Editing Perpetual motion (section)

=== Thought experiments ===
In some cases a [[thought experiment]] appears to suggest that perpetual motion may be possible through accepted and understood physical processes. However, in all cases, a flaw has been found when all of the relevant physics is considered. Examples include:
* [[Maxwell's demon]]: This was originally proposed to show that the [[second law of thermodynamics]] applied in the statistical sense only, by postulating a "demon" that could select energetic molecules and extract their energy. Subsequent analysis (and experiment) have shown there is no way to physically implement such a system that does not result in an overall increase in [[entropy]].
* [[Brownian ratchet]]: In this thought experiment, one imagines a paddle wheel connected to a ratchet. [[Brownian motion]] would cause surrounding gas molecules to strike the paddles, but the ratchet would only allow it to turn in one direction. A more thorough analysis showed that when a physical ratchet was considered at this molecular scale, Brownian motion would also affect the ratchet and cause it to randomly fail resulting in no net gain. Thus, the device would not violate the [[laws of thermodynamics]].
* [[Vacuum energy]] and [[zero-point energy]]: In order to explain effects such as [[virtual particles]] and the [[Casimir effect]], many formulations of [[quantum physics]] include a background energy which pervades empty space, known as vacuum or zero-point energy. The ability to harness zero-point energy for useful work is considered [[pseudoscience]] by the scientific community at large.<ref name="army">{{cite web|publisher=[[U.S. Army]] [[National Ground Intelligence Center]]|url=http://info.publicintelligence.net/USArmy-ZeroPointEnergy.pdf|title=Zero-Point Energy: Can We Get Something From Nothing?|first=Amber M. |last=Aiken |quote=Forays into 'free energy' inventions and perpetual-motion machines using ZPE are considered by the broader scientific community to be pseudoscience.}}</ref><ref name=saf>{{Scientific American Frontiers |8 |2}}</ref> Inventors have proposed various methods for extracting useful work from zero-point energy, but none have been found to be viable,<ref name="army"/><ref name=Gardner>[[Martin Gardner]], [http://www.csicop.org/si/show/dr._bearden_vacuum_energy/ {{"'}}Dr' Bearden's Vacuum Energy"]. ''[[Skeptical Inquirer]]''. January/February 2007. {{Webarchive|url=https://web.archive.org/web/20190403000514/http://www.csicop.org/si/show/dr._bearden_vacuum_energy/ |date=2019-04-03 }}.</ref> no claims for extraction of zero-point energy have ever been validated by the scientific community,<ref>{{cite news |title=What is the 'zero-point energy' (or 'vacuum energy') in quantum physics? Is it really possible that we could harness this energy? |first=Matt |last=Visser |url=http://homepages.mcs.vuw.ac.nz/~visser/general.shtml#what-zpe|newspaper=Phlogistin / Scientific American|date=3 October 1996
|archive-url=https://web.archive.org/web/20080714115109/http://homepages.mcs.vuw.ac.nz/~visser/general.shtml#what-zpe|archive-date= July 14, 2008
|access-date=31 May 2013}} [http://www.sciam.com/article.cfm?id=follow-up-what-is-the-zer Alternative link]</ref> and there is no evidence that zero-point energy can be used in violation of conservation of energy.<ref>{{cite journal |journal=[[Scientific American]]|date=18 August 1997|url=http://www.sciam.com/article.cfm?id=follow-up-what-is-the-zer|title=FOLLOW-UP: What is the 'zero-point energy' (or 'vacuum energy') in quantum physics? Is it really possible that we could harness this energy?}}</ref>
*[[Ellipsoid paradox]]: This paradox considers a perfectly reflecting cavity with two [[Black body|black bodies]] at points ''A'' and ''B''. The reflecting surface is composed of two elliptical sections ''E''<sub>1</sub> and ''E''<sub>2</sub> and a spherical section ''S'', and the bodies at ''A'' and ''B'' are located at the joint foci of the two ellipses and ''B'' is at the center of ''S''. This configuration is such that the black body at ''B'' heats up relative to ''A'': the radiation originating from the black body at ''A'' will land on and be absorbed by the blackbody at ''B''. Similarly, rays originating from point ''B'' that land on ''E''<sub>1</sub> and ''E''<sub>2</sub> will be reflected to ''A''. However, a significant proportion of rays that start from ''B'' will land on ''S'' will be reflected back to ''B''. This paradox is solved when the black bodies' finite sizes are considered instead of punctual black bodies.<ref>{{Cite journal|last1=Yoder|first1=Theodore J.|last2=Adkins|first2=Gregory S.|date=2011|title=Resolution of the ellipsoid paradox in thermodynamics|journal=American Journal of Physics|language=en|volume=79|issue=8|pages=811–818|doi=10.1119/1.3596430|bibcode=2011AmJPh..79..811Y|issn=0002-9505}}</ref><ref>{{Cite web|last=Mutalik|first=Pradeep|title=How to Design a Perpetual Energy Machine|url=https://www.quantamagazine.org/how-to-design-a-perpetual-energy-machine-20200401/|access-date=2020-06-08|website=Quanta Magazine|date=April 2020|language=en}}</ref>
[[File:Ellipsoid_paradox.svg|alt=|center|thumb|upright=1.6|Ellipsoid paradox surface and rays emitted by body {{mvar|A}} in the direction of body {{mvar|B}}. ({{mvar|a}}) When bodies {{mvar|A}} and {{mvar|B}} are point like, all rays from {{mvar|A}} must be incident on {{mvar|B}}. ({{mvar|b}}) When bodies {{mvar|A}} and {{mvar|B}} are extended, some rays from {{mvar|A}} will not be incident on {{mvar|B}} and may eventually return to {{mvar|A}}.]]