Criticisms I’ve read of many worlds focus on its baroque violation of Occam’s razor. Creating an essentially infinite number of other worlds to explain a few small effects in this one does seem over the top. But when you actually read about many worlds, you discover that the motivation is exactly the opposite.
Schrodinger’s equation describes the world with beautiful and amazing accuracy. If the equation says that some event A will occur 98.6% of the time, you can bet that when you set up the experiment 1 million times, 986 thousand of the trials will give you A. But what happens to the other 1.4%?
As David Deutsch points out in Chapter 12 of Beginning of Infinity, quantum physicists adopted a rule of thumb:
“Whenever a measurement is made, all the histories but one cease to exist. The surviving one is chosen at random, with the probability of each possible outcome being equal to the total measure of all the histories in which that outcome occurs.” The Beginning of Infinity, page 273
You might recognize this as the famous “collapse of the wave function” of the traditional Copenhagen interpretation.
And as Brian Greene points out in both Fabric of the Cosmos and The Hidden Reality, there is no mathematical justification for this rule of thumb.
“The collapse does not emerge from the mathematics of quantum theory: it has to be put in by hand, and there is no agreed-upon or experimentally justified way to do this.” – Fabric of the Cosmos, page 116
Many worlds, by contrast, simply accepts the mathematics as it is. The wave function does not collapse. Instead, we become part of the wave function, part of the probability it describes. We measure event A 98.6 % of the time because 98.6% of the time we find ourselves in an instance of the universe in which A occurs. The other 1.4% still exist, but we are forever separated from that outcome.
Deutsch again in Chapter 12 (page 274)
“(Q)uantum theory (was) clearly describing some sort of physical process that brought about the outcomes of experiments. Physicists, both through professionalism and natural curiosity, could hardly help wondering about that process. But many of them tried not to. Most of them went on to train their students not to. This counteracted the scientific tradition of criticism in regard to quantum theory.” (italics in original)
and on page 276
“And in Dublin in 1952 Schrodinger gave a lecture in which at one point he jocularly warned his audience that what he was about to say might ‘seem lunatic’. It was that, when his equation seems to be describing several different histories, they are ‘not alternatives but all really happen simultaneously’ . . . Here is an eminent physicist joking that he might be considered mad. Why? For claiming that his own equation – the very one for which he had won the Nobel prize – might be true.”
My point here is not to prove that many worlds is correct. Instead, I want to point out that in some sense many worlds is the most straightforward and least strange interpretation of the most successful theory in the history of science. To me, this revelation came as quite a shock.
It seems that many good explanations began as mathematical constructs that no one really took seriously. Copernicus’ sun-centered system, atoms, Faraday and Maxwell’s fields, the neutrino, and most recently quarks. Later, all turned out to be real, and many became crucial to technologies and discoveries that made us masters of the world. Could many worlds be of the same character? Could it move from useful fiction to a fundamental reality? And could that change again alter the world?