Through Two Doors at Once—The Elegant Experiment That Captures the Enigma of Our Quantum Reality

Author: Anil Ananthaswamy
Publisher: Dutton 2018

This is perhaps the best book there is that encapsulates current knowledge of the “double-slit experiment”.  It is not an easy book to read.  However, if you are willing to spend the time, then I recommend this book.

In the original double slit experiment an opaque sheet with two thin slits was placed between a light source and a screen.  Light can be considered as a beam of particles called photons.  “Normally”, each particle that goes through the slits should go through one slit or the other.  As a result, we should see two bigger slits on the screen, where the particles landed.  What was observed instead is an interference pattern as if photons behave like waves.  That is, photons pass through both slits at the same time like a wave.

This experiment was repeated by sending one photon at a time and even then (eventually) the interference pattern emerged.  That is, the peaks are the places where most photons landed and the valleys are the places where fewer photons landed.

If you try to find which slit the photon is going though by placing a non-disrupting detector behind the screen, the interference pattern disappears.  That is as if the photon knows that something is lurking behind the slits and decides to behave “normally”. 

There are many interpretations that tries to “explain” what happens.  However, no one has succeeded.  The first interpretation known as the Copenhagen interpretation is the foundation of the quantum mechanics.  Quantum mechanics is the most successful theory there is.  David Mermin, a theoretical physicist from Cornell quotes: “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be ‘shut up and calculate!’”  The problems of the Copenhagen interpretation are known as the “measurement problem” and the “non locality problem”.

There were other interpretations such as QBism, Everett interpretation, Bohmian mechanics, Penrose Interpretation, and GRW interpretation.  These interpretations are proposed to eliminate problems of the Copenhagen Interpretation.  But, they introduced new problems of their own.

The following is the ending of the book.  (This does not spoil anything for anybody.)

As Feynman put it in his Cornell lecture: “Any … situation in quantum mechanics, it turns out, can always be explained afterwards by saying ‘You remember the case of the experiment with the two holes?’”

Physics has yet to complete its passage through the double-slit experiment.  The case remains unsolved.