I am sad to report the death of Kenneth Nordtvedt Jr, on October 9, 2025 at the age of 86.
Along with Robert Dicke, Leonard Schiff, Irwin Shapiro and Joseph Weber, Ken was a pioneer of the resurgence of experimental gravity that began in the 1960s. In three amazing papers in 1968, he parametrized the post-Newtonian point-mass metric of general relativity; showed that, in alternative theories, self-gravitating bodies could violate what we now call the Strong Equivalence Principle, and pointed out that lunar laser ranging could test the effect (today called the Nordtvedt effect). They were a revelation to me as a beginning graduate student in Kip Thorne’s group in 1969, inspiring me to generalize his approach using Chandrasekhar’s post-Newtonian hydrodynamics. Later, during a 1971 summer workshop on experimental gravity organized by Ken in Bozeman, one of Kip’s students named Sandor Kovacs took it upon himself to nag Ken and me incessantly to merge our two approaches into one, even threatening to lock us in a room until we did so. The result was the canonical parametrized post-Newtonian (PPN) framework that became a standard tool for the subject.
In July 1969, Apollo astronaut Neil Armstrong deployed the first laser “retroreflector” on the lunar surface, and within a few weeks, lunar laser ranging began in earnest, and continues to the present day, reaching millimeter precision in measuring the Earth-Moon distance. The data has consistently shown no evidence of the Nordtvedt effect, in agreement with GR. As Ken frequently remarked, zero is as good a number as any other (and indeed, null experiments have been a mainstay of fundamental physics). Data on the pulsar in a triple system, J0337+1715, also found no evidence of the Nordtvedt effect, but now in the strong-field regime.
Ken and I devised the first “vector-tensor” alternative theory of gravity designed to show how one could get effective violations of Lorentz invariance and “preferred-frame” effects in gravity. Our theory was crude and deeply flawed; later Ken and Ron Hellings produced a better theory. More recent theories such as Einstein-Aether, Horava gravity, TeVeS and others, were built upon the foundation laid by Ken and Ron.
In recent years Ken collaborated with Eric Adelberger, Tom Murphy and others to create APOLLO, an improved lunar laser ranging facility at the Apache Point telescope in New Mexico. He continued publishing interesting papers on post-Newtonian theory and tests of gravity into his 70s.
Ken got his undergraduate degree at MIT and his PhD under Schiff (in solid-state physics) at Stanford, and had a postdoc as a Junior Fellow at Harvard. But dissatisfied with the lifestyle and politics of the two coasts, Ken accepted a faculty position at Montana State University in Bozeman in 1967. He characterized himself as a Libertarian, and even served a six-year term (1979 – 84) in the Montana State legislature (a part-time job, as in many rural states). In 1986 he was appointed by President Ronald Reagan to serve on the National Science Board, which oversees the NSF. An avid DIY person, he once built from scratch a house as well as an ocean-worthy sailboat, which he sailed to Europe and around the Mediterranean.
At MSU, he built a strong gravitational physics group, including over the years Ron Hellings, Sachiko Tsuruta, Bill Kinnersley, Lee Lindblom, Bill Hiscock, Neil Cornish, Nico Yunes, and Hang Yu, and many grad and post-doc alumni with distinguished careers of their own.
Clifford Will, University of Florida