The many worlds interpretation of quantum mechanics is the idea that all possible alternate histories of the universe actually exist. At every point in time, the universe splits into a multitude of existences in which every possible outcome of each quantum process actually happens.
So in this universe you are sitting in front of your computer reading this story, in another you are reading a different story, in yet another you are about to be run over by a truck. In many, you don't exist at all.
This implies that there are an infinite number of universes, or at least a very large number of them.
That's weird but it is a small price to pay, say quantum physicists, for the sanity the many worlds interpretation brings to the otherwise crazy notion of quantum mechanics. The reason many physicists love the many worlds idea is that it explains away all the strange paradoxes of quantum mechanics.
For example, the paradox of Schrodinger's cat--trapped in a box in which a quantum process may or may not have killed it-- is that an observer can only tell whether the cat is alive or dead by opening the box.
But before this, the quantum process that may or may not kill it is in a superposition of states, so the cat must be in a superposition too: both alive and dead at the same time.
That's clearly bizarre but in the many worlds interpretation, the paradox disappears: the cat dies in one universe and lives in another.
Let's put the many world interpretation aside for a moment and look at another strange idea in modern physics. This is the idea that our universe was born along with a large, possibly infinite, number of other universes. So our cosmos is just one tiny corner of a much larger multiverse.
Today, Leonard Susskind at Stanford University in Palo Alto and Raphael Bousso at the University of California, Berkeley, put forward the idea that the multiverse and the many worlds interpretation of quantum mechanics are formally equivalent.
But there is a caveat. The equivalence only holds if both quantum mechanics and the multiverse take special forms.
Let's take quantum mechanics first. Susskind and Bousso propose that it is possible to verify the predictions of quantum mechanics exactly.
At one time, such an idea would have been heresy. But in theory, it could be done if an observer could perform an infinite number of experiments and observe the outcome of them all.
But that's impossible, right? Nobody can do an infinite number of experiments. Relativity places an important practical limit on this because some experiments would fall outside the causal horizon of others. And that would mean that they couldn't all be observed.
But Susskind and Bousso say there is a special formulation of the universe in which this is possible. This is known as the supersymmetric multiverse with vanishing cosmological constant.
If the universe takes this form, then it is possible to carry out an infinite number of experiments within the causal horizon of each other.
Now here's the key point: this is exactly what happens in the many worlds interpretation. At each instant in time, an infinite (or very large) number of experiments take place within the causal horizon of each other. As observers, we are capable of seeing the outcome of any of these experiments but we actually follow only one.
Bousso and Susskind argue that since the many worlds interpretation is possible only in their supersymmetric multiverse, they must be equivalent. "We argue that the global multiverse is a representation of the many-worlds in a single geometry," they say.
They call this new idea the multiverse interpretation of quantum mechanics.
That's something worth pondering for a moment. Bousso and Susskind are two of the world's leading string theorists (Susskind is credited as the father of the field), so their ideas have an impeccable pedigree.
But what this idea lacks is a testable prediction that would help physicists distinguish it experimentally from other theories of the universe. And without this crucial element, the multiverse interpretation of quantum mechanics is little more than philosophy.
That may not worry too many physicists, since few of the other interpretations of quantum mechanics have testable predictions either (that's why they're called interpretations).
Still, what this new approach does have is a satisfying simplicity-- it's neat and elegant that the many worlds and the multiverse are equivalent. William of Ockham would certainly be pleased and no doubt, many modern physicists will be too.
Ref: arxiv.org/abs/1105.3796: The Multiverse Interpretation of Quantum Mechanics