Editing Levitation (physics)

{{Short description|Physical process by which an object is held aloft, without mechanical support, in a stable position}}
{{other uses|Levitation (disambiguation)}}
{{more footnotes needed|date=March 2017}}
[[Image:Magnet 4.jpg|thumb|A cube [[magnet]] levitating over a [[superconductor|superconducting]] material (known as the [[Meissner effect]])]]
'''Levitation''' (from [[Latin]] ''{{linktext|levitas}}'', {{literal|lightness}})<ref>''Levitate'', "to rise by virtue of lightness," from Latin ''levitas'' "lightness," patterned in English on ''gravitate'': [http://dictionary.reference.com/search?q=levitate Online Etymology Dictionary]</ref> 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]] that counteracts the pull of [[gravitation|gravity]] (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.{{Citation needed|date=December 2014}}
Levitation excludes [[Buoyancy|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.{{Citation needed|date=December 2014}}

==Physics==
Levitation (on [[Earth]] or any planetoid) requires an upward force that cancels out the [[weight]] 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.{{Citation needed|date=December 2014}} 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 ratio|damping]] effects. (In a turbulent flow, the object might oscillate indefinitely.){{Citation needed|date=December 2014}}

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.<ref name=Nordine2000>{{citation|author1=Paul C. Nordine |author2=J. K. Richard Weber |author3=Johan G. Abadie |journal=Pure and Applied Chemistry|title=Properties of high-temperature melts using levitation|date=2000|volume=72|issue=11 |pages=2127–2136|doi=10.1351/pac200072112127|doi-access=free}}</ref>

==Magnetic levitation==
{{main|Magnetic levitation}}

[[File:Stickstoff gekühlter Supraleiter schwebt über Dauermagneten 2009-06-21.jpg|thumb|A [[high-temperature superconductor]] levitating above magnet]]

Magnetic levitation is the most commonly seen and used form of levitation. This form of levitation occurs when an object is suspended using magnetic fields.

[[Diamagnetic]] materials are commonly used for demonstration purposes. In this case the returning force appears from the interaction with the [[screening current]]s. For example, a [[superconductivity|superconducting]] 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 cooled with liquid nitrogen to levitate on top of a magnet becoming super diamagnetic. In a powerful magnetic field utilizing [[diamagnetic levitation]], 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.<ref>{{citation|author=Waldron, Robert D.|title=Diamagnetic Levitation Using Pyrolytic Graphite|journal=Review of Scientific Instruments|volume=37|issue=1|pages=29–35|doi=10.1063/1.1719946|bibcode = 1966RScI...37...29W |year=1966|doi-access=free}}</ref> Researchers have even successfully levitated (non-magnetic) liquid droplets surrounded by paramagnetic fluids.<ref>{{citation|author=Singh, Chamkor|author2=Das, Arup K.|author3=Das, Prasanta K.|title=Levitation of non-magnetizable droplet inside ferrofluid|journal=Journal of Fluid Mechanics|year=2018|volume=857|pages=398–448|doi=10.1017/jfm.2018.733|url=https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/levitation-of-nonmagnetizable-droplet-inside-ferrofluid/DD631FF653EE08399CBB233301E381D5#|arxiv=1712.01500|bibcode=2018JFM...857..398S|s2cid=53607312}}</ref> The process of such inverse magnetic levitation is usually referred to as Magneto-Archimedes effect.

[[Image:A maglev train coming out, Pudong International Airport, Shanghai.jpg|thumb|A magnetically levitated (maglev) train departing [[Shanghai Pudong International Airport]] on the first commercial high-speed maglev line in the world]]

Magnetic levitation is in development for use for transportation systems. For example, the [[Maglev]] 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]] uses AC magnetic fields.

==Electrostatic levitation==
{{main|Electrostatic levitation}}
In electrostatic levitation an [[electric field]] is used to counteract gravitational force. Some spiders shoot silk into the air to ride Earth's electric field.

==Aerodynamic levitation==
{{main|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]] can be levitated with the stream of air from a vacuum cleaner set on "blow" - exploiting the [[Coandă effect]] 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]] by floating it on a thin [[gas]] film formed by gas flow through a [[porous]] membrane.  Using this technique, high temperature melts can be kept clean from contamination and be supercooled.<ref name=Nordine2000/>  A common example in general usage includes [[air hockey]], where the puck is lifted by a thin layer of air.  [[Hovercraft]] also use this technique, producing a large region of high-pressure air underneath them.

==Acoustic levitation==
{{main|Acoustic levitation}}
Acoustic levitation uses sound waves to provide a levitating force.

==Optical levitation==
{{main|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]] [[momentum]] transfer ([[radiation pressure]]).

==Buoyant levitation==<!-- [[Buoyant levitation]] redirects here. -->
Gases at high pressure can have a density exceeding that of some solids. Thus they can be used to levitate solid objects through [[buoyancy]].<ref>http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=12048&DID=275340&action=detail Materials Processing Through Levitation in High Gas Pressure</ref> [[Noble gas]]es are preferred for their non-reactivity. [[Xenon]] is the densest non-radioactive noble gas, at 5.894g/L. Xenon has been used to levitate [[polyethylene]], at a pressure of 154atm.

==Casimir force==
Scientists have discovered a way of levitating ultra small objects by manipulating the [[Casimir force]], which normally causes objects to stick together due to forces predicted by [[quantum field theory]]. This is, however, only possible for micro-objects.<ref>{{cite web|url=https://news.yahoo.com/s/afp/20070806/sc_afp/britainsciencelevitation_070806132733|title=Scientists reveal secret of levitation, Yahoo! News|website=yahoo.com}}</ref><ref>{{cite web|url=http://www.null-hypothesis.co.uk/science/news/item/levitation_magic_nanotechnology_particles_research|title=Levitation in Miniature, Null Hypothesis|website=null-hypothesis.co.uk|access-date=2007-08-23|archive-url=https://web.archive.org/web/20110717123841/http://www.null-hypothesis.co.uk/science/news/item/levitation_magic_nanotechnology_particles_research|archive-date=2011-07-17|url-status=dead}}</ref>

==Uses==

===Maglev trains===
{{main|Maglev}}
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===
[[Image:Frog diamagnetic levitation.jpg|thumb|[[Diamagnetism|Diamagnetic]] levitation of a live frog]]
Scientists have levitated frogs,<ref>{{cite web|title=Frogs Levitate in a strong enough magnetic field|url=http://www.physics.org/facts/frog-really.asp|publisher=physics.org|access-date=20 November 2014}}</ref> grasshoppers, and mice by means of powerful electromagnets utilizing superconductors, producing [[diamagnetic]] repulsion of body water. The mice acted confused at first, but adapted to the levitation after approximately four hours, suffering no immediate ill effects.<ref>{{cite journal|title=NASA Levitates a Mouse With Magnetic Fields|journal=Popular Science|date=September 9, 2009|url=http://www.popsci.com/scitech/article/2009-09/nasa-levitates-mouse|access-date=20 November 2014}}</ref><ref>[http://www.livescience.com/5688-mice-levitated-lab.html] Mice Levitated in Lab</ref>

==Further reading==
*{{cite journal |author1=Charles P. Strehlow |author2=M. C. Sullivan |title=A Classroom Demonstration of Levitation... |journal=American Journal of Physics |volume=77 |issue=9 |pages=847–851 |date=2008 |arxiv=0803.3090|doi=10.1119/1.3095809 |s2cid=119108808 }}.

==See also==
*[[Levitation (illusion)]]
*[[Levitation based inertial sensing]]
*[[Anti-gravity]]
*[[Flight]]
*[[Leidenfrost effect]]
*[[Telekinesis]]
*[[Weightlessness]]

==References==
{{reflist}}

==External links==
*{{wiktionary-inline}}
*[https://www.youtube.com/watch?v=ezXYE5iFM_o Diamagnetic Levitation (YouTube)]
*[https://web.archive.org/web/20161011232444/http://www.imp.kiev.ua/~kord/levitation/ Superconducting Levitation Demos]

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[[Category:Levitation| ]]
[[Category:Gravity]]