In the third of a series of articlescelebrating Einstein’s MiraculousYear, Cormac O’Raifeartaighdescribes Einstein’sgroundbreaking contribution toatomic theory

In the year 1905, a young AlbertEinstein published a number of

scientific works that changed physicsforever. The best known of these,now known as the Special Theory ofRelativity, established Einstein as ascientist of note and led to his GeneralTheory of Relativity, one of the pillarsof modern physics. In a second 1905paper, the young Einstein publisheda highly controversial proposalconcerning the nature of light, aproposal that later formed acornerstone of quantum theory, therevolutionary theory that underpinsmuch of modern science andtechnology. Incredibly, the youngEinstein made a third ground-breaking advance in 1905. Hepublished an analysis that pointedthe way towards a crucial test of the

reality of atoms, and of the validityof the laws of thermodynamics. Theoutcome of that test underpins muchof modern science, from our view ofthe atomic nature of matter to ourunderstanding of meteorology andother complex systems.

Atoms and chemistryThe idea that all matter is made up ofminute, indivisible entities called‘atoms’ was first put forward by thephilosophers of ancient Greece. The

concept gained much credibility inthe 19th century when scientists suchas John Dalton used it to establishlaws of chemistry that successfullydescribed how the chemical elementscombine to form molecules. A listingof the known elements in order ofincreasing atomic weight led to thedevelopment of The Periodic Table byMendeleyev, a development thatrevolutionized the study ofchemistry. It was widely assumedthat the properties of a given elementwere determined by the properties ofits constituent atoms. However, therewas no direct evidence of theexistence of atoms, and someeminent scientists simply did notbelieve in the ‘atomic hypothesis’.

Enter EinsteinGreatly interested in the atomic viewof matter, the young Einstein deviseda mathematical method of calculatingthe size of atoms and molecules inearly 1905. From an analysis of sugarmolecules dissolved in water, hecalculated both the diameter of the

Einsteinand theAtomicTheory

Albert Einstein: another 1905 paperfacilitated a test for atomic theory

SCIENCE SPIN Issue 13 Page x