Ganesh P Paudel
“If you can’t explain it simply, you don’t understand it well enough.”
That was a quote by an eminent theoretical physicist Albert Einstein. Even though he was a celebrated scientist, his works for many of us are too intimidating to be brought into our normal discussion. So, in this write-up, I will try to explain his two theories of relativity over and above their ramifications for the readers who have had, at least, a rudimentary understanding of school level science.
Not only in the scientific firmament but also among common people Einstein is known as one of the greatest scientists of the millennium. His equation E = mc^2 is what he is remembered by common people. This question, however, is a result of his special theory of relativity.
First and foremost, let’s try to delve a little deeper into the fact why his theories of relativity are called so. It is because of the simple reason that his theory is based on the fact that speed, space, time and position are all relative in nature, whereas the speed of light is not. That is to say, the speed of light is constant for all observers irrespective of their position of rest or uniform motion. That’s the second postulate of STR.
Before shedding more light on the second postulate, it is vital to know what a “frame of reference” is. Oxford Advanced Learner’s Dictionary states its literal meaning as “a particular set of beliefs, ideas or experiences in relation to which something is measured or judged. “In order to observe and judge an event we use frames of reference. For example, while you are seated on a chair you are said to be at rest from the earth’s frame of reference but when you are viewed from space you are in motion along with the rotation of earth. Likewise, if you and your friend are seated face to face on a train that is moving, neither speeding up nor slowing down i.e. with a constant speed in a straight line, then you and your friend are both said to be at rest from the train’s frame of reference. Such frame of reference which is neither accelerating nor decelerating nor changing its direction is known as inertial frame of reference. The reference frame which is not inertial are non-inertial frames of reference. In a nutshell, inertial frames of reference are the observation points which themselves are either at rest or in a state of constant speed in a fixed direction. Non-inertial frames of reference are the observation points whose speed themselves are either going up or slowing down in whatever direction.
Having understood the meaning of frames of reference, and its two types, inertial and non-inertial, we are now in a position to understand the first postulate of Special Theory of Relativity which states that the laws of physics hold true for all inertial frames of reference. This means: what experiment we perform in our lab on earth will be as much valid as when it is performed in a spaceship moving with a constant velocity. Therein lies the beauty of relativity theory.
Einstein came up with his general theory of relativity (GTR) ten years after his special theory of relativity was published in 1905. His general theory of relativity tries to cover the aspect which his special theory couldn’t. That’s gravity. Yes, when gravity comes into play, GTR starts working, so to speak. GTR is called so because it applies in the presence of gravity which is omnipresent (thus the word “general”) in the universe. It entails a new term space-time because space and time are interwoven. Time is not a rigid parameter as envisioned by Newton. It flows at different rate for different observers. GTR generalizes STR by considering gravity and accelerating frame of reference. It rules out Newtonian concept of gravity as a force. It tells that a mass warps space-time fabric around it and this causes gravity. American theoretical physicist John Wheeler was quoted “Space time tells matter how to move; matter tells space time how to curve.” Indeed! Bending of light around a massive object is the testimonial of GTR working.
Time dilation, twin paradox and the length contractions are the ramifications of special theory of relativity. Time slows down for the observers moving closer to the speed of light. This gives birth to the idea of “twin paradox”, out of two, one twin is sent to an hour-long space journey with the speed closer to the speed of light. By the time the twin returns earth after an hour long space journey, the twin staying on earth will have become very old. It is because time passes very slowly for the twin on space journey compared to the twin staying on earth. This situation is referred as twin paradox. The length of objects moving with the speed closer to the speed of light appear shorter in the direction of motion
To sum up, STR and GTR changed the way looked into our universe. The former applies to quantum level as well whereas the latter applies to huge bodies. Had Einstein not given importance to his imagination over the knowledge established by his predecessors, we wouldn’t have had his theories of relativity today. By the same token, it doesn’t mean that the humanity is supposed to rest at its laurels over the achievements made in science thus far. Sky is the limit for human imagination.
(Author is a schoolteacher and a science enthusiast)
