Quote Originally Posted by Plug Drugs View Post
There's relativistic mass which must tie in to energy (you know, e=mc^2) then there is mass in its classical sense: resistance to being accelerated by a force.
The concept of the former is reached by considering that there is no objective reference frame, and whether a particle is accelerating or not depends on its relation to other particles. Hence, the more a particle accelerates, the more it will interact with what's around it, leading to an observed increase in volume as well as time for the particle.
This is probably the sweetest shortest explanation of special and general relativity you're ever going to read; if it went too fast for you and you're not a strong enough reader to take it all in in one paragraph, go watch a video explaining it with diagrams