Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand. What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question. I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the earths surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?