**Einstein’s theory of special relativity :**

*1) Physical laws are the same in all inertial reference systems.** *

*2) The speed of light in a vacuum is a universal constant.** *

*3) Measurement of time and space are dependent on two different events occurring at the same time. ** *

*4) Space and time are affected by motion.*

*1) Physical laws are the same in all inertial reference systems.*

*2) The speed of light in a vacuum is a universal constant.*

*3) Measurement of time and space are dependent on two different events occurring at the same time.*

*4) Space and time are affected by motion.*

### An inertial reference system is a system of coordinates (anywhere in space) in which a body with mass moves at a constant velocity as long as no outside force is acting on it. From this concept, other components of the special theory of relativity pursue.

#### Albert Einstein’s special theory of relativity supplies an accurate and logical description of events as they take place in different inertial frames of reference in the physical world, with the provision that the changes in space and time can be measured.

He developed the special theory of relativity to account for problems with the classical mechanistic system of physics. Many people had (and still do have) difﬁculty understanding his theory. In essence he is not describing the nature of matter or radiation, although he recognized their association. His theory describes the world or event, as it might look to two individuals in different frames of reference.

### For example, in classical Earth-bound physics, a person in a car going in one direction at 50 miles per hour (mph) meets a car approaching from the opposite direction going 100 mph. This describes how the speeds of the two cars are observed and judged (measured) independently by another person standing by the side of the road and not moving. But this does not apply to the drivers of two approaching cars. The driver in a car going 50 mph would perceive a carapproaching him from the opposite direction at 100 mph, as going at 150 mph.

### Conversely, a driver who is going just 50 mph and is passed by another car going 100 mph in the same direction will perceive the passing car as going just 50 mph. This is just common sense and can be proved with classical equations of adding and subtracting velocities, which is known as Galilean transformations. However, this is not how it works with electromagnetic radiation waves such as the velocity of light.

### Einstein’s theory states that the time between the two events (of the cars) is dependent on the motion of the cars. **The special theory states that there is no absolute time or space.**

#### According to experiments by Albert Michelson and Edward Morley, the speed of light is independent of the motion (velocity) of its source or the observer.

#### For instance, if both cars are traveling at astronomical speeds in space and one car is going twice as fast as the other car and both turned on a spotlight toward the approaching car, the light would travel the same speed in both (either) directions. One driver would not perceive the light as coming toward him at a greater speed than would the other driver because they would judge the combined speeds of 50 and 100 mph of the two cars on earth. In contrast to Earth-bound car drivers, Einstein stated that despite how fast you are going, the speed of light will be constant for all frames of reference. The drivers of the two ‘‘space’’ cars, regardless of how fast they are going, will be in two different frames of reference of both time and space, but the speed of light will remain constant.

### Thus, from their individual frames of reference (points of view), they will not be aware of ‘‘Earth-bound commonsense’’ differences in their speeds. No matter how fast you go, the speed of light will always be the same, even if you are speeding in the same direction as the light being propagated. The theory later included the concept of the three Euclidean coordinates of space—width (x), height, (y), and depth (z)—with the addition of the coordinate of time to arrive at a space-time continuum as developed by __Hermann Minkowski.__ Hermann Minkowski

__Hermann Minkowski.__

### There is much confusion about the word ‘‘relativity.’’ In science, it is used as something ‘‘relative’’ to something else that can be measured mathematically or statistically.

### Speciﬁcally, Einstein’s special theory of relativity is related to frames of reference as measured for the four coordinates of space and time .

**Einstein’s principle of gravity :**

*Gravity is the interaction of bodies equivalent to accelerating forces related to their inﬂuence on space-time. Gravity measurably affects the space-time** ***continuum.**

*Gravity is the interaction of bodies equivalent to accelerating forces related to their inﬂuence on space-time. Gravity measurably affects the space-time*

**continuum.**

*There are two related concepts of gravity: the Newtonian classical concept and the Einsteinian concept related to his theories of relativity. Newton’s law states that the gravitational attraction between two bodies is directly proportional to the product of the masses of the two bodies and inversely proportional to the square of the distance between them, as expressed in F = Gm1m2 / d2. Following is an example that relates acceleration to the force of gravity on Earth. If you are in a train or car that is traveling on a perfectly smooth surface and cannot see out the windows, you cannot tell if you are going backward, forward, or not moving at all if the vehicle is traveling at a uniform speed. But if the vehicle accelerates or decelerates, your senses will react as if gravity is affecting you. A person also becomes aware of G forces (simulated gravity) when a car or airplane rapidly accelerates. To sum this up, classical physics stated that all observers, regardless of their positions in the universe, moving or stationary, could arrive at the same measurement of space and time.*

Einstein’s theories of relativity negate this concept because the measurements of space and time are dependent on the observers’ relative motions regardless of their inertial frames of reference within space coordinates. Einstein combined the ideas of several other physicists and mathematicians that dealt with non-Euclidean geometry, the space-time continuum, and calculus to formulate his gravitational theory. In essence, Einstein’s concept of gravity affected space and time, as in his theory of general relativity. Even so, Einstein’s concept of gravity was not quite correct because he did not take into account the information developed by quantum theory for very small particles and their interactions, even though these subatomic particles are much too small (or evenmassless) to be affected by Earth’s gravity. His concept dealt with the macro (verylarge) aspects of the universe. As with all other laws of physics, the laws concerning gravity are not exact. There still is room for statements that more precisely interpret the properties of nature. For Einstein, the interactions of bodies are really the inﬂuence of these bodies (mass) on the geometry of space-time.

For many decades, scientists have tried to explain gravitational waves in relation to the theories of relativity or some other principle. How gravity acts on bodies (mass) can be described, but exactly what gravity is or why it is has not been discerned. Another hypothesis is based on the theoretical particle called the graviton, proposed by quantum theory. Gravitons behave as if they have a zero electrical charge and zero mass. Although assumed to be similar to photons, they do have momentum (energy). The concepts of gravity waves and gravitons are still under investigation. Recently, there have been several experiments designed to detect the graviton.