How did the principle of equivalence get rid of the idea of absolute time?

Special Relativity persists to plague millions of high-school students with its convoluted formulas that are fit to dazzle extraterrestrials. However, the Special Relativity successfully enlightened mankind with the prospect that the laws of nature hold true for all bodies moving freely. For instance, if I were to measure the speed of light while riding a motorbike, it would be the same as the speed I would measure while lying flat on my back on a sunny day. However, one question remained unanswered and that concerned objects that were not moving freely. What happened to them? The question paved way for the ‘exotic’ Theory of Relativity and the edifice of scientific knowledge standing on the pillars of the ‘Principle of Equivalence’.

What is this principle of equivalence? It states that when in an empty patch of space, it is impossible to conclude whether you happen to be at rest in a gravitational field or whether you are constantly accelerating in empty space.  For example, if you happen to be ebulliently floating through space in an empty box and suddenly smack your face against one side of the box because you feel its pull, it is impossible for one to determine whether the box happens to be at rest in a gravitational field or whether you are constantly accelerating with the box in the direction where you feel the pull.  This principle of equivalence is true only if inertial mass and gravitational mass are the same. Therefore, the mass in Newton’s second law is the same as the mass in Newton’s law of gravity. If this equivalence was not true, then under the influence of gravity, some objects would accelerate faster than others. But this was proven wrong by Galileo Galilei long back. Surely, Einstein did not aim to disappoint another great scientist after Newton!

How do we assume that time is affected by gravity? Let me borrow an example from Stephen Hawking’s book Briefer History of Time – imagine that there is a rocket ship which is a light-second long. Suppose there are two observers – one at the ceiling and other at the floor. Both have identical clocks and the ceiling observer is supposed to flash light at intervals of one second. If the ceiling observer sends signals one second apart, the floor observer would receive them in the same time gap. However, gravity would tangle things up here. The Principle of Equivalence allows the effect of uniform acceleration to be used here instead of the effect of gravity. Let’s assume our ship is accelerating upwards. The signal will now have less distance to traverse as the ship accelerates upwards. Now, the principle of equivalence makes it clear that the same thing would happen if the rocket ship was at rest in gravitational field. Hence, General Relativity tells us that time runs differently for observers at different heights in a gravitational field. Even our biological clocks would run differently. The twin paradox happens to be based on this. Consider a pair of twins where one twin is sent to live at the top of mountain and the other remains at sea level. The first twin will age faster. Thus, using Principle of Equivalence and the Theory of Relativity, Einstein got rid of absolute time. In theory of relativity, it is safe to say that each individual has a personal measure of time!

What are the other predictions of the Theory of Relativity? Is gravity the same as Newtonian gravity in Theory of Relativity? How is Theory of Relativity different from Newtonian mechanism? To know more about Special relativity, look out for the next articles. So keep reading!!! And continue to speculate, innovate till you constipate!


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