“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.” -Albert Einstein
It should be noted that the above quote is NOT the principle of relativity.
Albert Einstein, working quietly at the patent office in Berne, without any contact with the professional physics community, managed to come up with his famous Special Theory of Relativity (STR). His 1905 paper makes two postulates. Namely,
-The speed of light is the same for all observers no matter how they move.
-The laws of physics are the same in any inertial frame of reference
The first statement defies our everyday common sense. How can a person on a moving train measure light to be traveling at the same speed as someone at rest? However, this fundamental property of light has now been confirmed experimentally by many precise experiments. In other words, all observers, regardless of their state of motion, will measure the same speed for light. This strange and counter-intuitive behavior lies at the very heart of relativity theory.
The second statement is based on common, everyday experiences. A ball thrown up in the air on a train moving at constant velocity would fall straight back down, (instead of being swept back). All physical phenomena occurring on that train would behave just as they would if they were at rest on the ground. This concept is not new however; Italian physicist and philosopher Galileo Galileii came up with this concept in 1632 showing that there is no absolute motion or absolute rest. Thus Einstein did not create the principle of relativity; rather his main contribution was to reinterpret it as being fundamental than either Newton’s laws or the conceived notion of absolute time, and to insightfully explore its implications regarding the nature of time, light and space.
The startling implication regarding the STR is that perceptions of length and time need not be the same for all observers and depends on the relative movement between the two observers. Both length and time are not absolute as Newton predicted. The closer you approach the speed of light, the more the passage of time seems to stretch or “dilate”.
For the first time, a legitimate form of time travel has been discovered. The faster you travel, the slower is your passage of time relative to the rest of the universe when your journey ends. You would have traveled to the future by virtue of aging slower than the world outside. The faster you travel, the more pronounced this effect becomes.
If you want to see what Earth will be like 1,000 years from now, all you have to do is get into a spaceship, travel to a nearby star slightly less than 500 light years away and return, traveling at 99.995% the speed of light. The Earth will be 1,000 years older but you will have only have aged 10 years. The particle accelerator at CERN, a laboratory near Geneva, has been able to propel electrons up to speeds of 99.999999999% the speed of light, so such speeds are possible. However, this is only a one-way ticket into the future. Once you get there, you must remain there, as Einstein’s theory offered no way back.