John is standing motionless on a stage when he sings a solo rendition of $$I Am The Walrus.$$

While he sings$,$ Paul runs westward across the stage at a speed of at $0\mathrm{.}5$c$.$ Additionally, Ringo

runs westward across the stage at a speed of $0\mathrm{.}7$c$,$ and George runs eastward across the stage

at a speed of $0\mathrm{.}5$c$.$ All four of them measure the amount of time that John spends singing.

$($a$)$ Who measures the proper time for John$$s song?