So far we have discussed what is called the special theory of relativity which can only explain the motion of bodies which are moving smoothly ie at a constant speed. That is why it is called special theory of relativity because it works only under special circumstances. What about bodies which are accelerating. We can definitely feel the motion, right. It would be possible to conclusively say we are moving.

Let me just briefly explain why we feel acceleration. Inertia is the property of matter which resists acceleration. This is stated in Newton’s law; “every body continues in it’s state of rest or of uniform motion along a straight line, until it is acted upon by a force which causes it to change it’s state”. So what we experience when we accelerate is inertia, the tendency of our body to continue in its state of rest. So can we conclude that theory of relativity works only when a body is in uniform motion?

Einstein says “no” and he goes about brilliantly proving that it works for all kinds of motion.. How many of you have been to the Disney world? What do you experience when you are on the “free fall” ride. Don’t you feel absolutely weight less. Even NASA uses this technique to simulate weightlessness. They use special planes which fly to a a certain height and do a free fall. Weightlessness implies that you don’t feel the pull of gravity although you are falling. This means that the pull is exactly right to cancel inertia.

Since gravity cancels inertia you do not feel the acceleration and hence it is not possible to tell if you are at rest or moving!!!! How does gravity achieve this – by curving light. Lets say some one throws a ball at you. You are not able to catch it but it just scrapes past your finger. The ball will change direction and slow down. That’s exactly what gravity does to light, it slows it down and makes light bend toward it. This phenomenon is more evident around high gravity areas like black holes.

As we know from our previous discussions, the speed of light determines the flow of time. Since light travels at uniform speed for all observers, the effect of gravity will slow down the flow of time. We can extend the “twin brother” analogy of Cosmos. One goes to a black hole on vacation for a week, on his return he will find that his brother is a 100 years old.

We now know that gravity, acceleration and speed shrinks space and slows time. With these conclusions we run into difficulty when we analyze circular motions. Lets assume that you are rotating a cycle wheel close to the speed of light. Because speed shrinks space, the diameter of the wheel should reduce. What about the spokes. The cannot reduce in length, they can only slim down (since the shrinking of space can happen only in one direction). How do we now fit spokes in the wheel? This difficulty is overcome when you imagine a curved space rather than a flat one. The diameter of the wheel can shrink, the spokes can remain the same length since the spokes curve in the curved space and fit into the wheel.

In fact, Einstein concluded that there is no need to bring in gravity at all. We can simply say that matter curves space. Matter slows time and bends space and thus curves, distorts, bends, warps space-time.

With this I am going to wrap-up my series on relativity. BTW, we don’t have to Einstein to understand time slowing down at black holes. Just observe how slowly time moves in meeting rooms.

References :

Albert Einstein by Dr Mike Goldsmith

Brief History of Time by Stephen Hawking

Theory of Everything by Stephen Hawking

on February 18, 2007 at 8:33 pm |artyfishallIt’s Stephen Hawking, not Stephan.

on February 19, 2007 at 1:30 am |archanaraghuramThank you. I have made the correction.