Friedrich Hasenöhrl (; 30 November 1874 – 7 October 1915) was an Austrian physicist. Life Friedrich Hasenöhrl was born in Vienna, Austria-Hungary in 1874. His father was a lawyer and his mother belonged to a prominent aristocratic family. 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. In 1907 he became Boltzmann's successor at the University of Vienna as the head of the Department of Theoretical Physics. 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". When the war broke out in 1914, he volunteered at once into the Austria-Hungarian army. He fought as Oberleutnant against the Italians in Tyrol. He was wounded, recovered and returned to the front. He was then killed by a grenade in an attack on Mount Plaut (Folgaria) on 7 October 1915 at the age of 40. Cavity radiation Since J. J. Thomson in 1881, many physicists like Wilhelm Wien (1900), Max Abraham (1902), and Hendrik Lorentz (1904) used equations equivalent to for the so-called "electromagnetic mass", which expresses how much electromagnetic energy contributes to the mass of bodies. And Henri Poincaré (1900) implicitly used the expression m=E/c2 for the mass of electromagnetic energy. Following this line of thought, Hasenöhrl (1904, 1905) published several papers on the inertia of a cavity containing radiation. This was an entirely classical derivation (no use of special relativity) and used Maxwell's equation for the pressure of light. Hasenöhrl specifically associated the "apparent" mass via inertia with the energy concept through the equation: , where μ is the apparent mass, E0 is the radiation energy, and the speed of light. Subsequently, he used the notation: , where hε0 is the radiation energy. He also concluded that this result is valid for all radiating bodies, i.e. for all bodies whose temperature is > 0°K. For this result