Everything about Perfect Vacuum totally explained
» This vacuum means "absent of matter"; for the cleaning appliance, see vacuum cleaner.
A
vacuum is a
volume of
space that's essentially empty of
matter, such that its gaseous
pressure is much less than
atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty. A
perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that's never observed in practice.
Physicists often 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 real vacuum. 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. 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 theory sets limits for the best possible quality of vacuum, predicting that
no volume of space can be perfectly empty.
Outer space is a natural high quality vacuum, mostly of much higher quality than 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 laboratory 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 widespread use was in the
incandescent light bulb to protect the filament from chemical degradation. Its chemical inertness is also useful for
electron beam welding,
cold welding,
vacuum packing and
vacuum frying.
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,
physical vapor deposition, and
dry etching which are essential to the fabrication of
semiconductors and
optical coatings, and to
surface science. The reduction of convection provides the thermal insulation of
thermos bottles. 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.
Vacuums are commonly used to produce
suction, which has an even wider 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
Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. It has effectively no
friction, allowing
stars,
planets and
moons 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. The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces; however, it's much less costly 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. The Earth's atmospheric pressure drops to about 1 Pa (10
-3 Torr) at 100 km of altitude, the
Kármán line 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 from the
sun and the
dynamic pressure of the
solar wind, so the definition of pressure becomes difficult to interpret. The
thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to
space weather. Astrophysicists prefer to use
number density 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 on
satellites. Most artificial satellites operate in this region called
low earth orbit and must fire their engines every few days to maintain orbit. The drag here's low enough that it could theoretically be overcome by radiation pressure on
solar sails, a proposed propulsion system for
interplanetary travel. Planets are too massive for their trajectories to be affected by these forces, although their atmospheres are eroded by the solar winds.
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
Humans and animals exposed to vacuum will lose
consciousness after a few seconds and die of
hypoxia within minutes, but the symptoms are not nearly as graphic as commonly shown in pop culture.
Blood and other body fluids do boil when their pressure drops below 6.3 kPa, (47 Torr,) the
vapour pressure of water at body temperature. This condition is called
ebullism. The steam 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 restrained 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 2 kPa (15 Torr). 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 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's consistent with animal data. Limbs may be exposed for much longer if breathing isn't impaired. Injuries caused by rapid decompression are called
barotrauma. A pressure drop as small as 100 Torr, (13 kPa,) which produces no symptoms if it's gradual, may be fatal if occurs suddenly. The
philosopher Al-Farabi (872 - 950
CE) appears to have carried out the first 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. The commonly held view that nature abhorred a vacuum was called
horror vacui. 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. In spite of this, 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.
Jean Buridan reported in the 14th century that teams of ten horses couldn't pull open
bellows when the port was sealed, apparently because of horror vacui. Berti's barometer produced a vacuum above the water column, but he couldn't explain it. The breakthrough was made by
Evangelista Torricelli in 1643. Building upon Galileo's notes, he built the first
mercury barometer 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's experiments that proved the top space really contained vacuum.
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.
Robert Hooke also helped Boyle produce an air pump which helped to produce the 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.
While outer space has been likened to a vacuum, early theories of the nature of
light relied upon the existence of an invisible, aetherial medium which would convey waves of light (
Isaac 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). 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.
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.
Even an ideal vacuum, thought of as the complete absence of anything, won't in practice remain empty. Consider a vacuum chamber that has been completely evacuated, so that the (classical) particle concentration is zero. The walls of the chamber will emit light in the form of
black body radiation. This light carries momentum, so the vacuum does have a radiation pressure. This limitation applies even to the vacuum of interstellar space. Even if a region of space contains no particles, the
Cosmic Microwave Background fills the entire universe with black body radiation.
An ideal vacuum can't exist even inside of a molecule. Each atom in the molecule exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Thus, even "between" the atoms there's a certain probability of finding a particle, so the space can't be said to be a 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. However, pressures as low as 5×10
-17 Torr have been indirectly measured in a 4
K cryogenic vacuum system.
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, 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 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.
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, but a typical distribution is as follows:
| Atmospheric pressure |
760 Torr |
101.3 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. 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 that we can create. 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 that can't be obtained in a laboratory, nor can it be found in outer space.
Examples
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