the.com/simultaneity
Two things happening at once — until Einstein asked, at once according to whom?
means The state of multiple events occurring at the same time, a concept that physics revealed depends entirely on the observer's motion.
from From Latin simul, meaning at the same time, the same root that gives us simulate and assemble — things brought together at once.
Relativity twistTwo events simultaneous for you aren't for a moving observer.
No universal nowEinstein abolished the idea of one cosmic clock.
Speed limitLight's finite speed makes absolute simultaneity impossible to define.