James D. Bjorken
Wolf Prize Laureate in Physics 2015
James D. Bjorken
Affiliation at the time of the award:
Stanford University, USA
“for predicting scaling in deep inelastic scattering, leading to identification of nucleon’s pointlike constituents”.
This year’s Wolf prize in physics is awarded to two researchers who made fundamental contributions toward understanding the structure of the Universe at the very smallest and the very largest sizes.
The strong force is responsible for the existence of protons and neutrons and for holding them together in atomic nuclei. It is also responsible for over 99% of the atoms’ mass. Bjorken made a crucial contribution for elucidating the nature of the strong force. In 1967 Bjorken predicted that electrons scattering violently off protons would exhibit the so-called scaling behavior, as if they were interacting with pointlike, charged and quasi-free particles inside the nucleon. At the time this was a very counterintuitive and radical idea. Yet, subsequent experiments, carried in 1968/69 at the Stanford Linear Accelerator Center (SLAC), provided a stunning confirmation for Bjorken’s scaling prediction. The leaders of the SLAC experiments, Jerome Friedmann, Henry Kendall and Richard Taylor were recognized by the 1990 Nobel Prize in Physics, for providing the experimental proof for the existence of quarks – the pointlike constituents of the nucleon.
Following the experimental success of Bjorken’s scaling laws, theorists embarked on a quest for a fundamental quantum theory which exhibits scaling. In 1973 David Gross, Frank Wilczek and H. David Politzer discovered that a theory now known as Quantum Chromodynamics (QCD) possesses the required property, namely that the force between quarks goes down as they get closer, so that at small distance they behave as if they were free. This property is now known as “asymptotic freedom”. In 2004 Gross, Wilczek and Politzer were awarded the Nobel Physics Prize for their discovery. QCD was validated in detail by extensive experiments as the theory of Strong Interactions.
The prevailing view today is that all fundamental interactions in Nature, with possible exception of gravity, are described by theories whose mathematical structure is analogous to QCD. Such theories are known as non-abelian gauge theories. Thus in retrospective, Bjorken’s scaling not only led to the discovery of quarks, but also pointed the direction toward the mathematical framework governing all fundamental interactions.