Detection of Gravitational Waves
Gravitational waves were first theorized by Albert Einstein in 1916 in his Theory of General Relativity, He believed that accelerating objects will produce ripples in space-time that would radiate away from the source like a wave. Even though he doubted the prediction in the 1936 paper ‘Do Gravitational Waves Exist’ but was found to incorrect by a referee and was later renamed to ‘On Gravitational Waves’ with Einstein saying “You don’t need to be so careful about this, there are incorrect papers under my name too” 1These waves can contain information about the source that travels at the speed of light through the universe. There are four different types of gravitational waves that we know of:
· Continuous, which are waves produced by two objects orbiting and thata a fairly constant frequency. Examples of binary stars and black holes this type of waves are very weak as they are produced over a very long period of time
· Burst, which comes from unknown sources but is thought to originate at supernovas and gamma-ray bursts.
· Stochastic, which originate from the early universe from random events that then contribute to the gravitational wave background. This type of waves is very interesting as the waves could tell us about events close to the formation of the universe as were produced fraction of a second after the Big Bang.
· Inspiral, waves which are the ones that we are detecting and are produced by objects that are orbiting into collision. This produces a chirping sound as the frequency increases and the intensity increases.
But for a number of years’ scientists believed that if gravitational waves were true they would be far too weak to detect, this was in the 1970’s and the technologies then were detector such as the Webber bar, a resonance bar which was produced by Joseph Webber in the University of Maryland. The detector would work by the passing gravitational waves causing the large cylinder to vibrate at its resonance frequency of 1660 Hertz with it then being transmitted into electrical signals. But this device was not nearly sensitive enough to detect gravitational waves, but in 1987 he announced he detected gravitational waves from a supernova SN 1987A however with the technology used to results was discredited as noise as his data would suggest supernova in close proximity to earth which would fill the sky with them.
Figure 1 – Interferometer
The first detection of gravitational waves was observed on the 14th of September 2015 by the Laser Interferometer Gravitational-Waves Observatory (LIGO) which consists of two observatories located in Livingston and Hanford in the USA. These consist of two merging beams that are produced from the same source with the beams being the same length but if the beams are offset (get longer) when they merge it will produce an interference pattern and it would cause superposition and it will produce either lighter or dimmer output wave, that change is what can be measured. LIGO was completed in 1999 in collaboration from the California Institute of Technology and Massachusetts Institute of Technology and started observing in 2002 until 2010 but did not observe any events, but this laid the groundwork for the technology. This led to the upgrade of the detectors that would make them much more sensitive and able to look further into the universe, which was finished in 2015 at a cost of $200 million.
After the upgrade, the first measurement was observed and was from a black hole binary about 410 Mpc with the black holes having masses of 36 and 29 solar masses which merged and formed a black hole of 62 solar mass with 3 solar masses radiated as gravitational waves. The gravitational wave chirp was detected over 2 seconds and in that time it had produced more energy than all stars produced in the observable universe (the equivalent of 200 solar masses per second).
This was the first example that these types of observatories worked which proved the theory of Albert Einstein nearly 100 years before and that it could detect gravitational waves with little error. The LIGO detector moved 1/1000 of a proton due to the gravitational waves, making it the most precise measurement ever. It was also the first time we had observed a binary black hole system.
There is also a Laser Interferometer in Europe called Virgo named after the galaxy cluster and is a collaboration between 6 European countries and is funded by the European space agency. Virgo and LIGO collaborate to analyze the data and detection, this is because the observatories are not directional and observe the whole sky so using multiple detectors will give us a better idea of the origins of the waves. In 2011 the Virgo detector was shut down to be upgraded in order to detect even weaker Gravitational waves and in 2017 Virgo detected gravitation waves GW170814 directly which was also detected by LIGO showing the ability of the Collaboration of LIGO and Virgo.
After the first detections at Virgo and LIGO investment started to come in from the National Science Foundation (USA) and the European Space Agency and also the funding of the lesser capable GEO 600 in Germany by the Science and Technology Facilities Council and the Max Plank Gesellschaft. This allowed these facilities to detect Gravitational waves on multiple occasions:
Figure 2 – GW170817 detection
GW170817 the most recent observation was produced by two neutron stars at a distance of 40 Mpc so is the closest event so far with a total system mass of 2.74 Solar Masses and was the first Gravitation wave event to be observed by non-gravitational methods. It was observed as a gamma ray event GRB170817A 1.7 seconds from the detection of the Gravitation Waves and 12 after AT 2017gfo which was an optical observation of the Electromagnetic spectrum of radio to X-Ray. This more or less pinpointed the exact origin of the event. As shown in figure 2 showing the overlapping of the different observatories. After discovering all these events 2scientists are wanting to push for bigger more exciting discoveries future projects such as ELISA are designed to such this. This is a project designed to be stationed in space with arms of 2.5 million kilometres compared to LIGO’s 4km arms. Stationing this observatory in space gives a variety of benefits such as much less interference due to the environment and also the ability to create large arms with the use of only 3 spacecraft with high power lasers. This allows the detector to observe lower frequencies of 1Hz to 0.1mHz compared to LIGO which detects between 10Hz to 10 kHz giving ELISA the ability to detect events that have greater orbits and which are made up of heavier objects. It will also allow us to look further back into the beginning of the universe as it can detect gravitational waves that are very weak. The project has recently had its goal for the launch in the 2030’s the go-ahead by the European space agency. Another project in development is the Einstein Telescope which has been funded recently by the European Commission. This detector is designed to fill the gap between LIGO and eLISA as it will be able to detect gravitational waves at 1Hz to 10’s kHz. This would give scientist a very large band of frequency’s in the future to detect with.
The Discovery of gravitational waves gives us insight into the early universe and allows scientist to see if Albert Einstein’s theory was right and if it can be improved, but because of the ongoing development and investment, it also can benefit other industries. The observatories are very complex machines and have needed the increased development of the engineering needed such as the suspension system to reduce the interference caused by the environment that the laser is in. They have also needed the advancement of data analysis to produce results from signals coming from the detector, Birmingham University has had a key role in LIGO, GEO 600 and eLISA and is also looking at the Limits that the current technology can take us.
1 D.Kennefick Traveling at the Speed of Thought. Page 89
Figure 1, B. P. Abbott et al (2016) Observation of Gravitational Waves from a Binary Black Hole Merger online Available at: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102
2 Christopher Berry (2017) GW170817—The pot of gold at the end of the rainbow online Available at: https://cplberry.com/tag/at-2017gfo/