Editing Proper length (section)

{{short description|Length of an object in the object's rest frame}}
{{For|the cosmological notion of proper distance|Comoving distance}} 

'''Proper length'''<ref name=fayngold>{{cite book |author=Moses Fayngold |title=Special Relativity and How it Works |location=John Wiley & Sons |year=2009 |isbn=978-3527406074}}</ref> or '''rest length'''<ref name=franklin>{{cite journal |author=Franklin, Jerrold |title=Lorentz contraction, Bell's spaceships, and rigid body motion in special relativity |journal=European Journal of Physics |volume=31 |year=2010 |pages=291–298 |doi=10.1088/0143-0807/31/2/006 |bibcode = 2010EJPh...31..291F |issue=2 |arxiv = 0906.1919 |s2cid=18059490 }}</ref> is the length of an object in the object's [[rest frame]].

The measurement of lengths is more complicated in the [[theory of relativity]] than in [[classical mechanics]].  In classical mechanics, lengths are measured based on the assumption that the locations of all points involved are measured simultaneously. But in the theory of relativity, the notion of [[Relativity of simultaneity|simultaneity]] is dependent on the observer. 

A different term, '''proper distance''', provides an invariant measure whose value is the same for all observers.

''Proper distance'' is analogous to [[proper time]]. The difference is that the proper distance is defined between two spacelike-separated events (or along a spacelike path), while the proper time is defined between two timelike-separated events (or along a timelike path).