Albert Einstein was a theoretical physicist and philosopher who developed the general theory of relativity, which explains the phenomenon of gravity and a number of related principles. He won the Nobel Prize in Physics in 1921 for his services to theoretical physics, and he is universally recognized as one of the greatest minds in history. Einstein’s contributions to physics are so vast that it’s difficult to summarize them in just a few sentences. But here are some of his most significant ideas.

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## Theory of Relativity

The theory of relativity is the theory that all physical laws are the same for every observer, no matter what their state of motion. It is a theory in the sense that it makes falsifiable predictions and has withstood many tests over the last century. A short list of predictions includes the fact that spacetime is curved, that light has a particle nature, that time is relative, and that matter can create gravitational fields. The theory of relativity is different from classical mechanics in that it does not assume an absolute space or absolute time. Rather, it says that spacetime is curved. Essentially, the theory of relativity says that the speed of light is the same for all observers, that time is relative, and that matter can create gravitational fields.

## The Photoelectric Effect

This phenomenon in which electrons are emitted from metals when they are illuminated by light was first observed in 1887 by the physicist Heinrich Hertz. At the time, the prevailing theory was that the energy of electrons is produced by their acceleration in the electric field of a light source. This theory had been confirmed by many experiments, but it failed to account for Hertz’s observations. Hertz’s experiments showed that when light rays strike a metal, electrons are emitted with very little energy loss. The electrons emerge with no less energy than would be required to break the metal nucleus into positively charged nuclei. This surprising result was explained only in 1905 by Albert Einstein, who showed that the phenomena are due to electromagnetic induction. In this explanation, the energy of electrons is not derived from the acceleration of electrons by an electric field, but from the light source itself. The electrons are emitted because light energy is transformed into the kinetic energy of electrons. This process is called the photoelectric effect.

## E=mc2 - A Short Summary of the Equation

Einstein’s famous equation E=mc2 says that the energy of something (E) is equal to the mass of something (m) times the speed of light squared. This equation has proven to be a very useful tool in physics, and has been used to make accurate predictions about how things work. For example, it allows scientists to measure the mass of things really accurately. The equation was built on the theory of relativity, which describes how particles and electromagnetic waves (such as light) behave. The equation says that all forms of energy are related to mass, which is a fundamental property of matter.

## Derivation of E=mc2 from Theory of Relativity

In order to derive the equation, let’s first look at a thought experiment in which an observer is in a moving train that is passing another train. The observer in the first train sees the other train moving at a constant velocity, and all the objects in that train are moving at the same velocity. Since the objects are moving at a constant velocity, their mass cannot be changing. Therefore, the observer in the first train will measure the same mass of the objects in the second train. Now, let’s examine what happens when the observer in the first train measures the mass of a photon. The observer in the first train who sees the second train moving at a constant velocity will measure the photon to have the same mass. Now, let’s examine what happens when the observer in the first train measures the mass of a moving electron. The observer in the first train who sees the second train moving at a constant velocity will measure the moving electron to have a greater mass. Thus, the observer in the first train sees the moving electron to have a greater mass than the electron would have if it were at rest. Therefore, the moving electron has more “energy” than a non-moving electron. In other words, the moving electron has a higher mass than the stationary electron.

## Discarding the "C" in E=mc2

The speed of light is only a constant if the source of light is stationary or if the source and observer are moving at constant velocities relative to one another. In other words, the speed of light is the same for all observers in this case. However, when the source and observer are moving at non-constant velocities relative to one another, the speed of light will be different for different observers. This is why the “c” in E=mc2 can be discarded and not used in any of the subsequent derivations.