E = mc² — Was It Really Einstein’s Alone?
Austria produced two towering figures in modern physics: Ludwig Boltzmann (1844–1906), father of statistical mechanics and author of the famous equation S = k log W, and Erwin Schrödinger (1887–1961), whose Schrödinger equation became an icon of quantum mechanics.
In 1906, when young Schrödinger entered the University of Vienna, Boltzmann—his intellectual hero—took his own life. Schrödinger was deeply disappointed to miss the chance to attend his lectures. But eighteen months later, his enthusiasm for physics was reignited when Fritz Hasenöhrl, newly appointed professor of theoretical physics, gave his inaugural lecture. Schrödinger would later choose Hasenöhrl as his doctoral advisor, recalling,
“No one influenced me more than Fritz Hasenöhrl—except my father. There was something chivalrous about him. His warmth removed all barriers of formality and age between professor and student.”
A student of both Boltzmann and Schrödinger, Hasenöhrl led Austrian physics for about a decade until his death in 1915.
Hasenöhrl’s Path to a Pre-Einstein Mass–Energy Formula
Born in Vienna in 1874 into a well-to-do family, Hasenöhrl studied under Franz Exner and Boltzmann. In 1898, Boltzmann so valued him that, when Leiden physicist Heike Kamerlingh Onnes requested a research assistant, Boltzmann recommended Hasenöhrl.
In 1904, Hasenöhrl published an important paper, On the Theory of Radiation of Moving Bodies, deriving the formula:
This was a full year before Einstein’s famous 1905 paper concluding that “if a body gives off energy L in the form of radiation, its mass decreases by L/c².”
Einstein, famously sparing with references, cited only his own special relativity paper from three months earlier—without mentioning Hasenöhrl. Was this mere coincidence?
The Road from 3/8 to 3/4 to 1
In January 2015, astrophysicist Stephen Boughn of Haverford College published Fritz Hasenöhrl and E = mc² in the European Physical Journal H. His analysis traced how Hasenöhrl developed his ideas—and how his classical assumptions led to the erroneous constants and later .
Earlier still, J. J. Thomson (1881) and Max Abraham had linked a moving electron’s mass to its electric field energy, yielding results like . Hasenöhrl extended this logic to blackbody radiation—light trapped inside a perfectly absorbing cavity.
He imagined a hollow cylinder with blackbody end caps moving at constant speed. To a stationary observer, light traveling forward would be blue-shifted (shorter wavelength) while light traveling backward would be red-shifted (longer wavelength). Calculating the work needed to maintain the cylinder’s constant velocity, Hasenöhrl found .
A year later, he revised the scenario in another thought experiment and obtained . Both results erred because they omitted the relativistic postulate of light’s constant speed. Einstein, by contrast, worked within the framework of special relativity and concluded with the exact factor of 1.
Did Einstein Know Hasenöhrl’s Work?
It seems likely. Hasenöhrl’s 1904 and 1905 papers appeared in Annalen der Physik, the same prestigious journal that published Einstein’s special relativity and mass–energy equivalence papers. Hasenöhrl’s 1905 paper, deriving , came out months before Einstein’s.
Physicist Thomas Rothman of Princeton, who co-researched Hasenöhrl with Boughn, remarked, “We can’t prove it, but Einstein almost certainly knew—and probably sought to improve upon—it.” The resemblance in setup between the two men’s thought experiments is striking.
Einstein was fiercely protective of his priority. In 1907, when Johannes Stark attributed to Max Planck, Einstein wrote to him expressing surprise that Stark failed to recognize his precedence. Stark later retracted the attribution after realizing Planck had cited Einstein’s work.
An Unfinished Proof and a Tragic End
Einstein himself was not wholly satisfied with his 1905 derivation, which assumed a point particle. He tried—and failed—to generalize it to extended bodies, as Hasenöhrl had attempted. The fully generalized proof came in 1911 from German physicist Max von Laue.
When World War I broke out in 1914, Hasenöhrl was 40 and exempt from service, yet volunteered. He was wounded in combat, recovered, returned to the front, and was killed in action on October 7, 1915.
