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Bend it like Einstein

Einstein’s theory of General Relativity is considered revolutionary because it changed our ideas of both space and time. With this theory, space became a trampoline curving under the weight of heavy objects, and time became personal, with each of us having our own version of time. But when he proposed this theory in 1915, it did not become known to the common man immediately. That had to wait for 1919 when there was an experiment confirming what Einstein’s theory had predicted.

We all know that light travels only in a straight line. That is why we cannot see a TV running in the next room since the wall in between prevents the TV’s light from reaching us. One of the consequences of Einstein’s theory is that you can, see an object even though your line of sight to the object is blocked by a barrier.

Imagine you are watching a TV with 2 of your friends, sitting on your either sides. Now, suppose your spouse, wanting to grab your attention, comes and stands in front of you hiding the TV from your view. Your two friends can continue watching the TV, because there are rays from the TV directed at your friends too which are not blocked by your spouse. In this situation there is no way you can watch TV without either you or your spouse moving.

But when you consider a similar situation on a cosmic scale, things are different. Imagine a star far away from Earth. But suppose there is a black hole in between the earth and the star blocking our view. Would you be able to see the star? Our experience with the TV tells us that we cannot see the star. But actualy you can see it; that too not once but multiple times at the same instant (as shown below).

Multiple images of the same star at the same time. (Picture from Wikipedia, originally by NASA)

What happens is this. The light that comes from the distant star towards you is blocked by the black hole and you cannot see it. But, the star is sending out light in all the directions, and some of it is directed at your neighbors too. But since the black hole is a massive object its gravitation attracts the light that was going away from the black hole. But that light is not completely absorbed. So what ends up happening is that the path of the light that started off towards your neighbors, is bent slightly so that it eventually reaches you. It is as if, the light is taking a slight detour to avoid being pulled in by the black hole.

Light from a star bending around a massive obect.

Light from a star bending around a massive object (Picture from Wikipedia, originally by NASA)

Since the light comes from the side of the black hole, we would see the star as if it is by the black hole’s side and not behind it. Moreover, since such light would have started off in all the different directions, you can see light all around the black hole bending towards you. Thus what you see is multiple images of a single star. In many cases, what we observe is only a brightening of the star since the light that was meant for different directions is grouped and directed towards the observer.

This phenomenon called Gravitational lensing was the key experiment. It is called lensing, because a lens too alters the direction of light. The amount of this bending that relativity predicted was different from that of what was predicted by Newton’s theory of gravity. Only when this test was done, was the theory experimentally confirmed and the theory shot into limelight.

  1. Tom Mowlam
    March 6, 2015 at 06:37
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