is it possible to reduce matter of enery completly? and if not why can't that happen? i guess not because of E = mc^2 huh. owel it was only a thought. because i thought if you can have matter without energy then if you time travelled then the conservation of enery wouldn't be affected? but light can be slowed down,because when it enters the atmosphere it slows slightly and if it eners more dense medium but u can't get speed of light to 0.