Atoms: from philosophical hypothesis to experimental evidence
The idea that matter is made of indivisible particles is Greek — Leucippus and Democritus, 5th century BC — but for over twenty centuries it remained a philosophical conjecture with no experimental basis. Newton, Boyle and the founders of modern science treated it with sympathy but caution: “atoms” are useful as a metaphor, not as measurable objects. Still at the end of the nineteenth century, first-rate physicists such as Mach and Ostwald openly argued that atoms were bookkeeping fictions, convenient for calculations but not, for that reason, real.
The debate closed decisively between 1900 and 1908. On one hand kinetic theory, developed by Maxwell, Boltzmann and Clausius, predicts the behaviour of gases in detail starting from simple hypotheses about the “little balls” that make them up, and the numbers match with a precision too good to be coincidence. On the other hand Einstein, in 1905, showed that Brownian motion (the erratic movement of pollen grains in water, observed by Brown in 1827 without anyone understanding its origin) is precisely the effect of collisions of invisible water molecules with the grains; and Perrin, in 1908, experimentally measured these effects, deriving from them a value of Avogadro’s number coinciding with those obtained by completely independent methods (Faraday’s law, light scattering, capillarity).
When various ways of “counting atoms” that have nothing to do with each other all give the same answer, the existence of atoms stops being a conjecture. It is an instructive case of how science establishes the existence of the unobservable: not through direct vision, but through inferential convergence. The same logic that today makes us believe in quarks, cosmological neutrinos and dark matter: no one has ever seen them, but independent lines of evidence point to the same story, and it is this agreement, not direct observation, that makes it credible.
Collegamenti
Argomenti: Teoria cinetica dei gas
Esercizi collegati: Worked exercise — Helium in a braking truck · Problem — Escape temperature of hydrogen · Problem — Ranking of root-mean-square speeds