Put your hand on a hot stove for a minute, and it seems like an hour.
· Sit with a pretty girl for an hour, and it seems like a minute.
· THAT's relativity.
"
A. Einstein

Twenty-six year old German-born Albert Einstein, while holding down a full time job as a patent clerk (third class) in Switzerland in 1905, set the physics world on its ear with the publication of no fewer than five breakthrough papers in physics. While he won the Nobel prize in 1921 for his explanation of the photoelectric effect, perhaps his most famous (and most controversial) work dealt with special relativity. The "special" in special relativity deals exclusively with those cases when objects move with uniform speed in straight lines.
He explained the nature of accelerated objects and gravity in his general theory published in 1915. The ten-year lapse in publication was probably due in part to: 1) the distraction caused by the acclaim he received from his early work; 2) the fact that he moved back to Germany; 3) the outbreak of WWI; and 4) some serious domestic discord which led to estrangement and divorce from Mrs. Einstein.

The interested student will be well-rewarded for reading one of the several biographies available describing the life and thoughts of this scientist-turned-icon.

 THE TRAILER TRUCK Consider a trailer truck with a trailer having transparent sides. Our assistant is ridiing in the trailer and holds a ball over an X marked on the trailer floor. On cue, she will drop a ball exactly once from rest. Describe the path of the ball as seen by three different observers. Observer one is in the trailer with our assistant. He says that the ball has no horizontal velocity so that the path is a straight line ceiling to floor Observer two is stationary at the roadside watching the trailer proceed past at a constant rate of speed. Observer two reasons that the ball has a forward velocity from the trailer. Hence the path is a parabola arcing forward. Observer three meanwhile is passing the trailer truck while riding in a convertible as the ball is dropped. Because she ascribes all of the motion to the trailer truck with herself at rest, she reasons that the truck is backing up and has a rearward velocity; hence, the path of the ball is arcing toward the rear of the truck. Which is the three observers is correct in describing this event? The answer of course is that all three of them are correct. That each description is different is due to the fact that each observer saw the events from a different point of view. This will be a very compelling argument in special relativity. Observers will differ on events because they are seeing things from a different point of view

The consequences of the special theory alter our concept of space and time by predicting that

• moving clocks run slow
• moving lengths are contracted in a direction parallel to their motion
• two events that occur simultaneously to one observer do not occur simultaneously to a second with respect to whom the events are moving.
• relative velocities of moving objects do not add as expected
• moving objects experience an increase in mass

•  Common sense is the collection of prejudices acquired by age 18. A.Einstein

The special theory of relativity is predicated on the validity of two postulates. If experimental evidence finds a discrepancy in either of these principles, then the conclusions derived from these principles will have to be changed. Despite all the challenges that have been mounted over the last 100 years, the two postulates stand as published in 1905.

POSTULATE ONE - THE PRINCIPLE OF RELATIVITY

• One cannot tell by experiment whether one is at rest or moving uniformly.
• The laws of nature are the same in a laboratory at rest as they are in any moving laboratory.
• If two experiments are done under identical conditions except that one is done in a laboratory at rest and the other in a uniformly moving laboratory, the two experiments will lead to exactly the same conclusions.
• Anybody moving uniformly with respect to somebody at rest is entitled to consider him/herself at rest and the other person to be moving uniformly.
• A system moving uniformly with respect to an inertial system is also an inertial system.

POSTULATE TWO - THE PRINCIPLE OF THE CONSTANCY OF THE VELOCITY OF LIGHT

http://www.pay.ntnu.edu.tw/~hwang/relativity/relativity.html

http://monet.physik.unibas.ch/~schatzer/space-time.html

http://www.physics.brown.edu/physics/userpages/students/Matthew_Hirsch/Time/relbomb.html