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Principle of Equivalence Led to the Formulation of Einstein’s General Theory of Relativity
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The mass of a body, when subjected to a gravitational attraction but no acceleration (i.e., its gravitational mass) is the same when it is subjected to an acceleration but no gravitational attraction (i.e., its inertial mass).
This gave Einstein the idea that a gravitational field can be imitated by a field of acceleration and this, ultimately, led to the formulation of his general theory of relativity, wherein if showed that a non-accelerating or inertial frame of reference in which there is a gravitational field is physically equivalent to a reference frame accelerating uniformly with reference to the inertial frame but in which there is no gravitational field. This means, in other words, that experiments carried out in the two frames, under the same conditions, will yield identical results. This is called the Principle of Equivalence.
Why do the earth and other huge celestial bodies possess gravity?
The Attraction between two bodies having mass has been defined as Gravitation. And as the word Gravitation suggests, it works on Point of Gravity of the bodies. From, as Newton Explained this attraction force,
F=GMm/r2
it is clear that this force depends upon the mass of two bodies (M &m, say) and the shortest distance (r) between bodies.
But the question, which influence mind that WHY do they perform this kind of attraction? As far I know, this is due to a ghost particle, which works on bodies, one to other and vice versa. This ghost particle has been named as Graviton. And the attraction has been explained on the basis of exchange of GRAVITONS similar to nuclear forces on one hand and on the curvature of the gravitational field on the other. We should remember that the concept of gravitons is just a fact. May be true or untrue.
As my best knowledge tells me, Graviton is a ghost (hypothetical) elementary particle that mediates the force of gravitation in the framework of quantum field theory.
If the concept of Graviton is true, then according to Quantum field theory:
¤The graviton must be massless because the gravitational force has unlimited range.
¤The graviton must have a spin of 2. This is because the source of gravitation is the stress-energy tensor , a second-rank tensor, compared to electromagnetism, the source of which is the four- current , a first-rank tensor.
Additionally, it can be shown that any massless spin-2 field would be indistinguishable from gravitation, because a massless spin-2 field must couple to ( interact with) the stress-energy tensor in the same way that the gravitational field does.
This suggests that if a massless spin-2 particle is discovered, it must be the graviton, so that the only experimental verification needed for the graviton may simply be the discovery of a massless spin-2 particle.
Gravitons are postulated because of the great success of quantum field theory (in particular, the Standard Model).
The hypothesis is that the gravitational interaction is likewise mediated by a – yet undiscovered – elementary particle, dubbed the graviton , instead of being described in terms of curved spacetime as in general relativity . In the classical limit , both approaches give identical results, which are required to conform to Newton’s law of gravitation.
