Fullscreen HTML5Med Res (??MB)HTML4

Detecting Gravitational Waves By Dr David Maddison The confirmation of the existence of gravity waves involved the most sensitive measurements ever made. This article describes the past, present, and future efforts to detect these unimaginably hard-tomeasure (and quite fascinating) phenomena. Illustration Credit: LIGO, NSF, Aurore Simonnet (Sonoma State U.) Source: https://apod.nasa.gov/apod/ap160211.html O ne of Einstein’s many predictions that has been proven correct was the existence of gravitational waves, predicted by Einstein in 1916 and first directly observed on the 14th of September 2015. The idea of gravity as mass distorting space-time was described in Einstein’s General Theory of Relativity, first presented to the Prussian Academy of Sciences in 1915. This theory includes refinements to Newton’s Law of Universal Gravitation. General Relativity is the currently accepted explanation of gravitation, describing gravity as a geometric property of space and time (space-time) in four dimensions – three of space and one of time. There had previously been other attempts to describe gravitational waves, but Einstein was the first to get the concept right. Einstein thought his prediction of the existence of gravity waves was of academic interest only, as he did not believe they could ever be detected 14 Silicon Chip due to being so slight. In 1935, he had second thoughts about the existence of gravitational waves. But the journal he presented his paper to, Physical Review, refused to publish it due to an error. Then in 1957, Richard Feynman said they must be real based on the theory and used his “sticky bead” argument to convince others that they were real. For details on this, see the website at siliconchip.com.au/link/ab9f Explanation of gravity waves Unlike Newton, Einstein did not describe gravity as a force. In General Relativity, space-time is ‘flat’ without matter, but the presence of matter causes space-time to curve, and this distortion is manifest as gravity. It is relatively easy to visualise this by considering a heavy ball placed on a taut rubber sheet or trampoline (see Fig.1). Suppose another ball is in the vicinity of the distortion caused by this object. In that case, it will either rotate Fig.1: a massive object distorts the surrounding space-time, represented by the grid, creating a ‘gravity well’ to which other objects are attracted. They may orbit, bypass or fall into the other object depending on their velocity. Australia’s electronics magazine siliconchip.com.au around (orbit), bypass or fall into the “gravity well” created by the first ball (plus make one of its own), depending upon its velocity. This means that any mass accelerating through space-time also generates gravitational waves analogous to waves on a pond (see Fig.2), with the waves being distortions in space-time. An orbiting object is under constant acceleration in the physics sense, although that does not necessarily mean a change in its speed. Technically, the velocity of an object in a stable orbit is constantly changing while its speed is constant, because the direction of the vector is continually varying, even though its magnitude remains essentially constant. Examples of two bodies under acceleration that generate gravity waves include two massive objects, such as black holes orbiting each other, or massive objects merging such as a black hole or neutron star (see Fig.3, the panel below and siliconchip.com. au/link/ab9t). A stationary (non-accelerating) object does not emit gravitational waves. All accelerating objects with mass, no matter how tiny the mass, emit gravitational waves, but the effect is so small as to not be measurable in any realistic sense. Thus, the observation of gravitational waves is only possible when supermassive objects like black holes and neutron stars orbit or merge. Even the orbit of Jupiter about the Sun does not emit realistically measurable gravitational waves, even though Jupiter is 318 times as massive as Earth. A gravitational wave causes physical dimensions to change as it passes through space, by either stretching or compressing the distance between objects, but the effect is unimaginably tiny. Relevant video and audio links In 2016, University of Western Australia Emeritus Professor David Blair spoke to the ABC about the first discovery of gravitational waves in 2015. You can listen to that program at siliconchip.com.au/link/ab9h Also see the video titled “OzGrav: A new wave of discovery” at https://youtu. be/jMwHppyQiZw Read articles about gravitational waves written by Professor David Blair at https://theconversation.com/profiles/david-blair-4285/articles There is an Australian initiative to explain Einsteinian physics to children, The Einstein-First Project: www.einsteinianphysics.com Fig.2: waves on a pond are a familiar analogy for gravitational waves, although they are (essentially) two-dimensional while gravity waves are threedimensional. Source: www.pexels.com/photo/water-drop-photo-220213/ Even the gravitational waves formed by the collision of two black holes might alter the distance between Earth and the nearest star system Alpha Centauri, 41,343,000,000,000km (4.37 light years) away, by about one part in 1020 or 0.041mm, depending upon how far away the black hole is. That is less than the thickness of human hair. Another way to look at it is that in the LIGO detector we will discuss, the length change is one-thousandth of the width of a proton (subatomic particle). No matter how near or far a black hole might be, the effect is incredibly small. The creation of gravitational waves involves the loss of energy from the originating system, such as by orbital decay (‘inspiral’), merger and ‘ringdown’ (as the union is consolidated) of massive objects like white dwarfs, neutron stars or black holes. Like electromagnetic radiation, such as light or radio waves, the energy carried by gravitational waves follows the inverse square law with distance. That is, if you double the distance, the signal strength is 1 ÷ 4 (1 ÷ 22); if you triple the distance, the strength is 1 ÷ 9 (1 ÷ 32) etc. However, also like electromagnetic radiation, the amplitude of the waves Fig.3: the orbit of two massive objects (in this case, white dwarf stars), leading to the emission of gravitational waves as their orbits decay toward a final merger. This might end in a supernova explosion, as shown in the third panel. These types of gravitational waves would be detectable with a space-borne instrument such as LISA. Source: NASA. siliconchip.com.au Australia’s electronics magazine October 2021  15 Multiple gravity waves detected in January 2021 Two important, independent gravitational wave events were recently published. Both involved the merger of a neutron star and a black hole, and were recorded ten days apart. One event was caught on both LIGOs and Virgo. The other was only picked up by one LIGO detector, as the other was down for maintenance and the signal-to-noise ratio on Virgo was inadequate. The original paper “Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences” can be viewed at siliconchip.com.au/link/ab9u diminishes according to an inverse law. So if the distance between the source of a gravitational wave and the detector is doubled, the amplitude is 1/2; if the length is tripled, the amplitude is reduced to 1/3 etc. The original ‘inflation’ of the universe when it rapidly expanded from an infinitesimally small ‘singularity’ is also thought to have generated gravity waves. Still, these would be so small now that it is believed that it will be many decades before the technology exists for these to be detected. They would be similar to the cosmic microwave background radiation (see below) but represent an earlier period, and are referred to as the gravitational wave background. Gravitational wave astronomy Fig.4: this diagram shows the characteristic frequencies and ‘strain’ (dilation of space) caused by the gravity waves of various cosmic events. The coloured bars and black lines show the capabilities of various types of detector. Events below the black lines cannot be detected. Original Source: C. Moore, R. Cole and C. Berry (CC-BY-SA 1.0). Fig.5: the gravity waves originating at the time of the Big Bang should still exist today. The cosmic microwave background is only visible to 379,000 years after the Big Bang. The relic gravitational waves from the Big Bang can penetrate through the dense matter from before then, right up to the instant the universe came into being. Source: NASA. 16 Silicon Chip Australia’s electronics magazine The ability to observe gravitational waves opens up a whole new field of astronomy and physics in general. It could answer questions about the nature and extent of so-called dark matter and dark energy (if they really exist), the gravitational wave “footprint” of the universe at the time of its creation and give a better understanding of the formation of neutron stars, black holes and their mergers. First indirect observation of gravity waves The first indirect evidence for gravitational waves was found in 1974 by R. A. Hulse and J. H. Taylor Jr. They received a Nobel Prize in 1993 for their discovery. Looking at a binary system consisting of a neutron star and a pulsar (see the panel on page 21) called PSR B1913+16, they noticed a decay in the orbital period of 76.5 microseconds per year, and a reduction of orbital radius of 3.5m per year, leading to what will be the final ‘inspiral’ event (coalescence of the two bodies) in 300 million years. The decay of the orbit is due to energy released as gravitational waves, and the amount was in precise agreement with Einstein’s General Theory of Relativity. The amount of power radiated in the form of gravitational waves here is 7.35 x 1024 watts, which is 1.9% of the energy emitted by our Sun in the form of light. Incidentally, the gravitational power radiated from our solar system due to the orbit of the planets about the Sun is about 5kW. siliconchip.com.au Gravitational wave frequencies An important aspect of the observation of gravitational waves is the frequency and ‘strain’ (dilation of space-time) of such waves. Different cosmic events cause gravitational waves of different frequencies and strains, and this determines the type of detector that is appropriate to use. Unfortunately, any one type of detector is not suitable for all events. Some characteristics of various cosmic events and their associated strains, along with specific detector capabilities, are shown in Fig.4. In that figure, any event with properties below the black line is beneath the noise floor of the detector and cannot be detected. Events above the black lines and represented by coloured areas can be detected. Fig.6: an example of what low-frequency ‘stochastic’ gravitational waves might look like, as produced 10-36 to 10-32 seconds after the Big Bang. These cannot be sensed with present detectors. They would sound much like radio static if played as audio. It is hoped that other types of low-frequency signals can be detected with projects such as the IPTA. Source: LIGO. Lowest frequencies There is believed to be evidence of the relic gravitational waves formed at the instance of the Big Bang, when the universe was thought to have sprung into being from an infinitesimally small singularity (see Figs.5 & 6). These are at the lowest frequencies, in the microhertz or nanohertz range or even lower. The microwaves that permeate the cosmos, the ‘cosmic microwave background’ radiation (Fig.7), can be viewed to a point about 379,000 years after the Big Bang. But the matter from before that time is too dense to allow observations of light or microwaves before that, as the microwaves or light energy would have been absorbed. The cosmic microwave background is the farthest we can look back to the beginning of the universe. However, nothing can shield gravitational waves, so these should be visible as the “gravitational wave background” starting at a time close to the universe’s beginning. Still, the effect is so tiny that detection (of the gravitational wave background) is thought to be decades away, At a slightly higher frequency are waves from supermassive black-hole binaries with masses billions of times that of our Sun, presumed to exist at the centres of galaxies, resulting from previous galactic mergers. This is what the International Pulsar Timing Array (IPTA) aims to detect – see Fig.8. siliconchip.com.au Fig.7: a map of the cosmic microwave background radiation, a relic of the time 379,000 years after the creation of the universe. Primordial gravitational waves predate this and may have influenced its structure. As measured in the microwave spectrum, the difference in temperature from the hottest to the coldest points is a mere 200 millionth of a degree. Source: NASA/WMAP Science Team. Fig.8: the gravitational wave spectrum, showing signal sources and relevant detectors (NS in the diagram stands for neutron star). Source: NASA Goddard Space Flight Center. Australia’s electronics magazine October 2021  17 Fig.9: Australia’s Parkes Observatory, a 64m radio telescope participating in the International Pulsar Timing Array (IPTA) to look for gravitational waves. Source: Wikimedia user Diceman Stephen West. The IPTA is a cooperative effort of the European Pulsar Timing Array (EPTA), North American Nanohertz Observatory for Gravitational Waves (NANOGrav), Indian Pulsar Timing Array (InPTA) and Australia’s Parkes Pulsar Timing Array (PPTA) – see Fig.9. As you can imagine, detecting a nanohertz gravitational wave signal can take many years, as 1nHz is only one cycle every 32 years or, for microhertz, one cycle every 11 or so days. However, one would not have to observe a complete cycle. Medium frequencies Medium-frequency gravitational waves of about 0.1mHz (millihertz) to 1Hz are created by inspiral events, where objects with extreme mass ratios (one much more massive than the others) spiral into each other and merge (see Fig.10). This includes massive binary star systems circling each other (see Fig.11); ‘resolvable galactic binaries’, that is, binary star systems within our own galaxy which are not too obscured by noisy signals from other sources, perhaps with Sun-sized stars; massive binary star systems within or outside the galaxy; and Type 1A supernovae (exploding stars). It has been proposed to pick up medium frequency gravitational waves with space-based detectors such as the joint NASA and European Space Agency evolved Laser Interferometer Space Antenna (LISA) scheduled 18 Silicon Chip Fig.10: the expected gravitational wave signal from an ‘inspiral’, resulting in the merger of two black holes. The frequency increases as the two objects get closer and closer, as a spinning ice skater goes faster when they move their arms closer to their body. The gravitational wave amplitude also increases as they move closer to merging. This was the type of event that LIGO first detected. Source: LIGO. for launch in 2034, and the Japanese DECi-hertz Interferometer Gravitational-wave Observatory (or DECIGO). High frequencies High-frequency gravitational waves are much easier to detect than the others, although it is still extremely difficult. They have a frequency of approximately 10Hz to 1kHz, or more. Phenomena that cause these waves include inspiral and merger of binary objects such as neutron stars and black holes and core collapse of supernovae. The first gravitational wave directly observed was in this frequency range. Gravitational wave observatories for this frequency range include LIGO (USA), Virgo (Italy), GEO600 (Germany) and KAGRA (Japan). Attempts to directly observe gravitational waves The main problem with detecting gravitational waves is their tiny magnitude, making their measurement the most challenging of all, as incredibly sensitive instruments are required. The primary detection methods have been resonant mass antennas, laser interferometers and pulsar timing arrays. There are some other methods under development. Resonant mass antennas Resonant mass gravitational wave antennas were the first type of detectors developed. They consist of a large metal mass isolated from vibrations and possibly cooled to a low temperature. They are designed to have a particular resonant frequency, much like a bell or a tuning fork. If a gravitational wave passes through them, they Australia’s electronics magazine should resonate, and that resonance could be amplified and detected. A resonant mass antenna at the University of Western Australia (UWA) called NIOBE consisted of a 1.5-tonne cylindrical niobium bar with a resonant frequency of 710Hz, cooled to 5K (-268°C) with superconducting electromechanical sensors – see Fig.12. This was one of five similar detectors which operated in the 1990s. NIOBE achieved world-record sensitivity. It was used in joint observations with other similar detectors from 1993-1998. This experiment was performed under the leadership of Professor David Blair. Today, it is believed that resonant mass antennas are not sufficiently sensitive to detect anything other than the most powerful gravitational waves. However, there are still two spherical resonant mass antennas in operation, MiniGRAIL (the Netherlands – see Fig.13) and Mario Schenberg (Brazil). The MiniGRAIL consists of a precisely machined 1400kg, 68cm sphere of aluminium-copper alloy cooled to 20mK (thousandths of a degree) above absolute zero, -273°C. It has a resonant frequency of 2.9kHz and a bandwidth of about 230Hz. Its sensitivity is relevant to detecting events such as instabilities in rotating single and binary neutron stars, small black-hole or neutron-star mergers etc. The Brazilian device is similar. Laser interferometers An interferometer is a device that uses the interference pattern of two light beams (or other types of electromagnetic beams) from a common source to measure distances, by siliconchip.com.au Fig.11: a continuous gravitational wave might be generated from two black holes or neutron stars in a stable orbit around each other, or a massive irregular object rotating on its axis (for a neutron star, the irregularity need only be centimetres high). A detector like LIGO could sense such events, but it would need to have its sensitivity increased. Image courtesy: LIGO. examining the interference pattern caused by selective reinforcement or cancellation of the beams when they are combined. When using light waves such as lasers, the distances measured can be extremely small, down to 1/1000th of a subatomic particle’s width! For gravitational wave detection, low-noise, high-sensitivity detectors are required, but these did not become available until the late 1990s. There have been attempts to build suitable interferometers since the 1960s. The operation of a laser interferometer is shown in Fig.14. In regular operation (1), a laser light source in the black box strikes a beam splitter (half-silvered mirror) at an angle, and it is split into the beams shown in blue and red. These beams reflect off the cyan mirrors at the end of the two arms. The beams recombine via the beam splitter. The recombined beams are in phase and create a certain interference pattern, indicated by the purple circle. In (2), a gravitational wave (yellow) passes through the detector, and this changes the length of one or both arms, and thus the interference pattern of the recombined beam (white circle), indicating the presence of a gravitational wave. In reality, the beam travels down each arm 280 times. The overall design of the LIGO Fig.13: the internal mechanism of the MiniGRAIL resonant mass gravitational wave detector, designed and built in the Netherlands. 1 2 Fig.12: a cross-section of the Australian NIOBE detector. It was built around a niobium metal bar weighing 1.5 tonnes. The bar had a resonant frequency of 710Hz, was cooled to 5K (-268°C) and fitted with superconducting electromechanical sensors. siliconchip.com.au Australia’s electronics magazine Fig.14: a simplified diagram showing how interferometric gravitational wave observation works. Any change in the relative lengths of the two arms causes a change in the interference pattern on the detector at the right; constructive interference in case (1) and destructive in case (2). Source: Wikimedia user Cmglee (CC-BY-SA 3.0). October 2021  19 Fig.15: the basic configuration of the LIGO laser interferometer. Original Source: Wikimedia user MOBle. Fig.16: one of the LIGO mirrors. These mirrors are suspended on fine glass fibres and are among the most perfect mirrors ever made. Their stability is the key to the operation of the instrument. There is a video on the mirrors titled “EPISODE 1 LIGO: A DISCOVERY THAT SHOOK THE WORLD” at https:// vimeo.com/203776385 Fig.17: the two 4km-long arms (in a V shape) of the LIGO Hanford Observatory at Richland, Washington, USA. Source: LIGO/Caltech. 20 Silicon Chip Australia’s electronics magazine interferometer is shown in Fig.15. Its design is based on the Michelson interferometer, which has been in use since 1887. LIGO also has light storage arms in the form of a so-called Fabry-Pérot optical resonance cavity, which stores light for about a millisecond before leaving the storage arm to recombine with the other arm at the beam splitter. Laser amplification is achieved in the light storage arm when it is “on resonance” and said to be “locked”, and constructive interference of the laser light occurs. When the laser is locked in this mode, it is extremely sensitive to length changes due to gravitational waves. The “test masses” in the diagram are mirrors that allow a small amount of light transmission. LIGO LIGO (The Laser Interferometer Gravitational-Wave Observatory; www.ligo.org) has a long history of development, funding and politics beyond the scope of this article. It consists of two separate observatories, one in Washington state, USA and the other in Louisiana, about 3000km away or 10ms at the speed of light – see Figs.16 & 17. Two observatories are needed to confirm that any observations are real and enable an estimate of the source of any event detected. Additional instruments elsewhere in the world would make the localisation of an event more accurate. The observatory is operated by Caltech and MIT. When it was first built, it made observations from 2002 until 2010, during which time no gravitational waves were detected. The instrument was then upgraded to the Advanced LIGO, and observations formally began again on the 18th of September, 2015. The first observation of a gravitational wave was confirmed to have been made on the 14th of September 2015, several days before formal observations had begun, although the instrument was still operational for testing before that – see Fig.18 and the video titled “The Sound of Two Black Holes Colliding” at https://youtu.be/ QyDcTbR-kEA Further events were detected on the 26th of December 2015, the 4th of January 2017, the 14th of August 2017 and more since then (Fig.19). Apart from US organisations and funding agencies, some foreign siliconchip.com.au Other Earth-based interferometric detectors Apart from LIGO, other operational interferometric gravitational wave observatories are Virgo (Italy, two 3km arms), GEO600 (Germany, two 600m arms) and KAGRA (Japan, two 3km arms). siliconchip.com.au Hanford, Washington (H1) Livingston, Louisiana (L1) 1.0 0.5 0.0 Strain (10­21) ­0.5 ­1.0 H1 observed L1 observed H1 observed (shifted, inverted) Numerical relativity Reconstructed (wavelet) Reconstructed (template) Numerical relativity Reconstructed (wavelet) Reconstructed (template) Residual Residual 1.0 0.5 0.0 ­0.5 ­1.0 0.5 0.0 ­0.5 512 Normalized amplitude Frequency (Hz) agencies from the UK, Germany and Australia’s Australian Research Council and universities make essential contributions to LIGO. Each LIGO observatory has two 4km-long interferometer arms at right-angles to each other. A laser beam passes up and down each 4km tube, which is under a very high vacuum. This vacuum is one-trillionth that of Earth’s atmosphere, eight times less dense than space, and this is the largest-volume sustained high vacuum on Earth. The beams travel up and down each tube 280 times to increase the effective arm length to 1120km, increasing sensitivity. If a gravitational wave passes through the arms, the local space-time is altered and the length of one or both arms changes depending on the direction and polarisation of the wave. This results in a slight change in the phase of the laser beam arriving at the detector, which shows up as a difference in the interference pattern. The change in length is much less than the wavelength of light, but the interferometer will respond to this fractional change. The observatory has multiple extremely advanced measures to reduce noise and vibration from sources such as earthquakes, vehicles and people walking and even the thermal noise from atoms vibrating in various components. There are ongoing plans to improve the sensitivity of LIGO even further. The more gravitational-wave observatories exist, the more accurately the source can be determined. LIGO had plans to build an observatory in Australia on the site of AIGO (see below), where there is a provision for land for the two required 4km-long arms. Western Australia was a preferred location for the third LIGO observatory for many reasons; however, the Australian Government of 2011 did not commit to funding it, so this observatory will now be built in India instead (see www.ligo.caltech.edu/ page/ligo-india). 256 128 64 32 0.30 0.35 Time (s) 0.40 0.45 0.30 0.35 Time (s) 0.40 0,45 Fig.18: the first observation of gravity waves, signal GW150914 on the 14th of September, 2015, showing the signals received at the two Advanced LIGO detectors in the USA. The difference of 7ms in the arrival of the signal between the two sites reflects the delay taken for the gravitational wave travelling at the speed of light. It is less than the 10ms taken for a straight line because the signal arrived at a 45° angle between the two sites (cosine(45°) ≈ 0.7). Source: B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration). Fig.19: a selection of gravitational waves with time-frequency spectrograms above; brighter colours represent a stronger signal. If played back as audio, the signal typically sounds like a chirp. Source: LIGO Scientific Collaboration and Virgo Collaboration/Georgia Tech/S. Ghonge & K. Jani. Cosmic Explorer (cosmicexplorer. org) is a proposed ground-based interferometer with 40km-long arms. The Einstein Telescope is a European observatory proposal under study, to be built underground with 10km long arms, achieving higher sensitivity than LIGO. The optics will also be cooled to -263°C (about 10K). Australia’s electronics magazine There are some other proposals, but they don’t seem to have widespread support at this stage. Space-based interferometric detectors Detectors like LIGO can sense higher frequency gravitational waves, but much longer arms are required to October 2021  21 Fig.20: an artist’s concept of one of the LISA satellites with the laser beam from the distant LISA satellite visible, and small thrusters being fired for station keeping. Source: AEI/MM/ exozet. Fig.21: the proposed arrangement of the three LISA satellites, with arm lengths of 2.5 million kilometres. Source: Max Planck Institute for Gravitational Physics (Albert Einstein Institute) / Milde Marketing Science Communication / Exozet Effects. detect those of medium frequency than can be achieved on Earth. LISA (Laser Interferometer Space Antenna) is intended to be put into space in 2034. It is a joint NASA and ESA (European Space Agency) project, but it is led primarily by the ESA. It will be used to observe such phenomena as mergers of massive black holes at the centres of galaxies, small objects orbiting massive black holes (with an extreme mass ratio) and binary star systems in our galaxy; possibly also gravitational waves from the Big Bang. The detector will be in the form of a Michelson interferometer, just like LIGO, but without the light storage capability. It will have a total of three satellites with two ‘arms’ of 2,500,000km extending from a master satellite, with light travelling to the other satellites through the vacuum of space (see Figs.20 & 21). There will be a free-floating mirror within each satellite so that the mirror is free of forces that the satellite is subject to. The satellite constellation will be in the same orbit as the Earth but trailing it by 50 million kilometres. DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is a proposed Japanese space-based detector designed to be sensitive to the frequency band 0.1Hz to 10Hz, thus filling the gap of the sensitive bands of LISA and LIGO. It is hoped to launch in 2027. The overall layout will be similar to LIGO, with 1000km-long arms, and it will be placed in Earth orbit, at an altitude of 2000km. Big Bang Observer (BBO) is a proposal from the ESA for four LISAlike triangles (a total of 12 spacecraft) in solar orbit with arms of about 50,000km. Its purpose will be to observe gravitational waves from the Big Bang. Pulsar Timing Arrays Fig.22: how IPTA works. This notto-scale image shows the fabric of space-time represented by the green grid distorted by gravity waves (grey and cloudy), millisecond pulsars (dark spheres) and the Earth. The millisecond pulsars emit spinning radio beams which are monitored. Source: David Champion, Max Planck Institute for Radio Astronomy. 22 Silicon Chip As mentioned above, the International Pulsar Timing Array (IPTA; www.ipta4gw.org) is an international cooperation that involves Australia’s Parkes Observatory. Instead of using 4km-long baselines like Earth-based projects such as LIGO (see below), it uses an array of millisecond pulsars throughout the universe, monitored by a system of radio telescopes – see Fig.22. Millisecond pulsars are extremely fast-spinning neutron stars (see panel) Australia’s electronics magazine that emit highly predictable and stable pulses. These can be used as the basis of a clock. If a gravity wave alters the distance between the pulsar and a radio telescope on Earth, the timing of that pulse will be altered. By monitoring variations in the arrival time of these pulses due to the stretching and compression of spacetime, gravity waves may be detected, and their origin determined. The pulsar frequencies selected are around 100ms (ie, ~10Hz), while the gravity wave frequencies that can be detected are of the order of microhertz and nanohertz. Australia’s contribution Fifty-six Australian scientists were involved in the first observation of gravitational waves, and Australia now has 45 years of experience in the field. Contributions to gravitational wave research continue to come via the Australian Consortium for Interferometric Gravitational Astronomy (www.aciga. org.au) and The Arc Centre Of Excellence For Gravitational Wave Discovery (www.ozgrav.org). Universities involved in these organisations include the ANU, Charles Sturt University, Monash University, Swinburne University, University of Adelaide, University of Melbourne and the UWA. The CSIRO is also involved. As related by Emeritus Professor David Blair (siliconchip.com.au/link/ ab9h), among the contributions made were: • Technology to measure distortions in the laser light waves passing through the mirrors • Technology for aligning the output beams • Technology for preventing the detectors from becoming unstable • Supercomputer-based data analysis to extract signals from the noise Professor Blair also indicated that part of the Australian experience were contributions in: • Learning to make quantum measurements on masses ranging from micrograms to tonnes • Making mirrors precise to atomic dimensions, to reflect light with unsurpassed perfection • Learning how to suppress natural vibrations of atoms due to heat, and larger vibrations from Earthquakes, vehicles and people siliconchip.com.au Neutron stars, pulsars and black holes Fig.23: a simulated image of a neutron star with accretion disk and gravitational lensing. Gravitational lensing occurs when the mass of the body distorts light coming from behind. Source: Wikimedia user Raphael.concorde. A neutron star starts as a star about 10-25 times more massive than our Sun. At the end of its life, it explodes in a supernova and most of its mass is blown away or converted into electromagnetic energy. What remains is the gravitationallycollapsed core of the star, which is incredibly dense and composed only of the subatomic particles known as neutrons; no atoms are present – see Fig.23. A matchbox-sized piece of a neutron star would weigh three billion tonnes, the same amount as a cube • Detection of signals that were one billion times (or more) lower than the ambient vibrations • The programming of supercomputers to mimic the human ability to pick complex sounds from background noise • Learning how to prevent spurious noise from powerful laser lights from affecting detectors He mentioned the following contributions to Advanced LIGO: • Gingin team: vibration-isolation systems, giving the world’s best performance • ANU: length-stabilisation system and technology that uses quantum entanglement to reduce noise in the detector’s laser • University of Adelaide: sensors siliconchip.com.au Fig.24: the features of a pulsar, including its spin axis, magnetic field axis (which does not necessarily correspond to the spin axis) and magnetic field lines. Pulsars are neutron stars with strong magnetic fields. Beams of light are emitted along the magnetic axis, and if it is aligned with Earth, a “lighthouse” effect is seen. There could also be an accretion disc from other matter falling into the pulsar. from the Earth measuring 800 x 800 x 800m. A neutron star has a radius of about 10km, and a mass of about 1.4 times that of our Sun. Some spin several hundred times per second, have magnetic fields and emit beams detectable on Earth, and are known as pulsars (Fig.24 & 25). They are much like a “cosmic lighthouse”. The fastest known pulsar spins 716 times per second. For stars that are sufficiently massive, or neutron stars that accumulate sufficient additional matter to enable errors in the laser to be corrected at the level of 1/20,000 of the wavelength • UWA: the team predicted (and was proven correct) that the laser light in Advanced LIGO would create sounds in the mirrors, which would cause the detectors to become unstable, and went on to develop methods to control these instabilities • Charles Sturt University: detector calibration and characterisation of detection methods • The CSIRO: provision of some of the optical coatings on the Advanced LIGO mirrors There is also a special need for a southern-hemisphere gravitational wave detector. This would allow very Australia’s electronics magazine Fig.25: an image of a pulsar from NASA’s Chandra X-ray Observatory satellite, showing its jet, an outflow of ionised matter along its axis of rotation. such as when the core remnant is 3-4 solar masses or more, it will undergo complete gravitational collapse. Rather than stopping at the stage of neutron star, a black hole will be formed. A black hole has such powerful gravity that not even light can escape, and it will swallow any object, including stars, that come too close. Most galaxies are thought to have a supermassive black hole at their centre, with a mass ranging from 100,000 to one million times that of the Sun (or more). Neutron stars and black holes are the smallest and densest known objects in the universe. Neutron stars, pulsars and black holes can form binary pairs, orbiting each other, in any combination. accurate mapping of the source and greater sensitivity. If the source location were accurately known, radio, X-ray and optical telescopes could also observe the source. Other present contributions include Swinburne’s supercomputer via OzGrav. Australian International Gravitational Observatory AIGO is an Australian gravitational wave facility near Gingin, Western Australia, about one hour from Perth. It is primarily used for developing instrumentation for gravitational wave detection. It has an interferometer with 80m-long arms, and should funding ever become available, sufficient land October 2021  23 Fig.26: the present and future AIGO facilities and other facilities on-site near Gingin, Western Australia. Fig.27: a simplified diagram of the proposed Australian NEMO gravitational wave observatory. PRM is power recycling mirror; BS is beam splitter; ITM is input test mass (mirror); ETM is end test mass (mirror); SRC is signal recycling cavity; and SRM is signal recycling mirror. to build two 4km-long interferometer arms as used by Advanced LIGO (see Fig.26). The site houses the Australian International Gravitational Research Centre and also the Gravity Discovery Centre, which you can visit at gravitycentre. com.au See the video from 2012 titled “AIGO Australian Interferometric Gravitational wave Observatory” at https://youtu.be/BLO1fgkqa6g NEMO The Neutron Star Extreme Matter Observatory (NEMO) is an exciting Australian proposal to build a gravitational wave observatory explicitly designed to observe the merging of neutron stars that form a black hole – see Fig.27. Such mergers are estimated to occur about once every five minutes somewhere in the universe. They involve transforming the nuclear matter of neutron stars into a black hole or singularity, which is essentially the opposite process of the Big Bang, when a singularity transformed into nuclear matter. Such observations would give great insight into what happened in the Big Bang plus other related phenomena. The proposed technology uses a powerful laser and ‘quantum squeezing’ of light to achieve a very high sensitivity at a fraction of the cost of other gravitational wave detectors. The detector is optimised to be most sensitive in the 1-4kHz band of interest for the mergers being studied. For a paper about NEMO – “A kHzband gravitational-wave detector in the global network” – see siliconchip. SC com.au/link/ab9g Things you can do at home You can volunteer to participate in the search for gravity waves and gamma ray and radio pulsars using idle time on your computer with Einstein<at>Home (see Fig.28). This is a global-distributed computing project, and the free software automatically downloads and analyses data from LIGO, GEO600, VIRGO and the Arecibo radio telescope and the Fermi Gamma-Ray Telescope satellite. You might be aware that the Arecibo radio telescope collapsed, but old data sets from it are still being analysed. As of September 2020, 55 radio pulsars and 25 gamma-ray pulsars have been discovered by Einstein<at>Home (see https://einsteinathome.org). You can also participate in Gravity Spy, which helps scientists sort data ‘glitches’ from real gravitational wave signals. This is done by looking at signals and deciding what category they fit into. See www.zooniverse.org/projects/ zooniverse/gravity-spy There is no chance of a hobbyist doing their own gravitational wave observations, but they can observe the cosmic microwave background (CMB) radiaFig.28: the Einstein<at>Home tion. This can be done using old analog TVs, or even modern TVs with an anascreensaver, showing its log reception option. computation status. A small proportion of the noise that can be seen when tuned to an unused channel is attributable to the CMB; similarly, with an FM radio tuned between channels, a small amount of the hiss is from the CMB. You can make measurements of the CMB using a satellite TV and dish according to the description at the following link, but you will probably need access to liquid nitrogen. This is used by some restaurants and bars as well as laboratories – but follow all safety precautions if you obtain some. See https://portia.astrophysik.uni-kiel.de/~koeppen/CMB.pdf 24 Silicon Chip Australia’s electronics magazine siliconchip.com.au