why do we use light signals to measure time? In flat spacetime Let's say we have two observers in one's rest frame the other one has velocity v. We suppose each of them sent light signals to other one with time seperation of Δt(their proper time) and measure time accordingly.we see time dilation here. so far everything is ok for me. But I think I'm missing something and I do not get that. Do we have to use light signals? In the same situtation what if observers throw rocks with velocity w again with time seperation Δt. Both should see the rocks thrown from other one with velocity w' with different signs.Shouldn't this method measure time dilation with same accuracy? if it can , can't we built this method to build coordinate lattices? I assumed that we know Lorentz transformations and limiting speed.