Editing Faster-than-light (section)

===Casimir vacuum and quantum tunnelling===
[[Special relativity]] postulates that the speed of light in vacuum is invariant in [[inertial frame]]s. That is, it will be the same from any frame of reference moving at a constant speed. The equations do not specify any particular value for the speed of light, which is an experimentally determined quantity for a fixed unit of length. Since 1983, the [[International System of Units|SI]] unit of length (the [[meter]]) has been defined using the [[speed of light]].

The experimental determination has been made in vacuum. However, the vacuum we know is not the only possible vacuum which can exist. The vacuum has energy associated with it, called simply the [[vacuum energy]], which could perhaps be altered in certain cases.<ref>{{cite magazine |title=What is the 'zero-point energy' (or 'vacuum energy') in quantum physics? Is it really possible that we could harness this energy? |url=https://www.scientificamerican.com/article/follow-up-what-is-the-zer/ |magazine=Scientific American |date=1997-08-18 |access-date=2009-05-27}}</ref> When vacuum energy is lowered, light itself has been predicted to go faster than the standard value ''c''. This is known as the [[Scharnhorst effect]]. Such a vacuum can be produced by bringing two perfectly smooth metal plates together at near atomic diameter spacing. It is called a [[Casimir effect#Vacuum energy|Casimir vacuum]]. Calculations imply that light will go faster in such a vacuum by a minuscule amount: a photon traveling between two plates that are 1 micrometer apart would increase the photon's speed by only about one part in 10<sup>36</sup>.<ref>{{Cite web |last=Scharnhorst |first=Klaus |date=1990-05-12 |title=Secret of the vacuum: Speedier light |url=http://www.nat.vu.nl/~scharnh/m16scine.htm |access-date=2009-05-27 |website=[[Vrije Universiteit Amsterdam]]}}</ref> Accordingly, there has as yet been no experimental verification of the prediction. A recent analysis<ref name="lib">{{Cite journal |last1=Liberati |first1=Stefano |last2=Sonego |first2=Sebastiano |last3=Visser |first3=Matt |year=2002 |title=Faster-than-c Signals, Special Relativity, and Causality |journal=[[Annals of Physics]] |language=en |volume=298 |issue=1 |pages=167–185 |arxiv=gr-qc/0107091 |bibcode=2002AnPhy.298..167L |doi=10.1006/aphy.2002.6233 |s2cid=48166}}</ref> argued that the Scharnhorst effect cannot be used to send information backwards in time with a single set of plates since the plates' rest frame would define a "[[preferred frame]]" for FTL signaling. However, with multiple pairs of plates in motion relative to one another the authors noted that they had no arguments that could "guarantee the total absence of causality violations", and invoked Hawking's speculative [[chronology protection conjecture]] which suggests that feedback loops of virtual particles would create "uncontrollable singularities in the renormalized quantum stress-energy" on the boundary of any potential time machine, and thus would require a theory of quantum gravity to fully analyze. Other authors argue that Scharnhorst's original analysis, which seemed to show the possibility of faster-than-''c'' signals, involved approximations which may be incorrect, so that it is not clear whether this effect could actually increase signal speed at all.<ref>{{Cite journal |last=Fearn |first=H. |year=2007 |title=Can light signals travel faster than ''c'' in nontrivial vacua in flat space-time? Relativistic causality II |journal=Laser Physics |language=en |volume=17 |issue=5 |pages=695–699 |arxiv=0706.0553 |bibcode=2007LaPhy..17..695F |doi=10.1134/S1054660X07050155 |issn=1054-660X |s2cid=61962}}</ref>

It was later claimed by Eckle ''et al.'' that particle tunneling does indeed occur in zero real time.<ref name="Eckle">{{cite journal |last1=Eckle |first1=P. |last2=Pfeiffer |first2=A. N. |last3=Cirelli |first3=C. |last4=Staudte |first4=A. |last5=Dorner |first5=R. |last6=Muller |first6=H. G. |last7=Buttiker |first7=M. |last8=Keller |first8=U. |title=Attosecond Ionization and Tunneling Delay Time Measurements in Helium |journal=Science |date=5 December 2008 |volume=322 |issue=5907 |pages=1525–1529 |doi=10.1126/science.1163439|pmid=19056981 |bibcode=2008Sci...322.1525E|s2cid=206515239 }}</ref> Their tests involved tunneling electrons, where the group argued a relativistic prediction for tunneling time should be 500–600 attoseconds (an [[attosecond]] is one quintillionth (10<sup>&minus;18</sup>) of a second). All that could be measured was 24 attoseconds, which is the limit of the test accuracy. Again, though, other physicists believe that tunneling experiments in which particles appear to spend anomalously short times inside the barrier are in fact fully compatible with relativity, although there is disagreement about whether the explanation involves reshaping of the wave packet or other effects.<ref name="WinfulHartman">{{cite journal |last=Winful |first=Herbert G. |title=Tunneling time, the Hartman effect, and superluminality: A proposed resolution of an old paradox |journal=Physics Reports |volume=436 |issue=1–2 |pages=1–69 |date=December 2006 |url=http://sitemaker.umich.edu/herbert.winful/files/physics_reports_review_article__2006_.pdf |doi=10.1016/j.physrep.2006.09.002 |bibcode=2006PhR...436....1W |access-date=2010-06-08 |archive-url=https://web.archive.org/web/20111218061131/http://sitemaker.umich.edu/herbert.winful/files/physics_reports_review_article__2006_.pdf |archive-date=2011-12-18 |url-status=dead }}</ref><ref name="WinfulArticle">For a summary of Herbert G. Winful's explanation for apparently superluminal tunneling time which does not involve reshaping, see {{cite journal|last1=Winful|first1=Herbert|title=New paradigm resolves old paradox of faster-than-light tunneling|journal=SPIE Newsroom|date=2007|doi=10.1117/2.1200711.0927}}</ref><ref name="Sokolovski">{{cite journal |last=Sokolovski |first=D. |title=Why does relativity allow quantum tunneling to 'take no time'? |journal=Proceedings of the Royal Society A |volume=460 |issue=2042 |pages=499–506 |date=8 February 2004 |doi=10.1098/rspa.2003.1222 |bibcode=2004RSPSA.460..499S|s2cid=122620657 }}</ref>