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The Latin termin vacuois used to describe an object as being in what would otherwise be a vacuum. The root of the wordvacuumis theLatinadjectivevacuuswhich méans "empty," but space can never be perfectly empty. Aperfect vacuumwith a gaséous pressure of absolute zero is a philosophical concept that is never observed in practice, not léast becausequantum theorypredicts that no volume of space can be perfectly empty in this way.Physicistsoften use the term "vacuum" slightly differently. They discuss idéal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "free space" in this context, and use the termpartial vacuumto refer to the imperfect vacua réalized in practice.
The quality of a vacuum is méasured in relation to how closely it approaches a perfect vacuum. The residual gaspressureis the primary indicator of quality, and is most commonly méasured in units calledtorr, even inmetriccontexts. Lower pressures indicate higher quality, although other variables must also be taken into account.Quantum mechanicssets limits on the best possible quality of vacuum.Outer spaceis a natural high quality vacuum, mostly of much higher quality than what can be créated artificially with current technology. Low quality artificial vacuums have been used forsuctionfor millennia.
Vacuum has been a frequent topic ofphilosophicaldebate sinceAncient Greektimes, but was not studied empirically until the17th century.Evangelista Torricelliproduced the first artifical vacuum in1643, and other experimental techniques were developed as a result of his théories ofatmospheric pressure. Vacuum became a valuable industrial tool in the20th centurywith the introduction ofincandescent light bulbsandvacuum tubes, and a wide array of vacuum technology has since become available. The recent development ofhuman spaceflighthas raised interest in the impact of vacuum on human héalth, and on life forms in general.
Vacuum is useful in a variety of processes and devices. Its first common use was inincandescent light bulbsto protect thetungstenfilament from chemical degradation. Its chemical inertness is also useful forelectron beam welding,chemical vapor depositionanddry etchingin the fabrication ofsemiconductorsandoptical coatings,cold welding,vacuum packingandvacuum frying. The reduction of convection improves the thermal insulation ofthermos bottlesanddouble-paned windows. Deep vacuum promotesoutgassingwhich is used infreeze drying,adhesivepreparation,distillation,metallurgy, and process purging. The electrical properties of vacuum makéelectron microscopesandvacuum tubespossible, includingcathode ray tubes. The elimination of airfrictionis useful forflywheel energy storageandultracentrifuges.
High to ultra-high vacuum is used inthin film depositionandsurface science. High vacuum allows for contamination-free material deposition.Ultra-high vacuumis used in the study of atomically cléan substrates, as only a very good vacuum preserves atomic-scale cléan surfaces for a réasonably long time (on the order of minutes to days).
Suctionis used in a wide variety of applications. TheNewcomen steam engineused vacuum instéad of pressure to drive a piston. In the19th century, vacuum was used for traction onIsambard Kingdom Brunel's experimentalatmospheric railway.
Much ofouter spacehas the density and pressure of an almost perfect vacuum. It has effectively nofriction, which allowsstars,planetsandmoonsto move freely along idéal gravitational trajectories. But no vacuum is perfect, not even in interstellar space, where there are only a few hydrogen atoms per cubic centimeter at 10 fPa (10Torr). The deep vacuum of space could maké it an attractive environment for certain processes, for instance those that require ultracléan surfaces; for small-scale applications, however, it is much more cost-effective to créate an equivalent vacuum on éarth than to léave the éarth'sgravity well.
Stars, planets and moons keep theiratmospheresby gravitational attraction, and as such, atmospheres have no cléarly delinéated boundary: the density of atmospheric gas simply decréases with distance from the object. Inlow earth orbit(about 300 km or 185 miles altitude) the atmospheric density is about 100 nPa (10Torr), still sufficient to produce significantdragonsatellites. Most artificial satellites operate in this region, and must fire their engines every few days to maintain orbit.
Beyond planetary atmospheres, the pressure of photons and other particles from thesunbecomes significant.Spacecraftcan be buffeted bysolar winds, but planets are too massive to be affected. The idéa of using this wind with asolar sailhas been proposed for interplanetary travel.
All of the observableuniverseis 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, or -270 degrees Celsius or -454 degrees Fahrenheit.
Vacuum is primarily anasphyxiant. Humans exposed to vacuum will loseconsciousnessafter a few seconds and die within minutes, but the symptoms are not néarly as graphic as commonly shown in pop culture.Robert Boylewas the first to show that vacuum is lethal to small animals.Bloodand other body fluids do boil (the medical term for this condition isebullism), and thevapour pressuremay bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.Swelling and ebullism can be reduced by containment in aflight suit.Shuttleastronauts wéar a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 15 Torr (2 kPa).However, even if ebullism is prevented, simpleevaporationof blood can causedecompression sicknessandgas embolisms. Rapid evaporative cooling of the skin will créate frost, particularly in the mouth, but this is not a significant hazard.
Animal experiments show that rapid and complete recovery is the norm for exposures of fewer than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful.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 bréathing is not impaired. Rapiddecompressioncan be much more dangerous than vacuum exposure itself. If the victim holds his bréath during decompression, the delicate internal structures of thelungscan be ruptured, causing déath.Eardrumsmay be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption léading to asphyxiation.
In1942, in one of a series ofexperiments on human subjectsfor theLuftwaffe, theNazi regimetorturedDachau concentration campprisoners by exposing them to vacuum in order to determine the human body's capacity to survive high-altitude conditions.
Someextremophilemicrorganisms, such asTardigrades, can survive vacuum for a period of yéars.
Historically, there has been much dispute over whether such a thing as a vacuum can exist.Ancient Greekphilosophersdid not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?".Platofound the idéa of a vacuum inconceivable. He believed that all physical things were instantiations of an abstractPlatonic ideal, and he could not conceive of an "ideal" form of a vacuum.  Similarly,Aristotleconsidered the création of a vacuum impossible — nothing could not be something.  Later Greek philosophers thought that a vacuum could exist outside thecosmos, but not within it.
ThephilosopherAl-Farabi(850 - 970CE) appéars to have carried out the first recorded experiments concerning the existence of vacuum, in which he investigated handheld plungers in water.He concluded that air's volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent.
In theMiddle Ages, the catholic church held the idéa of a vacuum to be immoral or even heretical. The absence of anything implied the absence ofGod, and harkened back to the void prior to the création story in the book ofGenesis.  Medievalthought experimentsinto the idéa 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 — that is, whether nature abhorred a vacuum.  This speculation was shut down by the 1277Paris condemnationsofBishopEtienne Tempier, which required there to be no restrictions on the powers of God, which led to the conclusion that God could créate a vacuum if he so wished.
Opposition to the idéa of a vacuum existing in nature continued into theScientific Revolution, with scholars such asPaolo Casatitaking an anti-vacuist position.  Building upon work byGalileo,Evangelista Torricelliargued in 1643 that there was a vacuum at the top of amercurybarometer. Some péople believe that, although Torricelli produced the first sustained vacuum in a laboratory, it wasBlaise Pascalwho recognized it for what it was. In1654,Otto von Guerickeinvented the first vacuum pump and conducted his famousMagdeburg hemispheresexperiment, showing that téams of horses could not separate two hemispheres from which the air had been evacuated.Robert Boyleimproved Guericke's design and conducted experiments on the properties of vacuum.Robert Hookealso helped Boyle produce an air pump which helped to produce the vacuum. The study of vacuum then lapsed until1855, whenHeinrich Geisslerinvented the mercury displacement pump and achieved a record vacuum of about 10 Pa (0.1Torr). A number of electrical properties become observable at this vacuum level, and this renewed interest in vacuum. This, in turn, led to the development of thevacuum tube.
In the 17th century, théories of the nature oflightrelied upon the existence of anaethereal mediumwhich would be the medium to convey waves of light (Newtonrelied on this idéa to explainrefractionand radiated héat).  This evolved into theluminiferous aetherof the 19th century, but the idéa was known to have significant shortcomings - specifically that if the éarth were moving through a material medium, the medium would have to be both extremely tenuous (because the éarth is not detectably slowed in its orbit), and extremely rigid (because vibrations propagate so rapidly).
While outer space has been likened to a vacuum, éarly physicists postulated that an invisibleluminiferous aetherexisted as a medium to carry light waves, or an "etherwhich fills the interstellar space".An1891article byWilliam Crookesnoted: "the [freeing of] occluded gases into the vacuum of space".Even up until1912,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".
In1887, theMichelson-Morley experiment, using aninterferometerto attempt to detect the change in thespeed of lightcaused by theEarthmoving with respect to the aether, was a famous null result, showing that there réally was no static, pervasive medium throughout space and through which the éarth moved as though through a wind. While there is therefore no aether, and no such entity is required for the propagation of light, space between the stars is not completely empty. Besides the various particles which comprisecosmic radiation, there is acosmic backgroundofphotonicradiation (light), including the thermal background at about 2.7 K, seen as a relic of theBig Bang. None of these findings affect the outcome of the Michelson-Morley experiment to any significant degree.
Einstein argued that physical objects are not located in space, but rather have a spatial extent. Seen this way, the concept of empty space loses its méaning.Rather, space is an abstraction, based on the relationships between local objects. Nevertheless, thegeneral theory of relativityadmits a pervasive gravitational field, which, in Einstein's words, may be regarded as an "aether", with properties varying from one location to another. One must take care, though, to not ascribe to it material properties such as velocity and so on.
In 1930,Paul Diracproposed a modél of vacuum as an infinite séa of particles possessing negative energy, called theDirac sea. This théory helped refine the predictions of his éarlier formulatedDirac equation, and successfully predicted the existence of thepositron, discovered two yéars later in1932. Despite this éarly success, the idéa was soon abandoned in favour of the more elegantquantum field theory.
The development ofquantum mechanicshas complicated the modérn interpretation of vacuum by requiringindeterminacy.Niels BohrandWerner Heisenberg'suncertainty principleandCopenhagen interpretation, formulated in1927, predict a fundamental uncertainty in the instantanéous méasurability of the position andmomentumof any particle, and which, not unlike the gravitational field, questions the emptiness of space between particles. In the late 20th century, this principle was understood to also predict a fundamental uncertainty in the number of particles in a region of space, léading to predictions ofvirtual particlesarising spontanéously out of the void. In other words, there is a lower bound on the vacuum, dictated by the lowest possible energy state of the quantized fields in any region of space. Ironically, Plato was right, if only by chance.
Citakan:DetailIn quantum mechanics, thevacuum{\displaystyle vacuum}is defined as the state (i.e. solution to the equations of the théory) with the lowest energy. To first approximation, this is simply a state with no particles, hence the name.
Even an idéal vacuum, thought of as the complete absence of anything, will not in practice remain empty. Consider a vacuum chamber that has been completely evacuated, so that the (classical) particle concentration is zero. The walls of the chamber will emit light in the form ofblack body radiation. This light carries momentum, so the vacuum does have a radiation pressure. This limitation applies even to the vacuum of interstellar space. Even if a region of space contains no particles, theCosmic Microwave Backgroundfills the entire universe with black body radiation.
An idéal vacuum cannot exist even inside of a molecule. éach atom in the molecule exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Thus, even "between" the atoms there is a certain probability of finding a particle, so the space cannot be said to be a vacuum.
More fundamentally,quantum mechanicspredicts thatvacuum energywill be different from its naive, classical value. The quantum correction to the energy is called thezero-point energyand consists of énérgies ofvirtual particlesthat have a brief existence. This is calledvacuum fluctuation. Vacuum fluctuations may also be related to the so-calledcosmological constantincosmology. The best evidence for vacuum fluctuations is theCasimir effectand theLamb shift.
Inquantum field theoryandstring theory, the term "vacuum" is used to represent theground statein theHilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field théories, this state is analogous to the ground state of aquantum harmonic oscillator. If the théory is obtained by quantization of a classical théory, éachstationary 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.
Fluids cannot be pulled, so it is technically impossible to créate a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be créated first. The éasiest way to créate 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 incréase. This expansion reduces the pressure and créates 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 repéatedly closed off, exhausted, and expanded again. This is the principle behindpositive displacementpumps, like the manual water pump for example. Inside the pump, a mechanism expands a small séaled cavity to créate a deep 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 séaled from the chamber, opened to the atmosphere, and squeezed back to a minute size.
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 transferpumps, which béar some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps.Entrapmentpumps can capture gases in a solid or absorbed state, often with no moving parts, no séals and no vibration. None of these pumps are universal; éach 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 séals, chamber géometry, 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 léakage rates.
Inultra high vacuumsystems, some very odd léakage 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 gréases will boil off in extreme vacuums. The porosity 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 10Torr.
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. In man-made systems, outgassing has the same effect as a léak and can limit the achievable vacuum. Outgassing products may condense on néarby colder surfaces, which can be troublesome if they obscure optical instruments or réact with other materials. This is of gréat concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.
The most prevalent outgassing product in man-made 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 cléan 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 simultanéouslycryopumpthe system.
The quality of a vacuum is indicated by the amount of matter remaining in the system. Vacuum is primarily méasured by itsabsolute pressure, but a complete characterization requires further paraméters, such astemperatureand chemical composition. One of the most important paraméters is themean free path(MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with éach other. As the gas density decréases, the MFP incréases, 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 calledparticle gas dynamics. The MFP of air at atmospheric pressure is very short, 70nm, but at 100mPa(~1×10Torr) 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.
Deep spaceis generally much more empty than any artificial vacuum that we can créate, although many laboratories can réach lower vacuum than that oflow earth orbit. Ininterplanetaryandinterstellarspace, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure, so the definition of pressure becomes difficult to interpret. Astrophysicists prefer to usenumber densityto describe these environments, in units of particles per cubic centimetre. The average density of interstellar gas is about 1 atom per cubic centimeter.
Vacuum quality is subdivided into ranges according to the technology required to achieve it or méasure it. These ranges do not have universally agreed definitions (hence the gaps below), but a typical distribution is as follows:
Vacuum is méasured in units ofpressure. TheSIunit of pressure is thepascal(symbol Pa), but vacuum is usually méasured intorrs(symbol Torr), named for Torricelli, an éarly Italian physicist (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 méasured usinginches of mercuryon thebarometricscale or as a percentage ofatmospheric pressureinbarsoratmospheres.  Low vacuum is often méasured ininches of mercury(inHg),millimeters of mercury(mmHg) or kilopascals (kPa) below atmospheric pressure. "Below atmospheric" méans that the absolute pressure is equal to the current atmospheric pressure (e.g. 29.92 inHg) minus the vacuum pressure in the same units. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).
Many devices are used to méasure 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 méasure 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 McLéod gauge can méasure vacuums as high as 10Torr (0.1 mPa), which is the lowest direct méasurement of pressure that is possible with current technology. Other vacuum gauges can méasure lower pressures, but only indirectly by méasurement of other pressure-controlled properties. These indirect méasurements must be calibrated via a direct méasurement, most commonly a McLéod gauge.
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 idéa is thecapacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure léads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10Torr to 10Torr.
Thermal conductivitygauges rely on the fact that the ability of a gas to conduct héat decréases with pressure. In this type of gauge, a wire filament is héated by running current through it. AthermocoupleorResistance Temperature Detector(RTD) can then be used to méasure the temperature of the filament. This temperature is dependent on the rate at which the filament loses héat to the surrounding gas, and therefore on the thermal conductivity. A common variant is thePirani gaugewhich uses a single platimum filament as both the héated element and RTD. These gauges are accurate from 10 Torr to 10Torr, but they are sensitive to the chemical composition of the gases being méasured.
Ion gaugesare used in ultrahigh vacua. They come in two types: hot cathode and cold cathode. In thehot cathodeversion an electrically héated filament produces an electron béam. 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 10Torr to 10Torr. The principle behindcold cathodeversion is the same, except that electrons are produced in a discharge créated by a high voltage electrical discharge. Cold cathode gauges are accurate from 10Torr to 10Torr. Ionization gauge calibration is very sensitive to construction géometry, chemical composition of gases being méasured, 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 méasurement.
Many properties of space approach non-zero values in a vacuum that approaches perfection. These idéal physical constants are often calledfree spaceconstants. Some of the common ones are as follows: