Vacuum

# Vacuum

Discussion

Encyclopedia
In everyday usage, vacuum is a volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

of space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

that is essentially empty of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

, such that its gaseous pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

is much less than atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in practice. Physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

s often discuss ideal test results that would occur in a perfect vacuum, which they 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
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

or in space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

. The Latin term in vacuo is also used to describe an object as being in what would otherwise be a vacuum.

The quality of a vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

means higher-quality vacuum. For example, a typical vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...

produces enough suction
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, 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 space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. However, even if every single atom and particle could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

, and other phenomena in quantum physics. In modern Particle Physics, the vacuum is considered as the ground state of matter.

Vacuum has been a frequent topic of philosophical debate since ancient Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

times, but was not studied empirically until the 17th century. Evangelista Torricelli
Evangelista Torricelli
Evangelista Torricelli was an Italian physicist and mathematician, best known for his invention of the barometer.-Biography:Evangelista Torricelli was born in Faenza, part of the Papal States...

produced the first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. A torricellian vacuum is created by filling with mercury a tall glass container closed at one end and then inverting the container into a bowl to contain the mercury.

Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulb
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

s and vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s, and a wide array of vacuum technology has since become available. The recent development of human spaceflight
Human spaceflight
Human spaceflight is spaceflight with humans on the spacecraft. When a spacecraft is manned, it can be piloted directly, as opposed to machine or robotic space probes and remotely-controlled satellites....

has raised interest in the impact of vacuum on human health, and on life forms in general.

## Etymology

From Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

vacuum (an empty space, void) noun use of neuter of vacuus (empty) related to vacare (be empty).

"Vacuum" is one of the few words in the English language
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

that contains two consecutive 'u
U
U is the twenty-first letter and a vowel in the basic modern Latin alphabet.-History:The letter U ultimately comes from the Semitic letter Waw by way of the letter Y. See the letter Y for details....

's.

## Uses

Vacuum is useful in a variety of processes and devices. Its first widespread use was in the incandescent light bulb
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

to protect the filament from chemical degradation. The chemical inertness produced by a vacuum is also useful for electron beam welding
Electron beam welding
Electron beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to the materials being joined. The workpieces melt as the kinetic energy of the electrons is transformed into heat upon impact, and the filler metal, if used, also melts to form part of the weld...

, cold welding
Cold welding
Cold or contact welding is a solid-state welding process in which joining takes place without fusion/heating at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no liquid or molten phase is present in the joint....

, vacuum packing
Vacuum packing
Vacuum packing or vacuum packaging is a method of packaging that removes air from the package prior to sealing. It can involve both rigid and flexible types of packaging...

and vacuum frying
Vacuum fryer
A vacuum fryer is a deep-frying device housed inside a vacuum chamber. It was originally developed for potato chip production.Vacuum fryers are fit to process low-quality potatoes that contain higher sugar levels than normal, as they frequently have to be processed in spring and early summer before...

. 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
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

, physical vapor deposition
Physical vapor deposition
Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...

, and dry etching
Dry etching
Dry etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface...

which are essential to the fabrication of semiconductors
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...

and optical coating
Optical coating
An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...

s, and to surface science
Surface science
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid-gas interfaces. It includes the fields of surface chemistry and surface physics. Some related...

. The reduction of convection provides the thermal insulation of thermos bottles. Deep vacuum lowers the boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

of liquids and promotes low temperature outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...

which is used in freeze drying
Freeze drying
Freeze-drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport...

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

preparation, distillation
Vacuum distillation
Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure causing evaporation of the most volatile liquid...

, metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...

, and process purging. The electrical properties of vacuum make electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

s and vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s possible, including cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

s. The elimination of air friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

is useful for flywheel energy storage
Flywheel energy storage
Flywheel energy storage works by accelerating a rotor to a very high speed and maintaining the energy in the system as rotational energy...

and ultracentrifuge
Ultracentrifuge
The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 2,000,000 g . There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge...

s.

### Vacuum driven machines

Vacuums are commonly used to produce suction
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

, which has an even wider variety of applications. The Newcomen steam engine
Newcomen steam engine
The atmospheric engine invented by Thomas Newcomen in 1712, today referred to as a Newcomen steam engine , was the first practical device to harness the power of steam to produce mechanical work. Newcomen engines were used throughout Britain and Europe, principally to pump water out of mines,...

used vacuum instead of pressure to drive a piston. In the 19th century, vacuum was used for traction on Isambard Kingdom Brunel
Isambard Kingdom Brunel
Isambard Kingdom Brunel, FRS , was a British civil engineer who built bridges and dockyards including the construction of the first major British railway, the Great Western Railway; a series of steamships, including the first propeller-driven transatlantic steamship; and numerous important bridges...

's experimental atmospheric railway
Atmospheric railway
An atmospheric railway uses air pressure to provide power for propulsion. In one plan a pneumatic tube is laid between the rails, with a piston running in it suspended from the train through a sealable slot in the top of the tube. Alternatively, the whole tunnel may be the pneumatic tube with the...

. Vacuum brake
Vacuum brake
The vacuum brake is a braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in those countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in...

s were once widely used on train
Train
A train is a connected series of vehicles for rail transport that move along a track to transport cargo or passengers from one place to another place. The track usually consists of two rails, but might also be a monorail or maglev guideway.Propulsion for the train is provided by a separate...

s in the UK but, except on heritage railway
Heritage railway
thumb|right|the Historical [[Khyber train safari|Khyber Railway]] goes through the [[Khyber Pass]], [[Pakistan]]A heritage railway , preserved railway , tourist railway , or tourist railroad is a railway that is run as a tourist attraction, in some cases by volunteers, and...

s, they have been replaced by air brakes.

Manifold vacuum
Manifold vacuum
Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere....

can be used to drive accessories on automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s. The best-known application is the vacuum servo
Vacuum servo
A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the USA it is commonly called a brake booster.-Background:...

, used to provide power assistance for the brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

s. Obsolete applications include vacuum-driven windscreen wipers and fuel pumps.

## Outer space

Outer space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

has very low density and pressure, and is the closest physical approximation of a perfect vacuum. It has effectively no friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

, allowing star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s and moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

s to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic centimeter.

Stars, planets and moons keep their atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

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 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

at 100 kilometres (62.1 mi) of altitude, the Kármán line
Karman line
The Kármán line lies at an altitude of above the Earth's sea level, and is commonly used to define the boundary between the Earth's atmosphere and outer space...

, 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
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

and the dynamic pressure of the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

, so the definition of pressure becomes difficult to interpret. The thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...

. Astrophysicists prefer to use number density
Number density
In physics, astronomy, and chemistry, number density is an intensive quantity used to describe the degree of concentration of countable objects in the three-dimensional physical space...

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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

on satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s. Most artificial satellites operate in this region called low earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

and must fire their engines every few days to maintain orbit. The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sail
Solar sail
Solar sails are a form of spacecraft propulsion using the radiation pressure of light from a star or laser to push enormous ultra-thin mirrors to high speeds....

s, a proposed propulsion system for interplanetary travel
Interplanetary travel
Interplanetary spaceflight or interplanetary travel is travel between planets within a single planetary system. In practice, spaceflights of this type are confined to travel between the planets of the Solar System....

. Planets are too massive for their trajectories to be significantly affected by these forces, although their atmospheres are eroded by the solar winds.

All of the observable universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

is filled with large numbers of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s. The current temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, or -270 degrees Celsius or -454 degrees Fahrenheit.

## Effects on humans and animals

Humans and animals exposed to vacuum will lose consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

after a few seconds and die of hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

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
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains above the internal body temperature of 37°C. Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known as ebullism
Ebullism
Ebullism is the formation of gas bubbles in bodily fluids due to reduced environmental pressure, for example at high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lowers, for example, liquids reach their boiling point...

, is still a concern. The steam 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 a flight suit
Flight suit
A flight suit is a full body garment, worn while flying aircraft such as military airplanes, gliders and helicopters. These suits are generally made to keep the wearer warm, as well as being practical , and durable . Its appearance is usually similar to a jumpsuit. A military flight suit may also...

. Shuttle
Space Shuttle program
NASA's Space Shuttle program, officially called Space Transportation System , was the United States government's manned launch vehicle program from 1981 to 2011...

astronauts wear 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. 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 Boyle
Robert Boyle
Robert Boyle FRS was a 17th century natural philosopher, chemist, physicist, and inventor, also noted for his writings in theology. He has been variously described as English, Irish, or Anglo-Irish, his father having come to Ireland from England during the time of the English plantations of...

was the first to show in 1660 that vacuum is lethal to small animals.

During 1942, in one of a series of experiments on human subjects
Nazi human experimentation
Nazi human experimentation was a series of medical experiments on large numbers of prisoners by the Nazi German regime in its concentration camps mainly in the early 1940s, during World War II and the Holocaust. Prisoners were coerced into participating: they did not willingly volunteer and there...

for the Luftwaffe
Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

, the Nazi regime
Nazi Germany
Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

experimented
Human experimentation
Human subject research includes experiments and observational studies. Human subjects are commonly participants in research on basic biology, clinical medicine, nursing, psychology, and all other social sciences. Humans have been participants in research since the earliest studies...

on prisoners in Dachau concentration camp by exposing them to low pressure.

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 prevent altitude sickness
Altitude sickness
Altitude sickness—also known as acute mountain sickness , altitude illness, hypobaropathy, or soroche—is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude...

in humans that did not undergo prior acclimatization
Acclimatization
Acclimatisation or acclimation is the process of an individual organism adjusting to a gradual change in its environment, allowing it to maintain performance across a range of environmental conditions...

, and spacesuits are necessary to prevent ebullism above 19 km. Most spacesuits use only 20 kPa (150 Torr) of pure oxygen, just enough to sustain full consciousness. This pressure is high enough to prevent ebullism, but simple evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

of blood can still cause decompression sickness
Decompression sickness
Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...

and gas embolisms
Air embolism
An air embolism, or more generally gas embolism, is a pathological condition caused by gas bubbles in a vascular system. The most common context is a human body, in which case it refers to gas bubbles in the bloodstream...

if 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 alveoli of the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s. Eardrum
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...

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. Injuries caused by rapid decompression are called barotrauma
Barotrauma
Barotrauma is physical damage to body tissues caused by a difference in pressure between an air space inside or beside the body and the surrounding fluid...

. A pressure drop of 13 kPa (100 Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.

Some extremophile
Extremophile
An extremophile is an organism that thrives in physically or geochemically extreme conditions that are detrimental to most life on Earth. In contrast, organisms that live in more moderate environments may be termed mesophiles or neutrophiles...

Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...

s, can survive vacuum for a period of days.

## Historical interpretation

Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers
Greek philosophy
Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...

did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

found the idea of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract Platonic ideal, and he could not conceive of an "ideal" form of a vacuum. Similarly, Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

considered the creation of a vacuum impossible — nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos
Cosmos
In the general sense, a cosmos is an orderly or harmonious system. It originates from the Greek term κόσμος , meaning "order" or "ornament" and is antithetical to the concept of chaos. Today, the word is generally used as a synonym of the word Universe . The word cosmos originates from the same root...

, but not within it. Hero of Alexandria
Hero of Alexandria
Hero of Alexandria was an ancient Greek mathematician and engineerEnc. Britannica 2007, "Heron of Alexandria" who was active in his native city of Alexandria, Roman Egypt...

was the first to challenge this belief in the first century AD, but his attempts to create an artificial vacuum failed.

In the Roman city of Pompeii
Pompeii
The city of Pompeii is a partially buried Roman town-city near modern Naples in the Italian region of Campania, in the territory of the comune of Pompei. Along with Herculaneum, Pompeii was destroyed and completely buried during a long catastrophic eruption of the volcano Mount Vesuvius spanning...

, a dual-action suction pump was found, proving that the ancient Romans had access to this kind of technology. Used for raising water, this pump had two cylinders, alternately operated by a walking-beam pump. In the suction phase, a lower valve opened, permitting the entry of water into the cylinder, while an upper valve remained closed. When the piston went down, the lower valve closed and the upper one opened.

In the medieval Islamic world
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

, the Muslim physicist
Islamic physics
Physics in medieval Islam is the development of physics in the medieval Islamic world in the history of physics. In the course of the expansion of the Islamic world, Muslim scholars encountered the science, mathematics, and medicine of antiquity through the works of Aristotle, Archimedes, Galen,...

and philosopher
Early Islamic philosophy
Early Islamic philosophy or classical Islamic philosophy is a period of intense philosophical development beginning in the 2nd century AH of the Islamic calendar and lasting until the 6th century AH...

, Al-Farabi
Al-Farabi
' known in the West as Alpharabius , was a scientist and philosopher of the Islamic world...

(Alpharabius, 872-950), conducted a small experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...

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. However, the Muslim physicist Ibn al-Haytham (Alhazen, 965-1039) and the Mu'tazili
Mu'tazili
' is an Islamic school of speculative theology that flourished in the cities of Basra and Baghdad, both in present-day Iraq, during the 8th–10th centuries. The adherents of the Mu'tazili school are best known for their having asserted that, because of the perfect unity and eternal nature of God,...

theologians
Kalam
ʿIlm al-Kalām is the Islamic philosophical discipline of seeking theological principles through dialectic. Kalām in Islamic practice relates to the discipline of seeking theological knowledge through debate and argument. A scholar of kalām is referred to as a mutakallim...

disagreed with Aristotle and Al-Farabi, and they supported the existence of a void. Using geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....

, Ibn al-Haytham mathematically
Islamic mathematics
In the history of mathematics, mathematics in medieval Islam, often termed Islamic mathematics or Arabic mathematics, covers the body of mathematics preserved and developed under the Islamic civilization between circa 622 and 1600...

demonstrated that place (al-makan) is the imagined three-dimensional void between the inner surfaces of a containing body. Abū Rayhān al-Bīrūnī also states that "there is no observable evidence that rules out the possibility of vacuum". The suction
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...

was described in 1206 by the Muslim engineer and inventor
Inventions in the Islamic world
A number of inventions were developed in the medieval Islamic world, a geopolitical region that has at various times extended from Spain and Africa in the west to the Indian subcontinent in the east. The inventions listed here were developed during the medieval Islamic world, which covers the...

, Al-Jazari
Al-Jazari
Abū al-'Iz Ibn Ismā'īl ibn al-Razāz al-Jazarī was a Muslim polymath: a scholar, inventor, mechanical engineer, craftsman, artist, mathematician and astronomer from Al-Jazira, Mesopotamia, who lived during the Islamic Golden Age...

. The suction pump later appeared in Europe from the 15th century.
In medieval Europe
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, the Catholic Church regarded the idea of a vacuum as against nature or even heretical; the absence of anything implied the absence of God
God
God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

, and harkened back to the void prior to the creation story in the Book of Genesis.
Medieval thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

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. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, as Walter Burley
Walter Burley
Walter Burley was a medieval English Scholastic philosopher and logician. He was a Master of Arts at Oxford in 1301, and a fellow of Merton College, Oxford until about 1310. He spent sixteen years at Paris until 1326, becoming a fellow of the Sorbonne by 1324. After that, he spent seventeen...

postulated, whether a 'celestial agent' prevented the vacuum arising. The commonly held view that nature abhorred a vacuum was called horror vacui
Horror vacui (physics)
In physics horror vacui, or plenism, is a theory first proposed by Aristotle in the Fourth book of Physics that nature abhors a vacuum, and therefore empty space would always be trying to suck in gas or liquids to avoid being empty. The theory was widely accepted for a long time and supported by...

. Speculation that even God could not create a vacuum if he wanted to was shut down by the 1277 Paris condemnations of Bishop
Bishop
A bishop is an ordained or consecrated member of the Christian clergy who is generally entrusted with a position of authority and oversight. Within the Catholic Church, Eastern Orthodox, Oriental Orthodox Churches, in the Assyrian Church of the East, in the Independent Catholic Churches, and in the...

Etienne Tempier
Étienne Tempier
Étienne Tempier was a French bishop of Paris during the 13th century...

, which required there to be no restrictions on the powers of God, which led to the conclusion that God could create a vacuum if he so wished.
René Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

also argued against the existence of a vacuum, arguing along the following lines: "Space is identical with extension, but extension is connected with bodies; thus there is no space without bodies and hence no empty space (vacuum)." In spite of this, opposition to the idea of a vacuum existing in nature continued into the Scientific Revolution
Scientific revolution
The Scientific Revolution is an era associated primarily with the 16th and 17th centuries during which new ideas and knowledge in physics, astronomy, biology, medicine and chemistry transformed medieval and ancient views of nature and laid the foundations for modern science...

, with scholars such as Paolo Casati
Paolo Casati
Paolo Casati was an Italian Jesuit mathematician. Born in Piacenza to a Milanese family, he joined the Jesuits in 1634. After completing his mathematical and theological studies, he moved to Rome, where he assumed the position of professor at the Collegio Romano...

taking an anti-vacuist position. Jean Buridan
Jean Buridan
Jean Buridan was a French priest who sowed the seeds of the Copernican revolution in Europe. Although he was one of the most famous and influential philosophers of the late Middle Ages, he is today among the least well known...

reported in the 14th century that teams of ten horses could not pull open bellows
Bellows
A bellows is a device for delivering pressurized air in a controlled quantity to a controlled location.Basically, a bellows is a deformable container which has an outlet nozzle. When the volume of the bellows is decreased, the air escapes through the outlet...

when the port was sealed, apparently because of horror vacui.
The belief in horror vacui was overthrown in the 17th century. Water pump designs had improved by then to the point that they produced measurable vacuums, but this was not immediately understood. What was known was that suction pumps could not pull water beyond a certain height: 18 Florentine yards according to a measurement taken around 1635. (The conversion to metres is uncertain, but it would be about 9 or 10 metres.) This limit was a concern to irrigation projects, mine drainage, and decorative water fountains planned by the Duke of Tuscany, so the Duke commissioned Galileo to investigate the problem. Galileo advertised the puzzle to other scientists, including Gasparo Berti
Gasparo Berti
Gasparo Berti was an Italian mathematician, astronomer and physicist. He was probably born in Mantua and spent most of his life in Rome. He is most famous today for his experiment in which he unknowingly created the first working barometer...

who replicated it by building the first water barometer in Rome in 1639. Berti's barometer produced a vacuum above the water column, but he could not explain it. The breakthrough was made by Evangelista Torricelli
Evangelista Torricelli
Evangelista Torricelli was an Italian physicist and mathematician, best known for his invention of the barometer.-Biography:Evangelista Torricelli was born in Faenza, part of the Papal States...

in 1643. Building upon Galileo's notes, he built the first mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

barometer
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

and wrote a convincing argument that the space at the top was a vacuum. The height of the column was then limited to the maximum weight that atmospheric pressure could support. Some people believe that although Torricelli's experiment was crucial, it was Blaise Pascal
Blaise Pascal
Blaise Pascal , was a French mathematician, physicist, inventor, writer and Catholic philosopher. He was a child prodigy who was educated by his father, a tax collector in Rouen...

's experiments that proved the top space really contained vacuum.

In 1654, Otto von Guericke
Otto von Guericke
Otto von Guericke was a German scientist, inventor, and politician...

invented the first vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...

and conducted his famous Magdeburg hemispheres
Magdeburg hemispheres
The Magdeburg hemispheres are a pair of large copper hemispheres with mating rims. When the rims were sealed with grease and the air was pumped out, the sphere contained a vacuum and could not be pulled apart by teams of horses...

experiment, showing that teams of horses could not separate two hemispheres from which the air had been (partially) evacuated. Robert Boyle
Robert Boyle
Robert Boyle FRS was a 17th century natural philosopher, chemist, physicist, and inventor, also noted for his writings in theology. He has been variously described as English, Irish, or Anglo-Irish, his father having come to Ireland from England during the time of the English plantations of...

improved Guericke's design and conducted experiments on the properties of vacuum. Robert Hooke
Robert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...

also helped Boyle produce an air pump which helped to produce the vacuum. The study of vacuum then lapsed until 1850 when August Toepler
August Toepler
August Joseph Ignaz Toepler was a German physicist known for his experiments in electrostatics. In 1864 he applied Foucault's knife-edge test for telescope mirrors to the analysis of fluid flow and the shock wave. He named this new method schlieren photography, for which he is justifiably famous...

invented the Toepler Pump
Toepler pump
A Toepler pump is a form of mercury piston pump, invented by August Toepler in 1850.The principle is illustrated in the diagram. When reservoir G is lowered, bulb B and tube T are filled with gas from the enclosure being evacuated . When G is raised, mercury rises in tube F and cuts off the gas in...

. Then in 1855 Heinrich Geissler
Heinrich Geissler
Johann Heinrich Wilhelm Geißler was a German physicist and inventor of the Geissler tube, a low pressure gas-discharge tube made of glass....

invented the mercury displacement pump and achieved a record vacuum of about 10 Pa (0.1 Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

). 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 the vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

. Shortly after this Hermann Sprengel
Hermann Sprengel
Hermann Sprengel FRS was a German chemist who discovered the explosive nature of picric acid in 1873. He also invented a generic class of materials called Sprengel explosives...

invented the Sprengel Pump
Sprengel pump
The Sprengel pump is a vacuum pump invented by Hanover-born chemist Hermann Sprengel in 1865 while he was working in London. The pump created the highest vacuum achievable at that time....

in 1865.

While outer space has been likened to a vacuum, early theories of the nature of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

relied upon the existence of an invisible, aetherial medium which would convey waves of light. (Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

relied on this idea to explain refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

and radiated heat). This evolved into the luminiferous aether
Luminiferous aether
In the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....

of the 19th century, but the idea was known to have significant shortcomings - specifically, that if the Earth were moving through a material medium, the medium would have to be both extremely tenuous (because the Earth is not detectably slowed in its orbit), and extremely rigid (because vibrations propagate so rapidly). An 1891 article by William Crookes
William Crookes
Sir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...

noted: "the [freeing of] occluded gases into the vacuum of space". Even up until 1912, astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

Henry Pickering
William Henry Pickering
William Henry Pickering was an American astronomer, brother of Edward Charles Pickering. He was elected a Fellow of the American Academy of Arts and Sciences in 1883.-Work:...

commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".

In 1887, the Michelson-Morley experiment
Michelson-Morley experiment
The Michelson–Morley experiment was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University in Cleveland, Ohio. Its results are generally considered to be the first strong evidence against the theory of a luminiferous ether and in favor of special...

, using an interferometer to attempt to detect the change in the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

caused by the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

moving with respect to the aether, was a famous null result. Many misinterpreted the results, which neither proved nor disproved the existence of the aether, as showing that there really was no static, pervasive medium throughout space and through which the Earth moved as though through a wind. As a simplification, one can assume there no aether, and no such entity is required for the propagation of light. Besides the various particles which comprise cosmic radiation, there is a cosmic background
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

of photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

), including the cosmic microwave background
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

(CMB), the thermal remnant of the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

. However, 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 meaning. Rather, space is an abstraction, based on the relationships between local objects. Nevertheless, the general theory of relativity admits 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 Dirac
Paul Dirac
Paul Adrien Maurice Dirac, OM, FRS was an English theoretical physicist who made fundamental contributions to the early development of both quantum mechanics and quantum electrodynamics...

proposed a model of vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea
Dirac sea
The Dirac sea is a theoretical model of the vacuum as an infinite sea of particles with negative energy. It was first postulated by the British physicist Paul Dirac in 1930 to explain the anomalous negative-energy quantum states predicted by the Dirac equation for relativistic electrons...

. This theory helped refine the predictions of his earlier formulated Dirac equation
Dirac equation
The Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928. It provided a description of elementary spin-½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity, and...

, and successfully predicted the existence of the positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

, discovered two years later in 1932. Despite this early success, the idea was soon abandoned in favour of the more elegant quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

.

The development of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

has complicated the modern interpretation of vacuum by requiring indeterminacy
Quantum indeterminacy
Quantum indeterminacy is the apparent necessary incompleteness in the description of a physical system, that has become one of the characteristics of the standard description of quantum physics...

. Niels Bohr
Niels Bohr
Niels Henrik David Bohr was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in...

and Werner Heisenberg
Werner Heisenberg
Werner Karl Heisenberg was a German theoretical physicist who made foundational contributions to quantum mechanics and is best known for asserting the uncertainty principle of quantum theory...

's uncertainty principle
Uncertainty principle
In quantum mechanics, the Heisenberg uncertainty principle states a fundamental limit on the accuracy with which certain pairs of physical properties of a particle, such as position and momentum, can be simultaneously known...

and Copenhagen interpretation
Copenhagen interpretation
The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics. It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta,...

, formulated in 1927, predict a fundamental uncertainty in the instantaneous measurability of the position and momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

of 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, leading to predictions of virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...

s arising spontaneously 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.

## In electromagnetism

In classical electromagnetism
Classical electromagnetism
Classical electromagnetism is a branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents...

, free space or perfect vacuum is a standard reference medium for electromagnetic effects. Some authors refer to this reference medium as classical vacuum, a terminology intended to separate this concept from the vacuum state
Vacuum state
In quantum field theory, the vacuum state is the quantum state with the lowest possible energy. Generally, it contains no physical particles...

s of quantum electrodynamics
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...

or the standard model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

where vacuum fluctuations can produce transient virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...

densities and a relative permittivity and relative permeability
Relative permeability
In multiphase flow in porous media, the relative permeability of a phase is a dimensionless measure of the effective permeability of that phase. It is the ratio of the effective permeability of that phase to the absolute permeability...

that are not identically unity.

In the theory of classical electromagnetism, free space has the following properties:
• Electromagnetic radiation travels where unobstructed at the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

, the defined value 299,792,458 m/s in SI units.
• The superposition principle
Superposition principle
In physics and systems theory, the superposition principle , also known as superposition property, states that, for all linear systems, the net response at a given place and time caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually...

is 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 the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

at any point around these two charges is found by calculating the vector
Vector (mathematics and physics)
In mathematics and physics, a vector is an element of a vector space. If n is a non negative integer and K is either the field of the real numbers or the field of the complex number, then K^n is naturally endowed with a structure of vector space, where K^n is the set of the ordered sequences of n...

sum of the two electric fields from each of the charges acting alone.
• The permittivity
Permittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...

and permeability
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...

are exactly the electric constant ε0 and magnetic constant μ0
Vacuum permeability
The physical constant μ0, commonly called the vacuum permeability, permeability of free space, or magnetic constant is an ideal, physical constant, which is the value of magnetic permeability in a classical vacuum...

, respectively (in SI units), or exactly 1 (in Gaussian units
Gaussian units
Gaussian units comprise a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs units. It is also called the Gaussian unit system, Gaussian-cgs units, or often just cgs units...

).
• The characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

(η) equals the impedance of free space Z0 ≈ 376.73 Ω.

## In quantum mechanics

In quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

and quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, the vacuum is defined as the state (i.e. solution to the equations of the theory) with the lowest possible energy (the ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...

of the Hilbert space
Hilbert space
The mathematical concept of a Hilbert space, named after David Hilbert, generalizes the notion of Euclidean space. It extends the methods of vector algebra and calculus from the two-dimensional Euclidean plane and three-dimensional space to spaces with any finite or infinite number of dimensions...

). This is a state with no matter particles (hence the name), and also no photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, no graviton
Graviton
In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton must be massless and must have a spin of 2...

s, etc. 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 blackbody photons.)

This hypothetical vacuum state often has interesting and complex properties. For example, it contains vacuum fluctuations (virtual particles that hop into and out of existence). It also, relatedly, has a finite energy, called vacuum energy
Vacuum energy
Vacuum energy is an underlying background energy that exists in space even when the space is devoid of matter . The concept of vacuum energy has been deduced from the concept of virtual particles, which is itself derived from the energy-time uncertainty principle...

. Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some readily-apparent effects of vacuum fluctuations include the Casimir effect
Casimir effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, like capacitors placed a few micrometers apart, without any external electromagnetic field...

and Lamb shift.

There can be more than one possible vacuum state. The starting and ending of 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
Stationary point
In mathematics, particularly in calculus, a stationary point is an input to a function where the derivative is zero : where the function "stops" increasing or decreasing ....

of the energy in the configuration space
Configuration space
- Configuration space in physics :In classical mechanics, the configuration space is the space of possible positions that a physical system may attain, possibly subject to external constraints...

gives rise to a single vacuum. String theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

is believed to have a huge number of vacua - the so-called string theory landscape
String theory landscape
The string theory landscape or anthropic landscape refers to the large number of possible false vacua in string theory. The "landscape" includes so many possible configurations that some physicists think that the known laws of physics, the standard model and general relativity with a positive...

.

In the superfluid vacuum theory the physical vacuum is described as the quantum superfluid
Superfluid
Superfluidity is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and...

which is essentially non-relativistic whereas the Lorentz symmetry is an approximate emerging symmetry valid only for the small fluctuations of the superfluid background.
An observer who resides inside such vacuum and is capable of creating and/or measuring the small fluctuations would observe them as relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

objects - unless their energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

and momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

are sufficiently high (as compared to the background ones) to make the Lorentz-breaking corrections detectable. It was shown that the relativistic gravity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

arises as the small-amplitude collective excitation mode whereas the relativistic elementary particles can be described by the particle-like modes in the low-momentum limit.

## Pumping and ambient air pressure

Fluids cannot generally be pulled, so a vacuum cannot be created by suction
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

. 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 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 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.

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 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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

, and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

.

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
Ultra high vacuum
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

systems, some very "odd" leakage paths and outgassing sources must be considered. The water absorption of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

becomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel or titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

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 10−13 torr (13 pPa). However, pressures as low as (6.7 fPa) have been indirectly measured in a 4 K cryogenic vacuum system. This corresponds to ≈100 particles/cm3.

## Outgassing

Evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

and sublimation into a vacuum is called outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean 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. In man-made systems, outgassing has the same effect as a leak and can 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 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 of rotary vane pump
Rotary vane pump
A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity. In some cases these vanes can be variable length and/or tensioned to maintain contact with the walls as the pump rotates. It was invented by Charles C...

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
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

to shut down residual outgassing and simultaneously cryopump
Cryopump
A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature...

the system.

## Quality

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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

and chemical composition. One of the most important parameters is the mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

(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
Fluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...

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 nm, but at 100 mPa (~1×10−3 Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement...

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 do not have universally agreed definitions, but a typical distribution is as follows:
pressure (Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

)
pressure (Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

)
Atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

760 101.3 kPa
Low vacuum 760 to 25 100 kPa to 3 kPa
Medium vacuum 25 to 1×10−3 3 kPa to 100 mPa
High vacuum 1×10−3 to 1×10−9 100 mPa to 100 nPa
Ultra high vacuum
Ultra high vacuum
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

1×10−9 to 1×10−12 100 nPa to 100 pPa
Extremely high vacuum <1×10−12 <100 pPa
Outer Space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

1×10−6 to <3×10−17 100 µPa to <3fPa
Perfect vacuum 0 0 Pa

• Atmospheric pressure is variable but standardized at 101.325 kPa (760 Torr)
• Low vacuum, also called rough vacuum or coarse vacuum, is vacuum that can be achieved or measured with rudimentary equipment such as a vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...

and a liquid column manometer
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....

.
• Medium vacuum is vacuum that can be achieved with a single pump, but the pressure is too low to measure with a liquid or mechanical manometer. It can be measured with a McLeod gauge, thermal gauge or a capacitive gauge.
• High vacuum is vacuum where the MFP
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

of residual gases is longer than the size of the chamber or of the object under test. High vacuum usually requires multi-stage pumping and ion gauge measurement. Some texts differentiate between high vacuum and very high vacuum.
• Ultra high vacuum requires baking the chamber to remove trace gases, and other special procedures. British and German standards define ultra high vacuum as pressures below 10−6 Pa (10−8 Torr).
• 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 as 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
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

, 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
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s and graviton
Graviton
In physics, the graviton is a hypothetical elementary particle that mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton must be massless and must have a spin of 2...

s, as well as dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

, virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...

s, and other aspects of the quantum vacuum.
• Hard vacuum and Soft vacuum are terms that are defined with a dividing line defined differently by different sources, such as 5 psia, one Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

, or 0.1 Torr the common denominator being that a hard vacuum is a higher vacuum than a soft one.

### Examples

pressure (Pa) pressure (Torr) mean free path molecules per cm3
Vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...

approximately 80 kPa 600 70 nm 1019
liquid ring
Liquid ring
A liquid ring pump is a rotating positive displacement pump. They are typically used as a vacuum pump but can also be used as a gas compressor....

vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...

approximately 3.2 kPa 24 1.75 μm 1018
freeze drying
Freeze drying
Freeze-drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport...

100 to 10 Pa 1 to 0.1 100 μm to 1 mm 1016 to 1015
rotary vane pump
Rotary vane pump
A rotary vane pump is a positive-displacement pump that consists of vanes mounted to a rotor that rotates inside of a cavity. In some cases these vanes can be variable length and/or tensioned to maintain contact with the walls as the pump rotates. It was invented by Charles C...

100 Pa to 100 mPa 1 to 100 μm to 10 cm 1016 to 1013
Incandescent light bulb
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

10 to 1 Pa 0.1 to 0.01 1 mm to 1 cm 1015 to 1014
Thermos bottle 1 to 0.01 Pa 10−2 to 10−4 1 cm to 1 m 1014 to 1012
Earth thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

1 Pa to 100 nPa 10−2 to 10−9 1 cm to 100 km 1014 to 107
Vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

10 µPa to 10 nPa 10−7 to 10−10 1 to 1,000 km 109 to 106
Cryopump
Cryopump
A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature...

ed MBE
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...

chamber
100 nPa to 1 nPa 10−9 to 10−11 100 to 10,000 km 107 to 105
Pressure on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

approximately 1 nPa 10−11 10,000 km 4
Interplanetary space     10
Interstellar space
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

1
Intergalactic space   10−6

### Relative versus absolute measurement

Vacuum is measured in units of pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

, 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 Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, 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
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 meters; a 9.8 meter 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.

### Vacuum measurements relative to 1 atm

The SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

unit of pressure is the pascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

(symbol Pa), but vacuum is usually measured in torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

s, named for Torricelli, an early Italian physicist (1608–1647). A torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured using inches of mercury on the barometric
Barometer
A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

scale or as a percentage of atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

in bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

s or atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

s. Low vacuum is often measured in inches of mercury (inHg), millimeters of mercury (mmHg) or kilopascals (kPa) below atmospheric pressure. "Below atmospheric" means 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).

In other words, most low vacuum gauges that read, for example, −28 inHg at full vacuum are actually reporting 2 inHg, or 50.79 Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of −30 inHg, or 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) 25 torr.
Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.

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
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

is 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 the McLeod gauge
McLeod gauge
A McLeod gauge is a scientific instrument used to measure very low pressures, down to 10-6 Torr. It was invented in 1874 by Herbert McLeod . McLeod gauges were once commonly found attached to equipment that operates under a vacuum, such as a lyophilizer...

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 10−6 torr (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.

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+3 torr to 10−4 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
A thermocouple is a device consisting of two different conductors that produce a voltage proportional to a temperature difference between either end of the pair of conductors. Thermocouples are a widely used type of temperature sensor for measurement and control and can also be used to convert a...

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
Pirani gauge
The Pirani gauge is a robust thermal conductivity gauge used for the measurement of the pressures in vacuum systems. It was invented in 1906 by Marcello Pirani.-Structure:...

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

Ion gauges are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In the hot cathode
Hot filament ionization gauge
The hot-filament ionization gauge, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure measuring device for the region from 10-3 to 10-10 torr...

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 10−3 torr to 10−10 torr. The principle behind cold cathode
Cold cathode
A cold cathode is a cathode used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube which is not electrically heated by the circuit to which it is connected...

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 10−2 torr to 10−9 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.

• Joining materials
• Decay of the vacuum (Pair production
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...

)
• False vacuum
False vacuum
In quantum field theory, a false vacuum is a metastable sector of space that appears to be a perturbative vacuum, but is unstable due to instanton effects that may tunnel to a lower energy state. This tunneling can be caused by quantum fluctuations or the creation of high-energy particles...

• Helium mass spectrometer
Helium mass spectrometer
A helium mass spectrometer is an instrument commonly used to detect and locate small leaks. It was initially developed in the Manhattan Project during World War II to find extremely small leaks in the gas diffusion process of uranium enrichment plants. It typically uses a vacuum chamber in which a...

- technical instrumentation to detect a vacuum leak
• Engine vacuum
Manifold vacuum
Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere....

• Pneumatic tube
Pneumatic tube
Pneumatic tubes are systems in which cylindrical containers are propelled through a network of tubes by compressed air or by partial vacuum...

- transport system using vacuum or pressure to move containers in tubes
• Rarefaction
Rarefaction
Rarefaction is the reduction of a medium's density, or the opposite of compression.A natural example of this is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.Another...

- reduction of a medium's density
• Suction
Suction
Suction is the flow of a fluid into a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Suction is popularly thought of as an attractive effect, which is incorrect since vacuums do not...

- creation of a partial vacuum
• Vacuum angle
Vacuum angle
In quantum gauge theories, in the Hamiltonian formulation, the wave function is a functional of the gauge Connection A and matter fields \,\phi. Being a quantum gauge theory, we have to impose first class constraints in the form of functional differential equations. Basically, the Gauss...

• Vacuum cementing
Vacuum cementing
Vacuum cementing or vacuum welding is the natural process of solidifying small objects in a hard vacuum. The most notable example is dust on the surface of the Moon....

- natural process of solidifying homogeneous "dust" in vacuum
• Vacuum deposition
Vacuum deposition
Vacuum deposition is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub-atmospheric pressure on a solid surface. The layers may be as thin as one atom to millimeters thick . There may be multiple layers of different materials...

- process of depositing atoms and molecules in a sub-atmospheric pressure environment
• Vacuum engineering
Vacuum engineering
Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure...

• Vacuum flange
Vacuum flange
A vacuum flange is a flange at the end of a tube used to connect vacuum chambers, tubing and vacuum pumps to each other.-Vacuum flange types:...