Einstein stated that the laws of physics are the same in all inertial reference frames. Lets do the old thought experiment There are a pair of twins each with identical watches. One remains on earth while the other travels out into space near the speed of light and back again. The twin on earth will see the effects of relativity on the travelling twin because they are not in the same inertial reference frames. To the twin inside the spaceship, his watch ticks at the same rate as his brother because the traveller and his watch are always at rest with respect to each other. The twin on earth is also at rest with respect to his watch. Hence both twins keep their own time at the same rate (although during the trip, they will see each other differently due to relativity) . When the travelling twin completes the trip and comes back to earth thereby reuniting with the reference frame of his brother, the amount of time ticked off by the travelling twin should be identical to the amount ticked off by his brother who was on earth all that time. This is because they have been keeing time in an inertial frame that is idential with each other. Hence the two twins have aged the same when they finall meet. The aging difference should only occur when they do not meet in other words, when the are at different inertial references frames with respect to each other. But experts have stated that I am wrong and the travelling twin will only have aged a small proportion compared to how much it has aged on earth.