Any space in which no matter is present. In the laboratory, near vacuum is achieved by pumping out air from an enclosed chamber. Vacua of between 10?4 and 10?10 Pa are needed in many experiments if results are not to be affected by unwanted gas atoms. Many physics experiments and standard techniques are only possible due to modern high vacuum technology. A perfect vacuum can never be attained; the closest is interstellar space.
A vacuum is a volume of space that is essentially empty of matter, so that gaseous pressure is much less than standard atmospheric pressure. A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept with no physical reality, not least because quantum theory predicts that no volume of space is 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 by how closely it approaches a perfect vacuum. 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 common topic of philosophical debate since Ancient Greek times, but it was not studied empirically until the 17th century. Vacuum became a valuable industrial tool in the 20th century with the introduction of the light bulb and vacuum tube, 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 life forms in general.
Uses
Vacuum is useful in a variety of processes and devices. Its chemical inertness is also useful for electron beam welding, for chemical vapor deposition and dry etching in semiconductor fabrication and optical coating fabrication, for cold welding, and for vacuum packing. 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 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. But no vacuum is perfect, not even in interstellar space, where there are 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, but for small scale applications it is much easier to create an equivalent vacuum on Earth than to leave the Earth's gravity well.
Beyond planetary atmospheres, the pressure from photons and other particles from the sun become significant.
While outer space has been likened to a vacuum, early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves, or an "ether which fills the interstellar space". An 1891 article by William Crookes noted: "the [freeing of] occluded gases into the vacuum of space". Lang, writing in 2000 noted, "Half a century ago, most people visualized our planet as a solitary sphere traveling in a cold, dark vacuum of space around the Sun".
Effects on humans and animals
See also: Human adaptation to spaceHumans exposed to vacuum will lose consciousness after a few seconds and will die within minutes from asphyxiation, but the symptoms are not nearly as graphic as commonly shown in pop culture. Robert Boyle was the first to show that vacuum was lethal to small animals. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at vacuums of 15 Torr (2 kPa). Rapid decompression can be much more dangerous than the vacuum exposure.
Some extremophile microrganisms can survive vacuum for a period of years, as can the Tardigrade.
Historical interpretation
Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not 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 could not imagine an "ideal" form of a vacuum. Similarly, Aristotle considered the creation of a vacuum impossible—nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not inside it.
In the Middle Ages, Christians held the idea of a vacuum to be immoral or even heretical. 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, following Walter Burley 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. Following 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. Robert Boyle later 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 led to the development of the vacuum tube.
In the 17th century, theories of the nature of light had required the idea 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).
In 1930, Paul Dirac proposed a model of vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea.
The development of quantum mechanics has complicated the modern interpretation of vacuum by requiring indeterminacy. In other words, there is a lower bound on vacuum which is dictated by the lowest possible energy state of the quantized fields in any region of space.
Quantum-mechanical definition
Even an ideal vacuum, thought of as the complete absence of anything, will not 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 are at a temperature of thousands of degrees, infrared light if they are cooler. Another reason that perfect vacuum is impossible is the Heisenberg uncertainty principle which states that no particle can ever have an exact position. Even the space between molecules is not a perfect vacuum.
More fundamentally, quantum mechanics predicts that vacuum energy can never be exactly zero. This is called vacuum fluctuation. Vacuum fluctuations may also be related to the so-called cosmological constant in the theory of gravitation, if indeed this entity were to be observed in nature on a macroscopic scale. The best evidence for vacuum fluctuations is the Casimir effect and the Lamb shift.
In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. If the theory is obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum.
Pumping
Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be created first. The easiest way to create an artificial vacuum is to expand the volume of a container. 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. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum.
The above explanation is merely a simple introduction to vacuum pumping, and is not representative of the entire range of pumps in use. Momentum transfer pumps, which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps.
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.
In ultra high vacuum systems, some very odd leakage paths and outgassing sources must be considered. Some oils and greases will boil off in extreme vacuums.
The lowest pressures currently achievable in laboratory are about 10
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.
The most prevalent outgassing product in man-made vacuum systems is water absorbed by chamber materials. High vacuum systems must be clean and free of organic matter to minimize outgassing.
Ultra-high vacuum are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system and boil them off.
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. 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.
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.
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 (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 Torr | 100 mPa to 1 µPa |
| Ultra high vacuum | 1×10 Torr | 100 nPa to 100 pPa |
| Extremely high vacuum | <1×10-12 Torr | <100 pPa |
| Outer Space | 1×10 Torr | 100 µPa to <3fPa |
| Perfect vacuum | 0 Torr | 0 Pa |
Examples
| Vacuum cleaner | approximately 80 kPa | (600 Torr) |
| liquid ring vacuum pump | approximately 3.2 kPa | (24 Torr) |
| freeze drying | 100 to 10 Pa | (1 to 0.1 Torr) |
| rotary vane pump | 100 Pa to 100 mPa | (1 Torr to 10−3 Torr) |
| Incandescent light bulb | 10 to 1 Pa | (0.1 to 0.01 Torr) |
| Thermos bottle | 1 to 0.1 Pa | (10 Torr) |
| Near earth outer space | approximately 100 µPa | (10−6 Torr) |
| Cryopumped MBE chamber | 100 nPa to 1 nPa | (10 Torr) |
| Pressure on the Moon | approximately 1 nPa | (10−11 Torr) |
| Interstellar space | approximately 1 fPa | (10−17 Torr) |
Measurement
Vacuum is measured in units of pressure. The SI unit of pressure is the pascal (unit) (abbreviation Pa), but vacuum is usually measured in torrs. Vacuum is often also measured using micrometers of mercury, the barometric scale, or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often measured in inches of mercury (inHg) below atmospheric. "Below atmospheric" means that the absolute pressure is equal to the atmospheric pressure (29.92 inHg) minus the vacuum pressure in inches of mercury. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of (29.92 - 26) or 4 inHg.
Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed. 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
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.
Thermal Conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. 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.
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