Vacuum is space that is devoid of matter.
The word stems from the Latin adjective vacuus
for "vacant" or "void". An approximation to
such vacuum is a region with a gaseous pressure
much less than atmospheric pressure. Physicists
often discuss ideal test results that would
occur in a perfect vacuum, which they sometimes
simply call "vacuum" or free space, and use
the term partial vacuum to refer to an actual
imperfect vacuum as one might have in a laboratory
or in space. The Latin term in vacuo is used
to describe an object as being in what would
otherwise be a vacuum.
The quality of a partial vacuum refers to
how closely it approaches a perfect vacuum.
Other things equal, lower gas pressure means
higher-quality vacuum. For example, a typical
vacuum cleaner produces enough suction to
reduce air pressure by around 20%. Much higher-quality
vacuums are possible. Ultra-high vacuum chambers,
common in chemistry, physics, and engineering,
operate below one trillionth of atmospheric
pressure, and can reach around 100 particles/cm3.
Outer space is an even higher-quality vacuum,
with the equivalent of just a few hydrogen
atoms per cubic meter on average. According
to modern understanding, even if all matter
could be removed from a volume, it would still
not be "empty" due to vacuum fluctuations,
dark energy, transiting gamma- and cosmic
rays, neutrinos, along with other phenomena
in quantum physics. In modern particle physics,
the vacuum state is considered as the ground
state of matter.
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. A torricellian
vacuum is created by filling with mercury
a tall glass container closed at one end and
then inverting the container into a bowl to
contain the mercury.
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.
Etymology
The word vacuum comes from Latin, meaning
"an empty space, void", noun use of neuter
of vacuus, meaning "empty", related to vacare,
meaning "be empty".
Vacuum is one of the few words in the English
language that contains two consecutive letter
ues.
Classical Field Theories
The strictest criteria to define a vacuum
is a region of space and time where all the
components of the stress–energy tensor are
zero. It means that this region is empty of
energy and momentum, and by consequence, it
must be empty of particles and other physical
fields that contain energy and momentum.
Gravity
In general relativity, a vanishing stress-energy
tensor implies, through Einstein field equations,
the vanishing of all the components of the
Ricci tensor. Vacuum does not mean that the
curvature of space-time is necessarily flat:
the gravitational field can still produce
curvature in a vacuum in the form of tidal
forces and gravitational waves. The black
hole is an eloquent example of a region completely
"filled" with vacuum, but still showing a
strong curvature.
Electromagnetism
In classical electromagnetism, the vacuum
of free space, or sometimes just free space
or perfect vacuum, is a standard reference
medium for electromagnetic effects. Some authors
refer to this reference medium as classical
vacuum, a terminology intended to separate
this concept from QED vacuum or QCD vacuum,
where vacuum fluctuations can produce transient
virtual particle densities and a relative
permittivity and relative permeability that
are not identically unity.
In the theory of classical electromagnetism,
free space has the following properties:
Electromagnetic radiation travels, when unobstructed,
at the speed of light, the defined value 299,792,458 m/s
in SI units.
The superposition principle is always exactly
true. For example, the electric potential
generated by two charges is the simple addition
of the potentials generated by each charge
in isolation. The value of the electric field
at any point around these two charges is found
by calculating the vector sum of the two electric
fields from each of the charges acting alone.
The permittivity and permeability are exactly
the electric constant ε0 and magnetic constant
μ0, respectively, or exactly 1.
The characteristic impedance equals the impedance
of free space Z0 ≈ 376.73 Ω.
The vacuum of classical electromagnetism can
be viewed as an idealized electromagnetic
medium with the constitutive relations in
SI units:
relating the electric displacement field D
to the electric field E and the magnetic field
or H-field H to the magnetic induction or
B-field B. Here r is a spatial location and
t is time.
Quantum mechanics
In quantum mechanics and quantum field theory,
the vacuum is defined as the state with the
lowest possible energy. In quantum electrodynamics
this vacuum is referred to as 'QED vacuum'
to distinguish it from the vacuum of quantum
chromodynamics, denoted as QCD vacuum. QED
vacuum is a state with no matter particles,
and also no photons, no gravitons, etc. As
described above, this state is impossible
to achieve experimentally. Nonetheless, it
provides a good model for realizable vacuum,
and agrees with a number of experimental observations
as described next.
QED vacuum has interesting and complex properties.
In QED vacuum, the electric and magnetic fields
have zero average values, but their variances
are not zero. As a result, QED vacuum contains
vacuum fluctuations, and a finite energy called
vacuum energy. Vacuum fluctuations are an
essential and ubiquitous part of quantum field
theory. Some experimentally verified effects
of vacuum fluctuations include spontaneous
emission and the Lamb shift. Coulomb's law
and the electric potential in vacuum near
an electric charge are modified.
Theoretically, in QCD vacuum multiple vacuum
states can coexist. The starting and ending
of cosmological inflation is thought to have
arisen from transitions between different
vacuum states. For theories 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 have a huge number of vacua — the
so-called string theory landscape.
Outer space
Outer space has very low density and pressure,
and is the closest physical approximation
of a perfect vacuum. But no vacuum is truly
perfect, not even in interstellar space, where
there are still a few hydrogen atoms per cubic
meter.
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 3.2 × 10−2
Pa at 100 kilometres 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 significantly 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.
Historical interpretation
Historically, there has been much dispute
over whether such a thing as a vacuum can
exist. Ancient Greek philosophers debated
the existence of a vacuum, or void, in the
context of atomism, which posited void and
atom as the fundamental explanatory elements
of physics. Following Plato, even the abstract
concept of a featureless void faced considerable
skepticism: it could not be apprehended by
the senses, it could not, itself, provide
additional explanatory power beyond the physical
volume with which it was commensurate and,
by definition, it was quite literally nothing
at all, which cannot rightly be said to exist.
Aristotle believed that no void could occur
naturally, because the denser surrounding
material continuum would immediately fill
any incipient rarity that might give rise
to a void.
In his Physics, book IV, Aristotle offered
numerous arguments against the void: for example,
that motion through a medium which offered
no impediment could continue ad infinitum,
there being no reason that something would
come to rest anywhere in particular. Although
Lucretius argued for the existence of vacuum
in the first century BC and Hero of Alexandria
tried unsuccessfully to create an artificial
vacuum in the first century AD, it was European
scholars such as Roger Bacon, Blasius of Parma
and Walter Burley in the 13th and 14th century
who focused considerable attention on these
issues. Eventually following Stoic physics
in this instance, scholars from the 14th century
onward increasingly departed from the Aristotelian
perspective in favor of a supernatural void
beyond the confines of the cosmos itself,
a conclusion widely acknowledged by the 17th
century, which helped to segregate natural
and theological concerns.
Almost two thousand years after Plato, René
Descartes also proposed a geometrically based
alternative theory of atomism, without the
problematic nothing–everything dichotomy
of void and atom. Although Descartes agreed
with the contemporary position, that a vacuum
does not occur in nature, the success of his
namesake coordinate system and more implicitly,
the spacial–corporeal component of his metaphysics
would come to define the philosophically modern
notion of empty space as a quantified extension
of volume. By the ancient definition however,
directional information and magnitude were
conceptually distinct. With the acquiescence
of Cartesian mechanical philosophy to the
"brute fact" of action at a distance, and
at length, its successful reification by force
fields and ever more sophisticated geometric
structure, the anachronism of empty space
widened until "a seething ferment" of quantum
activity in the 20th century filled the vacuum
with a virtual pleroma.
The explanation of a clepsydra or water clock
was a popular topic in the Middle Ages. Although
a simple wine skin sufficed to demonstrate
a partial vacuum, in principle, more advanced
suction pumps had been developed in Roman
Pompeii.
In the medieval Middle Eastern world, the
physicist and Islamic scholar, Al-Farabi,
conducted a small experiment 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. However, according
to Nader El-Bizri, the physicist Ibn al-Haytham
and the Mu'tazili theologians disagreed with
Aristotle and Al-Farabi, and they supported
the existence of a void. Using geometry, Ibn
al-Haytham mathematically demonstrated that
place is the imagined three-dimensional void
between the inner surfaces of a containing
body. According to Ahmad Dallal, Abū Rayhān
al-Bīrūnī also states that "there is no
observable evidence that rules out the possibility
of vacuum". The suction pump later appeared
in Europe from the 15th century.
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. Speculation that even God could not
create a vacuum if he wanted to 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. Jean Buridan reported
in the 14th century that teams of ten horses
could not pull open bellows when the port
was sealed.
The 17th century saw the first attempts to
quantify measurements of partial vacuum. Evangelista
Torricelli's mercury barometer of 1643 and
Blaise Pascal's experiments that both demonstrated
a partial 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 partially evacuated.
Robert Boyle improved Guericke's design and
with the help of Robert Hooke further developed
vacuum pump technology. Thereafter, research
into the partial vacuum lapsed until 1850
when August Toepler invented the Toepler Pump
and Heinrich Geissler invented the mercury
displacement pump in 1855, achieving a partial
vacuum of about 10 Pa. A number of electrical
properties become observable at this vacuum
level, which renewed interest in further research.
While outer space provides the most rarefied
example of a naturally occurring partial vacuum,
the heavens were originally thought to be
seamlessly filled by a rigid indestructible
material called aether. Borrowing somewhat
from the pneuma of Stoic physics, aether came
to be regarded as the rarefied air from which
it took its name,). Early theories of light
posited a ubiquitous terrestrial and celestial
medium through which light propagated. Additionally,
the concept informed Isaac Newton's explanations
of both refraction and of radiant heat. 19th
century experiments into this luminiferous
aether attempted to detect a minute drag on
the Earth's orbit. While the Earth does, in
fact, move through a relatively dense medium
in comparison to that of interstellar space,
the drag is so minuscule that it could not
be detected. In 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 1930, Paul Dirac proposed a model of the
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, confirmed
two years later. Werner Heisenberg's uncertainty
principle formulated in 1927, predict a fundamental
limit within which instantaneous position
and momentum, or energy and time can be measured.
This has far reaching consequences on the
"emptiness" of space between particles. In
the late 20th century, so-called virtual particles
that arise spontaneously from empty space
were confirmed.
Measurement
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 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 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 shown in the following table. As we travel
into orbit, outer space and ultimately intergalactic
space, the pressure varies by several orders
of magnitude.
Atmospheric pressure is variable but standardized
at 101.325 kPa.
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 the pressure 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.
Deep space is generally much more empty than
any artificial vacuum. 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 of no particles
at all. It cannot be achieved in a laboratory,
although there may be small volumes which,
for a brief moment, happen to have no particles
of matter in them. Even if all particles of
matter were removed, there would still be
photons and gravitons, as well as dark energy,
virtual particles, and other aspects of the
quantum vacuum.
Hard vacuum and soft vacuum are terms that
are defined with a dividing line defined differently
by different sources, such as 1 Torr, or 0.1
Torr, the common denominator being that a
hard vacuum is a higher vacuum than a soft
one.
Relative versus absolute measurement
Vacuum is measured in units of pressure, typically
as a subtraction relative to ambient atmospheric
pressure on Earth. But the amount of relative
measurable vacuum varies with local conditions.
On the surface of Jupiter, where ground level
atmospheric pressure is much higher than on
Earth, much higher relative vacuum readings
would be possible. On the surface of the moon
with almost no atmosphere, it would be extremely
difficult to create a measurable vacuum relative
to the local environment.
Similarly, much higher than normal relative
vacuum readings are possible deep in the Earth's
ocean. A submarine maintaining an internal
pressure of 1 atmosphere submerged to a depth
of 10 atmospheres is effectively a vacuum
chamber keeping out the crushing exterior
water pressures, though the 1 atm inside the
submarine would not normally be considered
a vacuum.
Therefore to properly understand the following
discussions of vacuum measurement, it is important
that the reader assumes the relative measurements
are being done on Earth at sea level, at exactly
1 atmosphere of ambient atmospheric pressure.
Measurements relative to 1 atm
The SI unit of pressure is the pascal, but
vacuum is often measured in torrs, named for
Torricelli, an early Italian physicist. A
torr is equal to the displacement of a millimeter
of mercury in a manometer with 1 torr equaling
133.3223684 pascals above absolute zero pressure.
Vacuum is often also measured on the barometric
scale or as a percentage of atmospheric pressure
in bars or atmospheres. Low vacuum is often
measured in millimeters of mercury or pascals
below standard atmospheric pressure. "Below
atmospheric" means that the absolute pressure
is equal to the current atmospheric pressure.
In other words, most low vacuum gauges that
read, for example 50.79 Torr. Many inexpensive
low vacuum gauges have a margin of error and
may report a vacuum of 0 Torr but in practice
this generally requires a two stage rotary
vane or other medium type of vacuum pump to
go much beyond 1 torr.
Measuring instruments
Many devices are used to measure the pressure
in a vacuum, depending on what range of vacuum
is needed.
Hydrostatic gauges consist of a vertical column
of liquid in a tube whose ends are exposed
to different pressures. The column will rise
or fall until its weight is in equilibrium
with the pressure differential between the
two ends of the tube. The simplest design
is a closed-end U-shaped tube, one side of
which is connected to the region of interest.
Any fluid can be used, but mercury is preferred
for its high density and low vapour pressure.
Simple hydrostatic gauges can measure pressures
ranging from 1 torr to above atmospheric.
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 10−6 torr, which
is the lowest direct measurement of pressure
that is possible with current technology.
Other vacuum gauges can measure lower pressures,
but only indirectly by measurement of other
pressure-controlled properties. These indirect
measurements must be calibrated via a direct
measurement, most commonly a McLeod gauge.
The kenotometer is a particular type of hydrostatic
gauge, typically used in power plants using
steam turbines. The kenotometer measures the
vacuum in the steam space of the condenser,
that is, the exhaust of the last stage of
the turbine.
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.
A variation on this idea is the capacitance
manometer, in which the diaphragm makes up
a part of a capacitor. A change in pressure
leads to the flexure of the diaphragm, which
results in a change in capacitance. These
gauges are effective from 10+3 torr to 10−4 torr,
and beyond.
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 or Resistance Temperature
Detector 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. A common variant is
the Pirani gauge which uses a single platinum
filament as both the heated element and RTD.
These gauges are accurate from 10 torr to
10−3 torr, but they are sensitive to the
chemical composition of the gases being measured.
Ion gauges are used in ultrahigh vacuum. 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.
Ionization gauge calibration is very sensitive
to construction geometry, chemical composition
of gases being measured, corrosion and surface
deposits. 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.
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. The
chemical inertness produced by a vacuum 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. 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
lowers the boiling point of liquids and promotes
low temperature 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.
Vacuum-driven machines
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. Vacuum brakes were once widely used
on trains in the UK but, except on heritage
railways, they have been replaced by air brakes.
Manifold vacuum can be used to drive accessories
on automobiles. The best-known application
is the vacuum servo, used to provide power
assistance for the brakes. Obsolete applications
include vacuum-driven windscreen wipers and
fuel pumps. Some aircraft instruments and
the Heading Indicator) are typically vacuum-powered,
as protection against loss of all instruments,
since early aircraft often did not have electrical
systems, and since there are two readily available
sources of vacuum on a moving aircraft—the
engine and an external venturi. Vacuum induction
melting uses electromagnetic induction within
a vacuum.
Maintaining a vacuum in the Condenser is an
important aspect of the efficient operation
of steam turbines. A steam jet ejector or
liquid ring vacuum pump is used for this purpose.
The typical vacuum maintained in the Condenser
steam space at the exhaust of the turbine
is in the range 5 to 15 kPa, depending on
the type of condenser and the ambient conditions.
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.
Pumping and ambient air pressure
Fluids cannot generally be pulled, so a vacuum
cannot be created by suction. Suction can
spread and dilute a vacuum by letting a higher
pressure push fluids into it, but the vacuum
has to be created first before suction can
occur. The easiest way to create an artificial
vacuum is to expand the volume of a container.
For example, the diaphragm muscle expands
the chest cavity, which causes the volume
of the lungs to increase. 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. This is the
principle behind positive displacement pumps,
like the manual water pump for example. Inside
the pump, a mechanism expands a small sealed
cavity to create a vacuum. Because of the
pressure differential, some fluid 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.
The above explanation is merely a simple introduction
to vacuum pumping, and is not representative
of the entire range of pumps in use. Many
variations of the positive displacement pump
have been developed, and many other pump designs
rely on fundamentally different principles.
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 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. 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 final pressure 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 adsorptivity of
hard metals such as stainless steel or titanium
must be considered. Some oils and greases
will boil off in extreme vacuums. The permeability
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. However,
pressures as low as 5×10−17 Torr have
been indirectly measured in a 4 K cryogenic
vacuum system. This corresponds to ≈100
particles/cm3.
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 depicted
in media and popular culture. The reduction
in pressure lowers the temperature at which
blood and other body fluids boil, but the
elastic pressure of blood vessels ensures
that this boiling point remains above the
internal body temperature of 37 °C. Although
the blood will not boil, the formation of
gas bubbles in bodily fluids at reduced pressures,
known as ebullism, is still a concern. The
gas may bloat the body to twice its normal
size and slow circulation, but tissues are
elastic and porous enough to prevent rupture.
Swelling and ebullism can be restrained by
containment in a flight suit. Shuttle astronauts
wore a fitted elastic garment called the Crew
Altitude Protection Suit which prevents ebullism
at pressures as low as 2 kPa. Rapid boiling
will cool the skin and create frost, particularly
in the mouth, but this is not a significant
hazard.
Animal experiments show that rapid and complete
recovery is normal 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. Robert Boyle was the first to
show in 1660 that vacuum is lethal to small
animals.
An experiment indicates that plants are able
to survive in a low pressure environment for
about 30 minutes.
During 1942, in one of a series of experiments
on human subjects for the Luftwaffe, the Nazi
regime experimented on prisoners in Dachau
concentration camp by exposing them to low
pressure.
Cold or oxygen-rich atmospheres can sustain
life at pressures much lower than atmospheric,
as long as the density of oxygen is similar
to that of standard sea-level atmosphere.
The colder air temperatures found at altitudes
of up to 3 km generally compensate for the
lower pressures there. Above this altitude,
oxygen enrichment is necessary to prevent
altitude sickness in humans that did not undergo
prior acclimatization, and spacesuits are
necessary to prevent ebullism above 19 km.
Most spacesuits use only 20 kPa of pure oxygen.
This pressure is high enough to prevent ebullism,
but decompression sickness and gas embolisms
can still occur if decompression rates are
not managed.
Rapid decompression can be much more dangerous
than vacuum exposure itself. Even if the victim
does not hold his or her breath, venting through
the windpipe may be too slow to prevent the
fatal rupture of the delicate alveoli of the
lungs. Eardrums and sinuses may be ruptured
by rapid decompression, soft tissues may bruise
and seep blood, and the stress of shock will
accelerate oxygen consumption leading to hypoxia.
Injuries caused by rapid decompression are
called barotrauma. A pressure drop of 13 kPa,
which produces no symptoms if it is gradual,
may be fatal if it occurs suddenly.
Some extremophile microrganisms, such as tardigrades,
can survive vacuum for a period of days or
weeks.
Examples
See also
Decay of the vacuum
Engine vacuum
False vacuum
Helium mass spectrometer - technical instrumentation
to detect a vacuum leak
Joining materials
Pneumatic tube - transport system using vacuum
or pressure to move containers in tubes
Rarefaction - reduction of a medium's density
Suction - creation of a partial vacuum
Vacuum angle
Vacuum cementing - natural process of solidifying
homogeneous "dust" in vacuum
Vacuum deposition - process of depositing
atoms and molecules in a sub-atmospheric pressure
environment
Vacuum engineering
Vacuum flange
Notes
Henning Genz. Nothingness: The Science Of
Empty Space. Da Capo Press. ISBN 0-7382-0610-5. 
Luciano Boi. The Quantum Vacuum: A Scientific
and Philosophical Concept, from Electrodynamics
to String Theory and the Geometry of the Microscopic
World. Johns Hopkins University Press. ISBN 1-4214-0247-5. 
External links
VIDEO on the nature of vacuum by Canadian
astrophysicist Doctor P
The Foundations of Vacuum Coating Technology
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.
Roberts, Mark D.. "Vacuum Energy". High Energy
Physics - Theory. arXiv:hep-th/0012062. Bibcode:2000hep.th...12062R. 
Vacuum, Production of Space
"Much Ado About Nothing" by Professor John
D. Barrow, Gresham College
Free pdf copy of The Structured Vacuum - thinking
about nothing by Johann Rafelski and Berndt
Muller ISBN 3-87144-889-3.
