Dictionary Definition
vacuum
Noun
1 the absence of matter [syn: vacuity]
2 an empty area or space; "the huge desert
voids"; "the emptiness of outer space"; "without their support
he'll be ruling in a vacuum" [syn: void, vacancy, emptiness]
3 a region empty of matter [syn: vacuity]
4 an electrical home appliance that cleans by
suction [syn: vacuum
cleaner] v : clean with a vacuum cleaner; "vacuum the carpets"
[syn: vacuum-clean,
hoover] [also: vacua (pl)]
User Contributed Dictionary
Pronunciation
- /ˈvækjuəm/, /"v
Extensive Definition
- This vacuum means "absent of matter", "An empty Area or space"; for the cleaning appliance, see vacuum cleaner.
A vacuum is a volume of space that is 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 is 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 many
years.
Vacuum has been a frequent
topic of philosophical debate since
Ancient
Greek times, but was not 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 is
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 is 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
see also Human adaptation to spaceHumans 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 is not 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 is consistent
with animal data. Limbs may be exposed for much longer if breathing
is not 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 is 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. René Descartes also argued against the
existence of a vacuum, arguing along the following lines:“Space is
identical with extension, but extension is connected with bodies;
thus there is no space without bodies and hence no empty space
(vacuum)”. 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 could not
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 could
not 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 could not
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 is not detectably slowed in
its orbit), and extremely rigid (because vibrations propagate so
rapidly). An 1891 article by
William
Crookes noted: "the [freeing of] 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 is
therefore no aether, and no such entity is required for the
propagation of light, space between the stars is not completely
empty. Besides the various particles which comprise cosmic
radiation, there is 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 is 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
detail vacuum state In quantum mechanics, the vacuum is defined as the state (i.e. 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, will not 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 cannot 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
is a certain probability of finding a particle, so the space cannot
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 is not 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 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, 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 do not have universally agreed
definitions, but a typical distribution is as follows:
- 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 cannot be obtained in a laboratory, nor can it be found in outer space.
Examples
Synonyms, Antonyms and Related Words
absence, absence of mind,
blank, blankness, broom, brush, brush off, clean slate,
deprivation,
emptiness, empty
space, empty-headedness, hollowness, inanity, jejunity, mental void, negation, negativeness, negativity, nihility, nonbeing, nonentity, nonexistence, nonoccurrence, nonreality, nonsubsistence,
not-being, nothing,
nothingness,
nullity, sweep, sweep out, sweep up, tabula
rasa, unactuality,
unreality, vacancy, vacuity, vacuousness, vacuum-clean,
vapidity, void, whisk