**3. Detection**

The interferometry system essentially works by measuring the variations that occur in light beams, which are arranged along two different arms. This analysis occurs when we observe the variations and interferences in the return of the light beams, which overlap, since according to the Theory of Relativity light always travels the same distance using the same time, this is our ideal ruler, eliminating the error of a form of measurement that also suffers from the geometric variations caused by ripples. All this technology must be sensitive enough to be able to detect variations of less than a thousandth of a proton.

A powerful laser beam passes through the beam splitter allowing the two generated beams to have the same phase and to be separated perpendicularly by the arms of 4 km each; at the end, they are reflected by the mirrors [13]. Everything was designed so that normally the phases of the waves of the originally emitted light beam and the reflected one generate a destructive effect, so nothing is detected by the photodetector. For the occasion of a gravitational wave passing by the Earth, causing spacetime expand and contract infinitesimally in one direction, thus generating interference arising from the physical property of the wave behavior of light when the phases produce a more constructive effect, thus a signal is detected. **Figure 2** shows the schematics of such a detector.

This is not so straightforward, as the gravitational wave passes through the detector, the gravitational wave also changes the spacetime between the two mirrors, if this

**Figure 2.** *Schematics of a laser-interferometric GW detector.*

space was made of some material, the length of this material will also change by the gravitational wave, but the passage of gravitational wave does not change the speed of light, then the laser will travel in a shorter time in one arm and a larger time in the other arm then changing the interference pattern in the photodetector.

The fact that there are two observatories is a way of circumventing the possibility of confusing the detection of small earthquakes or some other local source of noise since when detecting a signal, this signal will be compared with that detected by the other observatory. It is only confirmed that this jolt was generated by gravitational waves if the generated signal has the same characteristics, for example, exactly the same profile in frequencies, since the observatories are exactly the same. Importantly, this all takes place in a vacuum, thus ensuring that the light will not have an unstable medium that could alter it in some way. Among the improvements to the observatories, the laser was updated to generate a higher frequency, there was an implementation of fused silica in the mirrors to reduce random mirror movements, and also their suspension was improved to reduce thermal noise and seismic isolation, making the observatories more sensitive for detection [14].
