In this last entry on many worlds, I want to revisit free will. In The Fabric of Reality, Deutsch gives the best argument in favor of free will I’ve encountered. Using the full glory of many worlds, Deutsch completely turns free will on its head.

Classical physics is strictly deterministic. Pierre-Simon Laplace described the situation accurately in 1814 when he said that a being with perfect knowledge of the positions and velocities of all particles at any one time could completely predict the future (and past) of the entire universe. The advent of quantum uncertainty showed that Laplace’s idea, while correct for classical physics, didn’t apply to our universe. But Deutsch shows how completely at odds with reality Laplace’s idea actually is.

One must think of many worlds not as a small add-on to classical physics, but as completely reversing the conception. Everything that can happen does happen. In the many worlds multiverse of Deutsch, every event branches out into different results. Consider a galaxy.

A galaxy forms because of gravitational attraction. But if things had been just a little different at the galaxy’s inception, every detail of that galaxy, including its composition, its placement in relation to other galaxies, and its shape, would have been different, as well. In Deutsch’s view, were one to view the multiverse all at once, a single galaxy would transform into a uniform spread of stars, dust, and debris smeared across enormous distance. Everything that can happen does in some portion of the multiverse.

The key thing about a galaxy is that it doesn’t “kick back”. There’s nothing about the galaxy’s makeup that will cause it to resist any multiversal change in its placement, composition, etc. It just doesn’t care, and as a result ends up all over the place.

Contrast that, though, with a living thing. A living thing does care. It does “kick back.” A living thing is knowledge, and as Deutsch says in The Beginning of Infinity, “(K)nowledge is information which, when it is embodied in a suitable environment, tends to cause itself to remain so.” (BoI, p 123) When looked at in the multiverse, life does something that no non-living entity can do. It molds and shapes the multiverse. It makes great portions of the multiverse look alike. As Deutsch says in The Fabric of Reality, “such places stand out . . . as the location of processes – life, and thought – that have generated the largest distinctive structures in the multiverse.” Knowledge is like a crystal that stretches across the multiverse, differentiating itself from all other structures in the universe.

So what of free will? We see now that free will is not just the ability to make different things happen. Everything has that ability in the multiverse. Consider a single uranium atom. It might decay this second, or next second, or not for billions of years. It might fire an alpha particle north, south, up, down, or in any other direction. It is fundamentally unpredictable. So is every other microscopic process. Variability isn’t hard to achieve in the multiverse, it’s natural and automatic.

What is hard to achieve is consistency. And this, Deutsch argues, is the heart of free will. Here’s his argument, which he presents in a chart in Chapter 13 of FoR:

After careful thought I chose to do X; I could have chosen otherwise; it was the right decision; I am good at making such decisions. What do each of these statements mean when looked at through the multiversal lens?

After careful thought I chose to do X: in the multiverse view, this means that some proportion of all the versions of me, including the one speaking, chose to do X.

I could have chose otherwise: in the multiverse view, some other versions of me did choose otherwise.

It was the right decision, I am good at making such decisions: in the multiverse view, the vast majority of all the versions of me made this decision – I have molded the multiverse by my decision-making.

From a deterministic world in which we really have no choices, Deutsch has given us a uniform cloud called the multiverse. It’s up to us to mold and shape that cloud into the form we want. The multiverse is ours for the making.

One must be careful, of course, to not accept an idea simply because it is attractive. Beware the ideas you want to be true. I’m still not fully sold on the quantum multiverse. As Deutsch says again and again, our ideas are always fallible. Many worlds may well be wrong. What Deutsch and the other writers have done for me, however, is convince me that a working hypothesis, no matter how crazy its consequences might seem, is better than no hypothesis at all. If many worlds is wrong, I think someone needs to show why its wrong, rather than just dismissing it as a silly extravagance.

As quantum technology brings these strange effects more and more into the world of the macroscopic, we will have an answer. Already there are proposed experiments that will be possible in the near future. These experiments will make one prediction for many worlds, another for rival ideas. Soon we won’t have to speculate, we will know. I for one (or maybe for many?) can’t wait to find out.