 # Einstein's Theory of Special Relativity

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In 1905, Einstein's Theory of Special Relativity was proposed. The reason that it is so "special" is because it was part of the more complex and extensive Theory of General Relativity, which was published in 1915. His theory reshaped the world of physics when it contradicted all previous laws of motion erected by Galileo and Newton. By mathematically manipulating these previous laws of motion, physicists in the nineteenth century were able to explain such phenomena as the flow of the ocean, the orbits of planets around the sun, the fall of rocks, and the random behavior of molecules in gases. At first, Einstein faced great opposition when he came up with his radical new theory because the previous laws of motion proposed by Galileo and expanded upon by Newton had remained valid for over two hundred years. However, it wouldn't be long before the "cement" in the foundation of Newtonian and Galilean physics would begin to crumble.

Galileo had determined in 1608 that merely addition and subtraction could calculate relative speeds. Suppose that an observer stands on the side of the highway, and they watch two cars approach each other at 30 and 40 miles per hour. If they were to ask the question, "how fast is the 40 mile per hour car moving relative to the 30 mile per hour car?" They could solve the problem easily by adding the two speeds of the cars, which would equal 70 miles per hour. This means that the 40 mile per hour car sees the 30 mile per hour car advance at a speed of 70 miles per hour and vice versa.

At the core of Newtonian physics was the fact that space and time were absolute. Newton's absolute space was the space of everyday experience with its three dimensions: east-west, north-south, and up-down. This space gives us our sense of length, breadth, and height; according to Newton. We all, regardless of our motion, will agree on the length, breadth, and height of an object, so long as we make sufficiently accurate measurements. Newton's absolute time was the time that flows inexorably forward as we age. It is a time whose flow is experienced in common by all humanity.

The maximum speeds of birds in nature are regulated by air. No matter what direction a bird flies, it always has the same maximum speed. Newton had proposed something similar for light, which he referred to as the aether. He theorized that it was omnipresent and that it regulated the speed of light in any direction. Furthermore, since the aether was at rest in absolute space (according to Newtonian physics), anybody who is stationary will measure the same light speed in all directions, while anybody in motion will measure different light speeds.

Newton and Galileo would have assumed that like the speeds of the two cars in the previous example, the velocity of light could be calculated in the same fashion. For example: If a car is moving at a speed of 25 meters per second with its headlights on, what is the speed of the light emitted by the headlights? Newton and Galileo would have thought, "25 meters per second for the car plus 299,792,458 meters per second for the speed of light equals 299,792,483 meters per second for the speed of the light emitted by the headlights of the car."

This method of thinking would have been acceptable up until 1881. At this time, an experiment took place that would change physics forever. Albert Michelson wanted to test Newton's idea of variable speeds of light due to the existence of the aether. He knew that since the Earth moves in absolute space, that the speed of light should be measured differently in January than six months later in June when it is moving in an opposite direction in its orbit. This is because the speed of light and Earth would be additive. The difference, according to Newton and Galileo, would only be about 1 part in 10,000 since the earth moves slowly relative to the speed of light.

Michelson set up an extremely accurate test using a special device that he developed called an interferometer, which measured very small distances using the wave properties of light. After he performed a battery of tests, he was astonished to find that the speed of light is the same in all directions and seasons. This means that in the previous example, the speed of light would still be 299,792,458 meters per second, regardless of the speed and direction of the car. Therefore, Galileo's relative motion theories, which had been accepted for over two hundred had been definitively proven not to apply to light.

Einstein soon heard of the results of the Michelson experiment. Although a lot of physicists at the time were skeptical about the validity of the results due to their great ramifications, Einstein took the data from the experiment at face value. He concluded that Newtonian physics was flawed. Although Einstein forced to reject Newtonian Physics, he also came to two revolutionary principles that forever changed the world of physics. The principle of relativity states that the laws of physics are the same for observers in all uniformly moving reference frames. In Einstein's Theory of Special Relativity, a reference frame is simply the platform or framework from which one makes observations. This first postulate was already widely accepted by the physics world. The only difference between it and what Newton had proposed is that now Einstein is referring to reference frames instead of motion in general. For instance, if an observer was to study the motion of billiard balls in a rocket that is passing them, they would have to take into account the speed of the rocket. The principle of the absoluteness of the speed of light states that whatever the nature of space and time are, they must be constituted as to make them the same in all directions, and absolutely independent of the motion of the person who is measuring it. This theory is in agreement with the Michelson experiment, and no matter how accurate measuring devices may become in the future, the speed of light will always be the same. The second postulate is simply an exception to the first because it says that space and time must exist in a way that the speed of light is the same in all directions.

Einstein began writing his paper on Special Relativity by thinking what his two postulates implicated about motion. He knew that motion is described by using acceleration and speed. Speed is an example of how much distance is covered per unit time; therefore, in order to obtain speed, one needs distance and time. The ability to quantify motion with speed and time is rooted in the measurement of space and time, which Theory of Special

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