So if time dilation is what's left after correcting for the optical effect and we want to know our motion through the universe. We can't use light because we have no reference point to what not being in motion is and there's lots of attempts using light which doesn't work. Why don't we just take a few atoms we know the decay rate of and send them in different directions. As motion makes them last longer the one's travelling in a direction that reduces there overall motion should last the least amount of time and the one that lasts the least amount of time would not be in motion. Then deducting that motion from our own will give us a reference to what no motion throughout the universe is. Then everything would be a lot more relative =) Would that work ?