Gravitational Waves: Space Calling!

Feb 16, 2016 By Deepa Gopal
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To our ears, it is nothing more than a chirp. But, among the scientific community, the sound of the Universe has been generating a lot of excitement.

The chirp is being compared to the first telephone call by Alexander Graham Bell to his assistant, Watson, or the beeps made by Sputnik - the first spacecraft put into orbit by the USSR! 

What makes the chirp particularly interesting is that it is the sound of two black holes colliding and was predicted by Einstein almost a century ago! Fascinated? Let's dig deeper.

Einstein's Prediction

In 1915, Albert Einstein - the famous physicist, came up with his now celebrated 'Theory of Relativity'.

The theory explains that space and time are interconnected like a fabric, on which our planets rest. To imagine this better, think of a tarp that is held by its four corners and stretched by four of your friends. Now, drop a heavy bowling ball in the middle of this sheet, and you will see how the sheet sinks under the weight of the ball. This 'sinking' warps the space-time sheet.

Einstein predicted that any disturbance in this space-time sheet would cause ripples of gravitational waves to spread outwards. So, in our example, if the bowling ball collided with another ball, that would be considered a disturbance.

Einstein, however, did go back and forth on his prediction - twice taking it back, before proposing it again. His confusion is understandable for there were no devices to confirm the existence of these waves a century ago. It was all in Einstein's mind.

Enter LIGO

LIGO stands for Large Interferometer Gravitational Wave Observatory. There are two LIGOs in Washington State and Louisiana. LIGO is an interferometer - the word might be complex but what it does is really simple! A beam of light is split into two, both of which travel towards two mirrors that are placed 90 degrees to each other. The reflected beams travel back along the same path and merge with each other. 

Now, if the merged beam looks the same as the split beam, all is good. If, however, the two beams are out of sync with each other, it means there has been some kind of disturbance. Gravitational waves, as we mentioned earlier, capture ripples in space-time. By the time these waves reach the Earth, they are very faint. It takes a very sensitive interferometer to detect them, which is what makes LIGO amazing.

How do physicists know that the disturbance was caused by black holes? Well, it turns out that the disturbance of such a magnitude could only be caused by a burst of gamma rays, which are generated when two extremely massive black holes collide!