One of the organizers, Charles Édouard Guillaume, reporting to Nature, considered that the congress provided "the most complete representation of any science at a given epoch ever made.
"[2] Novel discoveries at the time were discussed, including radioactivity, the electron and cathode rays, and challenges related to the black-body radiation and to the Michelson–Morley experiment.
[2] Henri Poincaré held the initial plenary keynote on the overview of physics, turn of the century problems, and philosophy of science.
[2] He also raised the epistemological question of what should be the importance of mathematical physics, and discussed the possibly bankruptcy of science for lack of more fundamental principles.
[2] In Henri Poincaré's address, he promoted the idea of abandoning the hypothesis of the luminiferous aether and asserted that only relative motions would ever be observed.
[2] Similarly, in a keynote lecture of Lord Kelvin in the thermodynamics section of the congress, he addressed his work on trying to understand the aether and an elastic solid.
[2] However Kelvin found interest in the hypothesis of George Francis FitzGerald and Hendrik Lorentz of length contraction when moving through aether.
[1][2] Pyotr Lebedev presented for the first time his experiments that demonstrated that radiation pressure was a real phenomenon, as theorized by James Clerk Maxwell.
[6][7] Lord Kelvin is attested to having said:[7] "Maybe you know that all my life I struggled against Maxwell not admitting his light pressure, and now yours Lebedev make me to give up in front of his experiments.