In the mid-1930s the Austrian physicist Erwin Schrodinger proposed a thought experiment to highlight how crazy quantum mechanics was. He suggested taking a box in which we place a cat, some lethal poison, and a radioactive source. According to quantum mechanics we cannot say, unless we are checking, whether a radioactive atom has broken apart, or decayed, within a given time, so we must describe it as having both decayed and not decayed at the same time. Only when we check do we force it to be one or the other.Inside Schrodinger's box, the experiment is designed so that any decayed atom will have spat out a particle that triggers the release of the poison, killing the cat. Since the cat, said Schrodinger, is also made up of atoms (albeit trillions of them) then it too is presumably subject to the laws of quantum mechanics. So, until we open the box to look, we must describe the cat as being both dead and alive at the same time. Only when we open the box do we force everything inside into one or other state.
by Jim Al-Khalili, 30 Second Theory