The Equivalence Principle

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A better version of this article is — THEORY OF RELATIVITY IN BRIEF which is also a featured article on our blog. So, we recommend you to read that article instead of this, and, also that article describes RELATIVITY completely.


Sir Isaac Newton was best known for his work on GRAVITY.

According to Newton’s theory of gravity, every object in the universe attracts every other object in the universe.

The strength of gravity according to Newton’s theory is given by:

F is force of gravitation, G is universal gravitational constant whose value is about 0.00000000006M and m are gravitational masses of the two bodies and R is the distance between them.

This equation is so accurate, that we still use it for practical purposes like flying a rocket from Earth and landing it on moon or any other planet. But, there are two problems with Newton’s theory –

  1. It fails for strong gravitational fields like a neutron star.
  2. The way Newton described gravity was not correct.

The orbits of all planets (except Mercury), predicted by general relativity (the currently accepted model of gravity) are same as those predicted by Newton’s theory of gravity because sun’s gravitational field beyond planet MERCURY is weak (Actually, strength of gravitational field decreases with increase in distance). So, if Newton’s description of gravity is WRONG, so what is its correct description? Today, the currently accepted model of gravity is Einstein’s general theory of relativity, which is based on the PRINCIPLE OF EQUIVALENCE. The equivalence principle states:

The effects in a gravitational field are same as the effects in accelerated bodies or objects

or in other words you can’t be 100% sure that if you are at rest in a gravitational field or you are accelerating uniformly. We can understand it by the given two cases:

  1. A man is in a rocket at rest on Earth, he drops a ball in the rocket.
  2. The man, now, is in the same rocket without any contact with a gravitational field and the rocket is accelerating in ‘empty space’ at acceleration of 9.8 m/s2 (m/s2 is unit in which acceleration is measured.) (9.8 m/s2 is the acceleration due to gravity on Earth.) Now, again he drops a ball.

Einstein thought similarly like the above cases to develop the principle of equivalence as a FOUNDATION of his 1915 general relativity. He will see the ball fall in the same way in both cases. So, we can conclude that the effects in the accelerated rocket are same as the effects in a gravitational field.



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