Gravity and the Bending of Space-Time

Einstein’s General Theory of Relativity has changed the perception of how force acts between the two bodies. According to Newton, force acts because of interaction between two masses but Einstein changed the whole scenario by introducing his General Theory of Relativity.

Things become easier to understand when compared with things that we see around us. So, imagine a trampoline and let a small ball roll across it. How would it go? Well, it goes straight of course! What if I place a yellow ball at the centre of the stretched trampoline massive enough to cause a depression in it? And what if I now place a blue ball less massive than the yellow ball, what trajectory this blue ball will now go through? It would now orbit around the yellow body instead of passing straight through it.

This happens because of the warping of trampoline by the yellow body and any body that tries to pass by it will orbit around it.

Now let’s just go into more depth. What if I roll a red ball on it just after rolling the blue ball a bit farther from the yellow ball? The red ball will take more time to rotate around it as compared to time taken by blue ball because of being in the less warped region of trampoline.

This is how our sun also exerts force on the planets around it by warping spacetime around it so that every planet in the vicinity of it orbits without any interaction.

We were constantly told by our Physics teachers in school that light follows straight line path, so after learning General Theory of Relativity, a question comes to mind: Does warping of spacetime by the massive body affect the trajectory of light? Of course it would affect it!

Light bends because of warping of spacetime by heavy body. Due to this bending of its path, light takes less time to reach its destination and hence experiences a change in frequency. More the bending, less time it would take, and hence there would be a blue shift.

We can have an intuitive understanding of this by folding a paper and seeing whether an ant would reach the other side of the paper earlier than the time taken when the paper is not folded. The ant will take lesser time to reach the other side by traversing a curved path than by following straight line path.

Now as we all know, physicists can’t just sit around imagining things at an astronomical level and be satisfied by observations unless they could feel it by performing their own experiments at a practical level. Once such experiment was the very famous Pound Rebka experiment, which was performed to test Einstein’s General Theory of Relativity. It was proposed by Robert Pound and his graduate student Glen and Rebka in 1959 and was the last of the classical tests of General Theory of Relativity to be verified.