MrIn 1905, Einsteins 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 wouldnt 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. Newtons 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. Newtons 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 Newtons 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, Galileos 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 Einsteins 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 Relativity in intimately connected with.
The results of Einsteins postulates are described in terms of three effects, called the relativity of simultaneity, time dilation, and length contraction. Although these effects are all related, they are easiest to construe individually.
In order to explain simultaneity, once again, a hypothetical situation will be used. Adam is standing on a platform of a railroad station while a train passes. It just so happens that two lightning bolts strike the train; one strikes the front and one strikes the rear. The lightning bolts leave burn marks on both ends of the train and analogous marks on the platform where Adam is standing. At this point, he makes an assessment of the situation. He realizes that the lightning bolts both reached him at the same instant in time, and that the distances between him and the burn marks on the platform are the same. Thus, Adam is equidistant from both burn marks on the platform. From this, he can conclude the time that it took the light to travel from each burn mark on the platform is the same.
Now, lets look at this same series of events from an observer inside the car. This person will be Brett, who is positioned precisely at the center of the train. Because Brett is moving with the train, he is moving towards the light signal that is traveling to him from the front of the train, and away from the light signal that is traveling to him from the rear of the train. Because of this, he first sees the light signal from the front of the train, then the light signal from the rear of the train. Brett now notices that he is an equal distance from the two burn marks on the train. From this, he concludes that he is equidistant from the two burn marks, and the speed of light experienced by him is the same as Adam experienced. Because the light from the front of the train reached Brett first, and light travels at the same speed all the time; Brett concludes that the lightning bolt that struck the front of the train occurred before the lightning bolt that struck the rear of the train.
Einsteins answer to this situation is that both Adam and Brett are correct in their observations. This is due to Einsteins postulate of the relativity of simultaneity. This postulate states that the same event is not necessarily experienced the same way in two different moving reference frames.
Simultaneity is also connected with two other space and time effects called time dilation and length contraction. Time dilation means that an observer will see a clock on a rocket recording time slower than if it were stationary. Suppose that the Earth and rocket clocks are synchronized as the rocket whizzes around Earth at 2:00 P.M. An hour later, when the clock on Earth reads 3:00 P.M.; the clock on the rocket would read less than 3:00 P.M., depending on how fast the rocket was traveling. The faster the rate of travel, the more time is slowed down. Also, if someone in the rocket were to read the Earth clock when the clock on the rocket read 3:00 P.M., they would see it read slower than 3:00 P.M. Thus, this slowing in time works both ways. Each observer in a different frame of reference traveling at a different speed will see the others clock slowed down.
Finally, length contraction is apparent whenever an object is in motion. For instance, an observer on the Earth would measure the length of the rocket to be shorter when it is moving at its high speed as compared to its length at rest.
Simultaneity, time intervals, and length must all be relative. Two events that are observed to be simultaneous in one reference frame will not be simultaneous in any other reference frame that is moving with respect to the first frame.
If Newtonian physics is so flawed, then why is it still used today? The answer is very simple. When traveling at speeds that are far from the speed of light i.e. speeds typical of human experience, effects such as time dilation and length contraction are so minute, its not practical to use Einsteins more complex equations of Special Relativity in place of Newtons for these motions. The fastest a human being has ever gone in a spacecraft in space is nowhere remotely near the awesome speed of light. Perhaps in the future, when spacecraft capable of traveling just under speed of light is developed, will we encounter this phenomenon in a substantial quantity.