I understand that the larger the mass the greater gravity is and the slower time is, as well the faster an object is traveling the slower time passes. My question is that since the faster an object travels the more mass it has, is the increase in mass the reason for the change in time, or is it the velocity?
I think the question uses a piece of the General Theory of Relativity and a piece of the Special Theory of Relativity.
Perhaps this article about the Global Positioning System ( which uses both the Special and General Theory to keep time) will be helpful. I've talked to Astronomers that get the answer wrong! (They're amateur astronomers).