**2. Some history**

The possibility of gravitational waves was discussed in 1893 by Oliver Heaviside using the analogy between the inverse square law of distance in gravitation and electricity [7]. In 1905, Henri Poincaré proposed for the first time the existence of gravitational waves, which emanated from accelerated bodies and propagated at the same speed of light, this is dictated by the transformations of Lorentz [8] and implies an analogy that, accelerating electric charges produce electromagnetic waves, accelerating masses must emanate gravitational waves. When publishing his theory of gravitation (the general theory of relativity) in 1915, Einstein did not agree with Poincaré's proposal, as in his theory there are not gravitational dipoles, essential for the emission in the electromagnetism theory. However, based on a weak field approximation, he concluded that there should be three kinds of gravitational waves (named by Hermann Weyl as longitudinally-longitudinally, transversely-longitudinally, and transversely-transverse).

These approximations made by Einstein were criticized by several researchers and even Einstein had doubts. In 1922, Arthur Eddington wrote a paper entitled: "The propagation of gravitational waves" [8], in which he showed that two of the three types of waves proposed by Einstein were only mathematical artifacts produced by the system of coordinates and they were not really waves. This also cast doubt on the physicality of the third type (transversely transverse); however, Eddington proved that these would travel at the speed of light in all coordinate systems, so he did not rule out their existence.

In 1956, Felix Pirani corrected the confusion caused by the use of several coordinate systems by reformulating gravitational waves as the manifestation of the Riemann tensor observables. The Pirani work was ignored at that time mainly because the scientific community was concerned with another issue of whether gravitational waves could transport energy. This question was solved by Richard Feynman using a thought experiment presented at the first conference for General Relativity in 1957 known as the Chapel Hill Conference. His argument, known as the sticky bead argument, presents that: if a gravitational wave passes orthogonally to the beaded rod (a rod if some bead), the effect of it is to deform the bead and the rod, but as the rod is longer, the bead moves beads over the rod; this movement causes friction and then heat, which meant the passing gravitational wave would have energy. Afterward,

#### *Interferometric Gravitational Wave Detectors DOI: http://dx.doi.org/10.5772/intechopen.106417*

Hermann Bondi (who was skeptical of the existence of gravitational waves) published a more complete version of this argument.

After this conference, the scientific community took the existence of gravitational waves more seriously. Joseph Weber began to design and build a gravitational wave detector. It was the start of many gravitational wave detectors that are called Weber bars. Weber claimed to have detected gravitational waves in 1969 and 1970, the signals coming from the Galactic Center [9]. However, the high detection frequency quickly cast doubt on the validity of his observations, as the Milky Way's implied rate of energy loss would drain our galaxy's energy on a much shorter timescale than the galaxy's inferred age. It got worse when in the middle of the 1970 decade, the build of other Weber bar experiments by other groups around the world failed to detect such signs. By the end of the 1970 decade, the consensus was that Weber's detections were some kinds of noise.

The first indirect evidence of gravitational waves was discovered in 1974 by Russell Alan Hulse and Joseph Hooton Taylor Jr., using their discovery of the first binary pulsar. Results were published in 1979, showing the measure of the orbital period decay of the, so-called, Hulse-Taylor pulsar, which precisely describes the angular momentum and energy loss due to gravitational radiation emission predicted by general relativity. A discovery that gave them the 1993 Physics Nobel Prize.

This indirect detection of gravitational waves motivated further searches, despite Weber's discredited result. Some groups continued to improve on Weber's original concept. Using very low temperatures (cryogenic) for the bars, in high-vacuum systems and under vibrational isolation. There were many of these projects around the World. One of these groups built a Niobium bar resonant-mass gravitational wave detector [10]. In this detector, the vibrations caused by the passage of gravitational waves in the niobium bar are measured by a microwave parametric transducer. In this system, microwaves are pumped into a microwave cavity and the vibrations of the microwave cavity are connected to the niobium bar causing microwave signals in the microwaves to leave the cavity. This signal now must be amplified, but it mixes with the original microwave signal, which is too strong for the low-noise microwave amplifier; then the microwave carrier signal is removed by the use of an interferometer that cancels only the microwave carrier. A similar system can be seen in **Figure 1**. This is another use for interferometers in gravitational wave detectors; unfortunately, this kind of detector never made a detection, maybe because of a poor choice in the frequency range [11].

Other experimental groups pursued gravitational wave detection using laser interferometers. This idea of using appears to have been around for a long time by several independent groups, for example in 1962 ME Gertsenshtein and VI Pustovoit [12] and in 1966 by the group of Vladimir B. Braginskiĭ. The first prototype appeared in the 1970 decade built by Robert L. Forward and Rainer Weiss. In the following decades, increasingly sensitive detectors were constructed, culminating in LIGO and Virgo detectors.

After years and years of null results, the first detection of gravitational waves was made by LIGO on September 14, 2015, as the signal, named GW150914, probably came from the merger of two black holes [13, 14]. A year earlier, LIGO could have been brought down when scientists from the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization 2) experiment claimed to have detected a weak signal in the CMB (Cosmic Microwave Background) that appeared like evidence of gravitational waves originating in the beginning universe. This evidence, according to researchers, could have been a smoking gun proof of the theory of cosmic inflation, which

#### **Figure 1.**

*Schematics of a similar electronics of Niobè gravitational wave detector showing a microwave interferometer (between the cryogenic circulator and the cryogenic low-noise amplifier) used to cancel the microwave carrier signal that will degrade the performance of the cryogenic low-noise amplifier.*

postulates that very shortly after the Big Bang (10−32 seconds after), the expanding universe experienced a period of very rapid expansion (a factor of 1026 times). This fast expansion would have created ripples over the CMB, the fossil cosmic radiation that fills the universe being the first detectable electromagnetic radiation in Universe history. However, the BICEP2 signal detected could be explained also by Milky Way dust, making the scientists withdraw the claim that gravitational waves had been detected.
