Ch+26+-+Relativity

=__Relative to What?...__= Postulates for Special Theory of Relativity:
 * 1) The laws of physics are the same in all coordinate systems either at rest or moving at constant velocity with respect to one another.
 * 2) The speed of the light in a vacuum has the same value, 3e8 m/s, regardless of the velocity of the observer of the velocity of the source emitting the light.

This concept can be described using the wave understanding of light. Being a wave of electric and magnetic fields, Maxwell presented his famous equations which included the speed that this wave would travel. An observer of these fluctuating fields would calculate the change from his own perspective. Rather than imagining an observer moving, imagine being in that observers perspective. A little bit of wind will not affect Faraday's equations regarding the relationship between electric and magnetic fields. Therefore, any inertial observer should calculate the speed of light in a vacuum as c.

This "wind" idea was proven by the famous Michelson-Morley experiment, in which they showed that light does not have a medium. Although Einstein's relativity came later, this experiment initially showed the lack of the perfect observer.

What this means:

 * 1) Time Dilation
 * 2) Length Contraction
 * 3) Lack of Simultaneity
 * 4) Energy-Mass Equivalence

The key to understanding the following concepts is to not ask who is correct of the observers. What is shown is merely that one observer may calculate a different answer than another. They are both right in their own frame, so it is useless to ponder the absolute truth. In fact, a main result of these theories is precisely that: there is no absolute truth in measurement.

Time Dilation:
Imagine two mirrors facing each other, one above the other, and separated by a distance //d//. Knowing the speed of light, it should be easy to determine that the time it takes for a beam of light to travel from one mirror, to the other, and back to the first one will take 2d/c. The total distance travel is 2d, and its speed is c, so that time is a simple calculation. However, seeing that same "clock" pass by on a train will cause a difference. Now, if the light travels straight up a distance d, the second mirror will have moved, and the clock "won't" work. Therefore, the light must travel up at an angle, to reach the second mirror in its newly forward position. The new distance that the light must travel to reach the second mirror is naturally a function of the speed of the train. Avoiding the math and the tricky methods of wikispaces to defend itself against mathematical equations: Note that it is again doubled, because the same effect will occur as the light returns to its original mirror. Comparing this to the time calculated for one "tick" of the clock in its own rest frame: "Delta t proper," or "proper time," represents the time calculated in clocks reference frame. Proper is a term used to represent that the calculations were made in the same frame as that which was measured. Also, the new symbol gamma appears here, and will reappear often in the realm of special relativity. It is a unit-less constant based on relative velocity. The light clock is used as an example to use the constant speed of light as simple evidence that the apparent time is relative. However, this is true of any calculated time, and has been shown experimentally in the lifetime of particles from our upper atmosphere. Again, do not try to consider which one is "correct." the different times are merely how the different observers calculate the same event differently.

Mass-Energy Equivalence:
=__Postulates of Einstein:__= General Relativity:
 * 1) All laws of nature have the same form for observers in any frame of reference, whether accelerated or not
 * 2)  In the vicinity of any given point, a gravitational field is equivalent to an accelerated frame of reference without a gravitational field (Principle of Equivalence)