▁a ▁better ▁metric ▁for ▁air ▁travel . ▁( A ▁straight ▁line ▁would ▁go ▁through ▁the ▁earth ). ▁Another ▁example ▁is ▁planning ▁a ▁car ▁journey , ▁where ▁one ▁might ▁want ▁the ▁short est ▁journey ▁in ▁terms ▁of ▁travel ▁time ▁- ▁in ▁that ▁case ▁a ▁straight ▁line ▁is ▁a ▁poor ▁choice ▁of ▁metric ▁because ▁the ▁short est ▁distance ▁by ▁road ▁is ▁not ▁normally ▁a ▁straight ▁line , ▁and ▁even ▁the ▁path ▁nearest ▁to ▁a ▁straight ▁line ▁will ▁not ▁necessarily ▁be ▁the ▁quick est . ▁A ▁final ▁example ▁is ▁the ▁internet , ▁where ▁even ▁for ▁nearby ▁towns , ▁the ▁quick est ▁route ▁for ▁data ▁can ▁be ▁via ▁major ▁connections ▁that ▁go ▁across ▁the ▁country ▁and ▁back ▁again . ▁In ▁this ▁case ▁the ▁metric ▁used ▁will ▁be ▁the ▁short est ▁time ▁that ▁data ▁takes ▁to ▁travel ▁between ▁two ▁points ▁on ▁the ▁network . ▁ ▁In ▁cosm ology , ▁we ▁cannot ▁use ▁a ▁r uler ▁to ▁measure ▁metric ▁expansion , ▁because ▁our ▁r uler ▁internal ▁forces ▁easily ▁overcome ▁the ▁extremely ▁slow ▁expansion ▁of ▁space ▁leaving ▁the ▁r uler ▁int act . ▁Also ▁any ▁objects ▁on ▁or ▁near ▁earth ▁that ▁we ▁might ▁measure ▁are ▁being ▁held ▁together ▁or ▁pushed ▁apart ▁by ▁several ▁forces ▁which ▁are ▁far ▁larger ▁in ▁their ▁effects . ▁So ▁even ▁if ▁we ▁could ▁measure ▁the ▁tiny ▁expansion ▁that ▁is ▁still ▁happening , ▁we ▁would ▁not ▁notice ▁the ▁change ▁on ▁a ▁small ▁scale ▁or ▁in ▁every day ▁life . ▁On ▁a ▁large ▁inter gal actic ▁scale , ▁we ▁can ▁use ▁other ▁tests ▁of ▁distance ▁and ▁these ▁do