## MANY WORLDS INTERPRETATION

In a nutshell, the measurement problem in quantum mechanics arises because of the incompatibility between the microscopic world of elementary particles and the macroscopic world of the measuring apparatus. Objects in these two worlds behave in fundamentally different ways.

Earlier we discussed how in the quantum world an object can exist as a superposition of multiple states described by its wave function. The time evolution of the wave function is described by Schrödinger’s equation whose deterministic progression is interrupted when a measurement is made on the system causing its wave function to collapse into one of its member states—the outcome of a measurement cannot be predicted deterministically because the choice of the state of collapses is completely arbitrary. The discontinuity in progression of the wave function due to measurement (collapse) is sudden and irreversible. It is not governed by the Schrödinger equation but has to be added as an additional postulate that seems to violate its validity.

The Copenhagen interpretation was designed to deal with the measurement problem although the trouble with this model of reality is that it offers no explanation for how the collapse would occur. Observers can access microscopic systems only through macroscopic measuring devices that, along with the observers themselves, are subject to the laws of classical physics. Thus the universe is governed by different laws: quantum mechanics (explicitly, Schrödinger’s equation) for the microscopic world, and classical physics (Newton’s laws of motion) for the directly accessible macroscopic world, although the exact nature of the divide between the quantum and classical realms remains unclear. There are strong physical grounds for suspecting that the Copenhagen interpretation is fundamentally inadequate, though its empirical success to date is unquestionably impressive. Many physicists, including Einstein, believe that the quantum measurement paradox requires a satisfactory resolution in form of either a realistic interpretation of quantum mechanics or an appropriate modification of the formalism itself, or both.

The many worlds interpretation of quantum mechanics was proposed in 1956 by Hugh Everett III to address the measurement problem by merging the macroscopic and microscopic world views. Everett used the mathematics of quantum mechanics to make the external observer an integral part of the quantum system, introducing a universal wave function that linked objects with their observers. In other words, Everett was able to quantum mechanically include macroscopic objects in the framework of quantum superposition. This was a truly revolutionary approach although not a universally accepted one.

Everett considered whether the Schrödinger equation is applicable to objects and observers alike. In his model the wave function of an observer bifurcated at each interaction with a superposed object. The universal wave function contained branches of every alternative outcome with each branch having its own copy of the observer. Once formed, one branch cannot influence another, with each branch pursuing a different future outcome. The many-worlds picture requires inclusion of the observer into the system, although the branching process happens regardless of whether a “conscious” observer is present or not. Every measurement outcome “happens,” but each in a different branch called “universe” or “world.”^{5} There is no point where the wave function ever collapses within the universe, because that would imply that some portion of the universe doesn’t follow the Schrödinger wave function.

As is customary, we will use the Schrödinger’s cat to illustrate the features of the many worlds interpretation. A cat remains in a quantum superposition of living and dead inside a sealed box until it is opened for observation—an action that “collapses the wave function” into one of two possible measurement outcomes, a living or dead cat. This is the traditional way of looking at Schrödinger’s cat. In contrast, the many worlds picture proclaims that, the universe splits in two: one in which the cat is alive, and the other in which the cat is dead. Once split, the universes evolve independently, never to interact with each other again.

Unfortunately, Everett’s theory of multiple universes met with utter disdain, particularly from the Copenhagen school led by Niels Bohr. Disheartened and emotionally scarred, Hugh Everett abandoned academia and immersed himself into the secretive world of military research, becoming a heavy drinker and prematurely dying at the age of 51.

Everett’s many worlds interpretation has witnessed a resurrection in recent years, largely popularized by the efforts of physicist Bryce DeWitt. Some of the most popular work has been done by David Deutsch, who has applied the many worlds interpretation as part of his theoretical explanation of quantum computers.