**Six Impossible Things****The Mystery of the Quantum World***John Gribbin**(MIT, 2019)**101 p.*

The shelves of neighbourhood bookstores groan under the weight of popular science books about quantum theory. *Six Impossible Things* stands out from this crowd because it focuses not on the mechanics or the findings of quantum theory, but on how to *interpret* the theory. What is it saying the world is like?

Gribbin opens the book by describing two quantum phenomena in which the strangeness of quantum theory is most evident. The first is the famous double-slit experiment, and the second is entanglement. These form the main course in many popular books on quantum mechanics, but are here only the appetizers.

In the remainder of the book, Gribbin describes six different schools of interpretation of quantum theory, and discusses, in particular, what each of them says is happening in these two famous case studies. These six schools are really schools *of interpretation*; they all rely on exactly the same mathematics and make exactly the same predictions for experiments. They differ only in how they describe the entities and dynamics underlying the observations. They agree on what happens in the end; they differ on how and why it happens.

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So, for example, the most common school of interpretation among working physicists is the so-called Copenhagen interpretation. On this view, there is an entity called the wave function, the time-evolution of which is described by the Schrodinger equation. Quantum states evolve into superpositions of states, and when a measurement is made the wave function collapses into one state with predictable probability. Quantum mechanical systems don’t have definite properties until measured, on this view. The biggest problem for this school is the “measurement problem”: the measurement intervenes in the physics and itself has no physical description. What *is* a measurement anyway? Does it require a conscious agent to carry it out? Many physicists have thought so, but this has led to thought experiments like the one with Schrodinger’s cat, which strain credulity as a sound description of the world.

A second interpretation is the “pilot wave” theory, pioneered in the early days by de Broglie and developed at length by David Bohm. On this view, particles (like electrons or photons) are always and only particles with definite properties — there is no wave/particle duality or superposition of states — but the particles interact with a “pilot wave” which evolves dynamically, like the wave function, and which guides particles on different paths through space depending on small variations in initial conditions. The theory is deterministic, but specific outcomes are impossible to predict without more information. This view has its attractions: much of the quantum weirdness found in the Copenhagen interpretation is gone, and there is no measurement problem. But the theory has features not obviously attractive too: the pilot wave affects particles but is itself un-measurable, and the theory requires non-locality — the properties of a particle depend on the properties of all other particles with which it has ever interacted, and on their properties *now*, not just their past properties.

Third comes my own least favourite: the “many worlds” interpretation. On this view there is no collapse of the wave function so that the outcome of a measurement is one of the range of possible states. Instead *all* of the possible outcomes are realized each time a measurement is made, each outcome in a different universe. The universe we live in is constantly branching into myriad new universes each time a quantum mechanical system is reduced from a superposition of states to a specific state. Every possible universe is realized somewhere, provided it obeys the laws of physics. The contempt I feel for this interpretation would be difficult to overestimate. Practically the sole consolation it provides is the reassurance that somewhere a living, breathing Elizabeth Bennett is living her life just as was so memorably recounted by Miss Austen in her story.

Another possible interpretation is the ‘decoherence’ view, which takes entanglement, sometimes an odd and slightly annoying phenomenon in the mental universe of physicists, and makes it the key to everything. On this view, quantum systems become entangled with their environments, and these macroscopic entangled states behave, on average, like classical (ie. non-quantum) systems. The larger the distance scale, the faster this entanglement causes the quantum system to ‘decohere’. This view has the advantage of explaining why quantum mechanical systems are generally very small and relatively simple: if they become large or complex, they decohere through interactions with their environments. In fact, to the degree that I understand it, this interpretation of the theory is my own favourite: it is metaphysically modest and makes a lot of sense. However, I may not understand its implications adequately; Gribbin devotes a chunk of his chapter to arguing that the decoherence interpretation is equivalent to something called the Many Histories interpretation, which he encapsulates in this way: “Everything that could possibly happen already has happened, and we only noticed part of it.” That sounds bad, but I confess I don’t understand this alleged equivalence.

Next is the ‘ensemble interpretation’, favoured by Einstein, which attempts to do away with quantum weirdness by claiming that the probabilistic predictions of the theory pertain not to any one quantum system, but to an ensemble (or collection) of such systems, if they existed. On this view, each individual quantum system behaves deterministically, but duplicates of the system could behave differently, according to the probabilities predicted by the theory. It makes a kind of sense, but has trouble explaining why *this* particular quantum system behaves as it does. The idea has been rejuvenated by Lee Smolin in recent years, who has proposed a highly non-local interpretation whereby the degree of “quantumness” exhibited by a system depends on how many instances of that system exist in the universe, with all such instances together forming an ‘ensemble’ in the relevant sense. Hydrogen atoms behave quantum mechanically because there are many of them, but you and I behave classically because we are unique (or, if you prefer, we know we are unique because we behave classically). This is an interesting view, if you can abide the radical non-locality.

Finally, Gribbin describes the “transactional interpretation”, a view inspired by the fact that the equations of quantum theory allow for solutions travelling both forward *and backward* in time. The backward-travelling solutions are usually regarded as spurious, but in this interpretation they are taken seriously. The proposal is that physics depends not just on where things have been, but also on where they are going. Not just on where and how a quantum system originated, but also on where and how, in the future, it will be measured. When worked out in detail, this view, rather surprisingly, produces exactly the formalism of quantum mechanics. One hardly knows what to do with this fact.

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Gribbin summarizes the six interpretations he has described in this way:

- The world does not exist unless you look at it.
- Particles are pushed around by an invisible wave, but the particles have no influence on the wave.
- Everything that could possibly happen does, in an array of parallel realities.
- Everything that could possibly happen already has happened, and we only noticed part of it.
- Everything influences everything else instantly, as though space did not exist.
- The future influences the past.

Choose your poison.

A few closing remarks. First, there is no getting around the fact that quantum theory has upset our conventional views of what the world is like, but, as this books makes clear, just *how* it upsets them is not clear. Second, it strikes me as quite amazing that a single theory, the mathematical structure of which is uncontroversial, could produce such a wide variety of possible physical descriptions. Third, this is a very fine book, written without undue technicalities, and I recommend it to anyone with an interest in the subject.