“The last homework assignment from Einstein” has at last been completed: gravitational waves have been detected.
What impact will this have on science? In what ways, and using what mechanisms, can gravitational waves be detected? How is the KAGRA Project (Kamioka Gravitational Wave Telescope, now being constructed in Japan) progressing? We asked these questions to three researchers who study gravitational waves in the School of Science of the University of Tokyo.
How are gravitational waves born?
On February 11, 2016, announcement of the first detection of gravitational waves shook the entire world. Einstein had predicted the existence of gravitational waves as a consequence of the general theory of relativity 100 years ago, but no one had been able to confirm its existence. Now, mankind has finally succeeded in the first direct detection of gravitational waves.
A signal that seemed likely to be a sign of a gravitational wave had been caught earlier, on September 14, 2015. The research team examined the signal data for nearly five months and concluded that it was a gravitational wave signal. The source was identified to be a pair of black holes 1300 million light years away from Earth.
“Gravitational waves were produced by the merger of the two black holes,” explains Prof. Jun’ichi Yokoyama of the Research Center for the Early Universe (RESCEU), the School of Science.
“The two black holes respectively had about 36 times and about 29 times the mass of the Sun. The merger left behind a final black hole about 62 times the mass of the Sun. The difference in the total mass before and after the merger was about three times the mass of the Sun, meaning that energy equivalent to about three solar masses was emitted as gravitational waves during the collision.”
Why are gravitational waves produced? The general theory of relativity explains gravity as a distortion or curvature of space-time. All objects with mass generate a gravitational field by curving or warping the surrounding space. The larger the mass of an object, the greater the curvature of space. Objects with small mass are pulled into this curvature as if to fall into a hole. The reason why objects on Earth fall toward the Earth is because Earth’s mass is much larger than the object’s. The general theory of relativity encompasses Newton’s law of universal gravitation.
“Gravitational waves are ripples in space-time generated when objects with mass move with accelerating speed. As the object moves, the curvature of space also moves to reflect the changed locations of the object, and accelerating objects generate changes in the curvature causing waves that propagate at the speed of light.” (Prof. Yokoyama)
One hundred years that led to the “discovery of the century”
Gravitational waves propagate the curvature of space-time, but their effects are extremely subtle.
“Even those gravitational waves produced by massive cosmic events can make the distance between the Sun and Earth expand and contract over a distance no larger than the diameter of one hydrogen atom.”
Associate Prof. Masaki Ando of the Department of Physics says that Einstein himself thought that gravitational waves were so weak that they would never be detected.
Einstein’s prediction of gravitational waves was made in 1916. His general theory of relativity led to a number of predictions, most of which have been proved true during the 20th century. However, only his prediction of gravitational waves remained undetected into the 21st century, which is why it is called “the last homework assignment from Einstein.” By a curious coincidence, the first announcement of the detection came exactly 100 years after the prediction was first made. The sheer length of time it took testifies to how difficult the attempt to detect gravitational waves has been.
The discovery of the century was made by the Laser Interferometer Gravitational-Wave Observatory (LIGO), jointly operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT). The observatory consists of two interferometric gravitational wave detectors separated by 3000 km—one installed in Hanford, Washington (northwestern U.S.) and the other in Livingston, Louisiana (southern U.S.). The detectors have two L-shaped arms, each extending 4 km.
Gravitational waves could never have been observed without these huge devices. It was the culmination of advancements in science and technology as well as the inexhaustible passion and long years of effort by numerous scientists.
Specific attempts to detect gravitational waves date back to the 1960s, about half a century after Einstein’s prediction. Although an astronomic phenomenon that suggests the effect of gravitational waves was discovered in the 1970s (which led to the 1993 Nobel Prize in Physics), direct evidence of space-time curvature could not be obtained.
Signs of hope started to appear before the turn of the century, around the year 2000. Observatories that operate on the same principle as LIGO were opened in Japan, U.S., Germany and Italy, forming an international gravitational wave network, which led to the accumulation of a wealth of data and techniques. The detectors used at these observatories are called the “first-generation gravitational wave detectors.” LIGO was constructed at the turn of the century (Initial LIGO), then later drastically upgraded in the 2010s (Advanced LIGO, aLIGO). This “second-generation gravitational wave detector” started full operation in September 2015. Immediately after its first run, it captured gravitational waves generated approximately 1300 million years ago. In 2017, three scientists from Caltech and MIT who had founded and led the LIGO Project were awarded the Nobel Prize in Physics for this discovery.
Fighting noise that hinders the detection of gravitational waves
Associate Prof. Kipp Cannon, who was appointed to RESCEU in February 2016, has greatly contributed to the detection of gravitational waves at LIGO. He is a member of the LIGO Scientific Collaboration (LSC)* and developed GstLAL, a data analysis software which played a critical role in the detection of gravitational waves. He was the Canadian representative of LSC at the time of the first gravitational wave detection in September 2015 and was one of the authors of the research paper reporting the first detection of gravitational waves, which later led to the aforementioned Nobel Prize.
“The signal data caught by the gravitational wave telescope are converted into a form that can be processed using software. GstLAL examines the data and determines whether the signals are from a gravitational wave or not,” says Associate Prof. Cannon.
Because gravitational wave detectors have very high sensitivity, they are extremely sensitive to changes in the environment. They can detect the tiniest earthquake tremors that humans cannot sense, vibration in soil caused by vehicles running on a nearby road surface, and even the thermal vibration of the detector material at a finite temperature. All of these constitute noise that hinder the detection of gravitational waves. While the instruments are carefully designed to mitigate such vibrations, it is difficult to completely eliminate all vibrations.
That’s why the two LIGO detector sites are located 3000 km apart from each other—to verify whether a signal is from a gravitational wave or simply noise. If the detectors are located far apart enough, they won’t detect the same local vibrations. This means that signals detected by only one of the two detectors can be determined as noise, whereas signals simultaneously detected by both detectors are very unlikely to be noise. However, there is still the possibility that noise occurring at the two locations may coincide by chance. This is where GstLAL is called for. The software examines the signal data and calculates the probability of the coinciding signals being caused by noise.
The waveforms of the two signals captured at Hanford and Livingston on September 14, 2015 matched theoretically derived waveforms to an amazing extent.
“The probability that these waveforms could have been caused by coinciding noises was calculated to be less than once in 50,000 years. Based on the statistically significant probability, we concluded that the signals were gravitational waves.” (Associate Prof. Cannon)
Gravitational waves opened a new window to the mysteries of the universe
LIGO succeeded in detecting gravitational waves once again in December 2015 and four times in 2017, counting six times so far, of which five were gravitational waves generated by the collision and merger of two black holes.
“It was a great surprise to us that the first gravitational wave ever detected originated from a black hole., followed by a series of likewise detections. Most scientists had expected that the first gravitational wave to be detected would be from the collision of binary neutron stars.” (Associate Prof. Ando)
Black holes have such a huge mass and strong gravitational pull that nothing, not even light (electromagnetic waves), can escape. Although their existence was theoretically demonstrated, we had been unable to directly observe black holes because they don’t emit light (electromagnetic waves). The gravitational wave detection was the first direct evidence of their existence ever captured. Moreover, it was the first time that we found two black holes would form a binary system to eventually collide and merge, and that we confirmed the existence of black holes with masses tens of times that of the Sun. The discovery of the century brought many firsts to science.
“If we could see the universe through gravitational waves, we would be able to observe astrophysical phenomena and cosmic events that we could otherwise not see,” says Prof. Yokoyama, stressing the significance of this discovery.
Since Galileo first invented the telescope in the early 17th century, humans have utilized various means to observe the universe, from visible light to x-rays, ultraviolet rays, infrared rays and gamma rays, which are all electromagnetic waves. Next came the era of neutrino astronomy to study subatomic particles flying in from deep space, and now gravitational waves provide a whole new way to study our universe, opening up a new realm of astronomy.
Prof. Yokoyama has studied the origins and evolution of the universe for many years. In the primordial universe, light (electromagnetic waves) could not travel freely, and therefore light (electromagnetic waves) cannot guide us to explore back in time to the very beginning of the universe. Neutrinos and gravitational waves are the key to clearly understanding the physics of the early universe and such phenomena as black holes that have long been cloaked in mystery.
Another feature of gravitational waves absent in light or electromagnetic waves is their ability to pass through matter without being distorted. This means that even events that occurred behind and beyond massive astronomical bodies or brilliant galaxy centers can be captured using gravitational waves. Gravitational waves have opened a new window to uncovering hidden secrets of the universe.
Looking through a multifaceted eye: Combining gravitational waves and electromagnetic waves
The gravitational waves detected on August 17, 2017 opened up another window to astronomy. The source of the waves observed on this day was the collision and merger of binary neutron stars. Neutron stars emit light (electromagnetic waves) as well as gravitational waves, so this cosmic event was captured by numerous facilities across the world including the Japanese collaboration of Gravitational wave Electro-Magnetic follow-up (J-GEM). It marked the first cosmic event observed in both near infrared rays and visible light.
Associate Prof. Cannon’s GstLAL software played an important role here as well. There is a system in place to automatically send notifications to astronomical observatories across the globe whenever signals that seem to be gravitational waves are detected.
“It had been theoretically predicted that elements heavier than iron were generated by the merger of neutron stars and in that process electromagnetic waves would be released. The data of the electromagnetic waves observed on that day well matched the predicted waves, suggesting that we captured the process of nucleosynthesis of heavy elements.” (Associate Prof. Cannon)
Observing cosmic events in a multifaceted manner by combining gravitational waves, light (electromagnetic waves), neutrinos and other “messenger” signals from outer space is called “multi-messenger astronomy”. This coordinated approach to observation is expected to elucidate the unknown mechanisms of various cosmic phenomena.
In this electromagnetic waves observation, important contributions came from Virgo, an interferometric gravitational wave detector located in Pisa, Italy, which started operating following LIGO. As with LIGO, Virgo went into operation in the 2000s (Initial Virgo), then later upgraded to become a second-generation detector (Advanced Virgo). The more detectors involved in detecting a gravitational wave event, the more accurate the identification of the wave source will be. Observation accuracy improved significantly with the addition of Virgo in the international gravitational wave network in August 2017.
The next-generation of gravitational wave telescopes: KAGRA, a “2.5 generation” detector
Another longed-for addition to the international gravitational wave network is KAGRA, a new gravitational wave telescope now under construction in Kamioka-cho, Hida City, Gifu Prefecture. It is built near the Super Kamiokande neutrino telescope. KAGRA is hosted by the Institute for Cosmic Ray Research, The University of Tokyo, co-hosted by the National Astronomical Observatory of Japan (NAOJ) and High Energy Accelerator Research Organization (KEK), and operated with the cooperation of more than 60 institutions around the world. Multiple research departments of the School of Science of the University of Tokyo are closely collaborating in the operation of KAGRA.
One of the two main features of KAGRA will be its placement deep underground, where ground surface vibrations will be low. KAGRA is built underneath a former mine by hewing enormous tunnels out of solid rock to form an L-shaped cavity with two vertical 3-km-long arms. Vibrations have been reduced to less than one hundredth of that on the ground surface. The other feature is that the mirrors at the heart of the facility will be cooled to an ultra-low temperature of -253 °C (20K) to reduce thermal vibrations.
“We will be able to stably operate the telescope by reducing vibration to the lowest possible level. Even the tiniest vibrations can cause the telescope to go out of order as gravitational wave telescopes are extremely sensitive instruments. If it goes out of order, then the operation of the telescope must be discontinued for maintenance. We can increase run time and minimize down time by reducing vibration.” (Associate Prof. Ando)
The two main features of being placed deep underground and maintaining ultra-low temperature are also being considered for the next-generation gravitational wave telescopes planned to be built in Europe, aiming to start operation in the 2030s. KAGRA has adopted these features ahead of others and that is why it is called a “2.5 generation” telescope.
The last homework assignment from Einstein has not yet been completely solved
KAGRA is now undergoing preparation towards the start of full operations in 2019. Pilot operations are expected to start by the end of 2018.
The start of the operation of KAGRA is much anticipated by scientists around the world. Prof. Yokoyama explains why.
“It is very important that multiple gravitational wave telescopes are simultaneously in operation to realize more detailed identification of wave sources and in-depth analysis of the nature of gravitational waves. KAGRA is expected to make critical contributions particularly to the latter.”
Analysis of gravitational waves that have been detected so far show results that do not conflict with the predictions of Einstein’s general theory of relativity. A more complete explanation of gravity, however, would require an ultimate theory that encompasses the general theory of relativity and reaches beyond. In fact, multiple theories of gravity have been proposed, and an important key for determining whether those theories are correct or not is the polarization mode of gravitational waves. The general theory of relativity predicts only two modes of polarization, whereas more modes are predicted in other theories of gravity.
Verification of polarization modes requires three or more gravitational wave detectors to be in operation. The two LIGO detectors plus Virgo would make three, but the two LIGO detectors are only counted as one for verifying polarization modes. This is because the arms of the detectors need to be set at different angles for mode verification, but the two LIGO detectors were built in parallel to ensure the first detection of gravitational waves.
“Operation of KAGRA is indispensable for verifying the general theory of relativity. If the theory does not pass this test, the theory of gravity will need to be expanded.” (Prof. Yokoyama)
Thus, the last homework assignment from Einstein has not yet been completely solved.
With an eye towards KAGRA’s start of operation, Associate Prof. Cannon is now working to ensure smooth collaboration among LIGO, Virgo and KAGRA. He is busy preparing for the collaboration, aligning data formats to make data obtained by each observatory available to everyone.
Associate Prof. Ando is engaged in the planning of the next project coming up after KAGRA. This project, called DECIGO, will take place in outer space. Three satellites will be launched to detect the warping of space-time between the satellites. The main aim is to capture the primordial gravitational wave that is thought to have been generated immediately after the birth of the universe. This would require even larger detectors, but ground-based detectors will naturally be limited in size. Preparations are underway to launch the satellites and start operation during the 2020s.
Humans have gazed at the sky since ancient times, longing to see the truths of the universe. We have acquired the means, one after another, to observe the universe. And now, we have acquired the gravitational wave telescope. What secrets of the universe will it reveal for us to see? This tool may also help us resolve the mysteries of gravity.
Humans have come another step closer to the reality of the birth of the universe and the true nature of gravity.
Interview and text: Masatsugu Kayahara
Photography: Junichi Kaizuka
Originally published in The School of Science Brochure 2018