Gravity

[indranilhalder]

There are four basic interactions in nature through which particles interact. They are termed strong, weak, electromagnetic and gravitational. Out of these, strong and weak interactions are very ‘short-range’ interactions. These are responsible for building of nucleus and the decay of nuclei and other particles respectively.
The gravitational and electrostatic interactions are long range interactions and are responsible for most of the phenomena we observe in nature.
The word “GRAVITY” has came form the Latin word “GRAVITAS” which means ‘weight’.
Gravity or gravitation is a natural phenomenon by which all the things with mass or energy-including planets, stars, galaxies, even light are brought toward or in other words “gravitate toward” one another.
In 1687, Sir Isaac Newton published “Principia”, which hypothesises the inverse square law of universal gravitation. Newtons law of gravitation states that
“Every body in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of their distance apart”.
That means if the separation between two objects becomes twice as before then the gravitational attraction between the two objects will become 1/4 of the previous. So the objects attract each others based on their mass.
So at this point it seems that light can not be effected by gravity because light is massless.
But the source of gravity is not mass; it’s energy and momentum, which light certainly has, and of course regular matter does too. When light passing by a star, planet or even a black hole not only light gets bent but also in return the light attracts them too, it is a very very small amount but the small amount is not zero.
The problem is here Newton’s law of gravitation is just an approximation. It is good enough to describe most of the natural phenomenon, but it is not perfect.
Gravity is most accurately described by the General Theory of Relativity which was proposed by Albert Einstein in 1915.
The theory of general relativity states that any massive object warps the space-time around it. Since a photon travels by the shortest distance between two points, light appears to bend when it passes through the warped space-time around a massive object.
What this means is that gravity doesn’t directly bend light (by influencing the motion of photons); it’s just that the space-time around a massive object (a black hole) is warped and light takes the shortest path (which is a little curved), making it look like the black hole is affecting the motion of light. At the event horizon, the space-time is curved into itself. The upshot of this phenomenon is that light cannot escape the black hole.