MPS - Special Theory of Relativity
Special Theory of Relativity
Isaac Newton's explanations of the motion of objects held sway from the 1600's until the start of the 1900's. While Newton's mechanics could accurately describe and predict the motion of most objects, there were errors when dealing with objects moving at high speed. In 1905, Albert Einstein published several papers, one of which outlined his theory of relativity. This theory corrected Newtonian mechanics for objects moving at high speeds while simultaneously opening the eyes of physicists to a world much more fascinating than previously considered.
Einstein's theory deals with inertial reference frames, which are frames of reference where Newton's 1st law of motion, the law of inertia, is valid. According to Newton's mechanics, a certain relativity principle already existed. To illustrate, imagine you are riding a train that is traveling at a constant 70 miles per hour. If you walk forward with a speed of 5mph, relative to the train, you have a speed of 5 mph in your reference frame. However, to a person watching from outside the train, you are moving at your 5mph, plus the 70 mph the train is traveling forward. Even though the speed measurements are different, both you and the observer are correctly describing your speed based on your respective reference frames. Einstein took this concept a bit further. Imagine the same situation, but now you are carrying a flashlight. If you shine the flashlight forward, you would measure the speed of the light to be roughly the speed of light in a vacuum, 3x108m/s. Would the observer outside the train now see the light traveling at 3x108m/s plus the 70 mph of the train? The answer, Einstein tells us, is no. Even from a different reference frame, the speed of light will still be measured as 3x108m/s.
This leads us to the two postulates of the special theory of relativity:
- The laws of physics have the same form in all inertial reference frames.
- Light propagates through empty space with a definite speed c independent of the speed of the source or observer.
This idea that light had a definite speed was different. Even more interesting is that this speed of light acts like a universal speed limit. Nothing can travel faster than light. This may not seem that interesting on the surface, but it has some interesting consequences. One of these consequences is that time itself is relative. Different reference frames can experience time differently.
It is recommended that you read more about these consequences by following the resource links in the sidebar.
For these effects to be noticeable in every day life the object in question would have to be traveling at relativistic speeds, or speeds higher than about 0.10c. Newton's mechanics explain the motion of everyday objects well because most things we experience daily don't move at relativistic speeds. In this regard, special relativity is considered an addendum to Newton's laws of motion rather than a replacement.
Rest Mass Energy
An important consequence of the special theory of relativity is the relationship between mass and energy. Mathematically the relationship takes the form of one of the most famous of physics equations:
Where E represents the rest mass energy, m is the rest mass, and c is the speed of light in a vacuum. As the names suggest, rest mass and rest mass energy are associated with an object at rest (as opposed to moving at relativistic speeds).
The equation is simple enough, but what it implies was a monumental idea for the day. There is a connection between mass and energy. More importantly, through various processes, mass can be converted into energy and energy can be converted into mass.
Rest Mass Energy Practice
The fusion of hydrogen nuclei into helium nuclei is responsible for the heat and light produced by the Sun. During the several steps of this fusion, four hydrogen nuclei (protons) create a helium nucleus (two protons and two neutrons). How much energy is released in this fusion reaction?
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