The new book by Sean Carroll, Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime, is probably the single best argument for the Everett understanding of quantum mechanics.
That’s the approach named after physicist Hugh Everett III and which is often also called (although slightly misleadingly) the “Many Worlds Interpretation”.
In his book, Carroll makes clear the powerful attractions of the Everett understanding, and persuasively counters the objections that are commonly raised against it. He highlights how this approach is the natural, straightforward response to the extraordinary success of the quantum formalism. Despite its apparent profligacy of multiple worlds (multiple diverging branches of reality), it’s actually a lean and austere interpretation of quantum mechanics. Unique among interpretations of quantum mechanics, it adds in nothing beyond the wave equation itself.
Back in the 1980s I spent four years mulling the philosophical implications of quantum mechanics. Over time, against my initial inclinations (and hopes), I came to have an increasing respect for the Everett understanding – an outcome I wrote about here. Alongside my grudging respect for that interpretation, I retained the view that it still faced many hard questions. However, Carroll’s book has convinced me that these questions aren’t particularly daunting. In other words, the book has strengthened my conviction that these “Many Worlds” do come into being whenever quantum transactions are macroscopically magnified.
In terms of the history of the topics covered, and the pros and cons of the different interpretations reviewed, I see Carroll as being overwhelmingly correct. I particularly liked his demolition of the idea that there’s such a thing as a coherent “Copenhagen interpretation” of quantum mechanics. The only area where I wanted to see the argument extended was that more could have been said about how all the non-Everett interpretations of quantum mechanics have to accept one or other kind of radical non-locality (despite the attempts of various writers to “have their cake and eat it”).
The final third of the book may be the most important. It reviews the possibility for progress in an area of physics that has long experienced troubles: quantum gravity. Carroll argues that the best hopes for us obtaining a correct quantum theory of gravity (that works at all energy scales) is to take quantum mechanics itself more seriously. This part of the book is more speculative than the earlier parts, but it has raised my interest in delving more into these topics.
This final part of the book also underlines the difficulties faced by the non-Everett interpretations of quantum mechanics in dealing, not with particles, but with the relativistic fields which modern physics views as being more fundamental than particles. This part also reviews how space and time should emerge from the theory of quantum gravity, rather than being presupposed as the canvas upon which the theory would operate. Some of the potential implications for black holes (and maybe even the Big Bang) are mind-stretching.
It’s a shocking possibility that each of us exist alongside numerous different versions of ourselves, in the overall multiverse – versions that have increasingly divergent experiences. I see this possibility as one of the most remarkable insights to have arisen from humanity’s millennia-long exploration into science. It’s an insight that takes time to sink in. It’s a good question how much this insight should change our day-to-day behaviour. Carroll has an answer to that too: not as much as we might first think. Personally I find it a humbling realisation.
PS For another book that addresses some of the same topics – inside an even larger set of profound ideas – I recommend Our Mathematical Universe by Max Tegmark.
dw2 
I appreciate how you mention that your initial hopes and inclinations were not aligned with the “many worlds” theory. I too have a hard time accepting it. I realize it’s a totally different subject altogether, but if it boils down to either accepting that matter is an emergent property of consciousness, which would explain most every quantum conundrum, OR for every single action or decision is made, there is a parallel universe where the another decision or action was taken, resulting in infinite, multiplying infinities of ‘reality,’ it seems to me that the one that best adheres to Occam’s Razor is the consciousness argument. It just seems like a very long way for nature to go to accomplish what she needs to do to create a world where we exist, think, move, etc. I realize I am talking about metaphysics here, and hardcore scientists – especially materialists – always like to bash me about this stuff. But there is a gentleman named Bernardo Kastrup who has written an excellent book called “The Idea of the World” that makes a very solid argument for a non-materialist view of reality. Great review by the way David – thank you!
Comment by Kevin Wood — 28 October 2019 @ 12:02 am
Many thanks for your comments Kevin. I answer a few points below.
Thanks also for your pointer to the book by Bernardo Kastrup. I see it has a lot of 5-star endorsements. I’ve added it to my to-read list.
>”accepting that matter is an emergent property of consciousness, which would explain most every quantum conundrum”
I would regard that hypothesis, on the contrary, as explaining nothing. It’s a bit like saying, “we don’t understand x, but don’t worry, x can all be explained by the hypothesis that ‘God did it'”. Any hypothesis that explains everything explains nothing.
>”resulting in infinite, multiplying infinities of ‘reality,’”
In the Everett interpretation, there’s only one reality. But different branches of it can be perceived in many different ways.
Sean Carroll is careful to explain that it’s by no means obvious that the number of different branches is actually infinite. There are quantum gravitational arguments in favour of this number being finite (though very large).
>”for every single action or decision is made, there is a parallel universe where the another decision or action was taken”
Everett says that, whenever a quantum outcome scales to decoherence, the previous branch effectively splits. It’s misleading to conceive of there having been separate parallel branches all along. The former view is metaphysically lean, whereas the latter is, I accept, metaphysically extravagant.
Comment by David Wood — 28 October 2019 @ 1:10 am