Lets try this conceptual physical experiment. There exist two different and separated worlds, ‘o’ and ‘O’ where physical laws uphold. In each world a measuring rod, made from one atom of Hydrogen that enables the measure by comparison of masses, lengths and time interval. To keep it simple we will accept that only Hydrogen exists on both. As it is a conceptual experiment we must accept, although impossible, that the observers are made of H. The difference between worlds is that atoms at ‘O’ are greater than at ‘o’. Observers at ‘o’ and at ‘O’ with their respective measuring rods find the same physical laws. The observer at ‘o’ see the light emitted at ‘O’ and find that it as the same pattern characteristic of Hydrogen but, and it is a surprise, it is reddened. The observer at ‘o’ stays undecided between 3 conclusions: the world ‘O’ is equal to the world ‘o’, but moving away (i.e. it doesn’t recognize the existence of ‘O’) the world ‘O’ is different the world ‘O’ is different and moving The pertinent question is: How to discriminate between the hypotheses?