Theory of Relativity – The Basics

The Theory of Relativity, proposed by the Jewish physicist Albert Einstein (1879-1955) in the early part of the 20th century, is one of the most significant scientific advances of our time. Although the concept of relativity was not introduced by Einstein, his major contribution was the recognition that the speed of light in a vacuum is constant and an absolute physical boundary for motion. This does not have a major impact on a person’s day to day life since we travel at speeds much slower than light speed. For objects traveling near light speed, however, the theory of relativity states that objects will move slower and shorten in length from the point of view of an observer on Earth. Einstein also derived the famous equation, E = mc2, which reveals the equivalence of mass and energy. When Einstein applied his theory to gravitational fields, he derived the “curved space-time continuum” which depicts the dimensions of space and time as a two-dimensional surface where massive objects create valleys and dips in the surface. This aspect of relativity explained the phenomena of light bending around the sun, predicted black holes as well as the background radiation left from the Big Bang. For his work on relativity, the photoelectric effect and blackbody radiation, Einstein received the Nobel Prize in 1921.

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The perception and measurement of distant objects, especially those moving very quickly (near speed of light quickly) will be skewed by that object’s motion as well as our own motion; the object will appear shorter or longer than it actually is depending on how both us and it are moving. The reality doesn’t really change, just our perception and measurement of it.

Space and Time are enmeshed as one spacetime continuum. The addition of your motion through time and motion through space is equal to the speed of light. Therefore if you move faster through one, you must decrease motion through the other. Mass is simply “condensed” energy(E=mc^2). Gravity is curving of the spacetime continuum caused by mass and energy. Accelerated motion causes you to view spacetime in a curved manner equivalent to gravitational fields.

The Special Theory of Relativity was published in 1905 and deals with (frames of reference) objects separating with constant high velocities (close to the speed of light). In this first theory, Einstein stated that the speed of light was the absolute limit for all velocities and furthermore all observers, what ever their motion, must measure the speed of light to be exactly the same. Finally, only mass-less objects may attain the speed of light. However, massive objects contract in their direction of motion, their clocks slow down relative to observer’s clocks and they gain mass and thus inertia as they accelerate towards the speed of light.

Einstein’s theory of General Relativity (1915) is a cause and effect theory. The theory describes the fabric of space-time as a medium that may be distorted by the presence of mass and energy. From his earlier relativity theory, Einstein had related mass and energy in the famous equation: –

E = mcÂ²

From this concept, he described the curvature or distortion of space-time as due to the total sum of mass-energy present within the region of distorted space. It is the curvature of space-time we call gravity. Thus, our planet follows the ‘straightest’ path or path of least action whilst it orbits the mutually curved space around the Sun. An object falling to earth, under the pull of gravity, is following the curvature of space in the region around the surface of the Earth. Einstein described the relationship between space-time curvature and the mass-energy causing it, in the tensor field equation: –

G = 8Ï€T

Where ‘G’ is the Einstein tensor representing the ‘gravity’ or curvature of space-time and ‘T’ is the total energy tensor representing the mass-energy creating the gravitational curvature of the space-time.

The theory, accurately predicts many features of the solar system, such as the peculiar motion of Mercury’s perihelion, and may be used to account for the gravitational physics of black holes and neutron stars. The extreme high pressure physics at the core of a star may be described using this theory. General relativity also predicted the Big Bang origin of the universe, although Einstein refused to accept the conclusion. He later claimed that this erroneous conclusion was his biggest blunder. Overall, after ninety-three years, General Relativity is still the best theory of gravity that we have!

Special relativity – there is nothing at aboslute rest. The laws of physics are the same for all observers in constant motion (constant speed) therefore the speed of light is constant regardless of the relative motion of the person making the measurement. This leads to lack of simultaneity between moving observers – they will not agree on which events occur simultaneously. First geomteric view of space and time. Space and time are a four dimensional construct – space-time. But this space-time is “flat” – it is pseudo-Euclidean (Minkoskian really) in its geometry.

General relativity – the laws of physics must be the same for all observers regardless of their motion. THis now includes accelerations. Speed of light is still a constant value regardless of who make the measurement. Gravitational force is now a geometric curvature of space-time caused by mass and energy. Much more complicated geometry (Riemannian) in which line that are locally parallel can intersect in some other region of space-time. Since matter curves space-time, space-time dictates how the matter moves through it so description of dynamical systems is more complex. Background on which matter moves changes as matter moves on it!

Curvature of space-time leads to black holes – completely collapsed stars whose gravitation is so strong even light does not escape.

On a large scale, theory predicts expansion of te universe – leads to Big Bang theory.

Just some of the highlights