Probability is treated differently in the two main interpretations of quantum mechanics. In the traditional view, observing a quantum system yields just one outcome. This view, called the Copenhagen interpretation, is a bit baffling. An initial superposition of states in a given system collapses into just one state upon being measured. Exactly why this change happens, or how the system “chooses” to be in one state or another, is unclear.

An alternative, proposed by physicist Hugh Everett in the 1950s, suggests the initial mix of states never collapses. Instead, making a measurement splits our universe into parallel versions that exist in an abstract quantum realm, and all possible outcomes occur somewhere. If a system is a mix of two equally probable states, the universe splits into two when the system is observed. But what if Schrödinger’s cat is, say, 70 per cent more likely to be found alive? Does that mean the universe with the live cat would somehow be “more real” than the one in which the cat died?

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