No, Light doesnt have 'resting' mass. Yes, it does have relativistic mass, which means photons mass is affected by the speed it travels. No, Blackholes dont suck light. Light seemed sucked in when near black holes because space-time is distorted nearing its singularity. 2 Popular "UNSTABLE" element are uranium and plutonium. scientist would not want to mess with UNSTABLE element. If there are unstable reactions, scientist make sure its in a minimum. You need an element for a nuclear reaction not just atomic particles and neutrons colliding. The gravity produce by planets are the effect of the space being occupied by the planet. the gravity created by blackholes are from the nuclear reactions that produced singularity.