in the early 1930s , erwin schrodinger published a way of thinking about the circumstance of radioactive decay that is still usefull . we imagine an apparatus contaioning just one nitrogen - 13 atom and a detector that will respond when the atom decays . connector to the detector is relay connected to the hammer , and when the atom decays , the ralay releases the hammer , which then falls on a glass vial , containing poisonous gas . we take the enitire apparatus and put it in a box . we also place a cat in the box , close the lid and wait for 10 minutes ... is the cat dead .. or alive ????

the answer according to quantum mechanics is that th cat is 50 % dead and 50 % alive .
quantum mechanics describes the world in the form of a wave function . dewitt wrote abt the cat that .. " at the end of one half life , the total wave function will have a form in whichc the living cat and the  dead cat are mixed in equal proportions .."

when we opeen the box ,  we " collapse the wave funstion" or " collapse the state " and either have a live cat or a dead cat . imagine we have a frend waiting outside when we open the box . for us the wave function collapses and we have , say , a live cat . but our friends wave function does not collapse until he comes in the room . this leads to a strong solipsism , since our friend can then say that we owe our objective existence to his kind  intervention  in coming into the room and collapsing the state .

as heisenberg said , then " the wave funstion represents partly a fact and partly our knowledge of the fact " 

our frend neednt have come into the room to collapse the wave function . : if we have a cell fone we can call him and tell him the result of the experiment . ofcourse this assumes that we do not lie to him and tell him that the cat is dead when it is alive .

unexplained but apparently true is the fact that when a state collapses , it collapses into the same state for everybody . if we see a live cat everybody sees a live cat ( unless they or we are hallucinating 0
as de beayregard commented .. " finally , the need for consistency of the whole scheme leads me to thnk of the world we are living in as the leibnitzian world , where cats are rather high in the heiirerchy of monads "