Einstein's Theory of Relativity

Introduction

This chapter looks at Einstein's special and general theories of relativity. It also explains related terms like 'time-space', 'gravity' and 'gravitation'. This chapter also compares Newton's and Einstein's theories of gravitation.

Who is Einstein?

Albert Einstein was born in Germany in 1879. His family moved around Europe. When young Einstein finished school he was trained as a physics and mathematics teacher at the Swiss Federal Polytechnic School in Zurich. After graduation he could not find a job as a teacher so he accepted a position as a technical assistant in the Swiss Patent Office. In 1905 he was awarded a doctorate. Albert Einstein worked and taught in different scientific institutions in Europe and the United States until his death in 1955. His works and ideas have been influencing modern physics and astronomy ever since. Einstein is often called the greatest scientist of the twentieth century. See image 1.

Einstein's Theory of Relativity

Einstein's famous theory of relativity was developed as an attempt to unite the laws of mechanics with the laws of electromagnetic fields.

Special theory of relativity

Einstein's special theory of relativity is a description of interactions between moving objects. It is based on the concepts proposed by Galileo a few hundred years earlier. Galileo stated that all motion was relative and that there was no absolute and well-defined state of rest. A person seating in a moving car, for example, might consider himself motionless, but to an observer on the street he will appear to be moving
Einstein has stated that distance and time are not absolute parameters and depend on the observer. This statement led to the famous formula that connects the mass of an object and its potential energy:

E = mc²

The formula is based on the fact that light moves at a uniform speed, c = 300,000 km/s, in all frames of reference. According to Einstein c is an absolute speed limit in the universe. The theory was called 'special' because it applies only when objects are moving at constant speeds in straight lines. This theory does not cover accelerated motions.

General theory of relativity

Einstein's general theory is the generalised version of the special theory of relativity. This theory is also called the equivalence principle. It states that gravity pulling in one direction is equivalent to acceleration in the opposite direction, and both are equivalent to the curvature of space-time.

F = ma.

Space-time

In his general theory of relativity, Einstein views gravity not as a force but as a curvature of the fabric of space in the presence of a massive object like a black hole (see Topic 2, Chapter 1 of this unit). According to Einstein, the fabric of space is a four-dimensional space representing the universe. It consists of the commonly accepted three space dimensions plus the time dimension. This four-dimensional fabric of space is also referred to as space-time. The basic elements of space-time are events. In any given space-time, an event is a unique position at a unique time. The example of an event on a universal scale is a comet crashing into another celestial body. Einstein also stated that objects with large masses can warp time by speeding it up or slowing it down. See image 2.

How many dimensions are needed to describe the universe is still an open question. According to some modern theories, the universe can only be adequately described by using a system with many more dimensions than were originally proposed by Einstein.

Einstein versus Newton

Newton described gravity, but he did not know how it worked. He stated that gravity is caused by some constantly working element, but Newton was not sure whether this element is material or immaterial. A few hundred years later, after extensive studying of Newton's work, Einstein described gravity as a result of the interaction between the mass of an object and its surrounding space-time. According to Einstein, the mass of an object can warp the space-time around it. So, gravity is just a natural outcome of a mass's existence. According to most modern scientists, Einstein's general relativity theory explains everything Newton's theory did as well as fill in some 'gaps' in Newton's work. Einstein's theory has been extended and re-adjusted Newton's gravitation theory for a different level - the level of stars and planets.

According to Einstein, any object can generate gravitational waves (space 'warpage') but only extremely large ones, like stars or black holes, produce warps of space that are big enough to measure.

Even though Einstein predicted the gravitational waves, he doubted that scientists would ever detect them. It was not until the mid 1970s that astronomers came across some evidence of these waves while researching superdense and superheavy collapsed stars. The orbits of these stars were affecting each other at a rate predicted by Einstein in his theory of general relativity.

Today, scientists are searching for gravitational waves coming from large objects from space, like stars and galaxies. For that they use one of the most precise scientific instruments ever made, called LIGO, which stands for the Laser Interferometer Gravitational-wave Observatory. See image 3.

Like all scientific theories, Einstein's theory of relativity has been, and will probably remain, subject to constant research and re-evaluation. In most scientific institutions of the world, however, the current theory of gravitation is Einstein's theory of relativity. The theories of Einstein and Newton differ only if the velocity of an object can be compared to the speed of light or the gravitational fields are much larger than the Earth's. Under most conditions Newton's three laws and his theory of gravitation are correct.