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Look up Vacuum in Wiktionary, the free dictionary

For other uses, see vacuum (disambiguation)

A vacuum is a volume of space that is empty of matter, including air, so that gaseous pressure is much less than standard atmospheric pressure. The root of the word vacuum is the Latin word vacuus (pl. vacua) which means "empty," but space can never be perfectly empty. A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept with no physical reality; see sections below on Vacuum in Space and The Quantum Mechanical Vacuum. Physicists often discuss ideal test results that would occur in a perfect vacuum and use the term partial vacuum to refer to real vacuum.

Vacuum Quality

The quality of a vacuum is indicated by the amount of matter remaining in the system. For industrial purposes, 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 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, 7×10^-8m, but at 100 mPascal (~1×10^-3 Torr) the MFP of room temperature air is roughly 10cm, 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. 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. ^[1]

Vacuum quality is also 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: ^[2]

• 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 and 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 cannot be obtained in a laboratory, nor even in outer space.

Examples

Measurement

Vacuum is measured in units of pressure. The SI unit of pressure is the Pascal (abbreviation Pa), but vacuum is usually measured in Torrs. A Torr is equal to the displacment of a millimeter of mercury (mmHg) in a manometer, with 1 Torr equaling 133.3223684 Pascal above absolute zero pressure. Vacuum is often also measured using the barometric scale, or as a percentage of atmospheric pressure in bars or atms. Low vacuum is often measured in inches of mercury (inHg) below atmospheric. This means that the absolute pressure is equal to the atmospheric pressure (29.92inHg) minus the vacuum pressure in inHg. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of (29.92 - 26) or 3.92 inHg.

Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.^[3]

• Hydrostatic gauges (such as the oil or mercury manometer) consist of various liquids in some sort of tubing to measure pressure. The simplest of these is a closed-end u-shaped tube, one side of which is connected to the region of interest. The height of the liquid in the tube will rise or fall depending on the pressure in the region. Simple hydrostatic gauges are effective in the region of 0.1 torr (see McLeod gauge); more sophisticated versions can measure to about 10⁻⁴ Torr.
• Mechanical gauges depend on a diaphram, 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 diaphram makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphram, 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 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. Another method involves measuring the resistivity of the filament, which is dependent on its temperature. These gauges are accurate from 10 Torr to 10⁻³ Torr.
• 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.

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 vacuum welding, for chemical vapor deposition and Dry Etching in semiconductor fabrication and optical coating fabrication, and for ultra-clean inert storage. 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, steel manufacture, and process purging. The electrical properties of vacuum make vacuum tubes work, including cathode ray tubes.

In the 19th century, vacuum was used for traction on Isambard Kingdom Brunel's experimental atmospheric railway.

Vacuum Pumping

Main article: vacuum pump

The easiest way to create an artificial vacuum is to expand the volume of a container. For example, your muscles expand your lungs to create a partial vacuum inside them, and air rushes in to fill the vacuum. By repeatedly closing off a compartment of the vacuum and exhausting it, it is possible to pump air out of a chamber of fixed size. This is the principle behind positive displacement vacuum pumps. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some air from the chamber 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. This is how a simple manual water pump works.

Many variations of the positive displacement pump have been developed, and other principles have been used as well. 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, helium, and neon.

The ultimate vacuum 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 quality of vacuum 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 systems, some very odd leakage paths and outgassing sources must be considered. The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the absorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and greases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.

The lowest pressures currently achievable in laboratory are about 10^-13 Torr. ^[4]

Outgassing

Main article: 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 also 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 dessicating or baking the chamber, and removing absorbant materials. Outgassed water can condense in the oil of rotary vane pumps 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 are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system 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.

Vacuum in space

Much of outer space has the density and pressure of an almost perfect vacuum. It has no friction. The properties of the vacuum remain largely unknown.

A perfect vacuum is an ideal state that cannot practically be obtained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10⁻¹⁴ Pascal or 10⁻¹⁶ Torr.

All of the observable universe is also filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature is about 3 K, being merely 3 degrees above the absolute zero of temperature. Neither these photons nor the neutrinos produce a significant interaction with matter, so stars, planets and spacecraft move freely in this near perfect vacuum of interstellar space.

Stars, planets and moons keep their atmosphere by gravitational attraction, so atmospheres have no firm boundary. The density of gas decreases with distance from the object. In Low Earth Orbit (about 300 km altitude) the atmospheric density is still sufficient to produce significant drag on satellites. Most Earth satellites operate in this region, and they need to fire their engines every few days to maintain orbit. The atmosphere in Low Earth Orbit is increasingly being polluted with man-made debris. Studies have discovered that some satellites retrieved from orbit are coated with a very thin layer of urine and fecal matter evidently released from Russian and US space missions. [1]

Beyond planetary atmospheres, the pressure from photons and other particles from the sun become 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.

The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces.

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. ^[5]

See also interstellar medium

The quantum-mechanical vacuum

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. 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 everywheres in a given volume. Even the space between molecules is not a perfect vacuum.

More fundamentally, quantum mechanics predicts that vacuum energy can never be exactly zero. The lowest possible energy state is called the zero-point energy and consists of a seething mass of virtual particles that have brief existence. This is called vacuum fluctuation. While most agree that this represents a significant part of particle physics, it is a concept that would benefit from a deeper understanding than currently available. 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.

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. In free (non-interacting) quantum field theories, this state is analogous to the ground state of a quantum harmonic oscillator. 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. String theory is believed to be analogous to quantum field theory but one with a huge number of vacua - with the so-called anthropic landscape.

Effects on humans and animals

Humans 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. Blood and other body fluids do boil (medical term:ebullism) and the vapour pressure may be expected to bloat the body to twice its normal size and slow down 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 vacuums of 15 Torr. However, even if ebullism is prevented, simple evaporation can cause the bends and gas embolisms. Rapid evaporation cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard.

Animal experiments show that rapid complete recovery is the norm for exposures of less than 90 seconds, while longer full body exposures are fatal and resuscitation has never succeeded. (Ref.: Cook and Bancroft (1966)) There is limited data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if breathing is not impaired. Rapid decompression can be much more dangerous than the vacuum exposure. 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 surprise will accelerate oxygen consumption leading to asphyxiation.

During World War II, the Nazi regime tortured concentration camp prisoners by exposing them to simulated high altitude conditions. See Nazi human experimentation.

References

• Charles E. Billings, "Barometric Pressure," in Bioastronautics Data Book, Second edition, NASA SP-3006, edited by James F. Parker and Vita R. West, 1973.
• Webb P. "The Space Activity Suit: An Elastic Leotard for Extravehicular Activity", Aerospace Medicine. 1968; 39: 376-383.
• Cooke JP, RW Bancroft, "Some Cardiovascular Responses in Anesthetized Dogs During Repeated Decompressions to a Near-Vacuum," Aerospace Medicine, Nov. 1966, 37:1148-1152.

[2] [3]

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, the idea of a vacuum was thought to be immoral or even heretical. The absence of anything implied the absence of God, and hearkened back to the void prior to the story of creation 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, following William Burley whether a 'celestial agent' prevented the vacuum arising—that is, whether nature abhorred a vacuum. This speculation became irrelevant after the Paris condemnations of Bishop 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 vacuum, it was Blaise Pascal who recognized it for what it was. Robert Boyle later conducted experiments on the properties of vacuum. In 1654, Otto von Guericke conducted his famous Magdeburg hemispheres experiment, showing that teams of horses could not separate two hemispheres from which the air had been evacuated. The study of vacuum then lapsed until 1855 when Heinrich Geissler invented the mercury displacement pump and achieved a record vacuum of about 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). This evolved into the luminiferous aether of the 19th century, but the idea was known to have significant shortcomings - specifically that if the Earth is moving through a material medium, the medium would have to be both extremely tenuous (because the earth is not being detectably slowed in its orbit), and extremely rigid (because vibrations propagate so fast). 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.

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. (Ref: French Wikipedia, which cites appendix 5 of Relativity - the Special and General Theory, translated to French by Robert Lawson, 1961. Please replace this with a more direct reference.)

Notes

1. ↑ University of New Hampshire Experimental Space Plasma Group. What is the Interstellar Medium. The Interstellar Medium, an online tutorial. URL accessed on 2006-03-15.
2. ↑ American Vacuum Society. Glossary. (PDF) AVS Reference Guide. URL accessed on 2006-03-15.
3. ↑ John H., Moore; Christopher Davis, Michael A. Coplan and Sandra Greer (2002). Building Scientific Apparatus, Boulder, CO: Westview Press. ISBN 0813340071.
4. ↑ Ishimaru, H (1989). "Ultimate Pressure of the Order of 10^-13 Torr in an Aluminum Alloy Vacuum Chamber". J. Vac. Sci. Technol. 7 (3-II): 2439–2442.
5. ↑ (1913) “Polar Magnetic Phenomena and Terrella Experiments” The Norwegian Aurora Polaris Expedition 1902-1903, p. 720.

See also

• Cold cathode - device to measure a vacuum
• Cold trap - section to collect unwanted molecules in a vacuum
• Cryopump - device to create a vacuum
• Diffusion pump - device used to create a vacuum
• Evangelista Torricelli - unit of pressure named Torr
• Helium mass spectrometer - technical instrumentation to detect a vacuum leak
• Hot filament ionization gauge - device to measure a vacuum
• Ionization gauge - device to measure a vacuum
• Ion pump - device to create a vacuum
• Magdeburg hemispheres - interesting and historical experiment with vacuum
• Otto von Guericke - interesting and historical experiment with vacuum
• Engine vacuum
• Outgassing - properties in the process of a vacuum
• Rarefaction - reduction of a medium's density
• Suction - creation of a partial vacuum
• Turbomolecular pump - device to create a vacuum
• Ultra high vacuum - region of the vacuum spectrum
• Vacuum cementing - natural process of solidifying homogenous "dust" in vacuum
• Vacuum engineering - vacuum engineering
• Vacuum pump - device to create a vacuum
• Vacuum angle

External links

• American Vacuum Society
• Journal of Vacuum Science and Technology A
• Journal of Vacuum Science and Technology B
• FAQ on explosive decompression and vacuum exposure.
• Discussion of the effects on humans of exposure to hard vacuum.
• Vacuum Energy in High Energy Physics
• Scientist of vacuum
• http://www.mcallister.com/vacuum.html (Short History of Vacuum Terminology and Technology)

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