"The Age of Entanglement: When Quantum Physics was Reborn," by Louisa Guilder

"The Age of Entanglement: When Quantum Physics was Reborn," by Louisa Guilder tells the story of the fascinating and critical debate in theoretical physics that took place in pre-WWII Europe. While it's helpful to have a basic layman's understanding of the physics, the book is a history of the science and the scientists. In other words, it's a good read even for the math phobic!

The “EPR paper” (Einstein, Poldosky, and Rosen) was published in 1935, in the journal, “Physical Review.”  The title of the paper was “Can quantum mechanic description of physical reality be considered complete?”  The paper was a critique of the "Copenhagen Interpretation" supported by Bohr, Heisenberg, and Pauli. The new paper answered the title question with a firm "No," and derided the implications as "spooky action at a distance."

In the years leading up to this debate, Niels Bohr had founded the Institute for Theoretical Physics in Copenhagen in 1920.  He talked about “complementarity” – that is, he maintained that the complementarity of waves and particles, or position and momentum, meant that when one existed fully, its compliment did not exist at all.  He described waves and particles as abstractions observable only by means of their interactions with systems such as measuring apparatuses. In other words, both waves and particles exist in a complimentary way; what we observe depends on what we look for and how we look for it.

I ran across a link to this helpful little video on NPR today. It gives a short graphical presentation on the concept of wave/particle duality:

Schrodinger coined the term “entanglement” in 1935 to describe what he and Einstein saw as a paradox in quantum theory.  A rigorous application of the laws of quantum mechanics would lead to the conclusion that measuring one particle would affect the state of the other particle – however great the distance between them (the "spooky" part).

In the winter of 1926-27 Heisenberg worked out the "Uncertainty Principle."  The uncertainty in the exact momentum of a particle times the uncertainty in the exact position of the particle must be greater than or equal to Planck’s constant (later refined to Planck’s constant divided by 4 Pi).  

In the early 1930s, an alcoholic and depressed Wolfgang Pauli sought treatment with famous psychiatrist, Karl Jung.  Jung theorized a “universal unconscious" full of “archetypes” which uncannily echoed quantum theory.

Jung, in turn, had been “seduced” by the work of a botanist at Duke University, J.B. Rhine on Extra-sensory Perception or ESP (a term coined by Rhine). In 1934, Rhine completed his experiments with “ESP cards” – one student looking at the cards, turning over a deck of 25, one card at a time, while another student across campus tried to guess which symbol was on each card.  Rhine declared that the results were statistically significant, being correct 10% more often that would be predicted by chance alone.

Jung believed that:

“these experiments prove that the psyche at times functions outside of the spatio-temporal law of causality….This indicates that our conceptions of space and time, and therefore causality also, are incomplete.”

Pauli and Jung continued to correspond on the subject of a complementarity between causality and "synchronicity" (significant coincidences).  Pauli came to see physics as a one-dimensional section of a two-dimensional, more meaningful world, the second dimension of which could only be the unconscious and the archetypes.

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On a related note, if you ever get a chance to see the play, "Copenhagen," a version of what might have taken place during a fateful meeting between Bohr and Heisenburg in 1941, take it. Two of the greatest minds of their age in physics, at a pivotal time in history…unable to escape the politics of the world around them.