Hi. Recently I was watching youtube about time and spacetime. And I have few questions about it. If I well understood no one knows what is the time. Here are no formulas to describe time. If it is true how we know if other watches measure time and not our perception on time which can be subjective? Official definition of a second is 9,192,631,770 cycles of the radiation that gets an atom of the element called cesium to vibrate between two energy states. But this is based on assumption that cycles are constant and do not change changing physic parameters like gravity or object speed in space. From all videos I know that time slows down in more powerful gravity field and it slows down if objects speed in space increases. But how do we know if it's time slows down and not physics? It's like running in air and in water. Your running speed in water will be slower nor in air, because the water density greater than air. Spacetime in greater gravity and at greater speed should compress and may slow down all physics, but not time. As we don't know that time is. Such thinking may solve problems and discussions about time travel. Because you will not travel back in time if your speed greater nor c and wormholes will not help time travelers. As only perception of time is affected by gravity and speed not the time.