**Question: **What Is the Many Worlds Interpretation of Quantum Physics?

I've heard of the many worlds interpretation (MWI) of quantum physics. What is the many worlds interpretation? Do all physicists use it? What's the difference between it and other interpretations of quantum physics?

**Answer: **The many worlds interpretation (MWI) is a theory within quantum physics intended to explain the fact that the universe contains some non-deterministic events, but the theory itself intends to be fully deterministic.

In this interpretation, every time a "random" event takes place, the universe splits between the various options available. Each separate version of the universe contains a different outcome of that event. Instead of one continuous timeline, the universe under the many worlds interpretation looks more like a series of branches splitting off of a tree limb.

For example, quantum theory indicates the probability that an individual atom of a radioactive element will decay, but there is no way to tell precisely when (within those ranges of probabilities) that decay will take place. If you had a bunch of atoms of radioactive elements that have a 50% chance of decaying within an hour, then in an hour 50% of those atoms would be decayed. But the theory tells nothing precisely about when a given atom will decay.

According to traditional quantum theory (the Copenhagen interpretation), until the measurement is made for a given atom there is no way to tell whether it will have decayed or not.

In fact, according to quantum physics, you have to treat the atomas if it is in a superposition of states - both decayed and not decayed. This culminates in the famous Schroedinger's cat thought experiment, which shows the logical contradictions in trying to apply the Schroedinger wavefunction literally.

The many worlds interpretation takes this result and applies it literally, the form of the Everett Postulate:

Everett Postulate

All isolated systems evolve according to the Schroedinger equation

If quantum theory indicates that the atom is both decayed and not decayed, then the many worlds interpretation concludes that there must exist two universes: one in which the particle decayed and one in which it did not. The universe therefore branches off each and every time that a quantum event takes place, creating an infinite number of quantum universes.

In fact, the Everett postulate implies that the entire universe (being a single isolated system) continuously exists in a superposition of multiple states. There is no point where the wavefunction ever collapses within the universe, because that would imply that some portion of the universe doesn't follow the Schroedinger wavefunction.

### History of the Many Worlds Interpretation

The *many worlds interpretation* was created by Hugh Everett III in 1956 in his doctoral thesis, *The Theory of the Universal Wave Function*. It was later popularized by the efforts of physicist Bryce DeWitt. In recent years, some of the most popular work has been by David Deutsch, who has applied the concepts from the many worlds interpretation as part of his theoretical in support of quantum computers.

Though not all physicists agree with the many worlds interpretation, there have been informal, unscientific polls which have supported the idea that it is one of the dominant interpretations believed by physicists, likely ranking just behind the Copenhagen interpretation and decoherence. (See the introduction of this Max Tegmark paper for one example. Michael Nielsen wrote a 2004 blog post (at a website which no longer exists) which indicates - guardedly - that the many worlds interpretation is not only accepted by many physicists, but that it was also the most strongly **disliked** quantum physics interpretation. Opponents don't just disagree with it, they actively object to it on principle.) It is a very controversial approach, and most physicists who work in quantum physics seem to believe that spending time questioning the (essentially untestable) interpretations of quantum physics is a waste of time.

*Other Names for the Many Worlds Interpretation*

*The many worlds interpretation has several other names, though work in the 1960s & 1970s by Bryce DeWitt has made the "many worlds" name more popular. Some other names for the theory are relative state formulation or the theory of the universal wavefunction.*

Non-physicists will sometimes use the broader terms of multiverse, megaverse, or parallel universes when speaking of the many worlds interpretation. These theories usually include classes of physical concepts that cover more than just the types of "parallel universes" predicted by the many worlds interpretation.

### Many Worlds Interpretation Myths

In science fiction, such parallel universes have provided the foundation for a number of great storylines, but the fact is that none of these have a strong basis in scientific fact for one very good reason:

The many worlds interpretation does not, in any way, allow for communication between the parallel universes that it proposes.

The universes, once split, are entirely distinct from each other. Again, science fiction authors have been very creative in coming up with ways around this, but I know of no solid scientific work that has shown how parallel universes could communicate with each other.

Edited by Anne Marie Helmenstine