Vacuum

This article is about empty physical space or the absence of matter. For the appliance, seevacuum cleaner.For other uses, seeVacuum (disambiguation)."Free space" redirects here. For other uses, seeFree space (disambiguation).

Vacuum pump andbell jarfor vacuum experiments, used in science education during the early 20th century, on display in the Schulhistorische Sammlung ('School Historical Museum'),Bremerhaven,Germany

Avacuum(pl.:vacuumsorvacua) isspacedevoid ofmatter.The word is derived from the Latin adjectivevacuus(neutervacuum) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseouspressuremuch less thanatmospheric pressure.Physicists often discuss ideal test results that would occur in aperfectvacuum, which they sometimes simply call "vacuum" orfree space,and use the termpartial vacuumto refer to an actual imperfect vacuum as one might have in alaboratoryor inspace.In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure.The Latin termin vacuois used to describe an object that is surrounded by a vacuum.

Thequalityof a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gaspressuremeans higher-quality vacuum. For example, a typicalvacuum cleanerproduces enoughsuctionto reduce air pressure by around 20%.But higher-quality vacuums are possible.Ultra-high vacuumchambers, common in chemistry, physics, and engineering, operate below one trillionth (10⁻¹²) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm³.Outer spaceis an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space.

Vacuum has been a frequent topic ofphilosophicaldebate since ancientGreektimes, but was not studied empirically until the 17th century.Clemens Timpler(1605) philosophized about the experimental possibility of producing a vacuum in small tubes.Evangelista Torricelliproduced 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).

Vacuum became a valuable industrial tool in the 20th century with the introduction ofincandescent light bulbsandvacuum tubes,and a wide array of vacuum technologies has since become available. The development ofhuman spaceflighthas raised interest in the impact of vacuum on human health, and on life forms in general.

Etymology

The wordvacuumcomes fromLatin'an empty space, void', noun use of neuter ofvacuus,meaning "empty", related tovacare,meaning "to be empty".

Vacuumis one of the few words in the English language that contains two consecutive instances of the vowelu.

Historical understanding

Historically, there has been much dispute over whether such a thing as a vacuum can exist. AncientGreek philosophersdebated the existence of a vacuum, or void, in the context ofatomism,which posited void and atom as the fundamental explanatory elements of physics.Lucretiusargued for the existence of vacuum in the first century BC andHero of Alexandriatried unsuccessfully to create an artificial vacuum in the first century AD.

FollowingPlato,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.Aristotlebelieved 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 hisPhysics,book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continuead infinitum,there being no reason that something would come to rest anywhere in particular.

In the medievalMuslim world,the physicist and Islamic scholarAl-Farabiwrote a treatise rejecting the existence of the vacuum in the 10th century.He concluded that air's volume can expand to fill available space, and therefore the concept of a perfect vacuum was incoherent.According toAhmad Dallal,Abū Rayhān al-Bīrūnīstates that "there is no observable evidence that rules out the possibility of vacuum".Thesuctionpumpwas described by Arab engineerAl-Jazariin the 13th century, and later appeared in Europe from the 15th century.

Europeanscholarssuch asRoger Bacon,Blasius of ParmaandWalter Burleyin 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 calledhorror vacui.There was even speculation that even God could not create a vacuum if he wanted and the 1277Paris condemnationsofBishopÉtienne 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.From the 14th century onward increasingly departed from the Aristotelian perspective, scholars widely acknowledged that asupernaturalvoid exists beyond the confines of the cosmos itself by the 17th century. This idea, influenced byStoic physics,helped to segregate natural and theological concerns.

Almost two thousand years after Plato,René Descartesalso proposed a geometrically based alternative theory of atomism, without the problematic nothing–everythingdichotomyof void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of hisnamesake coordinate systemand 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.

Torricelli'smercurybarometerproduced one of the first sustained vacuums in a laboratory.

Medievalthought experimentsinto 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.There was much discussion of whether the air moved in quickly enough as the plates were separated, or, asWalter Burleypostulated, whether a 'celestial agent' prevented the vacuum arising.Jean Buridanreported in the 14th century that teams of ten horses could not pull openbellowswhen the port was sealed.

TheCrookes tube,used to discover and studycathode rays,was an evolution of theGeissler tube.

The 17th century saw the first attempts to quantify measurements of partial vacuum.Evangelista Torricelli'smercurybarometerof 1643 andBlaise Pascal's experiments both demonstrated a partial vacuum.

In 1654,Otto von Guerickeinvented the firstvacuum pumpand conducted his famousMagdeburg hemispheresexperiment, 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 Boyleimproved Guericke's design and with the help ofRobert Hookefurther developed vacuum pump technology. Thereafter, research into the partial vacuum lapsed until 1850 whenAugust Toeplerinvented theToepler pumpand in 1855 whenHeinrich Geisslerinvented the mercury displacement pump, achieving a partial vacuum of about 10 Pa (0.1Torr). 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 calledaether.Borrowing somewhat from thepneumaofStoic physics,aether came to be regarded as the rarefied air from which it took its name, (seeAether (mythology)). Early theories of light posited a ubiquitous terrestrial and celestial medium through which light propagated. Additionally, the concept informedIsaac Newton's explanations of bothrefractionand of radiant heat.19th century experiments into thisluminiferous aetherattempted 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,astronomerHenry Pickeringcommented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".Thereafter, however, luminiferous aether was discarded.

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

Classical field theories

The strictest criterion to define a vacuum is a region of space and time where all the components of thestress–energy tensorare 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

Ingeneral relativity,a vanishing stress–energy tensor implies, throughEinstein field equations,the vanishing of all the components of theRicci tensor.Vacuum does not mean that the curvature ofspace-timeis necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces andgravitational waves(technically, these phenomena are the components of theWeyl tensor). Theblack hole(with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.

Electromagnetism

Inclassical electromagnetism,thevacuum of free space,or sometimes justfree spaceorperfect vacuum,is a standard reference medium for electromagnetic effects.Some authors refer to this reference medium asclassical vacuum,a terminology intended to separate this concept fromQED vacuumorQCD vacuum,wherevacuum fluctuationscan produce transientvirtual particledensities and arelative permittivityandrelative permeabilitythat are not identically unity.

In the theory of classical electromagnetism, free space has the following properties:

• Electromagnetic radiation travels, when unobstructed, at thespeed of light,the defined value 299,792,458 m/s inSI units.
• Thesuperposition principleis always exactly true.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 theelectric fieldat any point around these two charges is found by calculating thevectorsum of the two electric fields from each of the charges acting alone.
• Thepermittivityandpermeabilityare exactly the electric constantε₀and magnetic constantμ₀,respectively (inSI units), or exactly 1 (inGaussian units).
• Thecharacteristic impedance(η) equals theimpedance of free spaceZ₀≈ 376.73 Ω.

The vacuum of classical electromagnetism can be viewed as an idealized electromagnetic medium with theconstitutive relationsin SI units:

${\boldsymbol {D}}({\boldsymbol {r}},\ t)=\varepsilon _{0}{\boldsymbol {E}}({\boldsymbol {r}},\ t)\,$

${\boldsymbol {H}}({\boldsymbol {r}},\ t)={\frac {1}{\mu _{0}}}{\boldsymbol {B}}({\boldsymbol {r}},\ t)\,$

relating theelectric displacementfieldDto theelectric fieldEand themagnetic fieldorH-fieldHto themagnetic inductionorB-fieldB.Hereris a spatial location andtis time.

Quantum mechanics

Further information:QED vacuum,QCD vacuum,andVacuum state

A video of an experiment showingvacuum fluctuations(in the red ring) amplified byspontaneous parametric down-conversion.

Inquantum mechanicsandquantum field theory,the vacuum is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (theground stateof theHilbert space). Inquantum electrodynamicsthis vacuum is referred to as 'QED vacuum' to distinguish it from the vacuum ofquantum chromodynamics,denoted asQCD vacuum.QED vacuum is a state with no matter particles (hence the name), and nophotons.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 theblackbody 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.As a result, QED vacuum containsvacuum fluctuations(virtual particlesthat hop into and out of existence), and a finite energy calledvacuum energy.Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some experimentally verified effects of vacuum fluctuations includespontaneous emissionand theLamb shift.Coulomb's lawand theelectric potentialin vacuum near an electric charge are modified.

Theoretically, in QCD multiple vacuum states can coexist.The starting and ending ofcosmological inflationis thought to have arisen from transitions between different vacuum states. For theories obtained by quantization of a classical theory, eachstationary pointof the energy in theconfiguration spacegives rise to a single vacuum.String theoryis believed to have a huge number of vacua – the so-calledstring theory landscape.

Outer space

Main article:Outer space

Structure of themagnetosphere- is not a perfect vacuum, but a tenuousplasmaawash with charged particles, free elements such ashydrogen,heliumandoxygen,electromagnetic fields.

Outer spacehas 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.

Stars, planets, and moons keep theiratmospheresby 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 32 millipascals (4.6×10⁻⁶psi) at 100 kilometres (62 mi) of altitude,theKármán line,which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared toradiation pressurefrom theSunand thedynamic pressureof thesolar winds,so the definition of pressure becomes difficult to interpret. Thethermospherein this range has large gradients of pressure, temperature and composition, and varies greatly due tospace weather.Astrophysicists prefer to usenumber densityto 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 significantdragonsatellites.Most artificial satellites operate in this region calledlow Earth orbitand must fire their engines every couple of weeks or a few times a year (depending on solar activity).The drag here is low enough that it could theoretically be overcome by radiation pressure onsolar sails,a proposed propulsion system forinterplanetary travel.

All of theobservable universeis filled with large numbers ofphotons,the so-calledcosmic background radiation,and quite likely a correspondingly large number ofneutrinos.The currenttemperatureof this radiation is about 3K(−270.15°C;−454.27°F).

Measurement

Main article: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 itsabsolute pressure,but a complete characterization requires further parameters, such astemperatureand chemical composition. One of the most important parameters is themean 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 offluid mechanicsdo not apply. This vacuum state is calledhigh 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, 70nm,but at 100mPa(≈10⁻³Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such asvacuum tubes.TheCrookes radiometerturns 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 Pa corresponds to 0.75 Torr; Torr is a non-SI unit):

Pressure range	Definition	The reasoning for the definition of the ranges is as follows (typical circumstances):
Prevailing atmospheric pressure (31 kPa to 110 kPa) to 100 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 to1×10−6Pa	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
<1×10−6Pato1×10−9Pa	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
below1×10−9Pa	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 pressureis variable but 101.325 and 100 kilopascals (1013.25 and 1000.00 mbar) are commonstandard or reference pressures.
• Deep spaceis 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 vacuumis an ideal state of no particles at all. It cannot be achieved in alaboratory,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 bephotons,as well asdark energy,virtual particles,and other aspects of thequantum vacuum.

Relative versus absolute measurement

Vacuum is measured in units ofpressure,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 ofVenus,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. Asubmarinemaintaining 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 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 atm

A glass McLeod gauge, drained of mercury

TheSIunit of pressure is thepascal(symbol Pa), but vacuum is often measured intorrs,named for an Italian physicist Torricelli (1608–1647). A torr is equal to the displacement of a millimeter of mercury (mmHg) in amanometerwith 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on thebarometricscale or as a percentage ofatmospheric pressureinbarsoratmospheres.Low vacuum is often measured inmillimeters 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 Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0 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 torr.

Measuring instruments

Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.

Hydrostaticgauges (such as the mercury columnmanometer) 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, butmercuryis preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 torr (100 Pa) to above atmospheric. An important variation is theMcLeod gaugewhich 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 10⁻⁶torr (0.1 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.

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.

Mechanicalorelasticgauges 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 thecapacitance 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 10³torr to 10⁻⁴torr, and beyond.

Thermal conductivitygauges 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. AthermocoupleorResistance 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 thePirani gaugewhich uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 torr to 10⁻³torr, but they are sensitive to the chemical composition of the gases being measured.

Ionization gaugesare used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In thehot cathodeversion 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 10⁻³torr to 10⁻¹⁰torr. The principle behindcold cathodeversion is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10⁻²torr to 10⁻⁹torr. 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.

Uses

Light bulbscontain a partial vacuum, usually backfilled withargon,which protects thetungstenfilament

Vacuum is useful in a variety of processes and devices. Its first widespread use was in theincandescent light bulbto protect the filament from chemical degradation. The chemical inertness produced by a vacuum is also useful forelectron beam welding,cold welding,vacuum packingandvacuum frying.Ultra-high vacuumis 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 behindchemical vapor deposition,physical vapor deposition,anddry etchingwhich are essential to the fabrication ofsemiconductorsandoptical coatings,and tosurface science.The reduction of convection provides the thermal insulation ofthermos bottles.Deep vacuum lowers theboiling pointof liquids and promotes low temperatureoutgassingwhich is used infreeze drying,adhesivepreparation,distillation,metallurgy,and process purging. The electrical properties of vacuum makeelectron microscopesandvacuum tubespossible, includingcathode ray tubes.Vacuum interruptersare used in electrical switchgear.Vacuum arcprocesses are industrially important for production of certain grades of steel or high purity materials. The elimination of airfrictionis useful forflywheel energy storageandultracentrifuges.

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 producesuction,which has an even wider variety of applications. TheNewcomen steam engineused vacuum instead of pressure to drive a piston. In the 19th century, vacuum was used for traction onIsambard Kingdom Brunel's experimentalatmospheric railway.Vacuum brakeswere once widely used ontrainsin the UK but, except onheritage railways,they have been replaced byair brakes.

Manifold vacuumcan be used to driveaccessoriesonautomobiles.The best known application is thevacuum servo,used to provide power assistance for thebrakes.Obsolete applications include vacuum-drivenwindscreen wipersandAutovacfuel pumps. Some aircraft instruments (Attitude Indicator (AI)and theHeading 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 meltinguses electromagnetic induction within a vacuum.

Maintaining a vacuum in thecondenseris an important aspect of the efficient operation ofsteam turbines.A steam jetejectororliquid ring vacuum pumpis 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 article:Outgassing

Evaporationandsublimationinto a vacuum is calledoutgassing.All materials, solid or liquid, have a smallvapour 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 ofrotary vane pumpsand 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 byliquid nitrogento shut down residual outgassing and simultaneouslycryopumpthe system.

Pumping and ambient air pressure

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 article:Vacuum pump

Fluids cannot generally be pulled, so a vacuum cannot be created bysuction.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, thediaphragm muscleexpands 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 behindpositive 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.

A cutaway view of aturbomolecular 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.Momentum transfer pumps,which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps.Entrapment pumpscan 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, especiallyhydrogen,helium,andneon.

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 calledvacuum 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.

Inultra high vacuumsystems, some very "odd" leakage paths and outgassing sources must be considered. The water absorption ofaluminiumandpalladiumbecomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel ortitaniummust 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 1×10⁻¹³torrs (13 pPa).However, pressures as low as 5×10⁻¹⁷torrs (6.7 fPa) have been indirectly measured in a 4 K (−269.15 °C; −452.47 °F) cryogenic vacuum system.This corresponds to ≈100 particles/cm³.

Effects on humans and animals

See also:Space exposureandUncontrolled decompression

This painting,An Experiment on a Bird in the Air PumpbyJoseph Wright of Derby,1768, depicts an experiment performed byRobert Boylein 1660.

Humans and animals exposed to vacuum will loseconsciousnessafter a few seconds and die ofhypoxiawithin 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 of37 °C.Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known asebullism,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.Swelling and ebullism can be restrained by containment in aflight suit.Shuttleastronauts wore a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa (15 Torr).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.A study by NASA on eight chimpanzees found all of them survived two and a half minute exposures to vacuum.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.Robert Boylewas 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 kPa) for about 30 minutes.

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 km generally compensate for the lower pressures there.Above this altitude, oxygen enrichment is necessary to preventaltitude sicknessin humans that did not undergo prioracclimatization,andspacesuitsare necessary to prevent ebullism above 19 km.Most spacesuits use only 20 kPa (150 Torr) of pure oxygen. This pressure is high enough to prevent ebullism, butdecompression sicknessandgas embolismscan 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 delicatealveoliof thelungs.Eardrumsand 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.Injuries caused by rapid decompression are calledbarotrauma.A pressure drop of 13 kPa (100 Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.

Someextremophilemicroorganisms,such astardigrades,can survive vacuum conditions for periods of days or weeks.

Examples

See also:Vacuum pump

	Pressure (Pa if not explained)	Pressure (Torr, atm)	Mean free path	Molecules per cm3
Standard atmosphere,for comparison	101.325 kPa	760 torrs (1.00 atm)	66 nm	2.5×1019
Intensehurricane	approx. 87 to 95 kPa	650 to 710
Vacuum cleaner	approximately 80 kPa	600	70 nm	1019
Steam turbineexhaust (Condenser backpressure)	9 kPa
liquid ring vacuum pump	approximately 3.2 kPa	24 torrs (0.032 atm)	1.75 μm	1018
Mars atmosphere	1.155 kPa to 0.03 kPa (mean 0.6 kPa)	8.66 to 0.23 torrs (0.01139 to 0.00030 atm)
freeze drying	100 to 10	1 to 0.1	100 μm to 1 mm	1016to 1015
Incandescent light bulb	10 to 1	0.1 to 0.01 torrs (0.000132 to 1.3×10−5atm)	1 mm to 1 cm	1015to 1014
Thermos bottle	1 to 0.01	1×10−2to 1×10−4torrs (1.316×10−5to 1.3×10−7atm)	1 cm to 1 m	1014to 1012
Earththermosphere	1 Pa to1×10−7	10−2to 10−9	1 cm to 100 km	1014to 107
Vacuum tube	1×10−5to1×10−8	10−7to 10−10	1 to 1,000 km	109to 106
Pressure on theMoon	approximately1×10−9	10−11	10,000 km	4×105
CryopumpedMBEchamber	1×10−6to1×10−10	10−8to 10−12	10 to 100,000 km	108to 104
Densenebula				10,000
Interplanetary space				11
Interstellar space				1
Intergalactic space				10−6

See also

• Decay of the vacuum(Pair production)
• Engine vacuum
• False vacuum
• Helium mass spectrometer– technical instrumentation to detect a vacuum leak
• 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– 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 ofvacuum systems

References

• Henning Genz (2001).Nothingness: The Science Of Empty Space.Da Capo Press.ISBN978-0-7382-0610-3.
• Luciano Boi (2011).The Quantum Vacuum: A Scientific and Philosophical Concept, from Electrodynamics to String Theory and the Geometry of the Microscopic World.Johns Hopkins University Press.ISBN978-1-4214-0247-5.

External links

• Leybold – Fundamentals of Vacuum Technology (PDF)
• VIDEO on the nature of vacuumby Canadian astrophysicist Doctor P
• The Foundations of Vacuum Coating Technology
• American Vacuum Society
• Journal of Vacuum Science and Technology A
• Journal of Vacuum Science and Technology B
• FAQ on explosive decompression and vacuum exposure.
• Discussion of the effects on humans of exposure to hard vacuum.
• Roberts, Mark D. (2000). "Vacuum Energy".High Energy Physics – Theory:hep–th/0012062.arXiv:hep-th/0012062.Bibcode:2000hep.th...12062R.
• Vacuum, Production of Space
• "Much Ado About Nothing" by Professor John D. Barrow, Gresham College
• Free pdf copy ofThe Structured Vacuum – thinking about nothingbyJohann Rafelskiand Berndt Muller (1985)ISBN3-87144-889-3.