But you can calculate the difference, which happens with GPS satellites all the time. GPS is based on synced clocks. Problem is that the satellites suffer the

But you can calculate the difference, which happens with GPS satellites all the time. GPS is based on synced clocks. Problem is that the satellites suffer the rules of relativity. But the good news is that we know the rules of relativity, and therefore can compensate for them. I always thought relativity was purely theoretical, but it is actually practically applied in GPS sattelites.