I'm not sure if the way I understand it is correct or if it is slightly inaccurate, so I will just write it as I understand it and hope you can all my point out my errors. As I understand it, there are 4 dimensions. 3 of space and 1 of time (first question. Is time actually considered an extra dimension?) which are collectively known as spacetime. Your speed in spacetime is always equal to the speed of light (I will use 300,000,000 m/s for the sake of simplicity.) If you are not moving at all in the first 3 dimensions then you will be moving at 300,000,000 m/s in time (second question. are distance units the right unit of measurement in this question?). If you are going at 150,000,000 m/s then time will go twice as slow as it would be as normal. Also, what is the name of this quantity that space and time are shared between?