Because every time you make a fucking phone call, the call is routed to a satellite. The satellite has to account for the fact that it's traveling at a different speed than you are. So that means that because of that time dilation, we know for a fact that E does equal MC squared. We know the theory of relativity is true because it has to account for that, otherwise it won't work. The clock on your phone, because it is right, means that Einstein was right.