▁refers ▁most ▁commonly ▁to ▁an ▁in ert ial ▁reference ▁frame . ▁In ▁such ▁cases ▁an ▁in ert ial ▁reference ▁frame ▁may ▁be ▁called ▁an ▁" in ert ial ▁observer " ▁to ▁avoid ▁ambigu ity . ▁Note ▁that ▁these ▁uses ▁differ ▁significantly ▁from ▁the ▁ordinary ▁English ▁meaning ▁of ▁" ob server ". ▁Reference ▁frames ▁are ▁inher ently ▁non local ▁construct s , ▁covering ▁all ▁of ▁space ▁and ▁time ▁or ▁a ▁n ont rivial ▁part ▁of ▁it ; ▁thus ▁it ▁does ▁not ▁make ▁sense ▁to ▁speak ▁of ▁an ▁observer ▁( in ▁the ▁special ▁relativ istic ▁sense ) ▁having ▁a ▁location , ▁except ▁for ▁den oting ▁the ▁origin ▁of ▁his ▁reference ▁frame . ▁Also , ▁an ▁in ert ial ▁observer ▁cannot ▁acceler ate ▁at ▁a ▁later ▁time , ▁nor ▁can ▁an ▁acceler ating ▁observer ▁stop ▁acceler ating . ▁ ▁General ▁relativ ity ▁In ▁general ▁relativ ity ▁the ▁term ▁" ob server " ▁refers ▁more ▁commonly ▁to ▁a ▁person ▁( or ▁a ▁machine ) ▁making ▁pass ive ▁local ▁measurements , ▁a ▁usage ▁much ▁closer ▁to ▁the ▁ordinary ▁English ▁meaning ▁of ▁the ▁word . ▁Comm only ▁used ▁are ▁F ID O ▁( f id u cial ▁observer , ▁ ▁station ary ), ▁F FO ▁( free ▁falling ▁observer ), ▁Z AM O ▁( zero ▁angular ▁momentum ▁observer , ▁cor ot ating ). ▁ ▁A ▁Schwar z sch ild ▁observer ▁is ▁a ▁far ▁observer ▁or ▁a ▁book keeper . ▁ ▁Quant um ▁mechan ics ▁ ▁In ▁quantum ▁mechan ics , ▁" observ ation " ▁is ▁syn onymous ▁with ▁quantum