Friedrich Hasenöhrl (German: [ˈhaːzn̩øːɐ̯l]; 30 November 1874 – 7 October 1915) was an Austrian physicist and professor of the University of Vienna.

After his elementary education, he studied natural science and mathematics at the University of Vienna under Joseph Stefan (1835–1893) and Ludwig Boltzmann (1844–1906).

In 1896, he attained a doctorate under Franz-Serafin Exner with a thesis titled "Über den Temperaturkoeffizienten der Dielektrizitätskonstante in Flüssigkeiten und die Mosotti-Clausius'sche Formel".

He worked under Heike Kamerlingh Onnes in Leiden at the low temperature laboratory, and there he also befriended H. A. Lorentz.

He had a number of illustrious pupils there and had an especially significant impact on Erwin Schrödinger, who later won the Nobel Prize for Physics for his contributions to quantum mechanics.

In an autobiography, Schrödinger claimed "no other human being had a greater influence on me than Fritz Hasenöhrl, except perhaps my father Rudolph".

Hasenöhrl's results (concerning apparent mass and thermodynamics) by using cavity radiation was further elaborated and criticized by Kurd von Mosengeil (1906/7) who already incorporated Albert Einstein's theory of relativity in his work.

[2][3][4] In some additional papers (1907, 1908)[H 4] Hasenöhrl elaborated further on his 1904-work and concluded that his new results were now in accordance to the theories of Mosengeil and Planck.

After providing a complete relativistic description and solution of the cavity problem (in the "constant velocity case" and "slow acceleration case"), they wrote: ... more generally the reason he [Hasenöhrl] achieved an incorrect result on both occasions is that he wants to rigorously equate the work performed to kinetic energy, as the work-energy theorem demands.

[...] Let us end by saying that Fritz Hasenöhrl attempted a legitimate thought experiment and tackled it with the tools available at the time.

Nevertheless, his basic conclusion remained valid and for that he should be given credit.The equations for electromagnetic mass, like those of Hasenöhrl's (for example, Oliver Heaviside (1889), Henri Poincaré (1900), Abraham (1902)), formally similar to the famous Einstein's (1905) equation for mass–energy equivalence, [12] that of which the special case of a stationary massive body is widely known as

, have often prompted uninformed questioning of Einstein's priority of the discovery, starting soon after his publication and continuing to this day.

Max von Laue clarified as early as 1921 that, while the inertia of electromagnetic energy had been known long before Hasenöhrlt, Einstein was indeed the first to establish the equivalence of real mass and the total energy-momentum content and understand the deep implications of this principle in relativity.