The LIGO Experiment

In the 19th century, before the days of theory of relativity, there was a struggle to locate an absolute frame of reference. Some scientists believed that Aether (luminiferous ether) was an absolute frame that filled the empty space. Since sound waves require a medium to travel, it was believed that light too travelled in a medium-Aether.

Now to find the presence and properties of this aether, in the 1880s Albert A. Michelson and Edward Morley designed a setup. Their experiment was based on an interference pattern observed due to the relative motion between Earth and aether, which was believed to change the resultant velocity of light. Unfortunately, the results of this experiment didn’t match with the predictions, and was deemed as one of the failed experiments in history. They might not have got their results but their setup set a foundation for many experiments.

One of the recent examples is LIGO (Laser Interferometer Gravitational-Wave Observatory). The purpose behind its development was to detect cosmic gravitational waves and utilize gravitational-wave observations as an astronomical tool. Two such observatories were built in the US for detecting Gravitational waves using laser interferometry. LIGO's interferometers are the largest of its kind. With arms spanning 4 km (2.5 mi.) in length, they are 360 times larger than the one in the original MichelsonMorley experiment (which had arms 11 m (33 feet) in length).

This is crucial in the search for gravitational waves because longer arms allow the laser to travel farther, thus increasing the instrument’s sensitivity. Attempting to measure a change in arm length of the order 1,000 times smaller than a proton implies that LIGO has to achieve paradigmshifting sensitivity, so the longer the better. But there are obvious limitations to how long an arm one can build in an interferometer. Even with 4 km long arms, if LIGO's interferometers were nothing more than the basicMichelson’s design, they would still not be long and sensitive enough to detect gravitational waves...and yet somehow, they are! What makes this possible?

The conundrum was fixed by adding to the basic Michelson design, what are known as "Fabry Perot cavities". The figure shows the modifications of a basic Michelson interferometer to include Fabry Perot cavities. Each arm has a Fabry Perot cavity which is created by adding mirrors near the beam splitter. The mirrors repeatedly reflect parts of each laser beam back and forth within the 4 km long arms for about 280 times before the beams mergebtogether again.

So when there is a cosmic disturbance of the likes of a neutron star or black hole merger, a gravitational wave passes through the interferometer. This causes the spacetime in the local area to be altered. The results of this are reflected in the change in the length of one or both the arms, the change depending on the source of the wave and its polarization.

If the laser light travels exactly the same distance down both the arms, the two combining light waves interfere destructively, nullifying each other so that the photodetector registers no light . But if a gravitational wave stretches one arm and compresses the other even in the slightest amounts, the two beams would no longer completely nullify each other, and thereby, produce an interference pattern at the detector. This pattern contains information on the amount of elongation or compression of the arms, which in turn helps the analysts to infer the features - amplitude, frequency, etc. - of the gravitational waves. In turn, this helps in the prediction of which phenomenon or cosmic object produced the gravitational waves.

In 2017, the Nobel prize in physics was divided, one half being awarded to Rainer Weiss, and the other half was bagged by Barry C. Barish and Kip S. Thorne "for decisive contributions to the LIGO detector and the observation of gravitational waves."On April 25, 2019, LIGO as well as the European-based Virgo detector registered gravitational waves originating most likely from a merger of two neutron stars—the dense remnants of supernova explosions. One day later, the LIGO-Virgo network spotted another candidate source of GW, supposedly arising from a cosmic event that had never been witnessed before - collision of a neutron star and a black hole. Besides the two new candidates involving neutron stars, the LIGO-Virgo network has spotted three likely black hole mergers during the first month of the third observing run (O3).

Very recent developments have led to the discovery of intermediate black holes, which were just a theory earlier.

The Michelson-Morley apparatus, once a huge failure, is now creating leaps in astrophysics.

REFERENCES

1. en.wikipedia.org/wikiLIGO

2. physicsworld.com/a/ligo-detects-first-ever-gravitationalwaves-from-two-mergingblack-holes/

3. /www.ligo.org/news/index.php#O3NSMay19

4. https://www.ligo.caltech.edu/page/ligos-ifo

5. en.wikipedia.org/wiki/Gravitational_waves

6. en.wikipedia.org/wiki/First_observation_of_gravitational_waves

7. Book referred: Introduction to special relativity by Robert Resnick