The fifth force: A personal history

On January 6, 1986, a paper written by our group appeared in Physical Review Letters entitled "Reanalysis of the Eotvos Experiment". In that Letter we reanalyzed a well-known 1922 paper by Eotvos, Pekar, and Fekete (EPF) which compared the accelerations of samples of different composition to the Earth. Our surprising conclusion was that "Although the Eotvos experiment has been universally interpreted as having given null results, we find in fact that this is not the case". Two days later a front page story appeared in the New York Times under the headline "Hints of 5th Force in Universe Challenge Galileo's Findings", and so was born the concept of a "fifth force". In this personal history I review the pre-history which motivated our paper, and discuss details of our reanalysis of the EPF paper that have not been presented previously. Our work led to illuminating correspondence with Robert Dicke and Richard Feynman which are presented here for the first time. I also discuss an interesting meeting with T.D. Lee, one of whose papers with C.N. Yang provided part of the theoretical motivation for our work. Although there is almost no support from the many experiments motivated by the EPF data for a fifth force with properties similar to those that we hypothesized in our original paper, interest in the EPF experiment continues for reasons I outline in the Epilogue.