Editor’s note:Cao Huy Tuong is a Vietnamese research assistant at the University of Adelaide. He sent this piece to Tuoi Tre News after three scientists won the 2017 Nobel Prize in Physics for their decisive contributions to the LIGO detector and the observation of gravitational waves.
About 1.4 billion years ago, two lonely massive black holes collided, spiraled and collapsed, undergoing one of the most powerful astrophysics phenomena ever known to humans.
It sent out its own message, something that we now call gravitational waves, which then traveled through the universe at the speed of light. But while it's traveling for 1.4 billion years, let's talk about the recent science history …
A brief history
In 1916, Albert Einstein predicted the existence of gravitational waves as a consequence of his theory of general relativity. In Einstein's theory, space and time are aspects of a single measurable reality called space-time. One can think space-time as a fabric. The presence of large amounts of masses such as planets and stars will distort this fabric. When these large masses suddenly move, they create space distortion in space-time like waves, rippling outwards like ripples of an agitated pond. As these waves travel out through the universe, they stretch and squeeze space-time like sound wave stretches and squeezes the distribution of air molecules, carrying information about their sources and waiting to be discovered. Unfortunately, often by the time these waves arrive at our Earth, the distortion would become too minuscule to detect. One would need an instrument that, if capable, measures down to half of the radius of a proton to detect gravitational waves. This caused Einstein himself to doubt whether such phenomena would ever be detected.
There have been numerous attempts to measure these cosmic signals since 1960s. Many have failed and many results were never reproducible. Rainer Weiss, who had been previously a college dropout given a second chance at MIT and went to establish himself as a respectable physicist working in microwave background fields, then came up with a design of a gravitational wave interferometer, which uses laser light to measure minuscule change in length, and eventually becomes what we know today as LIGO. Yet up until now, when asked about it, Weiss still humbly says that the idea had been floating around in literature a long time ago. Yet to anyone working in the field of gravitational wave detection, we were all aware that it was Weiss who was the first to come up with a detailed design which could work, the first to identify all the potential noise sources that could limit the detector's sensitivity, and the first to draw up a map of guidance, paving the way to the new territory of astronomy.
Like any big physics issue today, it's important to have an experimental instrument to perform the measurements, yet it is also equivalently important to understand the theory that describes the physics to optimize the instrument and to be able to interpret the data recorded and understand their implication on our knowledge of the universe. This is where Kip Thorne, professor of theoretical physics at Caltech, plays a vital role in the development of LIGO’s instrument. He created the theoretical gravitational wave research group at Caltech in 1968, and also the experimental group later on in the late 1970s together with Ronald Drever, who was originally from Glasgow. It was the trio Rainer Weiss, Kip Thorne and Ronald Drever who are now hailed as the fathers of the field. Unfortunately, Drever passed away earlier this year.
Yet the road to LIGO's success today had not been easy. There had been ups and downs. There was even a point when the National Science Foundation had almost decided to withdraw funding for this project due to poor management. It was Barry Barish who saved the project from this tragedy. Barish was appointed as LIGO's director in 1994. He started to reorganize the structure of LIGO, oversaw the construction phase as well as the installation of LIGO's facilities. He was also the one to establish the LIGO Scientific Collaboration, which brings together thousands of scientists from hundreds of institutes within America and across the globe, whose expertise spans across various areas from material science, quantum optics, to data analysis and astrophysics.
The daily challenges of gravitational wave detection
Having the big concept does not mean one can immediately detect gravitational waves. It feels like the whole universe is working against the detector when one desires to measure such minute change in distance.
Ground vibration becomes a serious problem since the tiniest movement can already move mirrors by microns. Thus all mirrors are hung on one thin glass fiber. These fibers create a pendulum system which significantly reduces the high frequency seismic noise of ground vibration. Yet, the whole detector is still severely affected when there is an earthquake. Sitting at LIGO's site, one can observe the effects of an earthquake across the globe, even if they are all the way across the Pacific Ocean from the remote corners of Nepal, or Tahiti. Dependent on their strengths or locations, earthquakes can leave severe consequences on a detector's sensitivity. Like any object, mirrors and glass fibers have their natural frequencies that, once excited, they can ring up. Seismic waves arriving from earthquakes can easily trigger this excitement that lasts for weeks, which puts the detectors out of observation and requires efforts from many scientists and engineers to re-calibrate the whole system. But an earthquake is not the only troublemaker. Every day going to work at the observatories, one can see noise caused by people's footsteps in the morning as everyone is heading to work in the nearby city. Every car and truck on the highway 10km away also leaves signature noise sources, which require years of study to understand their characteristics so one can eliminate them from the signals.
After ground motion, the tiny fluctuations of temperatures in the surroundings also impose noise coupling into the detector. The movement of air up and down the atmosphere also creates noise in the signals. Photons in laser beams “pushing” the mirrors by nanometers also create multiple issues that require weeks or months of study and calibration.
The daily life of a scientist at LIGO's observatories can be quite similar to a detective's work. There are noise sources popping up here and there, at different times, different places, sometimes having a regular pattern, sometimes occurring irregularly. The number of PhD theses taken to understand these noise sources over the years has almost become uncountable. Stories of refrigerators at the observatory injecting noise to the ground, or ravens knocking on metal pipes transporting liquid nitrogen outside the complex, causing strange noise coupling to signals, and many more have become legends being retold multiple times between people within the research community.
|A researcher takes a selfie in complicated laboratory attire at a LIGO lab. Photo: Cao Huy Tuong|
But thanks to these types of issue, LIGO has helped facilitate pushing the boundaries of science in multiple areas. Because of LIGO, we can now have a device that can measure the nanometer change in optics’ thickness due to ambient temperature fluctuation. Because of LIGO, we now have new detectors that can predict earthquakes 30 seconds earlier than current technology, a short yet crucial amount of time, just enough to help factories and power plants to carry on safe power-off and minimize damage caused by such tremors.
All of these hard work and efforts finally came to fruition on the night of the 14th of September 2015 during Advanced LIGO's first observation run. Gravitational waves coming from two massive black holes had finally arrived at Earth after 1.4 billion years of travelling, triggering two LIGO detectors in Hanford, Washington and Livingston, Louisiana.
For 5 months, hundreds of scientists worked on the signal, making sure that the signal was true, performing statistical analysis and calculating the chance that such a massive event could occur. The first month was shrouded in both excitement and nerves. There were thousands of data channels that every scientist working at the observatories had to go through to check that it was not a false signal. It was well known to everyone that LIGO often employs a procedure called false triggering to test both the detectors as well scientists' integrity during science run. A secret committee elected by LIGO's governance, whose identities are unknown, would decide on injecting a false signal that imitates gravitational signals to the detectors during this process. Other scientists had to work independently to recover this kind of signal and characterize the detectors' response. By this way, we could verify the performance of the observatory. This is also used as a test on whether all scientists collaborate as a group and determine if anyone will leak premature information to the public. After months of confirming that this was not the case for the first detection, everyone could finally sigh with relief and in February 2016, LIGO announced the detection to the world, thus becoming the very first direct observation of gravitational waves. For the very first time in history, we can measure the equivalence of sound in space-time in continuum. For the very first time, we are able to listen to the sound of the universe. And for one more time, we are able to prove Einstein's theory of general relativity, answering his 100-year-old question about the existence of gravitational waves. Since then, multitudes of detection have been made.
Into the future
The most exciting yet is the most recent announcement of gravitational waves detected by both LIGO's 2 detectors and LIGO's European counterpart, Virgo on the 27th of September. On August 14, 2017, a signal was seen at LIGO-Livingston. 8 milliseconds later, LIGO-Hanford reported a detection and 6 milliseconds after that, VIRGO also detected a signal. Statistical analysis shows this event must be a true one, the chance for a false signal to be detected by all three detectors is one in 27,000 years. The signal was found to come from a merger of black holes of 30 and 25 solar masses each and at a distance of between 1.4 and 2.2 billion light years away.
The combination of three detectors has made the localization of gravitational wave sources improved dramatically, reducing the area from which the signal came from to an area of sky measuring roughly 60 square degrees. This is still a large area, approximately twice the size of Vietnam. However, this is already 10 times better than any previous detection.
This localization is expected to improve further in coming years due to new detectors being built across the globe. One of the new detectors is KAGRA, which is LIGO's counterpart in Japan. The basic concept of KAGRA is similar, albeit some different technologies being employed. Unlike LIGO and VIRGO, whose detectors are on the ground, KAGRA is built inside a mountain to minimize the effects of atmospheric Newtonian noise. All KAGRA mirrors are also made from sapphire and kept cold at cryogenic temperature (less than minus 150oC) to minimize thermal noise. KAGRA is expected to enter observation by 2020. Another facility that will be built soon is IndIGO – a new observatory in India. This detector will inherit technology from LIGO. The Indian government officially unveiled this plan after the announcement of the first gravitational wave detection. Currently, the process of choosing a site for the detector has been finalized.
Further into the future, LIGO and VIRGO are currently planning for technologies used in the next 20-30 years. Two new designs for the third gravitational wave detector are currently under investigation: LIGO Voyager and Einstein Telescope, proposed by the European Union. LIGO Voyager calls for change of silica optics to silicon optics, all of which will be kept at minus 150oC, as well as a change from laser source 1064nm in wavelength to a 1.5 or 2 micron laser source. These will improve LIGO's thermal noise, which currently limits its sensitivity at the intermediate frequency band (from 10 up to tens of kHz). The Einstein Telescope, on the other hand, calls for the increase of the detector's arm length to 40km instead of the current 4km. This will impose challenges on building a large-scale high-quality vacuum system as well as designing a structure that is not limited by the curvature of the Earth. Beyond LIGO Voyager and Einstein Telescope, there is the Cosmic telescope, which is a combination of LIGO Voyager and Einstein Telescope technologies.
The story of gravitational wave detection does not just end there. Next step, detection will be brought from ground into space. This mission, called LISA (Laser Interferometer Space Antenna), is currently under intense research at the European Space Agency (ESA). LISA will consist of three spacecrafts, arranged in an equilateral triangle, flying along an Earth-like heliocentric orbit. The laser beam will link each spacecraft to one another over a distance of 2.5 million km, whose length is precisely controlled to detect gravitational waves passing by. By moving the detector from ground into space, we will be able to eliminate seismic noise, which limits sensitivity at low frequencies. LISA is expected allow us to detect gravitational waves emitted by binary systems on much smaller scales than black holes such as white dwarves and neutron stars. In 2016, ESA's mission LISA Pathfinder performed a test for new technologies for the future LISA. The results were extremely successful, exceeding ESA's initial expectations. Since then, NASA has also expressed interest in joining the LISA mission.
We are living in one of the most exciting times in science
“This year’s prize is about a discovery that shook the world,” said Goran Hanssen, the Swedish Academy's secretary general. LIGO's discovery certainly shook the world, not only for what it had been able to accomplish so far, but also for the promises it brings to the future of physics, to the knowledge of humankind of the vast universe that we are a part of. With traditional electromagnetic astronomy, we had only been studying the universe with our eyes, searching for light signals emitted by celestial objects. We had been guiding ourselves through the universe without the ability to hear. Various objects in the universe never emit light, such as black holes. Like sound, gravitational waves carry at least half the information about an object. With LIGO's fully functioning VIRGO joining the network of detectors, we can finally know what our universe is saying. We will be able to learn about things we have never known before, from the structures of black holes, neutron stars to the physical processes occurring in these extreme conditions. We will be able to confirm or reject theories that we have been trying to understand for decades. And more importantly, we now can enjoy the eagerness like a child, waiting to see what new physics we can learn, to unwrap the mysterious gift that is our universe.
As a student working in the LIGO scientific collaboration, it has been an honor for me to get the opportunity to work on such an exciting field of physics. We were all overjoyed by the news of the Nobel Prize awarded to Weiss, Thorne and Barish. Words can't describe the contributions they have made to the field. Their stories and success remain an inspiration for generations of physics students to come, to keep us going through the most challenging tasks, to stay up until 2:00 in the morning to solve some issues in the lab, to maintain curiosity and to be passionate, all for putting together the symphony of the universe.