Where Does the Weirdness Go? by David Lindley is a popular science book about quantum theory. It presents some explanations for why our everyday experience of the world seems to be generally consistent and reliable… and in particular why the objects we see around us don’t tend to have two incompatible properties at once.
The book’s subtitle is “Why Quantum Mechanics Is Strange, But Not As Strange As You Think”.
Notes
- The choice of the word “measurement” in the Copenhagen interpretation of quantum mechanics had some unexpected ramifications. It wasn’t given a definition. The word “observation” has been used instead, and this has led some people to conclude that a conscious observer is required. But there’s nothing in the Copenhagen interpretation that requires an observer. Instead, there’s just a gap where there should be an explanation of what is actually needed for a “measurement” to occur.
- The “Many Worlds” (or “Many Universes”) hypothesis developed by Hugh Everett attempts to deal with this omission by proposing that a “measurement” doesn’t cause one possibility to come into existence and the other possibility to disappear, as the Copenhagen interpretation implies. Instead, the current universe splits into two, with one of the possibilities occurring in one universe and the other possibility occurring in the other universe.
- Lindley makes it clear just how extravagant the Many Worlds hypothesis actually is. When a photon from the sun enters the Earth’s atmosphere, its path is scattered by interactions with electrons, and each new direction taken by the photon is a quantum event determined randomly. So, under this hypothesis, every time a person glances up at the sky, many many new universes are created. There’s no way for these nearly identical universes to interact with each other once the split happens.
- And Lindley points out an issue with the Many Worlds hypothesis that I think many people have missed: it doesn’t fill the gap in the Copenhagen interpretation. There’s still no definition of “measurement”, so there’s no explanation of exactly when (or why) the event happens that causes the universe to split into two. (Personally, I think the Many Worlds hypothesis was an ingenious idea that doesn’t seem to lead anywhere interesting.)
- I found the section on Bell’s theorem fascinating – this book was the first time I’d heard of it. Some simple arithmetic and a complex experiment (only successfully run in 1982) clearly demonstrate the menu of weirdness that we need to choose an option from. Perhaps there’s no underlying definite reality. Or we need to accept that some kind of influence can travel instantaneously across space (breaking the speed of light and relativity). Or we have to abandon the idea of free will and believe that we’re living in a completely deterministic universe.
- The Copenhagen interpretation plus the concept of decoherence gives a possible (and, to me, plausible) answer to the question of where the weirdness goes. A key point is the distinction between quantum states and other states. An electron’s spin is a quantum state – it can be up or down (after it’s been measured) or in an indeterminate superposition of the two states. A creature can be alive or dead, but these states are not quantum states. Once a measurement has been made, and a superposition collapses into a definite state, the particle can interact with its broader environment in different ways depending on the outcome of the measurement. This can influence complex, non-quantum states – and some states will be incompatible (or “incoherent”) with each other. A creature can be alive or it can be dead, but it has a zero probability of being both alive and dead at the same time because these two non-quantum states are incoherent. In general, the “measurement” occurs as soon as a particle interacts with the larger environment in some way – that is, as soon as its state can influence the larger non-quantum states. Note that macroscopic cases of superposition can occur if a system, such as a superconducting ring, is completely isolated from the larger environment.
Reflections
I first read this book years ago, and I’ve reread it several times. I’m particularly impressed by how David Lindley explains some very counter-intuitive topics.
And it’s a very entertaining book. The chapter titles give a sense of this – here’s a selection:
- “In which Niels Bohr is obscure, even by his own standards”
- “And how many universes did you say you’d be needing?”
- “More than you really wanted to know about dried peas”
- “In which Einstein’s Moon is restored”
The superconducting ring is an interesting example of an apparent exception to a rule that instead provides a lot of insight into what’s going on.
I have the impression that the Many Worlds hypothesis remains popular with some people despite its exorbitant pointlessness. What makes it appealing? In films and books, the concept of parallel worlds is often used to explore emotional questions of regret or simply curiosity about what would have happened if we made different choices. I wonder if some people strongly want an answer to the question “What would my life be like now if I’d made Choice B instead of Choice A?” to exist somewhere, even if it’s inherently unknowable to us.