Molecules and atoms
The next two papers were easier for the physics community to swallow. They validated the idea that matter was composed of atoms and of groups of atoms called molecules.
Though most scientists accepted the concept, there were significant holdouts. "At that time, there were people who doubted the existence of molecules," Stachel said.
The first of these papers, a doctoral thesis submitted in April, was Einstein's prediction that the size of molecules could be gauged by the effects of dissolving sugar in a liquid. Einstein argued that "the effect of the dissolution of sugar molecules would change the viscosity of fluid; you can measure the viscosity, and from that estimate the size of the molecules," Stachel said. His prediction proved to be not far from reality.
Second was a description of the mechanism underlying Brownian motion — a particle's small random movements named after botanist named Robert Brown who observed pollen grains jiggling in water. Einstein derived a theory that predicted how far a particle will move over time, given such buffeting — a theory that was confirmed a few years later and which demonstrated that properties such as temperature and pressure were reflections of the average behaviour of huge numbers of molecules.
Relativity
Einstein's final two 1905 papers concerned relativity, the mind-bending idea about the ticking of clocks and the speed of light that most people associate with Einstein.
In June came the first paper, describing special relativity. In it, Einstein proposed a solution to a problem that had plagued physicists concerned with the spread of light waves. The prevailing belief was that light waves travelled in a fixed medium called the ether, analogous to how water waves travel in the medium of water and sound waves travel in the medium of the air.
Under that belief, the speed of light would vary according to how fast an observer was travelling compared with the ether. Physicists Albert Michelson and Edward Morley famously failed to find that difference in an experiment to measure changes in the speed of light as the Earth moved in different directions compared with this theoretical ether.
Einstein's June paper simply did away with the idea of the ether and said light moves at the same speed — 300 million metres per second — regardless of the speed of the observer. The same beam of light will appear to be a different colour to two observers moving at different speeds relative to the source of the light, but the beam will still be moving at the same speed compared with either of them.
