Over the past 100 years every prediction that Albert Einstein made in his theory of general relativity has been directly observed, except for one. Now, on the centennial of the publication of Einstein’s magnum opus, a global team of researchers, including an astrophysicist from West Virginia University, have verified that Einstein’s final prediction is true.
Gravitational waves exist, and they are going to blow your mind.
Sean McWilliams, assistant professor of physics and astronomy in the Eberly College of Arts and Sciences, is a member of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, the research team that detected these invisible ripples in spacetime.
“This is a watershed moment for physics and for science,” said McWilliams. “The direct observation of gravitational waves will fundamentally change our understanding of the universe.”
“This discovery is an example of the exciting research work being conducted by faculty
and students at West Virginia University,” said
Fred King, vice president of research. “This sort of ground-breaking work in
a variety of areas is what has enabled the University to achieve the recent recognition
as a Doctoral University – Highest Research Activity by the Carnegie Classification
of Institutes of Higher Education.”
For more information about the gravitational waves detected 100 years after Einstein's prediction, click here.
A beautiful mind
Einstein’s theory of general relativity describes the inner workings of gravity, which is the most important force on the scale of stars, galaxies, and the universe as a whole. Although it is complex and detailed, there is beauty and elegance in the theory’s form.
This theory describes spacetime as a fabric permeating the universe. Einstein posited that spacetime is not a static and rigid stage on which celestial bodies act, but is instead flexible, able to be distorted and warped as large masses move through it.
Picture a trampoline and a bowling ball. If you place the ball in the center of the trampoline its mass will cause a dip in the fabric where the ball is placed.
If large, very dense masses move in a particular way, such as pair of black holes or neutron stars spiraling into one another, the disturbances in the gravitational field will travel outward as gravitational waves.
In the analogy, if a pair of bowling balls were set to roll around each other on the trampoline, the fabric of the trampoline would undulate and oscillate, like ripples on a pond, reacting to the movement. The bowling balls would lose energy as a result of causing these oscillations, and they would ultimately collide with each other at the center.
Proving a prediction
The sources of these events occur billions of light years away, so their impact on Earth is infinitesimal. In order to detect gravitational waves, scientists need the most high-tech, sensitive measurement tools in the world.
That’s where the LIGO interferometers come in.
The National Science Foundation has funded an observatory in Washington state and in Louisiana. The instruments at each location are identical. They split a single laser into twin beams, and shoot each beam down a pair of perpendicular two-and-a-half-mile long vacuum tubes, turning them into giant L-shaped listening devices.
The beams bounce back and forth off a series of mirrors, and ultimately return to the location where they were originally split. Since the laser is just a wave of light, he instrument can measure the behavior of beams relative to one another by letting them interfere with each other; in this way, LIGO scientists are able to detect even the smallest change in the relative distance traveled, down to smaller than one-ten-thousandth the diameter of a proton.
This means that if a gravitational wave passing by causes even the slightest change in the distance of the arms, the instrument will be able to detect it.
LIGO physicists determined that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed, making this not only the first discovery of gravitational waves, but also the first discovery of a binary black hole system.
They estimate that the black holes were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago.
Based on observations they also determined that three times the mass of the sun was converted into gravitational waves in a fraction of a second – with a peak power output about 50 times that of the whole visible universe.
According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes.
During the final fraction of a second, the two black holes collide with each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s famous formula E=mc2. This energy is emitted as a final strong burst of gravitational waves, whichLIGO scientists were able to observe.
The galaxy in code
It’s not every day that you can say you’ve played a role in the biggest scientific discovery of modern times.
A native of the Pocono Mountains in Pennsylvania, McWilliams is a self-described coder. His work with LIGO has focused on simulating and modeling the gravitational-wave emission in order to detect the incoming signals and infer details about their source. He has been a member of the LIGO Scientific Collaboration, or LSC, since 2005. He is WVU’s institutional principal investigator for the LSC and is a member of the LSC Council.
He was part of a team of collaborators who performed some of the earliest supercomputer simulations of merging black holes. Since then, he has worked extensively on simulating and developing models for these signals, which LIGO scientists expect to detect from across the universe.
McWilliams has most recently collaborated with Zachariah Etienne, assistant professor of mathematics at WVU, and WVU mathematics graduate student Caleb Devine to improve and optimize the most state-of-the-art models for the signal from a pair of spinning black holes, so that the model could be used to better characterize the parameters of the Event.
In addition to making contributions to the writing and editing of the main detection paper and several of the companion papers, McWilliams was also one of a small group of experts who monitored incoming LIGO data in round-the-clock shifts during the first observing run, deciding when “triggers,” or relatively loud phenomena in the instrument, were actually cosmic events and should therefore be investigated further with regular telescopes.
In addition to McWilliams, WVU has three active members of the LSC: Etienne, Devine and physics graduate student Belinda Cheeseboro.
We go to the moon
McWilliams concedes that the detection of gravitational waves won’t make your morning commute go faster or get you a bigger tax return, but he says the discovery will open up an entirely new channel of information about the universe, revealing information that was previously unknown.
He likens the detection of gravitational waves to sound.
“Everything we know about the universe up until now has been visual,” says McWilliams. “Now we can not only see what is going on, but we can ‘hear’ it, too. We’ll discover things that we didn’t know existed and things that we thought we understood will be revealed in different ways.”
Currently, everything we know about the universe is what we have seen through telescopes and particles of light. But unlike light, gravitational waves can pass through the universe unobstructed, so they carry information that we cannot obtain otherwise.
That allows scientists to observe areas of the universe that we have never been able to before, like black holes. Up to this point, scientists had only been able to detect the black holes indirectly.
The binary system of two black holes detected by LIGO is at the end of life, giving a sketch in time of the final fractions of a second of this binary system as it became a single black hole.
McWilliams also says that the technology necessary to detect gravitational waves – from supercomputing to precise measurements of time – could eventually advance tools in our daily lives, such as computers and cell phones.
He continues to say that the detection of gravitational waves will provide more information about the evolution of galaxies and black holes over cosmic history.
For example, in the 1970s, scientists discovered a binary neutron star system that will merge and form a black hole in 300 million years. The binary system of two black holes detected by LIGO is at the end of life, giving scientists a snapshot of the final fractions of a second for such dense binary systems as they become a single black hole.
Beyond the exciting phenomena that scientists expect to observe in the coming years withLIGO and with other gravitational wave detectors, McWilliams emphasized that the possibilities for new and unexpected discoveries is almost without limit.
“This discovery is akin to Galileo first looking through his telescope and seeing the moons of Jupiter. We are “hearing” the Universe now for the first time, and given how much we have learned by seeing the Universe since Galileo’s time, it’s a genuine thrill to imagine how much we will now be able to learn by listening to gravitational waves.”
A space for discovery
In addition to his work with LIGO, McWilliams is a member of the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, along with WVUprofessors Duncan Lorimer and Maura McLaughlin.
The NSF awarded NANOGrav $14.5 million to create a Physics Frontiers Center aimed at using radio timing observations of pulsars with the Green Bank Telescope and Arecibo Observatory to search for the existence of low-frequency gravitational waves by using millisecond pulsars, nature’s most precise celestial clocks.
Last summer WVU launched the Center for Gravitational Waves and Cosmology, bringing together researchers from departments across the University and the National Radio Astronomy Observatory in Green Bank.
McWilliams says that since coming to WVU in 2013 he has been able to make significant advances in his research because of the ability to collaborate with colleagues in his department and across disciplines. In addition, McWilliams says that access the University’s supercomputing facility has been indispensable to his work.
“Roughly 10 years ago WVU developed a new research area in astrophysics. Now that original investment has resulted in one of six NSF Physics Frontiers Centers, a major newNSF EPSCoR grant, collaboration between colleges and departments, and with this result from LIGO, major visibility on the international stage for WVU faculty. Careful investment and support by the University created the foundation for all this activity,” said Earl Scime, chair of physics and astronomy. “It is an exciting time to be at WVU.”
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