Levitation (from
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''levitas'' "lightness") is the process by which an object is held aloft in a stable position, without mechanical support via any physical contact.
Levitation is accomplished by providing an upward
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
that counteracts the pull of
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
(in relation to gravity on earth), plus a smaller stabilizing force that pushes the object toward a home position whenever it is a small distance away from that home position. The force can be a fundamental force such as magnetic or electrostatic, or it can be a reactive force such as optical, buoyant, aerodynamic, or hydrodynamic.
Levitation excludes
floating at the surface of a liquid because the liquid provides direct mechanical support. Levitation excludes hovering flight by insects, hummingbirds, helicopters, rockets, and balloons because the object provides its own counter-gravity force.
Physics
Levitation (on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
or any planetoid) requires an upward force that cancels out the
weight
In science and engineering, the weight of an object is the force acting on the object due to gravity.
Some standard textbooks define weight as a Euclidean vector, vector quantity, the gravitational force acting on the object. Others define weigh ...
of the object, so that the object does not fall (accelerate downward) or rise (accelerate upward). For positional stability, any small displacement of the levitating object must result in a small change in force in the opposite direction. the small changes in force can be accomplished by gradient field(s) or by active regulation. If the object is disturbed, it might oscillate around its final position, but its motion eventually decreases to zero due to
damping
Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples incl ...
effects. (In a turbulent flow, the object might oscillate indefinitely.)
Levitation techniques are useful tools in physics research. For example, levitation methods are useful for high-temperature melt property studies because they eliminate the problem of reaction with containers and allow deep undercooling of melts. The containerless conditions may be obtained by opposing gravity with a levitation force instead of allowing an entire experiment to freefall.
Magnetic levitation
Magnetic levitation is the most commonly seen and used form of levitation.
Diamagnetic
Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...
materials are commonly used for demonstration purposes. In this case the returning force appears from the interaction with the
screening currents. For example, a
superconducting
Superconductivity is a set of physical properties observed in certain materials where Electrical resistance and conductance, electrical resistance vanishes and magnetic field, magnetic flux fields are expelled from the material. Any material e ...
sample, which can be considered either as a perfect diamagnet or an
ideally hard superconductor, easily levitates in an ambient external magnetic field. The superconductor is first heated strongly, then cooled with liquid nitrogen to levitate on top of a diamagnet. In a very strong magnetic field by means of
diamagnetic levitation
Magnetic levitation (maglev) or magnetic suspension is a method by which an object is levitation, suspended with no support other than magnetic fields. Lorentz force, Magnetic force is used to counteract the effects of the gravitational force a ...
, even small live animals have been levitated.
It is possible to levitate pyrolytic graphite by placing thin squares of it above four cube magnets with the north poles forming one diagonal and south poles forming the other diagonal. Researcher have even successfully levitated (non-magnetic) liquid droplets surrounded by paramagnetic fluids. The process of such inverse magnetic levitation is usually referred to as Magneto-Archimedes effect.
Magnetic levitation is in development for use for transportation systems. For example, the
Maglev
Maglev (derived from '' magnetic levitation''), is a system of train transportation that uses two sets of electromagnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage ...
includes trains that are levitated by a large number of magnets. Due to the lack of friction on the guide rails, they are faster, quieter, and smoother than wheeled mass transit systems.
Electrodynamic suspension
Electrodynamic suspension (EDS) is a form of magnetic levitation in which there are conductors which are exposed to time-varying magnetic fields. This induces eddy currents in the conductors that creates a repulsive magnetic field which holds the t ...
uses AC magnetic fields.
Electrostatic levitation
In electrostatic levitation an
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
is used to counteract gravitational force. Some spiders shoot silk into the air to ride Earth's electric field.
Aerodynamic levitation
In aerodynamic levitation, the levitation is achieved by floating the object on a stream of gas, either produced by the object or acting on the object. For example, a
ping pong ball
Table tennis, also known as ping-pong and whiff-whaff, is a sport in which two or four players hit a lightweight ball, also known as the ping-pong ball, back and forth across a table using small solid rackets. It takes place on a hard table div ...
can be levitated with the stream of air from a vacuum cleaner set on "blow" - exploiting the
Coandă_effect
The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a convex surface. ''Merriam-Webster'' describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to ent ...
which keeps it stable in the airstream. With enough thrust, very large objects can be levitated using this method.
Gas film levitation
This technique enables the levitation of an object against
gravitational force
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
by floating it on a thin
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
film formed by gas flow through a
porous
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
membrane. Using this technique, high temperature melts can be kept clean from contamination and be supercooled.
[ A common example in general usage includes ]air hockey
Air hockey is a ''Pong''-like tabletop sport where two opposing players try to score goals against each other on a low-friction table using two hand-held discs ("mallets") and a lightweight plastic puck.
The air hockey table has raised edges ...
, where the puck is lifted by a thin layer of air. Hovercraft
A hovercraft, also known as an air-cushion vehicle or ACV, is an amphibious Craft (vehicle), craft capable of travelling over land, water, mud, ice, and other surfaces.
Hovercraft use blowers to produce a large volume of air below the hull ...
also use this technique, producing a large region of high-pressure air underneath them.
Acoustic levitation
Acoustic levitation uses sound waves to provide a levitating force.
Optical levitation
Optical levitation is a technique in which a material is levitated against the downward force of gravity by an upward force stemming from photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
transfer (radiation pressure
Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
).
Buoyant levitation
Gases at high pressure can have a density exceeding that of some solids. Thus they can be used to levitate solid objects through buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
. Noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
es are preferred for their non-reactivity. Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
is the densest non-radioactive noble gas, at 5.894g/L. Xenon has been used to levitate polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...
, at a pressure of 154atm.
Casimir force
Scientists have discovered a way of levitating ultra small objects by manipulating the so-called Casimir force
In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of the field. It is named after the Dutch physicist Hendrik Casimir, who pr ...
, which normally causes objects to stick together due to forces predicted by quantum field theory
In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...
. This is, however, only possible for micro-objects.
Uses
Maglev trains
Magnetic levitation is used to suspend trains without touching the track. This permits very high speeds, and greatly reduces the maintenance requirements for tracks and vehicles, as little wear occurs. This also means there is no friction, so the only force acting against it is air resistance.
Animal levitation
Scientists have levitated frogs, grasshoppers, and mice by means of powerful electromagnets utilizing superconductors, producing diamagnetic
Diamagnetic materials are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnetic materials are attracted ...
repulsion of body water. The mice acted confused at first, but adapted to the levitation after approximately four hours, suffering no immediate ill effects.
Mice Levitated in Lab
Further reading
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See also
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Levitation (illusion)
A levitation illusion is one in which a magician appears to defy gravity by making an object or person float in the air. The subject may appear to levitate unassisted, or it may be performed with the aid of another object (such as a silver ball fl ...
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Anti-gravity
Anti-gravity (also known as non-gravitational field) is a hypothetical phenomenon of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to ...
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Flight
Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
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Leidenfrost effect
The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this re ...
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Telekinesis
Psychokinesis (from grc, ψυχή, , soul and grc, κίνησις, , movement, label=ㅤ), or telekinesis (from grc, τηλε, , far off and grc, κίνησις, , movement, label=ㅤ), is a hypothetical psychic ability allowing a person ...
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Weightlessness
Weightlessness is the complete or near-complete absence of the sensation of weight. It is also termed zero gravity, zero G-force, or zero-G.
Weight is a measurement of the force on an object at rest in a relatively strong gravitational fi ...
References
External links
{{wiktionary-inline
Diamagnetic Levitation (YouTube)Superconducting Levitation Demos
Gravity