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Said Einstein, "Mein Herr, we must chat."
Schroedinger smiled. "Ja, that cat!
The law I derive
Says it's dead AND alive.
What's it mean to be both this and that?"
Schroedinger's cat lives!
That bold declaration does not refer to an actual pet of the great twentieth-century physicist Erwin Schroedinger but rather to a paradox in which, according to Schroedinger's Equation, that famous fictitious feline finds itself in an odd state of existence best described as a superposition of being alive and being dead.
The paradox of Schroedinger's cat, born in a 1930s correspondence between Schroedinger and Albert Einstein, lives on as the best known example of many counter-intuitive yet never disproved consequences of the quantum approach to understanding natural phenomena.
British science writer Jim Baggott describes that kitty conundrum in the 17th of 40 "moments" of his insightful and entertaining new history, The Quantum Story.
"The diabolical scenario runs as follows. A cat is placed inside the steel chamber together with a Geiger tube containing a small amount of a radioactive substance, a hammer mounted on a pivot and a phial of prussic acid.... [W]ithin one hour there is a 50 per cent probability that one atom has disintegrated. If an atom does indeed disintegrate, the Geiger counter is triggered, releasing the hammer which smashes the phial. The prussic acid is released, killing the cat."
According to quantum mechanics, the triggering atom is described by a wave function "expressed as a linear combination of the possible measurement outcomes, corresponding to the physical states of the intact atom and the disintegrated atom...."
"But what about the cat?" Baggott continues. Until we actually open the chamber to check, Schroedinger's equation "would seem to suggest that we should express the wavefunction of the system-plus-cat as a superposition of the products of the wavefunction describing a disintegrated atom and a dead cat and of the wavefuction describing an intact atom and a live cat. Prior to measurement, the physical state of the cat is therefore 'blurred'--it is neither alive nor dead but in some peculiar combination of both states."
With results like that, quantum theory seems to some too problematic to be useful. Yet the problem is not with quantum reality itself but rather with our limited human capacity to visualize it. As many of Baggott's 40 pivotal moments show, quantum theory has produced exquisitely accurate and productive descriptions of natural phenomena from the subatomic to the cosmic, succeeding where classical descriptions have failed.
This is a challenging book for readers and author alike. For readers, the challenges include encounters with other non-intuitive phenomena beyond Schroedinger's dead-and-alive cat. They need to grapple with Werner Heisenberg's famous uncertainty principle and the phenomenon known as quantum entanglement that Einstein derided as "spooky action at a distance" but has since been experimentally verified.
Fortunately, Baggott leavens the difficult scientific and philosophical concepts with human stories, including Schroedinger's complicated love life, the famous Copenhagen meeting between Heisenberg and Neils Bohr under the shadow of impending war just after the discovery of nuclear fission, a raft of quirky characters like Paul Dirac and Richard Feynman, and the amusing story of Bertlmann's socks.
The book is structured as a triple chronological sweep. The first 29 moments cover quantum theory's full development. It begins with Max Planck's first proposal of quanta in 1900 to explain blackbody radiation and culminates with the apparent completion of the Standard Model of particle physics in 2003.
The next six moments describe the evolution of "Quantum Reality" from 1951 to the present, tracing how physicists gradually restructured their understanding of nature by learning to abandon intuitive but inaccurate mental models of particles, waves, and measurement.
The final five moments trace the history of "Quantum Cosmology," where efforts to unify quantum theory with general relativity and gravity, including string theory, have, according to Baggott, shown more loose ends than promise.
That isn't necessarily a bad thing. At every point in The Quantum Story, unanswered questions point the way to great moments ahead. Loose ends are simply a sign that more history remains to be written.