In 1904, Einstein got a permanent position at the Swiss Patent Office and a year later, he finished his doctorate by presenting a thesis entitled “A new determination of molecular dimensions”.

That same year, he wrote four seminal articles in which he explained Brownian motion, the photoelectric effect, developed special relativity, and mass-energy equivalence. These articles were sent to the journal "*Annalen der Physik*" and are generally known as the "*Annus Mirabilis*" (Latin: Extraordinary Year) articles.

**Brownian movement**

The first of his papers, entitled "The motion required by the molecular kinetic theory of heat of small particles suspended in a stationary liquid," was based on his studies of Brownian motion.

This motion had baffled the scientific community ever since it was discovered, and Einstein's explanation, making use of the statistics of the thermal motion of the individual atoms that make up the fluid, provided experimental evidence for the actual existence of atoms.

Until this article, atoms were considered a useful concept in physics and chemistry, but most scientists disagreed about their actual existence.

Likewise, the article also gave a strong boost to two highly controversial topics at the time, statistical mechanics and the kinetic theory of fluids.

**Photoelectric effect**

The second article, titled "A Heuristic View on the Production and Transformation of Light", Einstein proposed the idea of the "quantum" of light (now called a photon) and showed how this concept could be used to explain the photoelectric effect that only a full explanation could be given when quantum theory was more advanced.

The theory of light quanta was giving a hint of the wave-particle duality that physical systems can display simultaneously.

This article was one of the basic pillars of quantum mechanics.

**Special relativity**

Einstein's third article was entitled "The electrodynamics of bodies in motion" and introduced the theory of special relativity by studying the movement of bodies and electromagnetism in the absence of gravity.

Special relativity solved the problems opened by the Michelson-Morley experiment in which it had been shown that the electromagnetic waves that make up light moved in the absence of a medium. Therefore, the speed of light is constant and not relative to the movement, having surprising consequences since the concepts of absolute space and time are denied.

It is questioned that this famous publication is an original work of Einstein because it does not mention that already in 1894, George Fitzgerald had studied this question showing that the Michelson-Morley experiment could be explained if bodies contract in the direction of his move. In fact, some of the fundamental equations in Einstein's paper had been introduced in 1903 by Hendrik Lorentz, a Dutch physicist, giving Fitzgerald's conjecture a mathematical form.

Einstein's reasoning was based on two simple axioms: the first, introduced by Galileo centuries before, that the laws of physics must be invariant for all observers moving at constant speeds between them, and the second, that the velocity of the light is constant for any observer.

The theory is called the special theory of relativity to distinguish it from the general theory of relativity that was introduced by Einstein in 1915 and into which gravity is introduced.

**Mass energy equivalence**

The last article, titled “Does the inertia of a body depend on its energy content?” collected a deduction of the equation of relativity that relates mass and energy. In this equation the energy of a body at rest (E) is equal to its mass (m) multiplied by the speed of light (c) squared: E = m x c2