# vacuum

In 1657 Otto von Guericke demonstrated the effect of vacuum with his Magdeburg hemispheres
The external air pressure presses the Magdeburg hemispheres together
a) hemispheres filled with air
b) airless hemispheres
1. Handle
2. airtight seal
3. Magdeburg hemisphere
4. Air pressure
5. Vacuum
Bell jar with piston vacuum pump for school experiments

In technical practice, a vacuum is a space with a substantial absence of matter - there are no solid objects or liquids in a vacuum, extremely little gas and therefore an extremely low gas pressure .

In physics , the vacuum is a theoretical concept that describes the (complete) absence of matter in a space.

Technical vacuum became important in the 20th century with the introduction of the incandescent lamp and electron tube. A wide field of vacuum technologies has developed. A technical vacuum is created by removing (gas) molecules from a container with a pump ; the pressure in it sinks. Gas pressure is created by collisions between the gas molecules and the container wall. The pumping creates a negative pressure, i.e. a pressure that is lower than the ambient pressure. If the pressure in the container falls below 300 mbar and if molecules are removed further and further from the room, then a rough vacuum, fine vacuum, high vacuum and finally ultra-high vacuum is obtained. Creating a vacuum also means evacuating.

## Possibility of vacuum

In the known universe there is no complete vacuum, and it cannot be created with known technical means. The space is, for example, with a thin hydrogen gas filled.

The question of the theoretical possibility of an empty space was first  posed in the West by the Greek philosophers before Socrates - the pre-Socratics . Their starting point was not the scientific question about empty space, but the more general philosophical question about nothing - whether it can be thought. Against the same philosophical background, Empedocles (around 433 BC) and Leukippus (around 450 to around 420 BC) as well as Democritus (around 460 to around 370 BC) turned to the question of empty space.

The question of the possibility of an absolutely empty space is still unsolved in physics. According to quantum field theory , virtual particles are constantly being created and destroyed everywhere . In general, vacuum is only understood to mean the absence of matter. Electromagnetic radiation and other physical fields could be present in the room under consideration.

## Definitions

In colloquial language, the word vacuum is used for a largely vacuum. (With vacuum packaging ("vacuuming"), however, generally not even a rough vacuum is achieved.) Technology and experimental physics also use the term in this sense: Vacuum describes the state of a gas in a volume at a pressure that is significantly lower than that Atmospheric pressure under normal conditions . Depending on the prevailing residual pressure, one speaks of rough vacuum , fine vacuum , high or ultra high vacuum .

“Vacuum is the state of a gas when the pressure of the gas and thus the particle number density in a container is lower than outside or when the pressure of the gas is lower than 300 mbar, i. H. lower than the lowest atmospheric pressure occurring on the earth's surface "

- DIN 28400 part 1 (May 1990): vacuum technology; Terms and definitions; General terms.

In quantum field theory , the vacuum is the state with the lowest possible energy. The particle numbers for all types of particles ( field quanta ) have the value zero. However, if the zero-point energy is derived from Planck's radiation formula, then due to the energy-time uncertainty relation it follows that virtual particles are constantly being created in the vacuum and then destroyed again. These are also called vacuum fluctuations .

In contrast to the natural sciences, philosophy sees vacuum as basically completely empty space.

## History of exploration

The idea of ​​the vacuum probably originated from Leukippus or his student Democritus and was a mainstay of the worldview of Epicurean philosophy. This states that matter is made up of indivisible smallest particles (plural: ατόμοι atómoi ), which move in empty space, i.e. in a vacuum, and only have the possibility of movement and interaction due to the emptiness of the space. This assumption was rejected both by Plato , who denied the existence of "non-being", and by Aristotle , since movement without a driving medium seemed impossible. The space between the stars was also thought to be filled with an ether . Aristotle postulated nature's aversion to emptiness, which was later denoted by the Latin expression horror vacui . In the Middle Ages and the early Renaissance, Aristotle was considered an authority. René Descartes was convinced of the impossibility of a vacuum, since, based on rationalist considerations, he came to the view that space and matter are essentially the same.

The idea of ​​a vacuum was only able to gain acceptance through demonstrations. The first earthly (or man-made) vacuum was created in 1644 by Evangelista Torricelli with the help of a column of mercury in a curved glass tube. Shortly afterwards, Blaise Pascal was able to prove for the first time with his famous experiment vide dans le vide in November 1647 that a vacuum can actually exist. The vacuum became popular through Otto von Guericke , the inventor of the air pump . In 1657 he hitched horses to two metal hemispheres (see Magdeburg hemispheres ), from which he had previously sucked the air. However, the observed effect is not a direct property of the vacuum, but rather due to the pressure of the surrounding air.

Robert Williams Wood first observed the tunnel effect in a vacuum during the field emission of electrons in 1897 , but was not yet able to correctly interpret this effect.

At the end of the 19th century, it was still assumed that light could not propagate in a vacuum, but in a medium, the so-called ether . The Michelson-Morley experiment attempted in vain to prove the existence of such an ether. Due to the general acceptance of Einstein's special theory of relativity from 1905, the ether concept is considered obsolete and the propagation of light in a vacuum has been proven.

The scattering tests carried out by Ernest Rutherford in 1911 showed that alpha radiation can cross a gold foil without resistance. This showed that the mass of atoms is concentrated in a tiny nucleus compared to their total size. Building on this, Niels Bohr designed a model according to which the electrons orbit the atomic nucleus like the planets the sun. So there seemed to be a vacuum inside and between the atoms. Although this view is still occasionally encountered in the literature, the interior of the atoms is now considered to be filled with the areas where the electrons stay ( atomic orbitals ).

According to today's understanding, however, as already described above, the vacuum is not empty either, since even the quantum mechanical ground state has a finite energy density, which is noticeable, for example, in the Casimir effect or spontaneous emission .

## properties

### Pressure areas

While a completely material-free space cannot be produced, technical vacuums can be produced in various qualities. In technology, a distinction is made between different qualities of the vacuum achieved according to the amount of remaining matter. By default, the pressure is specified in Pascal (Pa) or millibar (mbar). In July 2019, with ISO 3529-1: 2019, the vacuum ranges were divided as follows:

Classification based on the pressure areas
Pressure area Pressure in hPa (mbar) Molecules per cm³ mean free path (fictitious air particle)
Normal pressure 1013.25 2.7 · 10 19 68 nm
Low vacuum ambient air pressure… 1 10 19 … 10 16 0.1 ... 100 μm
Fine vacuum 1… 10 −3 10 16 … 10 13 0.1 ... 100 mm
High vacuum (HV) 10 −3 ... 10 −8 10 13 … 10 8 100 mm ... 10 km
Ultra high vacuum (UHV) 10 −8 ... 10 −11 10 8 … 10 5 10 ... 10 4  km
extremely high vacuum (XHV) <10 −11 <10 5 > 10 4  km
Ideal vacuum (IV) 0 0

The pressure range between ambient pressure and approx. 300 mbar is also often referred to as negative pressure instead of rough vacuum.

The term “maximum vacuum” for pressures below the high vacuum is also used out of date.

When pumping out a vessel, the mechanical load from the external air pressure builds up to a fine vacuum. The limit to the fine vacuum can still be easily reached with mechanical pumps. In the fine vacuum range, the free path reaches the typical dimensions of vacuum vessels, so that the viscous flow passes over the Knudsen flow into the molecular flow . The prevailing type of flow not only has a fundamental influence on the use of the vacuum, but also on the generation and measurement of the vacuum itself. In the broad range of the high vacuum, the duration in which every point on the surface is hit by a residual gas particle on average increases by one hour per year, which is sufficient for many experiments. In the area of ​​the UHV, the vapor pressures of construction materials begin to interfere, e.g. B. lead contamination in aluminum. An ideal vacuum is technically neither achievable nor measurable.

Occurrence and examples according to vacuum quality:

Measuring devices for determining the gas pressure in a vacuum are called vacuum gauges .

### Physical Properties

 Permittivity ${\ displaystyle \ varepsilon _ {0}}$ ${\ textstyle \ approx 8 {,} 8542 \ cdot 10 ^ {- 12} ~ {\ frac {\ mathrm {As}} {\ mathrm {Vm}}}}$ permeability ${\ displaystyle \ mu _ {0}}$ ${\ textstyle \ approx 1 {,} 2566 \ cdot 10 ^ {- 6} {\ frac {\ mathrm {N}} {\ mathrm {A} ^ {2}}}}$ Wave resistance ${\ textstyle Z_ {0} = {\ sqrt {\ mu _ {0} / \ varepsilon _ {0}}}}$ ${\ displaystyle \ approx 376 {,} 73 ~ \ Omega}$ Speed ​​of Light ${\ textstyle c_ {0} = {\ sqrt {\ frac {1} {\ varepsilon _ {0} \ mu _ {0}}}}}$ ${\ textstyle \ approx 299 ~ 792 ~ \ mathrm {\ frac {km} {s}}}$

Light , particles , electric , magnetic and gravitational fields spread in a vacuum; on the other hand, sound waves require a material medium and therefore cannot propagate in an ideal vacuum. Thermal radiation can also propagate as an electromagnetic wave in a vacuum. On the other hand, the lowering of the pressure leads to a reduction of the material-bound heat transfer by convection and, as soon as the mean free path is greater than the vessel diameter, also the conductive heat conduction.

The reduction of heat flow through a vacuum is used for thermal insulation (vacuum jugs , Dewar vessels , vacuum insulation panels ).

The high dielectric strength of the high vacuum is used in vacuum circuit breakers , in vacuum capacitors in high-performance electronics and in the high-voltage part of evacuated X-ray tubes . When the pressure is reduced, the dielectric strength initially drops considerably due to the formation of a low-pressure plasma . Only when the free path of the ions produced during the breakdown becomes greater than the electrode spacing does the dielectric strength rise steeply again and is only then limited again by the field emission. This can be estimated using Paschen's law .

### Biological effects

The vacuum is not a living space, since living things depend on matter for their metabolism. However, many living things (bacterial spores, plant seeds and spores) can survive in a vacuum for a certain period of time.

For a short time, higher living beings such as healthy humans can withstand the vacuum, experiments with birds were documented in the picture " The experiment with the bird in the air pump ". Contrary to popular belief, despite the pressure difference, the blood does not immediately begin to boil. Skin and tissue are usually able to withstand the vapor pressure of body fluids at less than 0.05 bar (normal air pressure is 1 bar). Regardless, reduced pressure can lead to decompression sickness or altitude sickness .

## generation

A vacuum can be created on earth by using suitable vacuum pumps to free an enclosed cavity, the recipient , from the gas it contains . The simplest device is the water jet pump ; it creates a rough vacuum that corresponds to the water vapor pressure at the prevailing water temperature (e.g. 23 hPa (or mbar) at 20 ° C).

### High vacuum

In physics and surface chemistry, several types of pumps are usually used to generate a high or ultra-high vacuum . First of all, one or more mechanically operating pumps (e.g. rotary vane pump , diaphragm pump or scroll pump ) generate a negative pressure (“pre-pressure”) in the cavity in the range from 1 to 100 Pa (0.01 to 1 mbar). Depending on the size of the cavity and the pumping capacity of the pumps, this takes a few minutes, for example. Afterwards, a turbo-molecular pump (or the cheaper oil diffusion pump for lower demands ) is inserted between this backing pump (s) and the cavity by means of valves , which  creates a high vacuum of around 10 −7 mbar in a process that takes up to several hours . This pressure can no longer be reduced by simply continuing pumping, as water and other substances with low vapor pressure adsorbed on surfaces are constantly being desorbed .

The desorption is accelerated in that the chamber is brought to a temperature by direct heating of the chamber walls and indirect thermal heating of the inner surfaces, which is at least above the boiling point of water, but if possible significantly higher. The built-in components, such as bushings for electrical connections and viewing windows, must be correspondingly temperature-resistant. Temperatures for this heating are typically between 130 ° C and over 200 ° C. Since vacuum apparatuses have to withstand the atmospheric oxygen present outside even at this temperature, they are often made of stainless steel or glass, with seals made of aluminum or Teflon .

Most of the desorbed water is pumped out by the turbo molecular pumps during the heating process, as is any hydrocarbon contamination . This typically takes 24 hours or more; In the case of chambers with complex internal surfaces due to attached equipment, the heating is often only turned down after two to three days.

### Ultra high vacuum

Non-mechanical pumps are used to achieve the ultra-high vacuum. An ion getter pump pumps through ionization and trapping the residual gas molecules in titanium tubes in a pressure range of 10 −10 to 10 −7 mbar. The pumping capacity is only sufficient here if the heating has previously reduced the residual gas pressure sufficiently. A titanium sublimation works with fresh aufsublimiertem a wall titanium , which is characterized by a high chemical reactivity is characterized and residual gas atoms binds to itself and the (cold) wall of the chamber, then the residual gas pressure that further reduced. The residual gas pressure that can be achieved with the method described above is in the range of 10 −11  mbar.

Additional residual gas can be temporarily bound by cold traps on the lower part of the chamber and the chamber pressure can be reduced to approximately 10 −12  mbar. If the entire chamber is immersed in liquid helium, pressures of less than 10 −16  mbar can be achieved.

## Applications

Technical vacuums are used in research, in electron microscopy , in the melting of metallic materials and in the production of microelectronics. A low vacuum is often used to hold and / or transport flat workpieces with the aid of suction grippers .

Vacuum is very often used in the heat treatment of metals ( hardening , tempering , nitriding , carburizing ) in order to prevent oxidation by oxygen that is in the air.

Incandescent lamps and thus electric light only became possible thanks to the vacuum. Especially with Edison's filament lamp with carbon filament, the vacuum prevented the filament from burning (see also carbon filament lamp ); only later were incandescent lamps made with a filling of nitrogen or other gas that does not support combustion.

In the interior of electron tubes and picture tubes there is a high vacuum in order to keep the scattering of electrons low. Remaining and later diffusing gas residues are bound with a getter . The high vacuum in electron tubes (including picture tubes), X-ray tubes , magnetrons , electron beam sources , particle accelerators , vacuum fluorescent displays and the like. Ä. increases the free path of the electrons to the order of magnitude of the entire vessel, so that there are hardly any collisions with gas residues that would otherwise disrupt the particle beam.

### Evacuation as a manufacturing process (DIN 8580)

According to DIN 8580 manufacturing processes - terms, classification , evacuation is one of the manufacturing processes , which means the creation of a vacuum in hollow bodies or cavities in a workpiece. This can be a permanent vacuum that is required for the workpiece to function, as is the case with electron tubes, or evacuation is the prerequisite for filling in a protective gas , as is used in electrical switching technology for spark extinction.

### Vacuum in process engineering

The degassing under vacuum, on the other hand, belongs to the field of process engineering as a so-called separation process . In the core step of plastination , forced impregnation , vacuum is used to extract acetone or dichloromethane from the specimen .

The freeze-drying removes substances water by frozen and subjected to a vacuum. When freeze-drying coffee, tea, vegetables, blood or even biological preparations , sublimation takes place, the ice goes directly into the gas phase, there is no liquid phase that could boil.

The crystallization process in sugar production takes place under vacuum in order to prevent caramelization due to the lower boiling point of the sugar solution when water is removed.

### Vacuum in chemistry

Since the boiling point of liquids also falls with falling ambient pressure, high-boiling substances can be distilled more gently in a vacuum at lower temperatures ( vacuum distillation ). As a rough rule, if the pressure is halved, the boiling point drops by around 10 to 15 K.

### Vacuum for preservation and vacuum cooking

Another area of ​​application is the packaging of food ( vacuum packaging ) and other perishable products under vacuum. The perishable products are enclosed in gas-tight plastic casings. Due to the small amount of oxygen remaining in the air, aging and decomposition processes ( metabolic and oxidation processes) can only take place to a very limited extent, which makes the product last longer.

In the household, food can be packed in bags and evacuated with vacuum sealers so that the bag film is applied to the packaged goods; this means that less oxygen reaches the food. In addition, the volume is reduced. The vacuum sealers used can only generate a rough vacuum.

When preserving / preserving the food, it is sterilized and any gases it may contain are expelled; The remaining "air space" in the mason jar can largely be taken up by water vapor. The sealing rings maintain a better rough vacuum over longer periods of time; the rest of the air is also sterilized.

With vacuum cooking , the vacuum-packed food (meat, vegetables, etc.) is cooked either in a water bath or in temperature-controlled steam at temperatures below 100 ° C and thus retain its structure and aroma better than with conventional cooking methods. When vacuum frying z. B. of potato chips is mainly about the lower temperatures when frying the formation of harmful by-products of the Maillard reaction such. B. to prevent or reduce acrylamide .

### Vacuum as a heat insulator

The low heat transfer in the vacuum is used in vacuum jugs and Dewar flasks , as well as in vacuum tube collectors and rarely also in multi-pane insulating glass and vacuum insulation panels .

There are double-pane insulating glass in which there is a vacuum instead of a noble gas between the panes. Because the two panes deform under the air pressure on one side of the panes, inconspicuous, transparent spacers are required between the panes distributed over the surface. The result is a comparatively thin and light glazing with very low thermal conductivity.

## Vacuum of space

The vacuum that prevails in space in interstellar space or in intergalactic space is better than any vacuum that can be produced on earth. However, space is not completely empty either, but contains an average of one particle per cm³, but within voids significantly less (up to 1 particle per cubic meter). Static electric and magnetic fields, gravitational fields as well as electromagnetic waves (photons) and particle flows ( neutrinos , cosmic rays , particles) also occur there (see also plenism ).

Environmental parameters of near-earth and distant space
Pressure in mbar Particles per m³ mean free path in km
Interplanetary space <10 −18 <10 4 < 10 11
GEO < 10 −17 < 10 5 > 10 10
outer Van Allen belt < 10 −13 ... 10 −9 < 10 90 … 10 13 < 10 6
LEO < 10 −70 ... 10 −8 < 10 15 … 10 14 < ≈ 2

Artificial satellites and space probes are therefore subject to special design requirements: The control of the heat balance (internal heat sources and solar radiation) can only take place through heat conduction and radiation; Heat sinks, heat pipes ).

In the shade of the sun, radiation can also produce very low temperatures in a targeted manner due to the vacuum (e.g. for infrared and radio wave radiation sensors ).

## literature

• Max Wutz, Hermann Adam, Wilhelm Walcher, Karl Jousten: Manual vacuum technology. Theory and practice . Vieweg, 2000, ISBN 3-528-54884-3 .
• Wolfgang Pupp , Heinz K. Hartmann : Vacuum technology: Basics and applications . Fachbuchverlag Leipzig, Leipzig 1991, ISBN 3-446-15859-6 .
• Henning Genz : Nothing but nothing. The physics of the vacuum . WILEY-VCH, Weinheim 2004, ISBN 3-527-40319-1 .
• Frank Close: Understand nothing . The search for the vacuum and the development of quantum physics. Spectrum Academic Publishing House, 2009, ISBN 978-3-8274-2095-4 .
• John D. Barrow : The Book of Nothing . Jonathan Cape, London 2000, ISBN 0-224-05962-9 .
• Karin Wey, Ralph Jürgen Peters: History of vacuum technology. In: Vacuum in Research and Practice , 14, No. 3, 2002, pp. 180-183, ( doi : 10.1002 / 1522-2454 (200206) 14: 3 <180 :: AID-VIPR180> 3.0. CO; 2-A ).
• Heinz-Dieter Bürger: The history of vacuum cooling. In: Vacuum in research and practice. 16, No. 2, 2004, pp. 67-70, ( doi: 10.1002 / vipr.200400217 ).
• C. Granda, RG Moreira, SE Tichy: Reduction of Acrylamide Formation in Potato Chips by Low-temperature Vacuum Frying . In: Journal of Food Science . tape 69 , no. 8 , 2004, p. 405-411 , doi : 10.1111 / j.1365-2621.2004.tb09903.x ( pkdiet.com [PDF]).
• IJR Aitchison: Nothing's plenty - The vacuum in modern quantum field theory. In: Contemporary Physics , 50, No. 1, 2009, pp. 261-319, .
• Christian Reidenbach: Emptiness . In: Stephan Günzel (Ed.): Lexicon of the philosophy of space . Scientific Book Society, Darmstadt 2012, ISBN 978-3-534-21931-5 . P. 230 f.
• Christian Reidenbach: The gap in the world. A history of ideas about emptiness in early modern France (=  Epistemata Philosophie . No. 591 ). Königshausen & Neumann, Würzburg 2018, ISBN 978-3-8260-6374-9 .

Wiktionary: vacuum  - explanations of meanings, word origins, synonyms, translations
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## Individual evidence

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