The thought experiment was created by Max Tegmark. In the thought experiment, a person stands in front of a gun which fires if it detects a subatomic particle as having an upward spin, or does not fire if the gun detects a downward spin. The gun does this repeatedly. From an outside perspective, both outcomes are equally likely. However, according to the thought experiment, if the many-worlds interpretation is true, the person in front of the gun finds that the gun never fires, even though this is very, very unlikely. In the many-worlds interpretation, the gun fires in one world and does not fire in another, but the person can only continue being conscious in a world in which the gun does not fire. According to the thought experiment, such a person would find themselves to be immortal.