This
book provides the reader with a detailed and captivating account of the story
where, for the first time, physicists ventured into proposing a new force of
nature beyond the four known ones - the electromagnetic, weak and strong
forces, and gravitation - based entirely on the reanalysis of existing
experimental data.  

Back in 1986, Ephraim
Fischbach, Sam Aronson, Carrick Talmadge and their collaborators proposed a
modification of Newton’s Law of universal gravitation. Underlying this proposal
were three tantalizing pieces of evidence: 1) an energy dependence of the CP (particle-antiparticle
and reflection symmetry) parameters, 2) differences between the measurements of
G, the universal gravitational constant, in laboratories and in mineshafts, and
3) a reanalysis of the Eötvos experiment, which had previously been used to
show that the gravitational mass of an object and its inertia mass were equal
to approximately one part in a billion. The reanalysis revealed that, contrary to Galileo’s
position, the force of gravity was in fact very slightly different for
different substances. The resulting Fifth Force hypothesis included this
composition dependence and also added a small distance dependence to the
inverse-square gravitational force.



Over the next four years
numerous experiments were performed to test the hypothesis. By 1990 there was
overwhelming evidence that the Fifth Force, as initially proposed, did not
exist. This book discusses how the Fifth Force hypothesis came to be proposed
and how it went on to become a showcase of discovery, pursuit and justification
in modern physics, prior to its demise.



In this new and significantly expanded
edition, the material from the first edition is complemented by two essays, one
containing Fischbach’s personal reminiscences of the proposal, and a second on
the ongoing history and impact of the Fifth Force hypothesis from 1990 to the
present.