vol III chap 10 sect 2
Previous: 10.1. The Michelson interferometer and the Michelson-Morley experiment.
10.2. Gravitational astronomy.¶
Until very recently the gravitational force had resisted any experimental demonstration that the boson representing its quantum field really exists: the graviton. One of the problems is that the estimated interaction time of this particle is of the order of years. Such times are much shorter for the bosons that correspond to the other three forces of nature. If we indicate these times in seconds, for the photon, the quantum of electromagnetic force, it is between \(10^{-14}\) and \(10^{-20}\), for the gluon, the quantum of the strong force, it is less than \(10^{-22}\) and for the \(W^+\), \(W^-\) and \(Z^0\) particles, the quanta of the weak force, it is between \(10^{-8}\) and \(10^{-13}\).
The quanta associated with gravitational, electromagnetic, and strong interactions have zero mass and infinite range, while the weak force messenger, the gluon, has finite mass and short range. Another fact characterizes the type of propagation of two of these interactions: while photons propagate through electromagnetic waves, gravitons do so through gravitational waves. According to Thorne, next Table 10.1 describes the main differences between these waves:
| Table 10.1. Differences between electromagnetic waves and gravitational waves. | |
|---|---|
| Electomagnetic waves | Gravitational waves |
| "The oscillations propagate through three-dimensional space." | "The oscillations propagate through the very space-time “fabric” or shape of which the universe is made." |
| "The waves are incoherent superposition of emissions produced by individual charged particles, atoms or molecules." | "The waves are emitted coherently by the bulk motion of matter or energy." |
| "They are too easily absorbed and scattered by matter between their source and Earth." | "They are never significantly absorbed or scattered by matter, even when emitted in the earliest moments of the Universe´s life." |
An interference pattern produced by gravitational waves was detected and measured on 14 September 2015, after more than forty years of collaboration of over one thousand researchers from nearly twenty countries. However, it was not reported until February 11, 2016 because its authors wanted to refine their calculations.
The observation of gravitational wave interferences indicated that very far away in the past a violent change in the structure of spacetime was originated by the collision of two massive black holes during a few tenths of a second, more than a thousand million years ago. The difference in masses before the collision (29 and 36 solar masses) and after the collision (62 solar masses) resulted in energy dissipated (3 solar masses) in the form of gravitational waves.
The LIGO detector was just a laser interferometer with two arms forming an L, a beam splitter in the corner of the L and mirrors at the beginning and end of each arm whose length was 4 km. It functioned as follows: light coming from an external laser arrived at the beam splitter after which two signals traveled in different directions and reflected back and forth many times along the interferometer arms. Finally, these signals overlapped in the beam-splitter from where it was registered in a photodetector (see steps 5 and 6 in Figure 10.2)
When the interferometer’s arms did not change length, both signals traveled equal distances, and no interference pattern was formed. Nevertheless, gravitational waves shrink and lengthen spacetime and therefore modify the distances traveled by the laser radiation in the arms of the interferometer. Under these conditions the component waves arrived out of phase and produced an interference pattern. The compressions and expansions of space-time generated the observed gravitational wave interference pattern so much expected (Figure 10.2).
Figure 10.2. Figure 3. "How to catch a gravitational wave. The world’s first captured gravitational waves were created in a violent collision between two black holes, 1.3 billion lightyears away. When these waves passed the Earth, 1.3 billion years later, they had weakened considerably: the disturbance in spacetime that LIGO measured was thousands of times smaller than an atomic nucleus."
According to the document Popular information, the detection of gravitational waves manifests three characteristics of how physics is done in our time:”
- Broad and deep theoretical advances
“The predictions of Einstein's theory of general relativity, according to which the dimensions and scope of three-dimensional space are interrelated with the fourth dimension of time, were revised and updated. This fact is a consequence of the presence of very massive objects that produce the curvature of space-time where gravitational waves can be produced and propagated. Because these waves carry energy, they can produce observable effects.”
- Multidisciplinary and international collaborations
“The work was recognized in 2017 with the Nobel Prize in Physics to only three scientists, but has involved the collaboration of many people for more than 40 years. Participation in what became the LIGO project was initiated by Weiss and Thorne around 1980; then Barish led the participation of more than a thousand scientists (theoretical and experimental physicists, engineers, technologists and administrators). In August 2017 LIG0 and the Italian detector VIRGO simultaneously recorded three new events.”
- Powerful detection and recording instruments
“The two arms of the LIGO interferometer are two tunnels perpendicular to each other. At the end of each tunnel of length \(d = 4\) km there is a mirror that reflects the light coming from a laser. The two beams of reflected light then overlap after traveling an effective distance on each arm that differs from \(d\) in much very small but significant amounts. By the effect of the gravitational wave, while the length in one arm of the interferometer is shortened, in the other arm it lengthens. Then the light travels different distances in each arm and the electromagnetic waves arrive out of phase to the meeting point where they produce interference. This effect is amplified if the laser light bounces off the mirrors many times to increase the total distance traveled and make the difference in the lengths of each arm more evident.”
In what follows we present some excerpts of the three 2017 Nobel Lectures. When the text of the Lecture contains subtitles, we include them. Furthermore, for each laureate we include the document called WORK describing his contributions.
2017 Physics Nobel Prize awarded to Weiss.
WORK: “One consequence of Einstein’s general theory of relativity is the existence of gravitational waves. These are like ripples in a four-dimensional spacetime that occur when objects with mass accelerate. The effects are very small. Beginning in the 1970s the LIGO detector was developed. In this detector laser technology is used to measure small changes in length caused by gravitational waves. Rainer Weiss (1932) has made crucial contributions to the development of the detector. In 2015 gravitational waves were detected for the first time.”
MLA style: Rainer Weiss – Facts. NobelPrize.org. Nobel Prize Outreach AB 2024. Sat. 3 Feb 2024. https://www.nobelprize.org/prizes/physics/2017/weiss/facts/
NOBEL LECTURE: LIGO and Gravitational Waves I.
…..
“The three of us will give talks with the same title, “LIGO and the Discovery of Gravitational Waves”, but focus on different aspects. I will discuss some of the early history of gravitational waves and develop the concepts to understand the detectors as well as the challenges faced in measuring strains as small as \(10^{–21}\). Barry will show how the LIGO project organized to make steady improvements and ultimately carry out a successful scientific program. He will describe the detections as well as ideas to improve the detectors driven by the new science we hope to learn. Kip will look at the broader aspects of the new field of gravitational wave astronomy. He will tell of the critical role numerical relativity and understanding the quantum mechanics of precision measurements has played. He will also give a vision of the science that could come from an investigation of the gravitational wave sky from periods of fractions of milliseconds to tens of billions of years.”
…..
“In 1915, a little over 100 years ago, Einstein published the General Theory of Relativity (GR), a new theory of gravitation which replaced the Newtonian force by the idea that mass distorts the geometry of space and the flow of time. Matter then moves in this new space-time along the shortest four-dimensional paths” …
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… “GR also provided some new phenomena open to measurement; small effects difficult to measure but profound in their importance. These were the prediction that clocks ran more slowly in strong gravitational fields than weak ones and that starlight passing the limb of the sun would be deflected toward the sun, the so-called bending of light.” ….
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“GR had more in it still. In principle, as we have now have found directly, one could carry out calculations of massive systems moving at relativistic speeds. Furthermore, gravitational information – gravitational waves – did not travel at infinite speed as implied by Newton’s gravity, but travelled at the speed of light as is necessary to be compatible with special relativity.”
“In 1916 Einstein wrote a paper showing some of the ideas discussed above by perturbation calculations using linearized versions of his full field equations. It is in this paper he first describes gravitational waves.”
“They travel at the speed of light and are transverse waves much as electromagnetic waves, but rather than exerting forces on charges, they distort space perpendicular to the direction along which they propagate. Alternately stretching space in the east-west direction while simultaneously compressing space in the north-south direction.” …. “The gravitational waves carry energy as well as linear and angular momentum. In the 1916 paper, Einstein describes the wave kinematics well but runs into trouble showing how accelerated masses radiate gravitational waves.” ….
….
“An example one might have expected Einstein to use is the gravitational radiation emitted by the collision of two trains, a man-made event of significant energy transfer. … Another possibility might have been to estimate the change one might be able to detect in observing a binary star system through a telescope. The two stars orbiting each other would lead to the loss of energy by gravitational radiation and would be changing their period as they fall toward each other.” …
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NOTE: Weiss´ Lecture contains detailed descriptions of experimental contributions leading to the organization of the LIGO Project. These aspects will be considered in the activities reported in Table 10.1, Section 10.3.
MLA style: Rainer Weiss – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Mon. 15 May 2023. https://www.nobelprize.org/prizes/physics/2017/weiss/lecture/
2017 Physics Nobel Prize awarded to Barish.
WORK: “One consequence of Einstein’s general theory of relativity is the existence of gravitational waves. These are like ripples in a four-dimensional spacetime that occur when objects with mass accelerate. The effects are very small. Beginning in the 1970s the LIGO detector was developed to record gravitational waves. Barry Barish (1936) had a leading role in the project from 1994 and made crucial contributions to the development of the detector. In 2015 gravitational waves were detected for the first time.”
MLA style: Barry C. Barish – Facts. NobelPrize.org. Nobel Prize Outreach AB 2024. Sat. 3 Feb 2024. https://www.nobelprize.org/prizes/physics/2017/barish/facts/
NOBEL LECTURE: LIGO and Gravitational Waves II.
INTRODUCTION
THE LIGO LABORATORY
THE LIGO SCIENTIFIC COLLABORATION (LSC)
INITIAL LIGO PERFORMANCE AND SCIENCE
ADVANCED LIGO
THE BLACK HOLE MERGER EVENT (GW150914)
MORE BLACK HOLE MERGERS
SCIENCE IMPLICATIONS OF THE OBSERVED BLACK HOLE MERGERS
VIRGO DETECTS GRAVITATIONAL WAVES
NEUTRON STAR BINARY MERGER
FUTURE DIRECTIONS FOR LIGO AND GRAVITATIONAL WAVES
THE FUTURE
FINAL REMARKS AND REFLECTIONS
LIGO – SOME KEY MEMBERS
“The observation of gravitational waves in the Laser Interferometer Gravitational-Wave Observatory (LIGO) was announced on February 11, 2016, one hundred years after Einstein proposed the existence of gravitational waves. This observation came after more than fifty years of experimental efforts to develop sensitive enough detectors to observe the tiny distortions in spacetime from gravitational waves.” ….
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“In these three lectures, Rai, Kip and I tell the story of LIGO and gravitational waves in three parts. Rai covers the physics of gravitational waves, the experimental challenges and some of the pioneering interferometer work. He highlights the experimental challenges and some important early innovations that were proposed, tested at small-scale and have been incorporated in the LIGO interferometers. In this lecture, I describe the LIGO project and, the improvements that led to detection of merging black holes in Advanced LIGO. I also describe some key features of the interferometers, some implications of the discoveries, and finally, how we envision the evolution of LIGO over the coming decade. Kip will talk about some early personal history, theoretical advances that were crucial to making and interpreting the LIGO, and finally, his vision of the future opportunities in this new field of science.”
…..
“The basic scheme for LIGO was to use a special high power stabilized single-line laser (neodymium-doped yttrium aluminum garnet = Nd:YAG) that entered the interferometer and was split into two beams transported in perpendicular directions.” …. “In the simplest version of the interferometer, the equal length arms are adjusted such that the reflected light from mirrors at the far ends arrive back at the same time, and inverting one, the two beams cancel each other, and no light is recorded in the photodetector.” …..
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“When a gravitational wave crosses the interferometer, it stretches one arm and compresses the other, at the frequency of the gravitational wave. Consequently, the light from the two arms returns at slightly different times (or phase) and the two beams no longer completely cancel. This process reverses itself, stretching the other arm and squeezing the initial arm at the frequency of the gravitational wave. The resulting frequency and time-dependent amount of light is recorded by a photo-sensor and recorded as the waveform from the passage of a gravitational wave. The experimental challenge is to make the interferometer sensitive to the incredibly tiny distortions of spacetime that come from a gravitational wave, while at the same time, suppressing the various background noise sources.”
“The spacetime distortions from the passage of an astrophysical source are expected to be of the order of \(h = ΔL/L\) ~ \(10^{-21}\), a difference in length of a small fraction of the size of a proton.”…..
…..
“LIGO has been a remarkable adventure, beginning with Einstein’s prediction in 1916, and some later wavering in his own belief in gravitational waves. There was a long road, bringing us into the 1950s before the theoretical community became fully convinced of gravitational waves. The experimental part of the story started in earnest in the 1950s when Joseph Weber began the first serious attempts to detect gravitational waves with resonant bars. Some of the early history was marred by false discoveries, but overall, much was learned from the resonant bar era that has been built into the way we approach using interferometers. The concept of doing coincidences from separated detectors to be confident of detections, the analysis of background noise sources is used extensively in LIGO, and finally analyzing off-coincident time slices to measure accidental background levels.”
NOTE: Barish´s Lecture contains detailed descriptions of activities leading to the organization of the LIGO Project. These aspects will be considered in the activities reported in Table 10.2, Section 10.3.
MLA style: Barry C. Barish – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Mon. 15 May 2023. https://www.nobelprize.org/prizes/physics/2017/barish/lecture/
2017 Physics Nobel Prize awarded to Thorne.
WORK: “One consequence of Einstein’s general theory of relativity is the existence of gravitational waves. These are like ripples in a four-dimensional spacetime that occur when objects with mass accelerate. The effects are very small. Beginning in the 1970s the LIGO detector was developed. In this detector laser technology is used to measure small changes in length caused by gravitational waves. Kip Thorne (1940) has made crucial contributions to the development of the detector. In 2015 gravitational waves were detected for the first time.”
MLA style: Kip S. Thorne – Facts. NobelPrize.org. Nobel Prize Outreach AB 2024. Sat. 3 Feb 2024. https://www.nobelprize.org/prizes/physics/2017/thorne/facts/
NOBEL LECTURE: LIGO and Gravitational Waves III.
We just present some excerpts corresponding to the last subtitle.
INTRODUCTION AND OVERVIEW
SOME EARLY PERSONAL HISTORY: 1962–1976
SOURCES OF GRAVITATIONAL WAVES
INFORMATION CARRIED BY GRAVITATIONAL WAVES, AND COMPUTATION OF GRAVITATIONAL WAVEFORMS
Observables from a Compact Binary’s Inspiral Waves
Post-Newtonian Approximation for Computing Inspiral Waveforms
Numerical Relativity for Computing Merger Waveforms
Geometrodynamics in BBH Mergers
THEORISTS’ CONTRIBUTIONS TO UNDERSTANDING AND CONTROLLING NOISE IN THE LIGO INTERFEROMETERS
Scattered-Light Noise
Gravitational noise
Thermal Noise
Quantum Noise and the Standard Quantum Limit for a Gravitational Interferometer
Quantum Fluctuations, Quantum Nondemolition, and Squeezed Vacuum
THE FUTURE: FOUR GRAVITATIONAL FREQUENCY BANDS
LISA: The Laser Interferometer Space Antenna
PTAs: Pulsar Timing Arrays
CMB Polarization
THE FUTURE: PROBING THE UNIVERSE WITH GRAVITATIONAL WAVES
Multi-Messenger Astronomy
Exploring Black Holes and Geometrodynamics with Gravitational Waves
Exploring the First One Second of our Universe’s Life
MLA style: Kip S. Thorne – Nobel Lecture. NobelPrize.org. Nobel Prize Outreach AB 2023. Sun. 4 Jun 2023. https://www.nobelprize.org/prizes/physics/2017/thorne/lecture/
CONCLUSION
“Four hundred years ago, Galileo built a small optical telescope and, pointing it at Jupiter, discovered Jupiter’s four largest moons; and pointing it at our moon, discovered the moon’s craters. This was the birth of electromagnetic astronomy.”
“Two years ago, LIGO scientists turned on their Advanced LIGO detector and, with the data-analysis help of VIRGO scientists, discovered the gravitational waves from two colliding black holes 1.3 billion light years from Earth.”
“When we contemplate the enormous revolution in our understanding of the universe that has come from electromagnetic astronomy over the four centuries since Galileo, we are led to wonder what revolution will come from gravitational astronomy, and from its multi-messenger partnerships, over the coming four centuries.”
Next: 10.3. Organization and evaluation of learning communities.