 | date = 1981
 | isbn = 978-0-521-22983-8
 | url = https://books.google.com/books?id=SidBQyFmgpsC
 }}</ref> There was much discussion of whether the air moved in quickly enough as the plates were separated, or, as [[Walter Burley]] postulated, whether a 'celestial agent' prevented the vacuum arising. [[Jean Buridan]] reported in the 14th century that teams of ten horses could not pull open [[bellows]] when the port was sealed.<ref name="genz" />

[[File:Crookes tube two views.jpg|right|thumb|The [[Crookes tube]], used to discover and study [[cathode ray]]s, was an evolution of the [[Geissler tube]].]]

The 17th century saw the first attempts to quantify measurements of partial vacuum.<ref>{{cite web |url=http://www.denmark.com.au/en/Worlds+Largest+Barometer/default.htm |title=The World's Largest Barometer |access-date=2008-04-30 |url-status=dead |archive-url=https://web.archive.org/web/20080417093648/http://www.denmark.com.au/en/Worlds%2BLargest%2BBarometer/default.htm |archive-date=2008-04-17 }}</ref> [[Evangelista Torricelli]]'s [[Mercury (element)|mercury]] [[barometer]] of 1643 and [[Blaise Pascal]]'s experiments both demonstrated a partial vacuum.

In 1654, [[Otto von Guericke]] invented the first [[vacuum pump]]<ref>{{Cite web |title=Otto von Guericke {{!}} Prussian physicist, engineer, and philosopher {{!}} Britannica |url=https://www.britannica.com/biography/Otto-von-Guericke |access-date=2022-08-11 |website=www.britannica.com |language=en}}</ref> and conducted his famous [[Magdeburg hemispheres]] experiment, showing that, owing to atmospheric pressure outside the hemispheres, teams of horses could not separate two hemispheres from which the air had been partially evacuated. [[Robert Boyle]] improved Guericke's design and with the help of [[Robert Hooke]] further developed vacuum pump technology. Thereafter, research into the partial vacuum lapsed until 1850 when [[August Toepler]] invented the [[Toepler pump]] and in 1855 when [[Heinrich Geissler]] invented the mercury displacement pump, achieving a partial vacuum of about 10&nbsp;Pa (0.1&nbsp;[[Torr]]). A number of electrical properties become observable at this vacuum level, which renewed interest in further research.

While outer space provides the most rarefied example of a naturally occurring partial vacuum, the heavens were originally thought to be seamlessly filled by a rigid indestructible material called [[aether (classical element)|aether]]. Borrowing somewhat from the [[pneuma]] of [[Stoic physics]], aether came to be regarded as the rarefied air from which it took its name, (see [[Aether (mythology)]]). Early theories of light posited a ubiquitous terrestrial and celestial medium through which light propagated. Additionally, the concept informed [[Isaac Newton]]'s explanations of both [[refraction]] and of radiant heat.<ref>[[Robert Hogarth Patterson]], ''Essays in History and Art 10'', 1862.</ref> 19th century experiments into this [[luminiferous aether]] attempted to detect a minute drag on the Earth's orbit. While the Earth does, in fact, move through a relatively dense medium in comparison to that of interstellar space, the drag is so minuscule that it could not be detected. In 1912, [[astronomer]] [[William Henry Pickering|Henry Pickering]] commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".<ref>{{cite journal | last1 = Pickering | first1 = W.H. | date =1912 | title = Solar system, the motion of the, relatively to the interstellar absorbing medium | journal = [[Monthly Notices of the Royal Astronomical Society]] | volume = 72 | issue = 9 |bibcode=1912MNRAS..72..740P | page = 740 | doi=10.1093/mnras/72.9.740| url = https://zenodo.org/record/1431891 | doi-access = free }}</ref> Thereafter, however, luminiferous aether was discarded.

Later, in 1930, [[Paul Dirac]] proposed a model of the vacuum as an infinite sea of particles possessing negative energy, called the [[Dirac sea]]. This theory helped refine the predictions of his earlier formulated [[Dirac equation]], and successfully predicted the existence of the [[positron]], confirmed two years later. [[Werner Heisenberg]]'s [[uncertainty principle]], formulated in 1927, predicted a fundamental limit within which instantaneous position and [[momentum]], or energy and time can be measured. This far reaching consequences also threatened whether the "emptiness" of space between particles exists.

== Classical field theories ==
{{more citations needed|subsection|date=April 2014}}
The strictest criterion to define a vacuum is a region of space and time where all the components of the [[stress–energy tensor]] are zero. This means that this region is devoid of energy and momentum, and by consequence, it must be empty of particles and other physical fields (such as electromagnetism) that contain energy and momentum.

=== Gravity ===
{{more citations needed|subsection|date=April 2014}}
In [[general relativity]], a vanishing stress–energy tensor implies, through [[Einstein field equations]], the vanishing of all the components of the [[Ricci tensor]]. Vacuum does not mean that the curvature of [[space-time]] is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and [[gravitational wave]]s (technically, these phenomena are the components of the [[Weyl tensor]]). The [[black hole]] (with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.

=== Electromagnetism ===
In [[classical electromagnetism]], the '''vacuum of free space''', or sometimes just ''free space'' or ''perfect vacuum'', is a standard reference medium for electromagnetic effects.<ref name=weig>{{cite book
 | title = Introduction to complex mediums for optics and electromagnetics
 | author = Werner S. Weiglhofer
 | editor= Werner S. Weiglhofer
 | editor2= Akhlesh Lakhtakia
 | publisher = SPIE Press
 | chapter=§&nbsp;4.1 The classical vacuum as reference medium
 | date = 2003
 | isbn = 978-0-8194-4947-4
 | pages = 28, 34
 | chapter-url = https://books.google.com/books?id=QtIP_Lr3gngC&pg=PA34
 }}</ref><ref>{{cite book
 | title = Progress in Optics
 | volume = 51
 | chapter = Electromagnetic Fields in Linear Bianisotropic Mediums
 | author = Tom G. MacKay
 | editor = Emil Wolf
 | publisher = Elsevier
 | date = 2008
 | isbn = 978-0-444-52038-8
 | page = 143
 | chapter-url = https://books.google.com/books?id=lCm9Q18P8cMC&pg=PA143
 }}</ref> Some authors refer to this reference medium as ''classical vacuum'',<ref name=weig /> a terminology intended to separate this concept from [[QED vacuum]] or [[QCD vacuum]], where [[vacuum fluctuation]]s can produce transient [[virtual particle]] densities and a [[relative permittivity]] and [[Permeability (electromagnetism)#Relative permeability and magnetic susceptibility|relative permeability]] that are not identically unity.<ref name=Grynberg>{{cite book |url=https://books.google.com/books?id=l-l0L8YInA0C&pg=PA341 |page=341 |publisher=Cambridge University Press |date=2010 |title=Introduction to Quantum Optics: From the Semi-Classical Approach to Quantized Light |quote=...deals with the quantum vacuum where, in contrast to the classical vacuum, radiation has properties, in particular, fluctuations, with which one can associate physical effects. |isbn=978-0-521-55112-0 |author=Gilbert Grynberg |author2=Alain Aspect |author3=Claude Fabre}}</ref><ref name=Susskind>For a qualitative description of vacuum fluctuations and virtual particles, see {{cite book
|author = Leonard Susskind
|title = The cosmic landscape: string theory and the illusion of intelligent design
|publisher = Little, Brown and Co.
|date = 2006
|isbn = 978-0-316-01333-8
|url = https://books.google.com/books?id=RIW9E1sOyxUC&pg=PP60
|pages = 60 ''ff''}}</ref><ref name=Holstein>The relative permeability and permittivity of field-theoretic vacuums is described in {{cite book |title=Concepts of particle physics |volume=2 |author=Kurt Gottfried |author2=Victor Frederick Weisskopf |url=https://books.google.com/books?id=KXvoI-m9-9MC&pg=PA389 |page=389 |isbn=978-0-19-503393-9 |date=1986 |publisher=Oxford University Press}} and more recently in {{cite book
|author = John F. Donoghue
|author2 = Eugene Golowich
|author3 = Barry R. Holstein
|title = Dynamics of the standard model
|publisher = Cambridge University Press
|date = 1994
|isbn = 978-0-521-47652-2
|url = https://books.google.com/books?id=hFasRlkBbpYC&pg=PA47
|page = 47}} and also {{cite book |title=QCD and collider physics |author=R. Keith Ellis |author2=W.J. Stirling |author3=B.R. Webber |url=https://books.google.com/books?id=TqrPVoS6s0UC&pg=PA27 |pages=27–29 |isbn=978-0-521-54589-1 |date=2003 |quote=Returning to the vacuum of a relativistic field theory, we find that both paramagnetic and diamagnetic contributions are present. |publisher=Cambridge University Press}} [[QCD vacuum]] is [[Paramagnetism|paramagnetic]], while [[QED vacuum]] is [[Diamagnetism|diamagnetic]]. See {{cite book |title=Nuclear physics in a nutshell |author=Carlos A. Bertulani |url=https://books.google.com/books?id=n51yJr4b_oQC&pg=PA26 |page=26 |isbn=978-0-691-12505-3 |date=2007 |publisher=Princeton University Press|bibcode=2007npn..book.....B }}</ref>

In the theory of classical electromagnetism, free space has the following properties:
* Electromagnetic radiation travels, when unobstructed, at the [[speed of light]], the defined value 299,792,458&nbsp;m/s in [[SI units]].<ref name=NISTc>{{cite web |title=Speed of light in vacuum, ''c, c''<sub>0</sub> |website=The NIST reference on constants, units, and uncertainty: Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?c |publisher=NIST |access-date=2011-11-28}}</ref>
* The [[superposition principle]] is always exactly true.<ref>{{cite book
|author = Chattopadhyay, D.
|author2 = Rakshit, P.C.
|name-list-style = amp
|title = Elements of Physics
|volume = 1
|publisher = New Age International
|date = 2004
|isbn = 978-81-224-1538-4
|url = https://books.google.com/books?id=tvkoopJMQQ8C&pg=PA577
|page = 577}}</ref> For example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. The value of the [[electric field]] at any point around these two charges is found by calculating the [[Vector (mathematics and physics)|vector]] sum of the two electric fields from each of the charges acting alone.
* The [[permittivity]] and [[Permeability (electromagnetism)|permeability]] are exactly the electric constant [[vacuum permittivity|{{math|''ε''<sub>0</sub>}}]]<ref name=NISTep0>{{cite web |title=Electric constant, ε<sub>0</sub> |website=The NIST reference on constants, units, and uncertainty: Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?ep0|publisher=NIST |access-date=2011-11-28}}</ref> and magnetic constant [[vacuum permeability|{{math|''μ''<sub>0</sub>}}]],<ref name=NISTmu0>{{cite web |title=Magnetic constant, μ<sub>0</sub> |website=The NIST reference on constants, units, and uncertainty: Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?mu0|publisher=NIST |access-date=2011-11-28}}</ref> respectively (in [[SI units]]), or exactly 1 (in [[Gaussian units]]).
* The [[characteristic impedance]] ({{mvar|η}}) equals the [[impedance of free space]] {{math|''Z''<sub>0</sub>}} ≈ 376.73&nbsp;Ω.<ref name=NISTz>{{cite web |title=Characteristic impedance of vacuum, ''Z''<sub>0</sub> |website=The NIST reference on constants, units, and uncertainty: Fundamental physical constants |url=http://physics.nist.gov/cgi-bin/cuu/Value?z0 |access-date=2011-11-28}}</ref>

The vacuum of classical electromagnetism can be viewed as an idealized electromagnetic medium with the [[Constitutive equation#Electromagnetism|constitutive relations]] in SI units:<ref name=E_Wolf>{{cite book |author=Mackay, Tom G |author2=Lakhtakia, Akhlesh |name-list-style=amp |editor=Emil Wolf |title=Progress in Optics  |volume=51 |isbn=978-0-444-53211-4 |date=2008 |publisher=Elsevier |chapter-url=https://books.google.com/books?id=lCm9Q18P8cMC&pg=PA143 |chapter=§&nbsp;3.1.1 Free space |page=143 }}</ref>
:<math>\boldsymbol D(\boldsymbol r,\ t) = \varepsilon_0 \boldsymbol E(\boldsymbol r,\ t)\, </math>
:<math>\boldsymbol H(\boldsymbol r,\ t) = \frac{1}{\mu_0} \boldsymbol B(\boldsymbol r,\ t)\, </math>
relating the [[electric displacement]] field {{math|'''''D'''''}} to the [[electric field]] {{math|'''''E'''''}} and the [[magnetic field]] or ''H''-field {{math|'''''H'''''}} to the [[magnetic field|magnetic induction]] or ''B''-field {{math|'''''B'''''}}. Here {{math|'''''r'''''}} is a spatial location and {{mvar|t}} is time.

=={{anchor|The quantum-mechanical vacuum}} Quantum mechanics==

{{Further|QED vacuum|QCD vacuum|Vacuum state}}

[[File:Vacuum fluctuations revealed through spontaneous parametric down-conversion.ogv|thumb|350px|A video of an experiment showing [[vacuum fluctuations]] (in the red ring) amplified by [[spontaneous parametric down-conversion]].]]

In [[quantum mechanics]] and [[quantum field theory]], the vacuum is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (the [[ground state]] of the [[Hilbert space]]). In [[quantum electrodynamics]] this vacuum is referred to as '[[QED vacuum]]' to distinguish it from the vacuum of [[quantum chromodynamics]], denoted as [[QCD vacuum]]. QED vacuum is a state with no matter particles (hence the name), and no [[photon]]s. As described above, this state is impossible to achieve experimentally. (Even if every matter particle could somehow be removed from a volume, it would be impossible to eliminate all the [[black-body radiation|blackbody photons]].) Nonetheless, it provides a good model for realizable vacuum, and agrees with a number of experimental observations as described next.

QED vacuum has interesting and complex properties. In QED vacuum, the electric and magnetic fields have zero average values, but their variances are not zero.<ref name=Craig>For example, see {{cite book |title=Molecular Quantum Electrodynamics |author=Craig, D.P. |author2= Thirunamachandran, T. |name-list-style=amp|url=https://books.google.com/books?id=rpbdozIZt3sC&pg=PA40 |page=40 |isbn=978-0-486-40214-7 |publisher=Courier Dover Publications |date=1998 |edition=Reprint of Academic Press 1984}}</ref> As a result, QED vacuum contains [[vacuum fluctuations]] ([[virtual particles]] that hop into and out of existence), and a finite energy called [[vacuum energy]]. Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some experimentally verified effects of vacuum fluctuations include [[spontaneous emission]] and the [[Lamb shift]].<ref name=Barrow />  [[Coulomb's law]] and the [[electric potential]] in vacuum near an electric charge are modified.<ref name=Zeidler>In effect, the dielectric permittivity of the vacuum of classical electromagnetism is changed. For example, see {{cite book |chapter=§&nbsp;19.1.9 Vacuum polarization in quantum electrodynamics |author=Zeidler, Eberhard |chapter-url=https://books.google.com/books?id=miwuxaEXvOsC&pg=PA952 |page=952 |isbn=978-3-642-22420-1 |publisher=Springer |date=2011 |title=Quantum Field Theory III: Gauge Theory: A Bridge Between Mathematicians and Physicists}}</ref>

Theoretically, in QCD multiple vacuum states can coexist.<ref name=Altarelli>{{cite book |chapter-url=https://books.google.com/books?id=lBCyYTobfJ8C&pg=PT19 |pages=2–3 |author=Altarelli, Guido  |title=Elementary Particles: Volume 21/A of Landolt-Börnstein series |chapter=Chapter 2: Gauge theories and the Standard Model |quote=The fundamental state of minimum energy, the vacuum, is not unique and there are a continuum of degenerate states that altogether respect the symmetry... |isbn=978-3-540-74202-9 |publisher=Springer |date=2008 }}</ref> The starting and ending of [[Inflation (cosmology)|cosmological inflation]] is thought to have arisen from transitions between different vacuum states. For theories obtained by quantization of a classical theory, each [[stationary point]] of the energy in the [[Configuration space (physics)|configuration space]] gives rise to a single vacuum. [[String theory]] is believed to have a huge number of vacua – the so-called [[string theory landscape]].

== Outer space ==
{{Main|Outer space}}

[[File:Structure_of_the_magnetosphere_LanguageSwitch.svg|lang=en|left|thumb|upright=1.75|Structure of the [[magnetosphere]] - is not a perfect vacuum, but a tenuous [[Plasma (physics)|plasma]] awash with charged particles, free elements such as [[hydrogen]], [[helium]] and [[oxygen]], [[electromagnetic field]]s.]]

[[Outer space]] has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.<ref name=tadokoro />

Stars, planets, and moons keep their [[atmosphere]]s 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 {{convert|32|mPa}} at {{convert|100|km|mi}} of altitude,<ref name=squire2000>{{cite journal | first=Tom | last=Squire | date=September 27, 2000 | title=U.S. Standard Atmosphere, 1976 | journal=Thermal Protection Systems Expert and Material Properties Database | url=http://tpsx.arc.nasa.gov/cgi-perl/alt.pl | access-date=2011-10-23  | url-status=dead | archive-url=https://web.archive.org/web/20111015062917/http://tpsx.arc.nasa.gov/cgi-perl/alt.pl | archive-date=October 15, 2011 }}</ref> the [[Kármán line]], which is a common definition of the boundary with outer space. 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]]s, 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.

But although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant [[Drag (physics)|drag]] on [[satellite]]s. Most artificial satellites operate in this region called [[low Earth orbit]] and must fire their engines every couple of weeks or a few times a year (depending on solar activity).<ref>{{Cite web|url=https://earthobservatory.nasa.gov/features/OrbitsCatalog/page3.php|title=Catalog of Earth Satellite Orbits|date=2009-09-04|website=earthobservatory.nasa.gov|language=en|access-date=2019-01-28}}</ref> The drag here is low enough that it could theoretically be overcome by radiation pressure on [[solar sail]]s, a proposed propulsion system for [[interplanetary travel]].<ref>{{cite journal|last1=Andrews|first1=Dana G.|last2=Zubrin|first2=Robert M. |url=http://pdfs.semanticscholar.org/9f10/d81e06d0f8515411bff54728029f0b5551dc.pdf|archive-url=https://web.archive.org/web/20190302064738/http://pdfs.semanticscholar.org/9f10/d81e06d0f8515411bff54728029f0b5551dc.pdf|url-status=dead|archive-date=2019-03-02|title=Magnetic Sails & Interstellar Travel |date=1990|journal=Journal of the British Interplanetary Society|doi=10.2514/3.26230|volume=43|pages=265–272|s2cid=55324095|access-date=2019-07-21}}</ref> 

All of the [[observable universe]] is filled with large numbers of [[photon]]s, the so-called [[cosmic background radiation]], and quite likely a correspondingly large number of [[neutrino]]s. The current [[temperature]] of this radiation is about {{convert|3|K|C F|lk=on}}.

== Measurement ==
{{Main|Pressure measurement}}

The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its [[absolute pressure]], but a complete characterization requires further parameters, such as [[temperature]] and chemical composition. One of the most important parameters is the [[mean free path]] (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of [[fluid mechanics]] do not apply. This vacuum state is called ''high vacuum'', and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70&nbsp;[[nanometer|nm]], but at 100&nbsp;[[millipascal|mPa]] (≈{{val|e=-3|u=[[Torr]]}}) the MFP of room temperature air is roughly 100&nbsp;mm, which is on the order of everyday objects such as [[vacuum tube]]s. The [[Crookes radiometer]] turns when the MFP is larger than the size of the vanes.

Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges were defined in ISO 3529-1:2019 as shown in the following table (100&nbsp;Pa corresponds to 0.75&nbsp;Torr; Torr is a non-SI unit):
{| class="wikitable"
! scope=col | Pressure range
! scope=col | Definition
! scope=col | The reasoning for the definition of the ranges is as follows (typical circumstances):
|-
|Prevailing atmospheric pressure (31&nbsp;kPa to 110&nbsp;kPa) to 100&nbsp;Pa
|low  (rough) vacuum
|Pressure can be achieved by simple materials (e.g. regular steel) and positive displacement vacuum pumps; viscous flow regime for gases
|-
|<100 Pa to 0.1 Pa
|medium  (fine) vacuum
|Pressure can be achieved by elaborate materials (e.g. stainless steel) and positive displacement vacuum pumps; transitional flow regime for gases
|-
|<0.1 Pa to {{val|1|e=-6|u=Pa}}
|high vacuum (HV)
|Pressure can be achieved by elaborate materials (e.g. stainless steel), elastomer sealings and high vacuum pumps; molecular flow regime for gases
|-
|<{{val|1|e=-6|u=Pa}} to {{val|1|e=-9|u=Pa}}
|ultra-high vacuum (UHV)
|Pressure can be achieved by elaborate materials (e.g. low-carbon stainless steel), metal sealings, special surface preparations and cleaning, bake-out and high vacuum pumps; molecular flow regime for gases
|-
|below  {{val|1|e=-9|u=Pa}}
|extreme-high vacuum (XHV)
|Pressure can be achieved by sophisticated materials (e.g. vacuum fired low-carbon stainless steel, aluminium, copper-beryllium, titanium), metal sealings, special surface preparations and cleaning, bake-out and additional getter pumps; molecular flow regime for gases
|}
* '''Atmospheric pressure''' is variable but {{convert|101.325|and|100|kPa|mbar|comma=off}} are common [[Standard temperature and pressure|standard or reference pressures]].
* '''Deep space''' is generally much more empty than any artificial vacuum. It may or may not meet the definition of high vacuum above, depending on what region of space and astronomical bodies are being considered. For example, the MFP of interplanetary space is smaller than the size of the Solar System, but larger than small planets and moons. As a result, solar winds exhibit continuum flow on the scale of the Solar System, but must be considered a bombardment of particles with respect to the Earth and Moon.
* '''Perfect vacuum''' is an ideal state of no particles at all. It cannot be achieved in a [[laboratory]], although there may be small volumes which, for a brief moment, happen to have no particles of matter in them. Even if all particles of matter were removed, there would still be [[photon]]s, as well as [[dark energy]], [[virtual particle]]s, and other aspects of the [[quantum vacuum]].

=== Relative versus absolute measurement ===
Vacuum is measured in units of [[pressure]], typically as a subtraction relative to ambient atmospheric pressure on Earth. But the amount of relative measurable vacuum varies with local conditions. On the surface of [[Venus]], where ground-level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible. On the surface of the Moon with almost no atmosphere, it would be extremely difficult to create a measurable vacuum relative to the local environment.

Similarly, much higher than normal relative vacuum readings are possible deep in the Earth's ocean. A [[submarine]] maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 metres; a 9.8-metre column of seawater has the equivalent weight of 1 atm) is effectively a vacuum chamber keeping out the crushing exterior water pressures, though the 1&nbsp;atm inside the submarine would not normally be considered a vacuum.

Therefore, to properly understand the following discussions of vacuum measurement, it is important that the reader assumes the relative measurements are being done on Earth at sea level, at exactly 1 atmosphere of ambient atmospheric pressure.

=== Measurements relative to 1&nbsp;atm ===
[[File:McLeod gauge 01.jpg|upright|thumb|A glass McLeod gauge, drained of mercury]]
The [[SI]] unit of pressure is the [[pascal (unit)|pascal]] (symbol Pa), but vacuum is often measured in [[torr]]s, named for an Italian physicist Torricelli (1608–1647). A torr is equal to the displacement of a millimeter of mercury ([[mmHg]]) in a [[manometer]] with 1&nbsp;torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on the [[barometer|barometric]] scale or as a percentage of [[atmospheric pressure]] in [[bar (unit)|bars]] or [[atmosphere (unit)|atmospheres]]. Low vacuum is often measured in [[millimeters of mercury]] (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure.

In other words, most low vacuum gauges that read, for example 50.79&nbsp;Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0&nbsp;Torr but in practice this generally requires a two-stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 1&nbsp;torr.

=== Measuring instruments ===
Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.<ref>{{cite book| first=Moore | last=John H. | author2=Christopher Davis | author3=Michael A. Coplan | author4=Sandra Greer | name-list-style=amp | title=Building Scientific Apparatus | publisher=Westview Press | location=Boulder, Colorado | date=2002 | isbn=978-0-8133-4007-4 | oclc=50287675 }}{{page needed|date=May 2013}}</ref>

'''Hydrostatic''' gauges (such as the mercury column [[manometer]]) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but [[Mercury (element)|mercury]] is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1&nbsp;torr (100&nbsp;Pa) to above atmospheric. An important variation is the [[McLeod gauge]] which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10<sup>−6</sup>&nbsp;torr (0.1&nbsp;mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge.<ref name=measure>{{cite book| first=Thomas G. | last=Beckwith | author2=Roy D. Marangoni | author3=John H. Lienhard V | name-list-style=amp | date=1993 | title=Mechanical Measurements | edition=Fifth | publisher=Addison-Wesley | location=Reading, Massachusetts | isbn=978-0-201-56947-6 | pages=591–595 | chapter=Measurement of Low Pressures }}</ref>

The kenotometer is a particular type of hydrostatic gauge, typically used in power plants using steam turbines. The kenotometer measures the vacuum in the steam space of the condenser, that is, the exhaust of the last stage of the turbine.<ref>{{cite web | url=http://www.ephf.ca/blog.asp?id=74 | title=Kenotometer Vacuum Gauge | publisher=Edmonton Power Historical Foundation | date=22 November 2013 | access-date=3 February 2014}}</ref>

'''Mechanical''' or '''elastic''' gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idea is the '''capacitance manometer''', in which the diaphragm makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10<sup>3</sup>&nbsp;torr to 10<sup>−4</sup>&nbsp;torr, and beyond.

'''Thermal conductivity''' gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A  [[Thermocouple#Thermocouple as vacuum gauge|thermocouple]] or [[Resistance Temperature Detector]] (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the [[Pirani gauge]] which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10&nbsp;torr to 10<sup>−3</sup>&nbsp;torr, but they are sensitive to the chemical composition of the gases being measured.

'''[[Ionization gauge]]s''' are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In the [[Hot filament ionization gauge|hot cathode]] version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10<sup>−3</sup>&nbsp;torr to 10<sup>−10</sup> torr. The principle behind [[cold cathode]] version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10<sup>−2</sup>&nbsp;torr to 10<sup>−9</sup>&nbsp;torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.<ref>{{cite encyclopedia | editor=Robert M. Besançon | encyclopedia=The Encyclopedia of Physics | edition=3rd | date=1990 | publisher=Van Nostrand Reinhold, New York | isbn = 978-0-442-00522-1 | pages = 1278–1284 | article=Vacuum Techniques}}</ref>

== Uses ==
[[File:Gluehlampe 01 KMJ.jpg|thumb|upright|[[incandescent light bulb|Light bulbs]] contain a partial vacuum, usually backfilled with [[argon]], which protects the [[tungsten]] filament]]

Vacuum is useful in a variety of processes and devices. Its first widespread use was in the [[incandescent light bulb]] to protect the filament from chemical degradation. The chemical inertness produced by a vacuum is also useful for [[electron beam welding]], [[cold welding]], [[vacuum packing]] and [[Vacuum fryer|vacuum frying]]. [[Ultra-high vacuum]] is used in the study of atomically clean substrates, as only a very good vacuum preserves atomic-scale clean surfaces for a reasonably long time (on the order of minutes to days). High to ultra-high vacuum removes the obstruction of air, allowing particle beams to deposit or remove materials without contamination. This is the principle behind [[chemical vapor deposition]], [[physical vapor deposition]], and [[dry etching]] which are essential to the fabrication of [[semiconductor fabrication|semiconductors]] and [[optical coating]]s, and to [[surface science]]. The reduction of convection provides the thermal insulation of [[thermos bottle]]s. Deep vacuum lowers the [[boiling point]] of liquids and promotes low temperature [[outgassing]] which is used in [[freeze drying]], [[adhesive]] preparation, [[vacuum distillation|distillation]], [[metallurgy]], and process purging. The electrical properties of vacuum make [[electron microscope]]s and [[vacuum tube]]s possible, including [[cathode-ray tube]]s. [[Vacuum interrupter]]s are used in electrical switchgear.  [[Vacuum arc]] processes are industrially important for production of certain grades of steel or high purity materials. The elimination of air [[friction]] is useful for [[flywheel energy storage]] and [[ultracentrifuge]]s.

[[File:L-Pumpe2.png|thumb|left|upright|This shallow water well pump reduces atmospheric air pressure inside the pump chamber. Atmospheric pressure extends down into the well, and forces water up the pipe into the pump to balance the reduced pressure. Above-ground pump chambers are only effective to a depth of approximately 9 meters due to the water column weight balancing the atmospheric pressure.]]

=== Vacuum-driven machines ===
Vacuums are commonly used to produce [[suction]], which has an even wider variety of applications. The [[Newcomen steam engine]] used vacuum instead of pressure to drive a piston. In the 19th century, vacuum was used for traction on [[Isambard Kingdom Brunel]]'s experimental [[atmospheric railway]].  [[Vacuum brake]]s were once widely used on [[train]]s in the UK but, except on [[heritage railway]]s, they have been replaced by [[Railway air brake|air brakes]].

[[Manifold vacuum]] can be used to drive [[Automobile ancillary power#Vacuum|accessories]] on [[automobile]]s. The best known application is the [[vacuum servo]], used to provide power assistance for the [[brake]]s. Obsolete applications include vacuum-driven [[windscreen wipers]] and [[Autovac]] fuel pumps. Some aircraft instruments ([[Attitude indicator|Attitude Indicator (AI)]] and the [[Heading indicator|Heading Indicator (HI)]]) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments, since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft, the engine and an external venturi.
[[Vacuum induction melting]] uses electromagnetic induction within a vacuum.

Maintaining a vacuum in the [[Condenser (steam turbine)|condenser]] is an important aspect of the efficient operation of [[steam turbine]]s. A steam jet [[Steam ejector|ejector]] or [[Liquid ring pump|liquid ring vacuum pump]] is used for this purpose. The typical vacuum maintained in the condenser steam space at the exhaust of the turbine (also called condenser backpressure) is in the range 5 to 15 kPa (absolute), depending on the type of condenser and the ambient conditions.

=== Outgassing ===
{{Main|Outgassing}}

[[Evaporation]] and [[sublimation (chemistry)|sublimation]] into a vacuum is called [[outgassing]]. All materials, solid or liquid, have a small [[vapour pressure]], and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. Outgassing has the same effect as a leak and will limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they obscure optical instruments or react with other materials. This is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.

The most prevalent outgassing product in vacuum systems is water absorbed by chamber materials. It can be reduced by desiccating or baking the chamber, and removing absorbent materials. Outgassed water can condense in the oil of [[rotary vane pump]]s and reduce their net speed drastically if gas ballasting is not used. High vacuum systems must be clean and free of organic matter to minimize outgassing.

Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by [[liquid nitrogen]] to shut down residual outgassing and simultaneously [[cryopump]] the system.

=== Pumping and ambient air pressure ===
[[File:Hand pump.png|thumb|left|upright|Deep wells have the pump chamber down in the well close to the water surface, or in the water. A "sucker rod" extends from the handle down the center of the pipe deep into the well to operate the plunger. The pump handle acts as a heavy counterweight against both the sucker rod weight and the weight of the water column standing on the upper plunger up to ground level.]]

{{Main|Vacuum pump}}

Fluids cannot generally be pulled, so a vacuum cannot be created by [[suction]]. Suction can spread and dilute a vacuum by letting a higher pressure push fluids into it, but the vacuum has to be created first before suction can occur. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the [[diaphragm (anatomy)|diaphragm muscle]] expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.

To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind [[vacuum pump#Positive displacement pump|positive displacement pumps]], like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.

[[File:Cut through turbomolecular pump.jpg|thumb|upright|A cutaway view of a [[turbomolecular pump]], a momentum transfer pump used to achieve high vacuum]]

The above explanation is merely a simple introduction to vacuum pumping, and is not representative of the entire range of pumps in use. Many variations of the positive displacement pump have been developed, and many other pump designs rely on fundamentally different principles. [[vacuum pump#Momentum transfer pump|Momentum transfer pumps]], which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps. [[vacuum pump#Entrapment pump|Entrapment pumps]] can capture gases in a solid or absorbed state, often with no moving parts, no seals and no vibration. None of these pumps are universal; each type has important performance limitations. They all share a difficulty in pumping low molecular weight gases, especially [[hydrogen]], [[helium]], and [[neon]].

The lowest pressure that can be attained in a system is also dependent on many things other than the nature of the pumps. Multiple pumps may be connected in series, called stages, to achieve higher vacuums. The choice of seals, chamber geometry, materials, and pump-down procedures will all have an impact. Collectively, these are called ''vacuum technique''. And sometimes, the final pressure is not the only relevant characteristic. Pumping systems differ in oil contamination, vibration, preferential pumping of certain gases, pump-down speeds, intermittent duty cycle, reliability, or tolerance to high leakage rates.

In [[ultra high vacuum]] systems, some very "odd" leakage paths and outgassing sources must be considered. The water absorption of [[aluminium]] and [[palladium]] becomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel or [[titanium]] must be considered. Some oils and greases will boil off in extreme vacuums. The permeability of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.

The lowest pressures currently achievable in laboratory are about {{convert|1e-13|torr|pPa}}.<ref>{{cite journal| author=Ishimaru, H | title= Ultimate Pressure of the Order of 10<sup>−13</sup> torr in an Aluminum Alloy Vacuum Chamber | journal= Journal of Vacuum Science and Technology | date=1989 | volume=7 | issue=3–II | pages= 2439–2442 | doi= 10.1116/1.575916 | bibcode= 1989JVSTA...7.2439I }}</ref>  However, pressures as low as {{convert|5e-17|torr|fPa}} have been indirectly measured in a {{convert|4|K|C F}} cryogenic vacuum system.<ref name=Gabrielse>{{cite journal | doi = 10.1103/PhysRevLett.65.1317| pmid = 10042233| title = Thousandfold improvement in the measured antiproton mass| journal = Physical Review Letters| volume = 65| issue = 11| pages = 1317–1320| year = 1990| last1 = Gabrielse | first1 = G.| last2 = Fei | first2 = X.| last3 = Orozco | first3 = L.| last4 = Tjoelker | first4 = R.| last5 = Haas | first5 = J.| last6 = Kalinowsky | first6 = H.| last7 = Trainor | first7 = T.| last8 = Kells | first8 = W.|bibcode = 1990PhRvL..65.1317G | url = https://cds.cern.ch/record/493773/files/cm-p00043126.pdf}}</ref> This corresponds to ≈100 particles/cm<sup>3</sup>.

== Effects on humans and animals ==
{{See also|Space exposure|Uncontrolled decompression}}
[[File:An Experiment on a Bird in an Air Pump by Joseph Wright of Derby, 1768.jpg|thumb|This painting, ''[[An Experiment on a Bird in the Air Pump]]'' by [[Joseph Wright of Derby]], 1768, depicts an experiment performed by [[Robert Boyle]] in 1660.]]

Humans and animals exposed to vacuum will lose [[consciousness]] after a few seconds and die of [[Hypoxia (medical)|hypoxia]] within minutes, but the symptoms are not nearly as graphic as commonly depicted in media and popular culture. The reduction in pressure lowers the temperature at which blood and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains above the internal body temperature of {{nowrap|37 °C.}}<ref name="Landis Vacuum Exposure">{{cite web|last=Landis|first=Geoffrey|author-link=Geoffrey A. Landis|date=7 August 2007|title=Human Exposure to Vacuum|url=http://www.geoffreylandis.com/vacuum.html|url-status=dead|archive-url=https://web.archive.org/web/20090721182306/http://www.geoffreylandis.com/vacuum.html|archive-date=21 July 2009|access-date=25 March 2006|publisher=geoffreylandis.com}}</ref> Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known as [[ebullism]], is still a concern. The gas may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture.<ref name="NASA Bio Data Book">{{cite book| first=Charles E. | last=Billings | editor=Parker, James F. | editor2=West, Vita R. | date=1973 | title=Bioastronautics Data Book| edition=Second | publisher=NASA | id=NASA SP-3006 | chapter= Chapter 1) Barometric Pressure | page=5| hdl=2060/19730006364 }}</ref> Swelling and ebullism can be restrained by containment in a [[flight suit]]. [[Space Shuttle program|Shuttle]] astronauts wore a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2&nbsp;kPa (15&nbsp;Torr).<ref name="Webb Space Activity Suit">{{cite journal| author=Webb P. | title= The Space Activity Suit: An Elastic Leotard for Extravehicular Activity | journal=Aerospace Medicine | date=1968 | volume=39 | issue= 4| pages= 376–383 | pmid=4872696}}</ref> Rapid boiling will cool the skin and create frost, particularly in the mouth, but this is not a significant hazard.

Animal experiments show that rapid and complete recovery is normal for exposures shorter than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful.<ref name="Cooke Cardiovascular Responses">{{cite journal
 | pmid = 5972265
| year = 1966
| last1 = Cooke
| first1 = J.P.
| title = Some cardiovascular responses in anesthetized dogs during repeated decompressions to a near-vacuum
| journal = Aerospace Medicine
| volume = 37
| issue = 11
| pages = 1148–1152
| last2 = Bancroft
| first2 = R.W.
}}</ref> A study by NASA on eight chimpanzees found all of them survived two and a half minute exposures to vacuum.<ref>{{Cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19650027167.pdf|title=The Effect on the Chimpanzee of Rapid Decompression to a near Vacuum|date=November 1965|last1=Koestler|first1=A. G.|website=NASA}}</ref> There is only a limited amount of data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if breathing is not impaired.<ref name="harding">{{cite book | last1 =Harding | first1 =Richard M. | date =1989 | title =Survival in Space: Medical Problems of Manned Spaceflight | place =London | publisher =Routledge | isbn =978-0-415-00253-0 | oclc =18744945 | url-access =registration | url =https://archive.org/details/survivalinspacem0000hard }}.</ref> [[Robert Boyle]] was the first to show in 1660 that vacuum is lethal to small animals.

An experiment indicates that plants are able to survive in a low pressure environment (1.5&nbsp;kPa) for about 30 minutes.<ref>{{cite journal |doi=10.1016/j.asr.2010.12.017 |title=Plants survive rapid decompression: Implications for bioregenerative life support |date=2011 |last1=Wheeler |first1=R.M. |last2=Wehkamp |first2=C.A. |last3=Stasiak |first3=M.A. |last4=Dixon |first4=M.A. |last5=Rygalov |first5=V.Y. |journal=Advances in Space Research |volume=47 |issue=9 |pages=1600–1607 |bibcode=2011AdSpR..47.1600W|hdl=2060/20130009997 |hdl-access=free }}</ref><ref>{{cite journal |pmid=11987308 |date=2002 |last1=Ferl |first1=RJ |last2=Schuerger |first2=AC |last3=Paul |first3=AL |last4=Gurley |first4=WB |last5=Corey |first5=K |last6=Bucklin |first6=R |title=Plant adaptation to low atmospheric pressures: Potential molecular responses |volume=8 |issue=2 |pages=93–101 |journal=Life Support & Biosphere Science}}</ref>

Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level atmosphere. The colder air temperatures found at altitudes of up to 3&nbsp;km generally compensate for the lower pressures there.<ref name="harding" /> Above this altitude, oxygen enrichment is necessary to prevent [[altitude sickness]] in humans that did not undergo prior [[acclimatization]], and [[spacesuit]]s are necessary to prevent ebullism above 19&nbsp;km.<ref name="harding" />  Most spacesuits use only 20&nbsp;kPa (150&nbsp;Torr) of pure oxygen. This pressure is high enough to prevent ebullism, but [[decompression sickness]] and [[air embolism|gas embolisms]] can still occur if decompression rates are not managed.

Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his or her breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate [[Pulmonary alveolus|alveoli]] of the [[lung]]s.<ref name="harding" /> [[Eardrum]]s and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia.<ref name="Landis Ebullism">{{cite web| author=Czarnik, Tamarack R.| date=1999|website=unpublished review by Landis, Geoffrey A. | title=EBULLISM AT 1 MILLION FEET: Surviving Rapid/Explosive Decompression | url=http://www.geoffreylandis.com/ebullism.html | publisher= geoffreylandis}}</ref> Injuries caused by rapid decompression are called [[barotrauma]]. A pressure drop of 13&nbsp;kPa (100&nbsp;Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.<ref name="harding" />

Some [[extremophile]] [[microorganisms]], such as [[tardigrade]]s, can survive vacuum conditions for periods of days or weeks.<ref>{{cite journal | title = Tardigrades survive exposure to space in low Earth orbit | journal = Current Biology | date = 9 September 2008 |author = Jönsson, K. Ingemar | author2 = Rabbow, Elke | author3 = Schill, Ralph O. | author4 = Harms-Ringdahl, Mats | author5 = Rettberg, Petra | name-list-style = amp | volume = 18 | issue = 17 | pages = R729–R731| doi=10.1016/j.cub.2008.06.048 | pmid=18786368| s2cid = 8566993 | doi-access = free | bibcode = 2008CBio...18.R729J }}</ref>

== Examples ==
{{See also|Vacuum pump}}
{| class="wikitable" style="text-align:left"
! !! Pressure (Pa if not explained) !! Pressure (Torr, atm) !! [[Mean free path]] !! Molecules per cm<sup>3</sup>
|-
! [[Atmospheric pressure|Standard atmosphere]], for comparison
| 101.325 kPa || {{convert|760|torr|atm}} || 66&nbsp;nm || {{val|2.5|e=19}}<ref>Computed using [http://www.luizmonteiro.com/StdAtm.aspx "1976 Standard Atmosphere Properties"] calculator. Retrieved 2012-01-28</ref>
|-
! Intense [[hurricane]]
| approx. 87 to 95 kPa || 650 to 710 || ||
|-
! [[Vacuum cleaner]]
| approximately 80 kPa || 600 || 70&nbsp;nm || 10<sup>19</sup>
|-
!  [[Steam turbine]] exhaust ([[Condenser (steam turbine)#Vacuum system|Condenser backpressure]])
| 9&nbsp;kPa ||   ||  ||
|-
! [[Liquid-ring pump|liquid ring vacuum pump]]
| approximately 3.2&nbsp;kPa|| {{convert|24|torr|atm}} || 1.75&nbsp;μm || 10<sup>18</sup>
|-
! [[Atmosphere of Mars|Mars atmosphere]]
| 1.155&nbsp;kPa to 0.03&nbsp;kPa (mean 0.6&nbsp;kPa) || {{convert|8.66| to|0.23|torr|atm}} || ||
|-
! [[freeze drying]]
| 100 to 10 || 1 to 0.1 || 100&nbsp;μm to 1&nbsp;mm || 10<sup>16</sup> to 10<sup>15</sup>
|-
! [[Incandescent light bulb]]
| 10 to 1 || {{convert|0.1| to| 0.01|torr|atm}} || 1&nbsp;mm to 1&nbsp;cm || 10<sup>15</sup> to 10<sup>14</sup>
|-
! [[Thermos bottle]]
| 1 to 0.01 <ref name="chambers" /> || {{convert| 1e-2|to| 1e-4|torr|atm}} || 1&nbsp;cm to 1&nbsp;m|| 10<sup>14</sup> to 10<sup>12</sup>
|-
! Earth [[thermosphere]]
| 1&nbsp;Pa to {{val|1|e=-7}} || 10<sup>−2</sup> to 10<sup>−9</sup> || 1&nbsp;cm to 100&nbsp;km || 10<sup>14</sup> to 10<sup>7</sup>
|-
! [[Vacuum tube]]
| {{val|1|e=-5}} to {{val|1|e=-8}} || 10<sup>−7</sup> to 10<sup>−10</sup> || 1 to 1,000&nbsp;km || 10<sup>9</sup> to 10<sup>6</sup>
|-
! Pressure on the [[Moon]]
| approximately {{val|1|e=-9}} || 10<sup>−11</sup> || 10,000&nbsp;km || {{val|4|e=5}}<ref>{{cite journal |bibcode=1962P&SS....9..211O |title=The lunar atmosphere |last1=Öpik |first1=E.J. |volume=9 |date=1962 |pages=211–244 |journal=Planetary and Space Science |doi=10.1016/0032-0633(62)90149-6 |issue=5}}</ref>
|-
! [[Cryopump]]ed [[molecular beam epitaxy|MBE]] chamber
| {{val|1|e=-6}} to {{val|1|e=-10}} || 10<sup>−8</sup> to 10<sup>−12</sup> || 10 to 100,000&nbsp;km || 10<sup>8</sup> to 10<sup>4</sup>
|-
! Dense [[nebula]]
| &nbsp;|| &nbsp;|| || 10,000<ref name="chambers" />
|-
! [[Interplanetary space]]
| &nbsp;|| &nbsp;|| || 11<ref name="chambers" />
|-
! [[Interstellar medium|Interstellar space]]
| &nbsp;|| &nbsp;|| || 1<ref>{{cite web | author=University of New Hampshire Experimental Space Plasma Group | title=What is the Interstellar Medium | website=The Interstellar Medium, an online tutorial | url=http://www-ssg.sr.unh.edu/ism/what1.html | access-date=2006-03-15 | archive-url=https://web.archive.org/web/20060217172614/http://www-ssg.sr.unh.edu/ism/what1.html | archive-date=2006-02-17 | url-status=dead }}</ref>
|-
! [[Outer space#Intergalactic space|Intergalactic space]]
| &nbsp;||  || || 10<sup>−6</sup><ref name="chambers" />
|}

== See also ==
{{cols|colwidth=21em}}
* [[Decay of the vacuum]] ([[Pair production]])
* [[Manifold vacuum|Engine vacuum]]
* [[False vacuum]]
* [[Helium mass spectrometer]] – technical instrumentation to detect a vacuum leak
* [[Brazing#Vacuum brazing|Vacuum brazing]]
* [[Pneumatic tube]] – transport system using vacuum or pressure to move containers in tubes
* [[Rarefaction]] – reduction of a medium's density
* [[Suction]] – creation of a partial vacuum
* [[Theta vacuum]] – vacuum state of semi-classical pure-Yang Mills theories
* [[Vactrain]]
* [[Vacuum cementing]] – natural process of solidifying homogeneous "dust" in vacuum
* [[Vacuum column (tape drive)|Vacuum column]] – controlling loose magnetic tape in early computer data recording tape drives
* [[Vacuum deposition]] – process of depositing atoms and molecules in a sub-atmospheric pressure environment
* [[Vacuum engineering]]
* [[Vacuum flange]] – joining of [[vacuum system]]s
{{colend}}

== References ==

{{Reflist|30em}}
* {{cite book |title=Nothingness: The Science Of Empty Space |url=https://books.google.com/books?id=TGm2ddkL4qkC |author=Henning Genz |isbn=978-0-7382-0610-3 |publisher=Da Capo Press |date=2001}}
* {{cite book |title=The Quantum Vacuum: A Scientific and Philosophical Concept, from Electrodynamics to String Theory and the Geometry of the Microscopic World |url=https://books.google.com/books?id=rAEVOLae_FoC |author=Luciano Boi |date=2011 |publisher=Johns Hopkins University Press |isbn=978-1-4214-0247-5}}

== External links ==
{{Wikiquote}}
{{Wiktionary}}
{{Commons category}}
* [https://www.leybold.com/en/downloads/download-documents/brochures/ Leybold – Fundamentals of Vacuum Technology (PDF)]
* [http://spacegeek.org/ep9_QT.shtml VIDEO on the nature of vacuum] by Canadian astrophysicist Doctor P
* [https://web.archive.org/web/20080216082832/http://www.svc.org/H/H_HistoryArticle.html The Foundations of Vacuum Coating Technology]
* [http://www.avs.org/ American Vacuum Society]
* [http://scitation.aip.org/jvsta/ Journal of Vacuum Science and Technology A]
* [http://scitation.aip.org/jvstb/ Journal of Vacuum Science and Technology B]
* [http://www.sff.net/people/Geoffrey.Landis/vacuum.html FAQ on explosive decompression and vacuum exposure].
* [http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html Discussion of the effects on humans of exposure to hard vacuum].
* {{cite journal |arxiv=hep-th/0012062 |first=Mark D. |last=Roberts |title=Vacuum Energy |journal=High Energy Physics – Theory |pages=hep–th/0012062 |date=2000|bibcode = 2000hep.th...12062R }}
* [http://void.mit.edu/~4.396/wiki/index.php?title=Main_Page Vacuum, Production of Space]
* [https://web.archive.org/web/20070930185128/http://www.gresham.ac.uk/event.asp?PageId=4&EventId=258 "Much Ado About Nothing" by Professor John D. Barrow, Gresham College]
* Free pdf copy of [http://www.physics.arizona.edu/~rafelski/Books/StructVacuumE.pdf The Structured Vacuum – thinking about nothing] by  [[Johann Rafelski]] and Berndt Muller (1985)    {{ISBN|3-87144-889-3}}.

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