7.1 Let’s Try It This Way1
Sarah woke up with a headache, a foul temper, and an empty bottle of chardonnay on the bedside table. A loud snore emanated from the pile of bedclothes next to her.
The faculty colloquia didn’t usually end this way, but Jim had brought home a group of friends last night to celebrate his promotion. Associate Professorship, no less. The deal had been clinched by his presentation: “Use of the Future Subjunctive in Pre-Chaucerian Couplets, circa 1333.” Standing applause. All that remained were a few formalities to be settled this morning.
She squinted at her out-of-focus watch. Ten past nine. Suddenly she was wide awake. Jim’s appointment was for ten-thirty. She grabbed his shoulder, and shook him. “Wake up!” she shouted. “Come on. Wake up, you drunken slob!”.
“Huh!”.
“Ten-thirty. Your appointment.”
“So what?”.
“It’s ten past nine already.”
Now she had his attention. He sat bolt upright, and peered at his own watch. Six-twelve. He reached his wrist across so that she could read the time. “Well obviously it’s stopped. Possibly something to do with you dropping it into Angela’s martini.”
They were both awake now. Jim got out of bed, and spooned instant coffee into two mugs. All this would change when his promotion came through. Then it would be an espresso machine and freshly roasted beans from Kenya. He switched on the T.V. to check the time. The screen was full of snow, and the hiss of white noise emanated from the speakers. Now this was unusual.
A voice broke through the crackle: “… all the reports coming in have the same basic theme. People went to bed in one place and woke up in another. One man claims that he woke up next to his best friend’s wife.”
Jim chuckled, thinking of Angela. His smirk vanished as Sarah joined him.
“What’s wrong with the T.V.?” she asked.
“Shh! Listen.”
“The nation’s transport system is in chaos. … One moment, please. We’re getting Professor Karl Profundis on the line.”
“Ach!” Jim scoffed. “He’s the science bozo from our college. Nutty as the proverbial.”
“Professor, you have an explanation for what’s going on?”.
“This is exactly what I said might happen. Read my book. Space–time is the fabric of the universe. In the neighbourhood of a black hole, it becomes stressed and warped. It’s starting to break down, just like any other material. Wormholes are a distinct possibility.”
Sarah pulled back the curtains and peered through the grimy window. Jim joined her, and together they stared at the scene outside.
Enormous holes were opening up everywhere in the street, and buildings were collapsing into them. An unearthly blackness existed in some directions, containing not even a flicker of light. In the sky, scores of people were floating around on a mad carousel, clinging as hard as they could to what was surely a fragment of space–time.
The carousel dipped and veered towards them. As it swung around Jim noticed a group of revellers, oblivious to what was happening, sloshing drinks and shouting to each other across a coffee table laden with unappetising finger food. They looked familiar.
Sarah turned to him. “But there are no black holes near the earth. … Surely.”
Jim shrugged. It wasn’t his field. “Not unless someone decided to make one.”
In the next instant, Jim’s world vanished. He felt himself falling into a black silence, so intense that it swallowed even the beating of his heart, where seconds and centuries, metres and kilometres swirled together in a cosmic potpourri.
***
We defer our discussion of black holes until later in this Chapter (Sect. 7.5). By then the reader will be in a better position to decide whether the story of Jim and Sarah is fantasy or prophecy.
7.2 A 20th Century Cathedral
As we have seen in the previous Chapter, in 1905 Einstein proposed new concepts of space and time that overturned the credos of centuries. Two hitherto different quantities, space and time, were combined into the single entity: space–time. Profound physical consequences were found to arise from one basic hypothesis: that the speed of light in a vacuum is the same for all non-accelerating observers.
Immediately, the restrictive nature of this hypothesis raises a question: what happens when the observer is in an accelerating frame of reference? We have intimated, when discussing the twin paradox in the last Chapter, that consequences do arise, which are not at all obvious. The other outstanding issue, untreated in Special Relativity, was gravity, probably the most ubiquitous source of acceleration throughout the cosmos.
It was clear to Einstein that Newton’s theory of gravity2 was incompatible with Special Relativity, and in need of modification or replacement. Newton’s equations contain no reference to time: the gravitational attraction between two objects depends only on their mass and their separation. According to Newton, a movement of one mass results in an instantaneous change in the force being felt by the other. This perturbation in the force propagates at an infinite velocity, which violates the limit of c imposed by Special Relativity. Einstein worked on these problems for a decade, and finally in 1916 published the results of his deliberations in a paper that has become a classic in the literature of physics.
While Special Relativity is very important, both for its scientific relevance and for its applications, General Relativity owes its significance mostly to its philosophical value. In fact there were very few practical applications of General Relativity in the first fifty years of its existence, at which time, as we shall see, solutions of Einstein’s equations were found for black holes, and other celestial objects. Prior to this period, the vanguard of physics lay with Quantum Mechanics, the other great 20th Century revolution. Purely as an intellectual monument to human ingenuity, General Relativity can be compared to other momentous achievements of humankind, such as the ninth symphony of Beethoven, Leonardo’s Giaconda and the Egyptian Pyramids.
7.3 The Principle of Equivalence
Passenger in rising elevator (1) and free falling elevator (2)
If the speed of ascension in the first example in Fig. 7.1 is constant, then the passenger is in an inertial frame of reference, and the theory of Special Relativity applies, as we saw in the last Chapter. However, suppose that the elevator motor is applying a constant acceleration to the elevator. The passenger will feel in his legs that he is becoming heavier. It is as though the force of gravity has increased.
Now imagine that the elevator is allowed to fall freely, as in the second example in Fig. 7.1. The passenger will drift about inside the elevator, as if gravity has suddenly been turned off. It hasn’t, of course; it is just that gravity accelerates the cabin and the passenger downwards by equal amounts. Another way of looking at this is to say that the elevator and passenger are both falling at the same rate. This is what is happening in those videos we have all seen of astronauts floating and tumbling around inside satellite space laboratories.
These ideas owe their origin to Galileo, who noted that, ignoring air resistance, different objects dropped from a height at the same time would hit the ground together, irrespective of their mass. He realised that mass appears in two different guises: gravitational mass, which is responsible for an object’s weight, and inertial mass, which is a measure of its resistance to changes in its state of motion. If you weigh an object, you are measuring its gravitational mass; if you push an object to set it in motion, you are resisted by the object’s inertial mass. The fact that objects fall at the same rate, irrespective of their gravitational mass, told Galileo that their gravitational masses and inertial masses have the same value. This property is known as the Principle of Equivalence.
The genius of Einstein lay in his ability to pursue the consequences of his hypotheses, wherever they happened to lead him, with no regard to conventional thinking or common sense. His tool was rigorous logic, of the type that we have discussed in Part 1 of this book. In formulating Special Relativity, Einstein had asserted that there was no experiment observers could perform to determine whether they were in motion or not. He now maintained that there was no experiment observers could perform to determine whether they were in an accelerating elevator or under the influence of gravitational fields. This hypothesis became the basis of his theory of General Relativity.
However, in this case we must attach a caveat. Einstein is assuming here that the elevator and its immediate surrounds are small compared with the distance to the centre of the external gravitational field. If our elevator had a size of many kilometres and were located near the surface of the earth, then clearly the gravitational field experienced by the observer would be greater at the bottom of the elevator than at the top. This difference could be detected by the observer, as it results in a stretching force, or tidal force, acting on the observer.
With a small elevator, the difference in the gravitational field between the top and bottom of a normal elevator is too small to be measurable, and Einstein’s hypothesis holds. It is analogous to the assertion that a football field is flat, even though we know it to be a part of the earth’s curved surface. The curvature over the length of the field is too small to measure easily.
7.4 Bending of Rays of Light Under Gravity
As an example of the application of Einstein’s hypothesis, let us now explore the question, raised by Newton, of whether light is deflected by a gravitational field: “Do not Bodies act upon Light at a distance, and by their action bend its Rays, and is not this action … strongest at the least distance? [1]” We know that, when we throw a ball, it falls to the ground, no matter how hard we try to throw it. What about the beam of light from our flashlight? Does it fall to the ground too?
Passage of light ray across an accelerating elevator
An external observer will see the light traversing from left to right, as the elevator accelerates upwards. This is shown in the left half of Fig. 7.2, where each drawing shows the elevator at equally spaced intervals of time. However, a passenger in the cabin will see the ray falling down, following a parabola, as shown in the right half of Fig. 7.2.
This all seems fairly straightforward. However, suppose now that we remove the accelerating drive from the elevator, and replace it by an equivalent downwards gravitational force. An external observer will see that the elevator is now no longer accelerating, but what about the elevator’s passenger? According to Einstein, since there is no difference between the effects of acceleration and gravity, the ray of light will travel on the same parabolic path as before.
Of course, because of the enormous speed of light compared with the velocity of the elevator, the distance that the ray is deflected by the gravitational field in this instance would be infinitesimally small. However, in astronomical examples—e.g., a ray of light from a distant star passing close by the sun—the effect should be observable. This argument was seized upon as a possible check on the validity of General Relativity.
Before discussing astronomical observations, let us recall what Newton’s law of gravity states on the same subject. As we saw in the last Chapter, the debate about the particle versus wave theory of light had been going on for centuries when Einstein began working on General Relativity. Maxwell’s equations (see Chap. 5) represent the zenith of the wave theory’s popularity, before the advent of Quantum Mechanics. There is no place for gravity in Maxwell’s theory, a failing it shares with QM. There is therefore no mechanism within it for calculating any deflection of a ray of light by a massive object.
However, Newton was a believer in the corpuscular theory of light: “Are not the Rays of Light very small Bodies emitted from shining Substances?” [2]. The corpuscles of light (now called photons) have zero rest mass. As, since Galileo, it has been known that the path of objects in a gravitational field is independent of their mass, we would expect all particles, even those with zero mass, to fall along the same trajectory.
Indeed, Einstein in his first calculation of this effect in 1911 followed precisely this argument. However, by the time of his 1916 paper, he had realised that he needed to include the effect of space–time curvature (see Sect. 7.4), induced by the presence of the gravitational mass, and corrected his earlier calculation [2] The amended value for the deflection was twice as large as that of his first (and Newtonian) result.
Solar eclipse, 29th May 1919, observed by Sir Arthur Eddington, and providing the first evidence for Einstein’s Theory of General Relativity. The horizontal bars to the top-right of the image indicate the position of stars. Image, public domain due to age (https://commons.wikimedia.org/wiki/File:1919_eclipse_positive.jpg (accessed 2020/05/26))
Headline from the New York Times, November 10, 1919, announcing the results of Eddington’s expedition. Image: public domain due to age (https://commons.wikimedia.org/wiki/File:Einstein_theory_triumphs.png (accessed 2020/05/26))
One should not gain the impression that the bending of light predicted by General Relativity is something anyone can observe in their backyard with a pair of binoculars. The deflection of the stars of the Hyades cluster was predicted to be 1.75 arc seconds. This is equivalent to the angle subtended by a 2 cm diameter coin placed at a distance of 2.4 km. Eddington’s observations were 1.61 ± 0.30 arc seconds, which is within observational error of the predicted value. A controversy (since resolved) developed over Eddington’s data analysis. For a more accurate analysis of Eddington’s observations, see [3].
Confirmation of Eddington’s results was sought by a second expedition in 1922 to observe the solar eclipse on September 21. A proposed observation at Wallal, in northwest Australia, was vigorously supported by Alexander Ross of the University of Western Australia, but met strong opposition in Britain. Ross pointed out that it had only rained at Wallal in September twice in the previous 25 years, surely a relevant factor in the choice of a location for such an important observation. British and Dutch-German expeditions, however, duly proceeded to a Christmas Island observation site, where it clouded over on the day, preventing any eclipse photography. Ross took part in a US Lick Observatory expedition to Wallal, successfully obtaining measurements for over 100 stars, and providing further confirmation of Einstein’s theory.
Donkey team hauling equipment from a schooner during the 1922 Solar Eclipse Expedition at Wallal. Image: courtesy of State Library of Western Australia (Image: Sourced from the collections of the State Library of Western Australia and reproduced with the permission of the Library Board of Western Australia. https://purl.slwa.wa.gov.au/slwa_b4792891_1 (accessed 2020/05/26))
Observation of sun’s image through 5-foot cameras at Wallal Downs Station. Image: courtesy of State Library of Western Australia (Image: Sourced from the collections of the State Library of Western Australia and reproduced with the permission of the Library Board of Western Australia. https://purl.slwa.wa.gov.au/slwa_b4833949_1 (accessed 2020/05/26))
Eddington was a strong supporter of General Relativity. Shortly after the publication of Einstein’s paper, he delivered a lecture presenting the new theory. At that time it was said that only three people in the world understood Einstein’s work.3 One of the audience asked him if he was one of these three.4 Eddington stopped and seemed to puzzle a while, until urged by the questioner not to be modest. “On the contrary,” he quipped. “I'm trying to think who the third person might be.”
7.5 Einstein’s Field Equations
Following his recognition of the importance of the Principle of Equivalence, Einstein’s next step in the development of General Relativity was his realisation that space–time, which he had introduced in his Special Theory, was not necessarily flat. By this we mean that the geometry of Euclid, that we are all familiar with from high school, need not apply.
Distortion of space–time by the presence of a heavy mass
In effect, what Einstein proposed was that the mysterious “action at a distance” of Newton’s gravity was actually the result of a distortion of space–time, the fabric of the cosmos. He applied a branch of mathematics, called tensor analysis, to his formulation of General Relativity. The equations he developed, now known as Einstein’s Field Equations, supplant the equations of Newton in the formulation of a theory of gravity.
Why then are Newton’s equations still one of the first things that students of physics learn at the beginning of their courses? Firstly, they are very much simpler than Einstein’s to apply. This would be of no consequence, if they gave the wrong results. However, as we have seen in Chap. 2, “wrong” to a physicist means something quite different from “wrong” to a mathematician. In terms of accuracy, for most applications, the results of the two theories are indistinguishable within the limits of observational error. Only when very high velocities are involved (approximating that of light) and/or very large masses are present, do we need to turn to Einstein.
The major area of current application of General Relativity is in the development of theories for the origin and evolution of the universe, as we will see in Chaps. 10 and 11. Here we present only a few special examples of simple minded solutions.5 However, a qualitative description of some of the results is important for an understanding of modern cosmology. Obtaining these solutions generally requires some simplification of the problem, usually by choosing examples with particular symmetries that reduce the number of variables involved.
Examples of three symmetrical 3D figures, with axes of symmetry shown
The advantage of symmetries in physics is that they reduce the complexity of the mathematics involved in the application of physical laws to the problem. For example, to specify the surface of a sphere, one needs to know only its radius and the location of its centre, while to specify a general surface in 3D space, one needs to know how the surface varies in all three spatial directions, i.e. a virtually infinite number of data.
The first solution to the Field Equations was provided by Karl Schwarzschild in 1916 for a spherically symmetrical mass with no electric charge and no angular momentum. Think of a heavy ball that is not spinning. This model provides a useful description of stars and planets. In the limiting case of small masses, the solution reverts to Newton’s law, as it must if the Field Equations are correct.
However, Schwarzschild’s solution reveals a singularity6 at the origin (i.e. at the centre of the gravitational mass), if the matter comprising the sphere is dense enough. Singularities are common in mathematics, but they are causes of concern in physics. In this case, the singularity is interpreted as a black hole. At a distance, called the Schwarzschild Radius, from this central singularity lies a surface known as the event horizon. Any physical object at a distance less than the Schwarzschild Radius (i.e., within the event horizon) will collapse under the intense gravitational field and become captured by the black hole. Nothing, not even light, can escape from inside this region. The very natures of time and space become intermingled here, as in the plot of a far-fetched science fiction movie. However, as we shall see later in this Chapter, black holes have been detected by astronomers, and are now an accepted consequence of the theory of General Relativity.
The possibility of creating a mini black hole on earth in an accelerator, such as the Large Hadron Collider in Geneva, has been raised, with some concern that it might escape and devour the earth. At first it was thought that the accelerator had insufficient energy for black hole production. More recent work shows that such an event is a real possibility. Luckily the calculations indicate that such tiny black holes would pose no danger, since they would soon shrink and disappear before being able to gobble up any matter [4, 5].
The second solution of the Field Equations that we wish to consider here is the one for a homogeneous, isotropic universe. This is just physicists’ jargon for a universe that is not “lumpy”, and looks the same in all directions. One might well argue that our universe, comprising, as it does, stars, galaxies of stars and clusters of galaxies, is indeed “lumpy”. From our perspective here on earth, it most certainly is. However, by moving far enough away and taking a big picture, these inhomogeneities merge into each other in the same way that the pixels on a TV screen merge together, unless one sits too close. They should not affect the results of the model too much.7
Solutions for this model of the universe were developed independently by Alexander Friedmann, Georges Lemaître, Howard P. Robertson and Arthur Geoffrey Walker in the 1920s and 1930s. The model is generally known, rather unimaginatively, as the FLRW model, and is discussed in detail later in Chap. 10. One of its characteristics is the presence of a singularity which is identified with the big bang at the origin of the universe.
A third important solution for the Field Equations was developed for rotating massive objects, including black holes, by New Zealand mathematician, Roy Kerr, in 1963. His work inspired a flurry of activity on the physics of black holes by Stephen Hawking, and others. One of the unusual effects arising from Kerr’s solution is frame-dragging. Objects in the vicinity of a rotating black hole become caught up in the rotation because of the curvature of space–time that the rotation generates. At close enough distances, even light itself must rotate with the black hole.
As we have already remarked, exact solutions of the Field Equations are hard to find. However, even in classical physics, not all problems that appear on the surface to be simple, can be solved precisely. The three-body gravitational problem, where the earth, sun and moon interact with each other, is a well-known example that we have discussed in earlier chapters. However, the enormous progress in computational technology since the latter half of the 20th Century has facilitated an alternative approach. Physicists now use numerical techniques and fast computers to obtain the trajectories of the planets and space probes with the desired accuracy. Likewise, numerical techniques are now being employed in “numerical relativity”, which is a burgeoning research field.
Being as it is, a theory of gravity, General Relativity is of most importance in domains where gravity overweighs the other forces of nature, i.e. in the cosmos at large. In this domain, experimentation, as it usually pertains to physics, is very difficult, and astronomers largely rely on observations of events over which they have no control. Nevertheless, by chance, natural phenomena do arise from time to time that provide an opportunity to subject the predictions of General Relativity to observational test. We shall discuss some examples of these in the following Sections.
7.6 Further Observational Evidence for General Relativity
Changes in Orbit of Mercury
Precession of the orbit of mercury about the sun (schematic)
The vast majority of this precession is accounted for by Newton’s theory, and the effects of the outer planets (see Appendix 7.1). However, there remains a 40 arcseconds/century discrepancy, which is very well explained by General Relativity, when one includes the distortion of space–time caused by the gravitational mass of the sun.
The explanation of the hitherto mysterious anomaly in Mercury’s orbit, combined with Eddington’s observation of the bending of light by gravity, remained virtually the only experimental tests of General Relativity for the best part of half a century. In 1916 Einstein had proposed three possible tests of his theory [6]. The third, known as the gravitational redshift, was not carried out successfully until 1954.
Gravitational Redshift of Light
Consider the situation where we fire a projectile upwards against a strong gravitational field. As the projectile rises, it loses kinetic energy because it must do work against the gravitational force that is trying to pull it back to the ground. Now consider what happens if we replace the projectile by a series of photons (aka a beam of light) from a laser. We have already seen in Sect. 7.3 that photons are deflected by a gravitational field in the same manner as other particles. We might therefore expect them in this example to lose energy as they struggle to overcome the gravitational force retarding them.
There is, however, an important difference between photons and other projectiles: photons cannot lose energy by slowing down, as does a bullet fired vertically. Instead they are condemned always to travel at speed c. However, this does not imply that the Law of Conservation of Energy is somehow broken in this example. We have already seen in our Chapter on Quantum Mechanics that the energy of a photon is proportional to its frequency. When a beam of light travels from a region of strong gravitational field to a weaker one, the photons manage to achieve energy conservation by reducing their frequency, while still travelling at the velocity c. This phenomenon is known as the gravitational redshift. It is called a redshift because the light is shifted to lower (i.e. redder) frequencies.
The first reliable verification of the gravitational redshift was made in 1954 by Popper [7], who measured a frequency shift of 0.007 percent in light emitted from the white dwarf star, 40 Eridani B. His result was within experimental error of the value predicted by General Relativity. Since this first ground breaking observation, numerous more recent experiments have confirmed the existence of gravitational redshift (aka gravitational time dilation) (See Appendix 7.2).
Gravitational Lensing
The galaxy cluster, SDSS J0146-0929. The graceful arcs near the centre are an example of an Einstein Ring (see text). Image, courtesy of ESA/Hubble & NASA (Acknowledgment: Judy Schmidt. https://www.nasa.gov/image-feature/goddard/2018/hubble-finds-an-einstein-ring (accessed 2020/05/27))
The photograph shows a galaxy cluster containing hundreds of galaxies interacting with each other through their gravitational fields. The galaxies are the fuzzy objects distributed throughout the image. Some are clearly spiral, similar to our Milky Way. Others are seen as thin discs, and are probably spiral galaxies with their edges pointing towards us.
In the centre of the image are the graceful arcs of an Einstein Ring. This phenomenon is caused by the bending of light from a distant star or galaxy by the gravitational field within the cluster. As a consequence, the distant object will appear to lie in a different location from its true position.
The effect is known as gravitational lensing, and is explained in more detail in Appendix 7.3.
7.7 Gravitational Waves
We have mentioned earlier that Newton’s law of gravity contains no reference to time; it therefore implies that if a mass distribution somewhere out in space changes suddenly, the gravitational effect is felt instantaneously throughout the universe. According to Einstein’s Theory of Relativity, however, the propagation of such effects cannot take place at a speed greater than that of light.
One of the consequences of General Relativity predicted by Einstein is that the effect of any sudden change in the distribution of mass is propagated through space in the form of a gravitational wave. This wave is a ripple in the fabric of space–time. In our analogy with a heavy mass on a trampoline in Fig. 7.7, imagine the mass being suddenly displaced up or down by a small amount. A wave induced by the movement in the elastic trampoline surface will propagate across the trampoline at a speed determined by the nature and tension of the material comprising the trampoline. In the case of gravity, the gravitational wave is expected to propagate across the space–time “fabric” at the speed of light. However, the magnitude of such space–time distortions was predicted to be so small as to be unobservable with the technology available in the first half of the 20th Century (i.e., during Einstein’s lifetime.) During this period, gravitational waves were generally considered an intellectual curiosity of little relevance to mainstream physics.
This situation changed dramatically in the 1960s when Joseph Weber at the University of Maryland announced the detection of gravitational waves in a series of scientific papers. He used “antennas” made from aluminium bars two metres long and one metre in diameter. They were located at two sites separated by approximately 1000 km, and detections were triggered when perturbations to the bars were detected simultaneously at the two locations.
The initial excitement of the physics world soon became tempered by a feeling that all was not well. Weber’s detections raised more questions than they answered. Theorists began examining what sort of event could produce gravitational waves of a magnitude sufficient to register on Weber’s detectors. Calculations indicated that all the stars in the universe would need to fall into black holes to explain his detections. Over the next decade, other experimentalists designed and carried out more sensitive observations than Weber’s but were unable to reproduce his results. His anomalous findings were attributed to over-enthusiasm and a lack of healthy scepticism.9
Although over the next forty years no success was achieved in detecting gravitational waves directly, indirect evidence of their presence soon began to accumulate. A new astronomical object emitting regular pulses of radiation was discovered in 1967. At first there was a flurry of excitement as it was argued that the extreme regularity of the pulses indicated that they must be artefacts produced by an extra-terrestrial intelligence. They were designated by the acronym LGM, for “little green men”. Sanity soon prevailed, and the radiation was explained as a beam projecting into space, focussed by intense magnetic fields surrounding a mystery object. As the object rotated, the beam swept across space, intersecting the earth regularly in an analogous fashion to a lighthouse beam sweeping across a ship.
Similar objects were discovered regularly in the following years. They were named pulsars. Their high speed of rotation indicated that they must be relatively small, about 20 km in diameter. It was suggested that they might be an object that had long been predicted but not yet observed: a neutron star.10 The mass of a neutron star is found to be about 1.4 solar masses, which leads to a density so high that on earth one teaspoonful of neutron star material would weigh about a billion tonnes.
According to General Relativity, such extremely compact objects revolving in binary systems might be expected to perturb space–time significantly as they rotate about each other. The result should be the emission of a gravitational wave, which would carry energy away from the system, and radiate it throughout space. An example of such a binary pulsar system is discussed in Appendix 7.4. The confidence of physicists in the reality of gravitational waves therefore grew, and motivated the construction of even more sensitive apparatus in an attempt to detect these elusive ripples in space–time on their passage through the earth.
Besides binary pulsar systems, other stronger sources of gravitational waves were known to occur, and offered more hope of detection. For instance, nearby exploding supernovas might be detectable. However, such events are very rare; the last two supernova explosions in our galaxy occurred four centuries ago. Another possibility consisted of two black holes orbiting about each other, and passing into the stage of a final merger. However, the question was still open on whether such systems even existed.
To pursue this question, extremely sensitive detectors were developed, taking the technology off in a new direction. A detector was constructed by splitting a beam of light from a laser into two components, then sending these beams along paths several kilometres long, perpendicular to each other. An incoming gravitational wave would be expected to change the length of one path slightly, compared to the other.
Aerial photograph of the Virgo Detector near Pisa, Italy. Image courtesy of Virgo Collaboration (Image: The Virgo collaboration/CCO 1.0 https://www.ligo.caltech.edu/image/ligo20170927e)
On September 14, 2015, half a century of perseverance since the time of Weber finally paid off, when a gravitational wave was detected at the Hanford and Livingstone observatories [8]. The Virgo observatory was offline at this time undergoing upgrades. The signal arrived at Livingstone 0.007 s before Hanford, indicating the source lay in the southern hemisphere.
Great care was taken to eliminate any possibility of error. The wave was interpreted as arising from the final fraction of a second of the merger of two black holes into a single, more massive, spinning hole, and provided the first evidence that such phenomena exist. LIGO scientists estimated that the black holes had masses of approximately 29 and 36 times that of the sun, and the event occurred 1.3 billion years ago.
Since that historic date, detections have become almost commonplace. The next twist in this exciting tale occurred on August 17, 2017 [9]. This time it was not merging black holes that were detected, but two neutron stars spiralling in and colliding with each other, only some 130 million light years away. The masses of the neutron stars were 1.1 to 1.6 solar masses. (A similar event was detected on 25th April, 2019.)
Hubble telescope photograph of Galaxy NGC 4993 on 22nd August, 2017. The box in the image (and enlarged inset) shows the optical flare from two colliding neutron stars. Within 6 days, the flare had faded to invisibility. Image: NASA/Swift (The image was released by NASA/Swift. https://www.nasa.gov/press-release/nasa-missions-catch-first-light-from-a-gravitational-wave-event (accessed 2020/05.27))
Bringing so many different types of observation platforms to bear on the same source enables details of competing gravitational theories to be checked against each other. The fact that electromagnetic signals (gamma-rays) and gravitational waves arrive at the same time, (i.e. GW travel with the velocity of light) is in agreement with Einstein’s theory and excludes some of the alternative contenders for a theory of gravity.
With the development of further, more powerful, LIGO observatories, gravitational waves have now emerged as a possible means of exploring the physics of the early years of the universe, at a time period less than 380,000 years after the Big Bang, when electromagnetic radiation cannot penetrate (see Chap. 11). This new field of astronomy has been named multi-messenger astronomy, as it utilises widely different technologies. The future will be watched closely to see what it brings.
To end on a human note, this recent discovery of gravitational waves is a triumph of perseverance for many, now elderly, physicists, who have spent their whole professional lives on what many believed would be a fruitless search.