Gravitational waves
But it would still be another 34 years until their detection. This detection was made possible with the cunning genius of the laser (something whose fundamental principles Einstein had discovered). In 1967, during a thought experiment posed to his university students MIT professor Rainer Weiss proposed the idea of using lasers to detect gravitational wave by having laser light travel perpendicular directions in long arms and then recombine so that they meet and cancel each other out. [9] He was essentially describing a Michelson interferometer (see fig.6). If they did not cancel out, that would be due to gravitational waves. That is because, when gravitational waves pass through matter, they cause distortions in it (see fig.7). They cause matter to stretch and squeeze infinitesimally, in the x and y direction. Due to this minute effect, it would change the interference between the two paths light travelled. When the peaks of light line up, they interfere constructively, meaning that they add to make a brighter light but when the peak meets the trough interfere destructively which results in no output (See Fig. 8). This explanation satisfied Weiss and his students at the time. He would not revisit it until a year later. [9] A contemporary of Weiss called Joseph Webber had already produced a method for detecting gravitational waves previously and had claimed to have gotten a positive result. However, no other scientists were able to reproduce this result. Seeing this discrepancy, Weiss started investigating with the method he had originally proposed. [9] By 1972 Weiss had completed the invention of the interferometric gravitational wave detector and thus showed that in principle it could work . [10] This was no trivial feat, as Weiss would have to figure out how to account for things like seismic activity, traffic, people walking by, etc. in order to avoid incorrect readings. [11] In 1968 another scientist at Caltech named Kip Thorne was simultaneously studying gravitational waves and also ideas on how to address outside noise. [9] The National Science Foundation then funded the construction of a 40-metre prototype interferometer in 1980 after Kip Thorne had founded an experimental gravitational wave group in the late 1970s. [9] With the prototype showing success in 1984 there was a joint deal between MIT and Caltech to co-operate in working to build LIGO( Laser Interferometer Gravitational-wave Observatory).
Figure 6: Michaelson Interferometer Credit: LIGO
Figure 7: Effect of Gravitational Wave on matter Credit: ResearchGate
Figure 8: Constructive and Destructive Interference of waves Credit: LIGO
This was quite a logistical challenge. Whilst in principle it was just a Michelson interferometer (see Fig.6), there were many more operational difficulties. The world’s second largest vacuum had to be created, second only to the Large Hadron Collider (LHC). [12]
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