Gravitational waves (GWs) travel at the speed of light and are caused by a disturbance in spacetime by an accelerated mass.[footnoteRef:2] The idea of these waves was first suggested by French theoretical physicist Henri Poincare in 1905, and later predicted by Albert Einstein in 1916.[footnoteRef:3] For many years the existence of these waves was unproven. The evidence for the gravitational wave theory was finally proven to be true in 2015 by a laser interferometer gravitational-wave observatory (LIGO).[footnoteRef:4] Many different science facilities around the world communicated and collaborated to develop LIGO. [2: (What is Gravitational Waves?, 2020)] [3: (What are Gravitational Waves?, n.d.) (LIGO, 2020)] [4: (LIGO, 2020)]
Background Physics
Einstein predicted that when two bodies orbit each other in space, a ripple effect would occur, and a gravitational wave produced. Gravitational waves travel at the speed of light. The prediction was linked to Einstein’s general theory of relativity in which the gravitational effect between masses results from their warping of space-time.[footnoteRef:5] Some examples of this warping of space-time could be when a supernova (explosion of a star) occurs, or when two black holes in orbit of each other merge. Figure 1 shows how a mass in space can distort space-time. These GWs are first massive in size, but since they are so far from earth, these waves are very weak by the time they get to earth, meaning the observatory must be very large and sensitive. LIGO can detect gravitational waves by recording when a change in space occurs.[footnoteRef:6] The change in space is due to the interference from the ripple effect of the GWs. The LIGO observatories have two arms which are constructed of lasers and mirrors and stretch over 4 kilometres long. This setup is shown in figure 2. When a gravitational wave passes through the arms of the laser interferometers, the length of these arms changes slightly, which is identified by the light detector. The light detector works by using the principle of interference between waves, which is shown in figure 3.[footnoteRef:7] In the unaffected LIGO setup, the light waves from the laser reflecting against the mirrors causes destructive interference and no light is produced. When a GW passes through the observatory the length of the arm changes slightly causing the wave interference to become partially constructive allowing for some light to be produced, this is recorded by the light detector.[footnoteRef:8] [5: (What are Gravitational Waves?, 2020)] [6: (Gravitational waves, Einstein’s ripples in spacetime, spotted for first time, 2016)] [7: (LIGO, 2020)] [8: (What is an Interferometer?, 2020)]
Figure 3: Principle of Wave Interference
Figure 2: Design of LIGO
Figure 1: Warping of Space Time
SHE
Many different physicists have collaborated to develop LIGO, from many different fields including: classical and Quantum mechanics, electromagnetism and relativity. The development of LIGO consisted of over 1200 scientists making up the LIGO Scientific Collaboration. The high level of interdisciplinary collaboration in LIGO allows for three different areas of research to occur: analysis of the source of the gravitational waves, how detectors operate and how larger-scale applications of the detectors could be developed.[footnoteRef:9] The collaboration to improve the development of LIGO is evident in the development of the initial LIGO interferometers, with input from MIT and Caltech based on their kilometre-scale interferometer studies and together refining to create the design of LIGO.[footnoteRef:10] [9: (When The Past Becomes the Future of Physics In The 21st Centurary, 2020)] [10: (LIGO Discovery Press Kit, 2011)]
The development of LIGO was first completed in 2007. LIGO was built with two identical observatories measuring over 3000km apart. This is since the Interferometers can pick up any interference such as an earthquake which will cause detection in one interferometer but not the other. Both Interferometers pick up interference from GWs, so disturbance can be tracked and compared.[footnoteRef:11] The biggest limitation of LIGO relates to the degree of sensitivity required to detect GWs. In 2009, through increased technology capabilities the sensitivity of LIGO was improved by using increased laser power, Homodyne detection, Output mode cleaner and In-vacuum readout hardware. However, this progress was not enough and LIGO continued to be limited by lack of sensitivity. Over the next couple of years the interferometers still picked up no gravitational waves, so the original arms were rebuilt and project advanced LIGO came about.[footnoteRef:12] The advanced LIGO was developed over five years with collaboration from countries such as the United Kingdom, Germany and Australia, with over 900 scientists working together on the project. The development involved installing new detectors which were almost 22 more times more sensitive to waves than the original interferometers used in 2007. What allowed the advanced LIGO to be much more sensitive than the original LIGO was a heavier and larger mirror (test mass) to be used in conjunction with a quadruple pendulum suspension as shown in figure four, rather than a singular pendulum. Using this advanced LIGO, two days after it was turned online the first gravitational wave was detected. This was on September the 14th 2015. Since then approximately 50 gravitational waves have been detected.[footnoteRef:13] [11: (Gravitational Waves Detected 100 Years After Einstein's Prediction, 2016)] [12: (Technology Transfer Case Studies, 2020)] [13: (LIGO Magazine - Construction: Advance LIGO, 2012)]
Figure 4: Advance LIGO Mirror Design
After the successful outcome of the first two LIGO setups, the importance of collaboration in the research of GWs was made clear. The accuracy of the interferometers is limited and dependent on the number of Interferometers in use. With only one Interferometer no GW’s can be detected, with two interferometers, GW’s can be detected but with more than two the source of the waves can also be found. This influenced the development of the “sister facilities” of LIGO, made up of the interferometers: Virgo in Italy, GEO600 in Germany, and KAGRA in Japan with future plans for LIGO India to be built.[footnoteRef:14] The increase of the Interferometers in the world allows for a global network to be created, hence increasing the amount of research able to be gathered on GW’s. The increase of research will allow for gravitation and relativity theories to be tested and improve the ability to find sources of GW’s, allowing the knowledge of our universe to grow.[footnoteRef:15] [14: (Interferometer techniques for gravitational-wave detection, 2016)] [15: (OBSERVATION OF GRAVITATIONAL WAVES FROM A BINARY BLACK HOLE MERGER, 2016)]
There are two key limitations to the working ability of LIGO. The first one is due to the quantum nature of light. Light is made up of an elementary particle called a photon which causes electromagnetic radiation. This radiation can cause changes in the positioning of the mirrors in the LIGO design, which limits the ability of the interferometers to accurately gather information on gravitational waves that pass through. To overcome this limitation, the design of LIGO has been developed to include a concept called “squeezing”. In this process, light is sent through a squeezed vacuum decreasing the amount of radiation fluctuations.[footnoteRef:16] The second key limitation is the sensitivity of the LIGO structure. The sensitivity is affected by the length of the arms and the power of the lasers used in the design. The arms of the interferometer have been developed using the Fabry Perot cavity, which involves the reflection of the lasers on mirrors. This increases the distance travelled by each laser, increasing sensitivity. This also increases the amount of laser light and power of the lasers, hence also increases the sensitivity, the squeezed vacuum design also helps to improve this. These key limitations have not been fully resolved and any further developments to remove these limitations in full are very costly since the technology is new.[footnoteRef:17] [16: (LIGO R & D, 2020)] [17: (LIGO's Interferometer, 2020)]
The detection of these gravitational waves allows for the Universe to be further studied. Unlike light waves on the electromagnetic spectrum which the Universe is currently studied through, gravitational waves cannot be changed by the atmosphere of the Universe. As a result more information about the Universe can be discovered from the study of gravitational waves. LIGO can compare the information produced from the detection of GWs to the electromagnetic observations on light made by facilities such as the Hubble Space Telescope.[footnoteRef:18] An example of this was on August 17th when LIGO detected a gravitational wave, and then moments later the NASA Fermi Gamma-ray Space Telescope measured a quick release in gamma rays. Then using the Hubble Space Telescope an image of two neutrons stars colliding was produced as shown in figure 5. Another theory, which has not been proven, is that the world is made up of approximately 27% dark matter. By following the sources of GW’s the creation of dark matter, may be able to be tracked in future.[footnoteRef:19] [18: (Why Detect Them?, 2020)] [19: (NASA Missions Catch First Light from a Gravitational-Wave Event, 2017)]
Figure 5: Neutron Stars Colliding
The Laser Interferometer Gravitational-Wave Observatories can detect gravitational waves. The successful development of LIGO included collaboration from many different fields to overcome the limitations of LIGO. These limitations include lack of sensitivity and accuracy of the detection can be overcome by further developments, however these are costly. Using LIGO allows for more information and research to be discovered on the Universe than ever before and LIGO will positively impact society and allow the Universe to be further understood.
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