I wonder if anyone here can help me with this, i am refreshing my physics knowledge ( long lost ). I re-read Steven Hawkins Brief History of Time, and a many many issues jumped out of me, but lets start with the one titled in the the thread - how does energy differ from mass? Let me state the issue more clearly, as i understand it ( i maybe wrong) a. E=mc2 implies that energy and mass are interchangeable. Thus a photon has 'effective mass' - and therefore is acted upon by gravity to curve its trajectory in space If mass and energy are interchangeable - how do we know when to call energy just 'energy' and other times call it 'mass'? Hawkins says there is 'intrinsic mass' which is limited to less than speed of light, because as speed increases, its energy incrases to infinity - whereas 'energy' is said to have 'effective mass'. So why is a photon not subject to the same effect? i.e. what is 'intrinsic mass' that is different to 'energy effective mass' of say a photon In summary - what exactly is 'mass' - all matter is simply 'energy', so i cannot fathom it. IS is that energy in a particular arrangement is called mass? if so, what arrangement? and why?