Einstein's worlds collide

Einstein's work on the nature of light helped lay the foundations for the development of lasers Keystone Archive

When Bern-based Albert Einstein produced a series of groundbreaking papers in 1905, little did he realise how much he would change accepted theories in Physics.

This content was published on April 19, 2005 minutes

While he is best remembered for his mad hair and his famous E=mc² equation on relativity, it was his work on the so-called Photoelectric Effect that would convince the Nobel committee to award him the 1921 Physics prize.

The Photoelectric Effect

The Photoelectric Effect occurs when light shines on a metal surface. Depending on the colour of the light, the surface may emit electrons. Blue light always causes electrons to be displaced, whereas red light has no effect.

One way to imagine the photoelectric effect is to think of drivers trying to knock a parked car out of its space. A large blue van has the energy to knock the parked car away, but a tiny red mini (or even several red minis) just can’t manage it.

In other words, however bright a red light gets, red photons will not be able to budge a single electron while a dim blue light will always shift some.

Einstein realised that the only way to explain the photoelectric effect was to say that light sometimes behaved like a wave, and sometimes like a stream of particles. Until then, light had been thought of only as a wave.

The particle theory suggested that light delivers its energy in small packets called photons.

Only if there is a strong enough packet – from a short enough wavelength or high enough frequency – does light have the energy to emit an electron from the surface of metal.

The paper, which confirmed a suggestion made five years earlier by the German physicist, Max Planck, would eventually win Einstein the 1921 Nobel Prize in Physics.

It was a vital contribution to quantum theory where light can be either a particle or a wave, depending on how it is measured.

The title of the paper: On a heuristic viewpoint concerning the production and transformation of light (March 1905).

Brownian Motion

In 1905 scientists were still debating whether atoms really existed. Einstein not only gave them proof but solved another long-standing mystery.

Back in 1827 biologist Robert Brown had peered through a microscope at pollen grains in water and noticed that the pollen jiggles about. Nobody really knew what caused this motion.

Einstein realised that molecules of water, which couldn’t be seen, were hitting the grains. Even more significant, he worked out how fast the water molecules were moving and how many were hitting a single pollen grain.

Brownian motion not only showed the existence of atoms but also laid statistical physics on its course. It was a completely new way of applying statistical mechanics to observable phenomena.

The paper borrowed material from Einstein’s thesis for Zurich University. This described a new method to work out the number and size of atoms and molecules in a given space. It still has widespread industrial applications today.

The Brownian motion paper was titled: On the motion of small particles suspended in liquids at rest required by the molecular-kinetic theory of heat (May 1905).

A new determination of molecular dimensions was the title of Einstein’s doctoral dissertation to Zurich University (April 1905).

swissinfo, Vincent Landon

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