A
vacuum is a
volume of
space that's essentially empty of
matter, such that its gaseous
pressure is much less than standard
atmospheric pressure. The Latin term
in vacuo is used to describe an object as being in what would otherwise be a vacuum. The root of the word
vacuum is the
Latin adjective
vacuus which means "empty," but space can never be perfectly empty. A
perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that's never observed in practice, not least because
quantum theory predicts that no volume of space can be perfectly empty in this way.
Physicists often use the term "vacuum" slightly differently. They discuss ideal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "
free space" in this context, and use the term
partial vacuum to refer to the imperfect vacua realized in practice.
The quality of a vacuum is measured in relation to how closely it approaches a perfect vacuum. The residual gas
pressure is the primary indicator of quality, and is most commonly measured in units called
torr, even in
metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account.
Quantum mechanics sets limits on the best possible quality of vacuum.
Outer space is a natural high quality vacuum, mostly of much higher quality than what can be created artificially with current technology. Low quality artificial vacuums have been used for
suction for millennia.
Vacuum has been a frequent topic of
philosophical debate since
Ancient Greek times, but wasn't studied empirically until the
17th century.
Evangelista Torricelli produced the first artifical vacuum in
1643, and other experimental techniques were developed as a result of his theories of
atmospheric pressure. Vacuum became a valuable industrial tool in the
20th century with the introduction of
incandescent light bulbs and
vacuum tubes, and a wide array of vacuum technology has since become available. The recent development of
human spaceflight has raised interest in the impact of vacuum on human health, and on life forms in general.
Uses
Vacuum is useful in a variety of processes and devices. Its first common use was in
incandescent light bulbs to protect the
tungsten filament from chemical degradation. Its chemical inertness is also useful for
electron beam welding,
chemical vapor deposition and
dry etching in the fabrication of
semiconductors and
optical coatings,
cold welding,
vacuum packing and
vacuum frying. The reduction of convection improves the thermal insulation of
thermos bottles and
double-paned windows. Deep vacuum promotes
outgassing which is used in
freeze drying,
adhesive preparation,
distillation,
metallurgy, and process purging. The electrical properties of vacuum make
electron microscopes and
vacuum tubes possible, including
cathode ray tubes. The elimination of air
friction is useful for
flywheel energy storage and
ultracentrifuges.
High to ultra-high vacuum is used in
thin film deposition and
surface science. High vacuum allows for contamination-free material deposition.
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).
Suction is used in a wide variety of applications. The
Newcomen steam engine used vacuum instead of pressure to drive a piston. In the
19th century, vacuum was used for traction on
Isambard Kingdom Brunel's experimental
atmospheric railway.
Outer space
Much of
outer space has the density and pressure of an almost perfect vacuum. It has effectively no
friction, which allows
stars,
planets and
moons to move freely along ideal gravitational trajectories. But no vacuum is perfect, not even in interstellar space, where there are only a few hydrogen atoms per cubic centimeter at 10 fPa (10
−16 Torr). The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces; for small-scale applications, however, it's much more cost-effective to create an equivalent vacuum on Earth than to leave the Earth's
gravity well.
Stars, planets and moons keep their
atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. In
low earth orbit (about 300 km or 185 miles altitude) the atmospheric density is about 100 nPa (10
-9 Torr), still sufficient to produce significant
drag on
satellites. Most artificial satellites operate in this region, and must fire their engines every few days to maintain orbit.
Beyond planetary atmospheres, the pressure of photons and other particles from the
sun becomes significant.
Spacecraft can be buffeted by
solar winds, but planets are too massive to be affected. The idea of using this wind with a
solar sail has been proposed for interplanetary travel.
All of the observable
universe is filled with large numbers of
photons, the so-called
cosmic background radiation, and quite likely a correspondingly large number of
neutrinos. The current
temperature of this radiation is about 3
K, or -270 degrees Celsius or -454 degrees Fahrenheit.
Effects on humans and animals
Vacuum is primarily an
asphyxiant. Humans exposed to vacuum will lose
consciousness after a few seconds and die within minutes, but the symptoms are not nearly as graphic as commonly shown in pop culture.
Robert Boyle was the first to show that vacuum is lethal to small animals.
Blood and other body fluids do boil (the medical term for this condition is
ebullism), and the
vapour pressure may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a
flight suit.
Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 15 Torr (2 kPa). However, even if ebullism is prevented, simple
evaporation of blood can cause
decompression sickness and
gas embolisms. Rapid evaporative cooling of the skin will create frost, particularly in the mouth, but this isn't a significant hazard.
Animal experiments show that rapid and complete recovery is the norm for exposures of fewer than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful. There is only a limited amount of data available from human accidents, but it's consistent with animal data. Limbs may be exposed for much longer if breathing isn't impaired. Rapid
decompression can be much more dangerous than vacuum exposure itself. If the victim holds his breath during decompression, the delicate internal structures of the
lungs can be ruptured, causing death.
Eardrums may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to asphyxiation.
In
1942, in one of a series of
experiments on human subjects for the
Luftwaffe, the
Nazi regime tortured
Dachau concentration camp prisoners by exposing them to vacuum in order to determine the human body's capacity to survive high-altitude conditions.
Some
extremophile microrganisms, such as
Tardigrades, can survive vacuum for a period of years.
Historical interpretation
Historically, there has been much dispute over whether such a thing as a vacuum can exist.
Ancient Greek philosophers didn't like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?".
Plato found the idea of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract
Platonic ideal, and he couldn't conceive of an "ideal" form of a vacuum. Similarly,
Aristotle considered the creation of a vacuum impossible — nothing couldn't be something. Later Greek philosophers thought that a vacuum could exist outside the
cosmos, but not within it.
The
philosopher Al-Farabi (850 - 970
CE) appears to have carried out the first recorded experiments concerning the existence of vacuum, in which he investigated handheld plungers in water. He concluded that air's volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent.
In the
Middle Ages, the catholic church held the idea of a vacuum to be immoral or even heretical. The absence of anything implied the absence of
God, and harkened back to the void prior to the creation story in the book of
Genesis. Medieval
thought experiments 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 postulated, whether a 'celestial agent' prevented the vacuum arising — that is, whether nature abhorred a vacuum. This speculation was shut down by the 1277
Paris condemnations of
Bishop Etienne Tempier, 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.
Opposition to the idea of a vacuum existing in nature continued into the
Scientific Revolution, with scholars such as
Paolo Casati taking an anti-vacuist position. Building upon work by
Galileo,
Evangelista Torricelli argued in 1643 that there was a vacuum at the top of a
mercury barometer. Some people believe that, although Torricelli produced the first sustained vacuum in a laboratory, it was
Blaise Pascal who recognized it for what it was. In
1654,
Otto von Guericke invented the first vacuum pump and conducted his famous
Magdeburg hemispheres experiment, showing that teams of horses couldn't separate two hemispheres from which the air had been evacuated.
Robert Boyle improved Guericke's design and conducted experiments on the properties of vacuum. The study of vacuum then lapsed until
1855, when
Heinrich Geissler invented the mercury displacement pump and achieved a record vacuum of about 10 Pa (0.1
Torr). 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.
In the 17th century, theories of the nature of
light relied upon the existence of an
aethereal medium which would be the medium to convey waves of light (
Newton relied on this idea to explain
refraction and radiated heat). This evolved into the
luminiferous aether 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 isn't detectably slowed in its orbit), and extremely rigid (because vibrations propagate so rapidly).
While outer space has been likened to a vacuum, early physicists postulated that an invisible
luminiferous aether existed as a medium to carry light waves, or an "
ether which fills the interstellar space". An
1891 article by
William Crookes noted: "the [freeingof] occluded gases into the vacuum of space". Even up until
1912,
astronomer Henry Pickering commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".
In
1887, the
Michelson-Morley experiment, using an
interferometer to attempt to detect the change in the
speed of light caused by the
Earth moving with respect to the aether, was a famous null result, showing that there really was no static, pervasive medium throughout space and through which the Earth moved as though through a wind. While there's therefore no aether, and no such entity is required for the propagation of light, space between the stars isn't completely empty. Besides the various particles which comprise
cosmic radiation, there's a
cosmic background of
photonic radiation (light), including the thermal background at about 2.7 K, seen as a relic of the
Big Bang. None of these findings affect the outcome of the Michelson-Morley experiment to any significant degree.
Einstein argued that physical objects are not located in space, but rather have a spatial extent. Seen this way, the concept of empty space loses its 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 proposed a model of vacuum as an infinite sea of particles possessing negative energy, called the
Dirac sea. This theory helped refine the predictions of his earlier formulated
Dirac equation, and successfully predicted the existence of the
positron, discovered two years later in
1932. Despite this early success, the idea was soon abandoned in favour of the more elegant
quantum field theory.
The development of
quantum mechanics has complicated the modern interpretation of vacuum by requiring
indeterminacy.
Niels Bohr and
Werner Heisenberg's
uncertainty principle and
Copenhagen interpretation, formulated in
1927, predict a fundamental uncertainty in the instantaneous measurability of the position and
momentum 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 particles arising spontaneously out of the void. In other words, there's a lower bound on the vacuum, dictated by the lowest possible energy state of the quantized fields in any region of space. Ironically, Plato was right, if only by chance.
Quantum-mechanical definition
In quantum mechanics, the
is defined as the state (for example solution to the equations of the theory) with the lowest energy. To first approximation, this is simply a state with no particles, hence the name.
However, even an ideal vacuum, thought of as the complete absence of anything, won't in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of
black-body radiation: visible light if they're at a temperature of thousands of degrees, infrared light if they're cooler. If this soup of photons is in
thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. Another reason that perfect vacuum is impossible is the Heisenberg
uncertainty principle which states that no particle can ever have an exact position. Each atom exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Even the space between molecules isn't a perfect vacuum.
More fundamentally,
quantum mechanics predicts that
vacuum energy will be different from its naive, classical value. The quantum correction to the energy is called the
zero-point energy and consists of energies of
virtual particles that have a brief existence. This is called
vacuum fluctuation. Vacuum fluctuations may also be related to the so-called
cosmological constant in
cosmology. The best evidence for vacuum fluctuations is the
Casimir effect and the
Lamb shift.
Outgassing
Evaporation and
sublimation into a vacuum is called
outgassing. All materials, solid or liquid, have a small
vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. 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 pumps and reduce their net speed drastically if gas ballasting isn't used. High vacuum systems must be clean and free of organic matter to minimize outgassing.
Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by
liquid nitrogen to shut down residual outgassing and simultaneously
cryopump the system.
Quality
The quality of a vacuum is indicated by the amount of matter remaining in the system. Vacuum is primarily measured by its
absolute pressure, but a complete characterization requires further parameters, such as
temperature and chemical composition. One of the most important parameters is the
mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of
fluid mechanics don't 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) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as
vacuum tubes. The
Crookes radiometer turns when the MFP is larger than the size of the vanes.
Deep space is generally much more empty than any artificial vacuum that we can create, although many laboratories can reach lower vacuum than that of
low earth orbit. In
interplanetary and
interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure, so the definition of pressure becomes difficult to interpret. Astrophysicists prefer to use
number density to describe these environments, in units of particles per cubic centimetre. The average density of interstellar gas is about 1 atom per cubic centimeter.
Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges don't have universally agreed definitions (hence the gaps below), but a typical distribution is as follows:
| Atmospheric pressure |
760 Torr |
101 kPa |
| Low vacuum |
760 to 25 Torr |
100 to 3 kPa |
| Medium vacuum |
25 to 1×10-3 Torr |
3 kPa to 100 mPa |
| High vacuum |
1×10-3 to 1×10-9 Torr |
100 mPa to 100 nPa |
| Ultra high vacuum |
1×10-9 to 1×10-12 Torr |
100 nPa to 100 pPa |
| Extremely high vacuum |
<1×10-12 Torr |
<100 pPa |
| Outer Space |
1×10-6 to <3×10-17 Torr |
100 µPa to <3fPa |
| Perfect vacuum |
0 Torr |
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 and a liquid column manometer.
- Medium vacuum is vacuum that can be achieved with a single pump, but 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 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.
- Deep space is generally much more empty than any artificial vacuum that we can create.
- Perfect vacuum is an ideal state that can't be obtained in a laboratory, nor can it be found in outer space.
Examples
Measurement
Vacuum is measured in units of
pressure. The
SI unit of pressure is the
pascal (symbol Pa), but vacuum is usually measured in
torrs (symbol Torr), 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 scale or as a percentage of
atmospheric pressure in
bars or
atmospheres. 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 (for example 29.92 inHg) minus the vacuum pressure in the same units. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).
Many devices are used to 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 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 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's 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.
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 or
Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the
Pirani gauge which uses a single platimum filament as both the heated element and RTD. These gauges are accurate from 10 Torr to 10
−3 Torr, but they're sensitive to the chemical composition of the gases being measured.
Ion gauges are used in ultrahigh vacua. They come in two types: hot cathode and cold cathode. In the
hot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10
−3 Torr to 10
−10 Torr. The principle behind
cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10
−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.
Properties
Many properties of space approach non-zero values in a vacuum that approaches perfection. These ideal physical constants are often called
free space constants. Some of the common ones are as follows:
The speed of light approaches 299,792,458 m/s, but is always slower
Index of refraction approaches 1.0, but is always higher
Electric permittivity () approaches 8.8541878176x10-12 farads per meter (F/m).
Magnetic permeability (μ0) approaches 4π×10−7 N/A2.
Characteristic impedance () approaches 376.73 Ω.
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