Martin L. Perl’s speech at the Nobel Banquet, December 10, 1995

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

On behalf of my companion in the world of lepton physics, Frederick Reines, and myself I want to express our gratitude to the Royal Swedish Academy of Sciences and the Nobel Foundation for being honored for our discoveries in physics. It is a great pleasure for us to be here today, indeed it is a special pleasure because the physics world may not show much gratitude in the future for our work. Our discoveries may have brought up more problems than we solved. I will explain.

Before Frederick Reines and Clyde Cowan proved the physical existence of the neutrino it was possible to think of the neutrino as a vague sort of entity, perhaps not quite physical, perhaps only a mathematical concept. But their discovery forced the physics world to confront the question of what is the mass of the neutrino. If the mass is zero, how can a particle of zero mass carry information about its properties? If the neutrino mass is not zero, then what is its mass, and what law of physics fixes its mass? There have been many attempts to find the mass of the neutrino and such experiments continue. But still the mass has not been measured. Thus the discovery for which Frederick Reines is being honored today has raised more questions than he answered.

I must confess that my discovery of the tau lepton has left us with a similar situation. I began my research trying to find the connection between the electron and the muon, the two charged leptons which were known before 1975. Since my colleagues and I could not find the connection, I decided to look for an additional charged lepton, a heavier lepton, hoping that the heavier lepton would teach us about the connections between charged leptons. With the help of my colleagues, particularly Gary Feldman and Paul Tsai who are here tonight, the heavy lepton was found, and we called it the tau. Since this discovery a great deal of experimental work has been done on the tau lepton. And there has been a great deal of theoretical work, particularly by Haim Harari who is also here tonight. But none of us have been able to penetrate the secret of what is now a more complicated question. What is the connection between the electron, the muon, and the tau? Thus the discovery of the tau lepton also raised more questions than it answered.

Perhaps Frederick Reines and I are not working in the right directions, perhaps some young woman or some young man will point lepton physics in the right direction and will answer the multitude of questions we have raised.