This glossary provides useful information on terms from the world of physics which play a key role in Einstein's work. Einstein's positions are included in this collection of terms and are explained in clear terms.

Speed which is always constant, irrespective of the reference system under which it is measured. Einstein made the absolute speed of light the basis for his Special Theory of Relativity. This made the idea of an "absolute rest" (rest regarded under every reference system) untenable.

A universal time measurement which is the same for all observers in the universe irrespective of their movement. In Newtonian Mechanics absolute time plays a decisive role – Einstein was the first person to demonstrate that time must also be understood relative to a point of reference.

Acceleration is the measurement of the change in speed: A freely moving body which experiences an effect of force shows an accelerated movement. Negative acceleration is also called deceleration (or slowing-down).

Precision clocks with a deviation of approximately one second in three to thirty million years. The atomic clocks made it possible to confirm by experiment the effects on time predicted by the Special Theory of Relativity. The first atomic clock was constructed in the USA in 1949 using technology developed by the American chemist and physicist Isidor Isaac Rabi. The first cesium-beam atomic clock was introduced into operation by the National Physical Laboratory in Teddington, England in 1955.

The scientific postulation, testing and discussion of principles of natural science, which does not have to be motivated by any practical application. Einstein's work was pure theoretical physics. His theories show best how basic research can become useful even if no practical application can be derived at first.

In 1827, the botanist Robert Brown observed an irregular motion of pollen under a microscope. At first, a comparison to the migration of spermatozoa seemed obvious. Then, however, he also noted the phenomenon in the smallest particles of inanimate matter. The cause of motion is therefore of a physical and not a biological nature and it is dependent on temperature.

Science of the movement of objects and the impact of forces on them. The basic laws of mechanics were developed by Galileo Galilei (1564-1642), Christiaan Huygens (1629-1695) and Isaac Newton (1643-1727). Until well into the 19th century, people believed that all physical phenomena originated from mechanical processes. Einstein's theory of relativity shows that many areas of physics have their own laws and that the mechanics in Newton's formulation are only an approximation for systems in which the velocity of motion is extremely small in comparison to the speed of light.

The lightest electrically charged stable elementary particle. Electron (Greek) means amber because electricity was first observed in amber. The electron and its properties in electric and magnetic fields are important for the development of the theory of relativity.

The term "energy" designates the ability of a physical system to perform work. In this process, the generated energy can be transferred to other bodies. In 1905, Einstein was the first to show that mass and energy are the same in essence.

Measure of the quantity of energy in a system which is not available for work. As work presupposes order, entropy is a measure of the disorder or randomness in a system. In his first article in 1900, Einstein already dealt with entropy and the scientific field of which it forms a part, i.e. thermodynamics. His main intention was to find an explanation at the level of atoms for certain observations of thermodynamics, for example that heat always flows towards a cold object, never vice versa.

Until the end of the 19th century, it was assumed that ether as a hypothetical substance penetrated all matter and space. The propagation of electro-magnetic waves was also explained with the aid of the light-ether hypothesis. Theorists such as James Clerk Maxwell (1831-1879) were firmly convinced that ether occurs as a substance which is in a state of absolute rest and penetrates all other objects invisibly. Einstein's Special Theory of Relativity refuted ether as a reference system and described it as "metaphysical speculation".

System developed by Stephen W. Hawking in 1974. It states that there are particles around a black hole, a part of which can escape into the infinite. An observer located far away from the black hole gets the impression that the black hole glows with radiation. Until his death, Einstein fought against applying the general theory of relativity to extreme gravitation situations around black holes. He said that there was no proof that the theory of relativity could be applied to this area.

Newton’s mechanics, an object remains at rest or moves at constant velocity unless it is influenced by an external force. This is called inertial system. The special theory of relativity no longer differentiates between systems at rest and systems in constant motion. Here, all systems are in constant motion, i.e. are moving at constant velocity.

Basic quantity of physics and the resistance of an object to a change in motion in response to an external force. In classical mechanics, mass is considered to be invariant, i.e. as independent of velocity. Only since Einstein's theory of relativity is the mass of an object no longer seen as constant.

One of the two central statements of the special theory of relativity: If speed of light is almost reached, a "conversion factor" is needed for Newton's laws of motion. This factor depends on the velocity of the mass. I.e. that mass and energy are the same in essence. The mass of an object is a measure of its energy content. Einstein was able to demonstrate that energy must equal mass times the speed of light squared (E = mc²).

Einstein did not experiment in a laboratory but primarily in his thoughts. This approach is called "mathematism" and was criticized by colleagues like Johannes Stark and Philipp Lenard: According to them, mathematism as well as trends of "westernization" and liberalization in general could be blamed for the "relativation" and thereby the blurring of physics, and, in this context, they considered Einstein was a representative of "Jewish thinking".

Conception according to which there must be an underlying theoretical framework behind individual observations which means that individual phenomena can be explained independent of their state. After 1933, Einstein was practically the only physicist who still believed in an objective reality – his colleagues believed in the quantum theory which describes individual phenomena without an underlying theoretical framework.

Smallest unit of electromagnetic radiation and thus also of light. In his 1905 publication on the photoelectric effect, Einstein introduced light as consisting of light quanta with some properties of particles. There was long resistance against the idea of light particles, mainly by the Danish physicist Niels Bohr. When experiments showed that light can be scattered by electrons, Einstein's hypothesis was considered to be proven right.

A cause is an objective standard quantity creating an effect. Classical mechanics only recognizes a sequence of states, with the preliminary being understood as the cause of the subsequent state. In quantum theory, however, the principle of cause and effect is revised. A quant can simultaneously have different properties or occur at different places at the same time. An individual event is not a sequence of cause and effect. Only statements on probabilities are possible. Einstein was never happy about the idea of probabilities in quantum theory. He believed that there ultimately was a principle of cause and effect, because "God doesn't play dice".

The principle of relativity has been used in classical mechanics for a very long time, particularly, for example, by Galileo Galilei and Christiaan Huygens, who calculated the rules concerning the collision of billiard balls on the basis of this principle in the 17th century. The principle postulates that the laws of nature are independent of the state of motion of the reference object. In the 19th century it became clear that the principle of relativity does not apply to Maxwell’s equations because electromagnetic waves travel at an absolutely constant velocity. Einstein's theory of relativity merges the principle of relativity from classical mechanics with Maxwell’s equations.

Physical theory which was developed between 1900 and 1930. It explains and quantifies effects of microscopic objects in a way which radically breaks with the classical physics of Newton and its ideal of a complete, deterministic description of all objects in space and time. Einstein was personally involved in the development of this theory.

The speed at which light and other electromagnetic waves travel. Ever since the 17th century, scientists knew from astronomic observations that light travels about 300,000 kilometres per second. Today, we know that the speed of light in a vacuum is exactly 299,792,458 metres per second. Einstein was the first to develop the absolutely constant speed of light into a theoretical concept. In this concept, the speed of light is constant even if the point from which the source of lighting is observed moves rapidly. This was one of the two fundamental statements of the special theory of relativity which Einstein formulated in 1905. As space and time are variable in the theory of relativity, the speed of light becomes the only absolute unit for space.

If systems of many microscopic particles (in general atoms, molecules, elementary particles) are studied, statements on the macroscopic behaviour of the system can be made by using statistical methods. Statistical mechanics no longer describes individual particles but the probability with which they are in a specific microscopic state. Until 1905, Einstein's ideas evolved strongly within statistical mechanics. He introduced new concepts and ideas and was able to use statistical mechanics for particle physics.

Cf. General theory of relativity

The general theory of relativity describes the gravitational force as geometric change of the structure of spacetime. It postulates that mass bends spacetime in its environment. An object to which no force is applied always moves along a straight line between two points in spacetime. The general theory of relativity therefore geometrizes the gravitational force; what we call gravitation is a bending of spacetime. Einstein formulated the general theory of relativity in the years 1907-1915.

Einstein's most famous theory, which he formulated in 1905. It states that the structure of time and space is not the same everywhere but varies according to the position of the observer. Two assumptions are essential: 1. Absolute space is replaced by space dependant on the motion of a reference object, absolute time is replaced by time relative to the motion. The laws of nature hold true for all reference objects, independent of the state of motion (principle of relativity). 2. The speed of light, however, is always measured as constant in all frames of reference.

Systematic theory on the relation between heat, work, energy and temperature. Thermodynamics is a sub-area of classical physics and evolved during the course of the 19th century. It turned out to be versatile and applicable to chemistry, biology and technology. Einstein dealt intensively with thermodynamics. In 1924, he described together with Satyendra Nath Bose how atoms behave close to absolute zero. The novel physical state is called "Bose-Einstein condensate".

The phenomenon that a moving clock loses time in relation to a stationary clock. The difference depends on the speed. If the speed is low in comparison to the speed of light, the effect is not noticeable. Einstein described time dilation for the first time in his work on the theory of relativity in 1905. Only in 1971 was the effect experimentally demonstrated with atomic clocks.

This theory tries to reconcile all fields of force and the entire matter of the universe in one formula, a "unified field" or "single field". In the 1920s, Einstein was the first to speak of it and subsequently tried in vain to formulate it.

Quantity whose numeric value does not change because a constant size is ascribed to it. Planck's quantum, the speed of light, the elementary charge and the mass of a single electron or proton are such physical constants.

In the strict sense of the word, a vacuum describes an empty space in which there are no particles. The idea of a vacuum is ultimately not understood in the considerations of the Quantum Field Theory and the General Theory of Relativity.

In 1905, Einstein stirred the world when he demonstrated that light has particle properties. In 1921 he was awarded the Nobel Prize for this work. But how can the particle properties be reconciled with the wave properties of light? Around 1920, first efforts were made to explain the phenomenon stating that particles also show some characteristics of waves. With wave mechanics, Erwin Schrödinger and Max Born produced the right physical interpretation for the double characteristics of particles.