David I. Kaiser is an American physicist and historian of science. He is Germeshausen Professor of the History of Science at the Massachusetts Institute of Technology (MIT) and a full professor in MIT's department of physics. He also served as an inaugural associate dean for MIT's cross-disciplinary program in Social and Ethical Responsibilities of Computing.

Kaiser is the author or editor of several books on the history of science, including Drawing Theories Apart (2005), How the Hippies Saved Physics (2011), and Quantum Legacies (2020). He received the Apker Award from the American Physical Society in 1993 and was elected a Fellow of the American Physical Society in 2010. His historical scholarship has been honored with the Pfizer Award (2007) and the Davis Prize (2013) from the History of Science Society. In March 2012 he was awarded the MacVicar fellowship, a prestigious MIT undergraduate teaching award. In 2012, he also received the Frank E. Perkins Award from MIT for excellence in mentoring graduate students.

Kaiser received his bachelor's degree in physics at Dartmouth College in 1993. He then earned two PhDs from Harvard University. The first was in physics in 1997 for a thesis on cosmology and the second in the history of science in 2000 for a dissertation on physics education and research in the United States after World War II.

Kaiser's physics research mostly focuses on primordial cosmology, including topics such as cosmic inflation, post-inflation reheating, and primordial black holes. In particular, he and colleagues have studied a wide range of initial conditions under which inflation will begin, as well as constructing models of inflation that include features motivated by high-energy particle physics, such as multiple interacting fields with nonminimal couplings to spacetime curvature.

This work includes some of the first calculations of predictions from such models for observable features such as the spectral index of primordial perturbations measured in the cosmic microwave background radiation, the first demonstration that resonant particle production during the reheating phase can persist amid an expanding universe, and the first demonstration of attractor behaviors in multifield models. More recent work has identified distinct processes within the late stages of the reheating phase, which ultimately yield the conditions for standard Big Bang evolution: a hot plasma of Standard Model particles in thermal equilibrium.

Some of Kaiser's research focuses on primordial black holes, especially as a viable candidate for dark matter. Unlike various hypothetical particles, such as weakly interacting massive particles (WIMPs) or ultralight particles such as axions, primordial black holes would not require any new particles beyond the Standard Model in order to account for the measured dark matter abundance. Kaiser also explores the possibility of identifying primordial black holes by detecting exceptionally high-energy neutrinos, emitted as part of these objects' Hawking radiation.

Kaiser and his colleagues have studied mechanisms by which a population of primordial black holes could have formed during the very early universe in models that preserve the close fit between predictions and observations of the cosmic microwave background radiation. They have also identified a possible subpopulation of primordial black holes that would have formed with significant QCD color charge, constituting a novel state of matter. Additionally, they have proposed a new observable test to help establish whether primordial black holes exist and contribute significantly to dark matter abundance, based on high-precision measurements of visible objects within the Solar System, such as the planet Mars.

Kaiser has also helped to design and conduct novel experimental tests of quantum mechanics. In one such test, Kaiser and colleagues demonstrated how measurements of neutrino oscillations could be used to test whether quantum objects really persist in superposition states—akin to Schrödinger's cat—between preparation and measurement. By applying the neutrino measurements to the Leggett-Garg inequality, their long-baseline test showed clear evidence of quantum superpositions over a distance of 450 miles (720 km).