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Something Deeply Hidden

Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime

Sean Carroll
362 pp.
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Quantum mechanics is not for the faint of heart. The calculations are daunting, and concepts such as superpositions, uncertainty, and entanglement can be perplexing. But the most difficult aspect to grasp is what it means about the nature of reality.

At the heart of the conundrum is the “measurement problem.” Niels Bohr’s “Copenhagen interpretation,” developed almost a century ago, says that physical systems exist as quantum wave functions that can simultaneously occupy mutually exclusive states (a superposition). The famous, if macabre, example of Schrödinger’s cat posits a quantum superposition leading to the eponymous cat being both alive and dead at the same time, until observed. This seems to imply that reality does not come into being until the system is measured, whereupon the wave function collapses, with some probability distribution, into one and only one “real” state.

This view is quietly accepted by most physicists, who often consider “quantum foundations” issues, such as what constitutes a measurement, out of bounds for serious physics. Sean Carroll, however, has decided to tackle the issue head on. Something Deeply Hidden is Carroll’s ambitious and engaging foray into what quantum mechanics really means and what it tells us about physical reality.

He begins by taking the reader though a crash course in basic quantum physics, underscoring his “irritation” with the idea of the wave function collapse, which mars the simplicity of quantum theory and introduces unexplained randomness. There is plenty of humor in this section and no serious math, but prior knowledge of the subject will help.

Carroll then lays out his manifesto: There is no collapse, he insists, and no single true state after a measurement. Rather, the interaction of a measuring device and a quantum system (such as an electron) leads to “multiple worlds, each of which contains a single person with a very definite idea about where the electron was seen.” This “many worlds” interpretation, first developed by Hugh Everett in the 1950s, is predicated on the idea that copies of the entire world split off from one another every time quantum events occur. Carroll emphasizes that as strange as it sounds, “the other possible measurement outcomes still exist and are perfectly real, just as separate worlds.”

As Carroll admits, there is no clear experimental test for this or other interpretations of quantum mechanics. In the final chapters, he dangles tantalizing hints from the study of quantum gravity that suggest that the many worlds hypothesis may help us resolve long-standing puzzles about apparent contradictions between quantum mechanics and general relativity. If we have the “courage” to discard classical thinking and embrace the reality of quantum mechanics, he argues, we may someday “learn how to extract our universe from the wave function.”

As is unavoidable in such discussions, the book takes a philosophical turn, delving into questions of ethics, consciousness, and the nature of reality itself. Many physicists avoid such subjects because they just do not feel like physics. Ultimately, Carroll’s goal is to convince both the public and his fellow physicists that these questions are worth asking. Does he succeed? Like so much in quantum mechanics, it depends on your point of view.

About the author

The reviewer is at Walsh School of Foreign Service, Georgetown University, Washington, DC 20057, USA.