Editing Faster-than-light (section)

===Phase velocities above ''c''===
The [[phase velocity]] of an [[electromagnetic wave]], when traveling through a medium, can routinely exceed ''c'', the vacuum velocity of light. For example, this occurs in most glasses at [[X-ray]] frequencies.<ref>
{{cite book
|last=Hecht |first=E.
|year=1987
|title=Optics
|page=62 |edition=2nd
|publisher=[[Addison Wesley]]
|isbn=978-0-201-11609-0
}}</ref> However, the phase velocity of a wave corresponds to the propagation speed of a theoretical single-frequency (purely [[monochromatic]]) component of the wave at that frequency. Such a wave component must be infinite in extent and of constant amplitude (otherwise it is not truly monochromatic), and so cannot convey any information.<ref>
{{cite journal
|last=Sommerfeld |first=A.
|year=1907
|title=An Objection Against the Theory of Relativity and its Removal
|journal=[[Physikalische Zeitschrift]]
|volume=8 |issue=23 |pages=841–842
|title-link=s:Translation:An Objection Against the Theory of Relativity and its Removal
}}</ref>
Thus a phase velocity above ''c'' does not imply the propagation of [[signal (information theory)|signals]] with a velocity above ''c''.<ref name="phase">
{{cite journal
|title=Phase, Group, and Signal Velocity
|url=https://www.mathpages.com/home/kmath210/kmath210.htm
|journal=American Journal of Physics
|bibcode=1954AmJPh..22..618W
|access-date=2007-04-30
|last1=Weber
|first1=J.
|year=1954
|volume=22
|issue=9
|page=618
|doi=10.1119/1.1933858
}}</ref>