Editing Perpetual motion (section)

=== Impossibility ===
[[Image:Perpetual Motion by Norman Rockwell.jpg|thumb|right|October 1920 issue of ''[[Popular Science]]'' magazine, on perpetual motion. Although scientists have established them to be impossible under the laws of physics, perpetual motion continues to capture the imagination of inventors.{{notetag|The device shown is a "mass leverage" device, where the spherical weights on the right have more leverage than those on the left, supposedly creating a perpetual rotation. However, there are a greater number of weights on the left, balancing the device.}}]]

"[[Epistemic possibility|Epistemic impossibility]]" describes things which absolutely cannot occur within our ''current'' formulation of the physical laws. This interpretation of the word "impossible" is what is intended in discussions of the impossibility of perpetual motion in a closed system.<ref name=barrow>{{cite book |last= Barrow |first= John D. |title= Impossibility: The Limits of Science and the Science of Limits |publisher= [[Oxford University Press]] |year= 1998 |isbn=978-0-19-851890-7}}</ref>

The conservation laws are particularly robust from a mathematical perspective. [[Noether's theorem]], which was [[Mathematical proof|proven mathematically]] in 1915, states that any conservation law can be derived from a corresponding continuous symmetry of the [[Action (physics)|action]] of a physical system.<ref name="goldstein">{{Cite book| last1 = Goldstein | first1 = Herbert |author1-link=Herbert Goldstein
| last2 = Poole	| first2 = Charles |author2-link=Charles P. Poole
| last3 = Safko	| first3 = John
| title = Classical Mechanics
| url = https://archive.org/details/classicalmechani00gold_919 | url-access = limited | publisher = Addison Wesley
| year = 2002
| location = San Francisco
| pages = [https://archive.org/details/classicalmechani00gold_919/page/n594 589]–598
| isbn = 978-0-201-65702-9
| edition = 3rd
}}</ref> The symmetry which is equivalent to conservation of energy is the [[Time translation symmetry|time invariance]] of physical laws. Therefore, if the laws of physics do not change with time, then the conservation of energy follows. For energy conservation to be violated to allow perpetual motion would require that the foundations of physics would change.<ref>{{Cite news
 | url = http://news.bbc.co.uk/1/hi/technology/6283374.stm
 | title = The perpetual myth of free energy
 | work = BBC News
 | date = 9 July 2007
 | quote = In short, law states that energy cannot be created or destroyed. Denying its validity would undermine not just little bits of science – the whole edifice would be no more. All of the technology on which we built the modern world would lie in ruins.|access-date=16 August 2010}}</ref>

Scientific investigations as to whether the laws of physics are invariant over time use telescopes to examine the universe in the distant past to discover, to the limits of our measurements, whether ancient stars were identical to stars today. Combining different measurements such as [[spectroscopy]], direct measurement of the [[SN1987A|speed of light in the past]] and similar measurements demonstrates that physics has remained substantially the same, if not identical, for all of observable time spanning [[billion]]s of years.<ref>[http://www.talkorigins.org/indexcc/CE/CE410.html "CE410: Are constants constant?"] ''The TalkOrigins Archive''.</ref>

The principles of thermodynamics are so well established, both theoretically and experimentally, that proposals for perpetual motion machines are universally dismissed by physicists. Any proposed perpetual motion design offers a potentially instructive challenge to physicists: one is certain that it cannot work, so one must explain ''how'' it fails to work. The difficulty (and the value) of such an exercise depends on the subtlety of the proposal; the best ones tend to arise from physicists' own [[thought experiment]]s and often shed light upon certain aspects of physics. So, for example, the thought experiment of a [[Brownian ratchet]] as a perpetual motion machine was first discussed by [[Gabriel Lippmann]] in 1900 but it was not until 1912 that [[Marian Smoluchowski]] gave an adequate explanation for why it cannot work.<ref name="Harmor">{{cite web
  | last1 = Harmor
  | first1 = Greg
  | first2 = Derek
  | last2 = Abbott
  | title = The Feynman-Smoluchowski ratchet
  | work = Parrondo's Paradox Research Group
  | publisher = School of Electrical & Electronic Engineering, University of Adelaide
  | year = 2005
  | url = http://www.eleceng.adelaide.edu.au/Groups/parrondo/ratchet.html
  | access-date = 2010-01-15
  | archive-date = 2009-10-11
  | archive-url = https://web.archive.org/web/20091011194353/http://www.eleceng.adelaide.edu.au/Groups/parrondo/ratchet.html
  | url-status = dead
  }}</ref> However, during that twelve-year period scientists did not believe that the machine was possible. They were merely unaware of the exact mechanism by which it would inevitably fail.

{{blockquote|The law that entropy always increases – the second law of thermodynamics – holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations – then so much the worse for Maxwell's equations. If it is found to be contradicted by observation – well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.|Sir [[Arthur Stanley Eddington]], ''The Nature of the Physical World'' (1927)}}

In the mid-19th-century [[Henry Dircks]] investigated the history of perpetual motion experiments, writing a vitriolic attack on those who continued to attempt what he believed to be impossible:

{{blockquote|There is something lamentable, degrading, and almost insane in pursuing the visionary schemes of past ages with dogged determination, in paths of learning which have been investigated by superior minds, and with which such adventurous persons are totally unacquainted. The history of Perpetual Motion is a history of the fool-hardiness of either half-learned, or totally ignorant persons.<ref>{{cite book|url=https://books.google.com/books?id=lEWF9o2Ba4kC&pg=PA1|title=Perpetuum Mobile: Or, A History of the Search for Self-motive|first=Henry|last=Dircks|page=354|year=1861|access-date=17 August 2012}}</ref>|Henry Dircks, ''Perpetuum Mobile: Or, A History of the Search for Self-motive'' (1861)}}