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How can you tell if a train has moved: you can be looking out the window, seeing if the landscape moves. If there are no windows, you can hear the train perhaps. You would actually not be able to tell if the train has moved or not, provided that it"s going at a constant speed. You cannot tell the difference between you moving and everything standing still, or the opposite; you may be staying still and everything around you could be moving. You can only measure motion relative to a given frame of reference, both on earth and in the universe. If you fall asleep on a floating space ship you would not be able to whether the spaceship is still floating or is now coasting through space. If you consider the scenario that there are two people, one on a plane and another watching the plane from the ground.