Everyone knows we live in a partisan age. It's hard to find any issue these days that people aren't ready to square off on, with sharp, snarky barbs.
While no one will be surprised to find these kinds of arguments playing out about immigration or the importance of NATO, finding it among staid physicists — and about the nature of physical reality — might not be so expected. But all too often over the last 100 years, this has been the case, as scientists have disagreed sharply over the meaning of their greatest and most potent theory known as quantum mechanics.
That's the fraught territory best-selling author and physicist Sean Carroll dives into with his new book Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime. What makes Carroll's new project so worthwhile, though, is that while he is most certainly choosing sides in the debate, he offers us a cogent, clear and compelling guide to the subject while letting his passion for the scientific questions shine through every page.
Quantum mechanics, the study of molecules, atoms and even smaller stuff, is the theory behind computer technology and other modern miracles. But while physicists are experts at using the equations of quantum mechanics to make predictions or build things, they have no experimentally verified agreement about what those equations say about reality. In the face of that dilemma, most physicists have chosen to ignore the problem by "shutting up and calculating," while a small handful have battled over quantum interpretations.
The problem with "interpreting" the equations lies in the many ways quantum mechanics violates our common sense about the world. Carroll's book focuses on one of the basic most forms of quantum weirdness called "superposition." In the standard view of the field — called the Copenhagen interpretation — a quantum particle like an electron can be in many places at once. That's what superposition means. It's only the act of making a measurement — looking at the electron — that forces the particle to take on an existence at just one place.
If that seems weird to you, welcome to the club.
Sean Carroll, however, doesn't like the Copenhagen interpretation of quantum mechanics. In particular, Carroll doesn't like vague ideas of observers "collapsing" the smeared out superposed electron into a single position just by looking at it. Early on in the book, Carroll asks a series of pointed questions: What precisely do you mean by a "measurement?"; How quickly does it happen?; What exactly constitutes a measuring apparatus?; Does it need to be human or have some amount of consciousness ...?"
The idea that measurements somehow change reality introduces a "spookiness" onto discussions of quantum physics that Carroll thinks is both unnecessary and wrong. Instead, his intention is to prize "clarity over mystery" which leads him, and his readers, to what's known as the Many Worlds interpretation of quantum mechanics. The bulk of Something Deeply Hidden is a really carefully reasoned argument for taking the Many Worlds interpretation seriously as the true path to understanding nature's deepest structures.
But what sets Many Worlds apart from the Copenhagen way of looking at quantum mechanics? It all goes back to that basic weirdness of superposition. The Many Worlds interpretation also begins with the electron superposed, existing many places all at once. But when a measurement is made, it's not the other, unobserved versions of the particle which disappear. Instead, it's the universe branching off into multiple parallel copies of itself.
These copies are the "many worlds" of the Many Worlds interpretation — and each world has an observer in it seeing the electron in a different position. After the split occurs, each of these cosmic copies (with its own copy of the observer) goes on its merry way with new branches occurring each time something quantum mechanical happens. But zillions of quantum mechanical events are occurring zillions of times a second throughout the universe. That means there are a lot of copies of you, me and everything else out there all living parallel lives and all continuously branching off into new parallel versions.
Now if that seems weird to you, welcome to the club.
I count myself among those who find the Many Worlds interpretation a solution to quantum weirdness that's worse than the problem. And, I'm sad to admit, on a bad day I would be a snarky partisan. What makes Something Deeply Hidden so excellent is that, in straight-forward language, Carroll keeps his justification for the Many Worlds view grounded in principles like simplicity and economy of description that scientists should all agree on.
As his previous books have demonstrated, Carroll is an excellent guide through the frontiers of physics for interested laypeople. Those skills are on ample display in the new book as well. Carroll expertly takes his readers through the conundrum quantum mechanics dumps into the laps of scientists in terms of superposition (as well as another form of weirdness called "entanglement"). Then he lays out the argument for why the Many Worlds offers not only a resolution to those difficulties but also a path forward to solving some of physics' most vexing challenges — like the nature of space and time.
It is worth noting, however, that this book does not seem aimed at folks who are entirely new to the subject. It works at a slightly higher level and might prove challenging for those who've never seen the topic at all.
I'd like to be able to say that I came away convinced of Carroll's argument that the Many Worlds interpretation is the right way to view the world. But that didn't happen. I remain someone who believes that quantum physics is trying to teach us that we, the experimenters, are always part of the story (I lean towards what's called Quantum Baysianism or QBism for short. This theory states that the equations of quantum mechanics are always about our knowledge of the electron not the electron by itself).
But convincing people is not Carroll's only intention — which is the books' greatest charm. In a remarkable chapter, Carroll presents a number of the Many World's competitor interpretations (including QBIsm) in a wonderfully fair and balanced way. Most important, though, Carroll wants readers to see how remarkable the questions quantum mechanics poses are in-and-of-themselves. In my favorite chapter, Carroll imagines a gentle debate between a philosopher of science and her physicist father. The skeptical dad wants his daughter to explain why anyone would believe in the Many Worlds interpretation. After doing an admirable job of countering her father's criticisms, Carroll puts these words in her mouth:
I couldn't agree more. If you read Something Deeply Hidden, you will too.
Adam Frank is an astrophysics professor at the University of Rochester and author of Light of the Stars: Alien Worlds and the Fate of the Earth. You can find more from Adam here: @adamfrank4.
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