It is one of the most storied anecdotes in science, one that most of us learnt in school, and that our children and children’s children will probably hear as well — of how in the 17th century, Isaac Newton was resting under a tree in his garden when an apple fell on his head, resulting in the eureka moment that led him to discover the law of gravity.
“Newton’s Theory of Gravity stood for about 200 years...it was probably the most successful theory of physics ever,” says Professor Barry Barish, an experimental physicist from the California Institute of Technology and University of California, Riverside. Prof Barish was in Singapore last week for the 7th Global Young Scientists Summit.
And yet Newton’s theory had a major flaw, he says, one that Newton himself admitted. It assumes that gravitational force is transmitted instantaneously, even if it were acting at huge distances between planets. “But it’s unrealistic to think how gravitational effects would get to us immediately.”
“It’s not that Newton’s theory was wrong,” says Prof Barish. “It was just incomplete and didn’t describe all of nature,” he says.
Enter Albert Einstein in 1915, with his general theory of relativity. Einstein proposed a radical shift in thinking: that gravity isn’t an ordinary force, but rather the result of massive objects distorting space and time around it. Einstein’s calculations also showed that when massive objects, such as black holes or neutron stars, accelerate, they create ripples in space-time — similar to how placing a bowling ball on a trampoline distorts the space around it. These ripples are known as gravitational waves.
“But space and time are very stiff so the amount of distortion is incredibly small and Einstein concluded that it would probably never be measured,” explains Prof Barish.
Einstein even rejected his own idea 20 years after he proposed it, writing to his friend and fellow physicist Max Born: “I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation.”
“But what Einstein couldn’t foresee was modern science, where we have powerful lasers and instruments,” says Prof Barish. “So he didn’t believe it was more than a theoretical concept at that time.”
A hundred-year scientific quest
It would take scientists a century after Einstein’s initial prediction to prove that gravitational waves did indeed exist, with Prof Barish playing an instrumental role in its discovery.
In 1994, Prof Barish took on the role of director at the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO comprises two L-shaped detectors, called interferometers, in Washington and Louisiana, which use a series of laser beams and mirrors to pick up distortions caused by gravitational waves. As director, he helped revive what was until then, a fledgling project. He reorganised the management team, hired more scientists, developed a steady R&D programme for future upgrades, and established the independent LIGO Scientific Collaboration. He also made technical improvements to LIGO’s interferometers, upgrading the vacuum chambers housing them, making the switch from argon to solid-state lasers, and from analog to digital controls.
On the morning of 14 September 2015, Prof Barish woke to find an email directing him to look at two graphs on an internal LIGO website. The Washington and Louisiana detectors had both picked up a signal in the early hours of the morning, suggesting a gravitational wave had been detected. The blip only lasted two-tenths of a second, and caused a movement so tiny it only measured one-ten-thousandth the diameter of a proton, recalls Prof Barish.
“The story actually starts 1.3 billion years ago, when two black holes coalesced and merged, releasing a gravitational wave,” says Prof Barish. “It then passed through the universe and managed to come to Earth...and we detected that distortion.”
Scarce to believe it wasn’t just an instrumental error, Prof Barish and the large team of over a thousand scientists who work on LIGO, spent months meticulously checking to confirm that what the interferometers had detected was indeed caused by a gravitational wave — the first proof of its existence. For his efforts, Prof Barish was awarded the 2017 Nobel Prize in Physics, a prize he shared with two others.
Revealing the secrets of our Universe
Since then, ten other gravitational waves have been detected at LIGO and the Virgo observatory near Pisa, Italy. All but one of the waves were formed from the merger of black holes, with the exception the result of a merger of neutron stars (collapsed core of giant stars). The most recent announcement of gravitational wave detection in December included an event that to date, is the largest and most distant collision observed — a merger from about five billion years ago that created a black hole 80 times more massive than the Sun, which released an amount of gravitational energy equivalent to five solar masses.
Thanks to these detections, scientists now have a greater understanding of how black holes were created. Gravitational waves allow us to study black holes in a way that was previously impossible using X-rays, optical light, and radio waves. We can now infer that nearly all stellar-mass black holes weigh less than 45 times the mass of the sun — anything more than that and they’ll be unstable, says Prof Barish — and that heavier black holes were probably created earlier in the Universe and during the Big Bang.
Gravitational waves can also tell us how fast the Universe is expanding, because scientists can calculate, based on how strong the signal is, how far the waves had to travel before reaching Earth. They might even shed light on the physics of the Big Bang itself — gravitational waves are thought to have been produced when electromagnetic force and the weak nuclear force, two fundamental forces which were indistinguishable in the beginning, separated.
In a bid to detect more gravitational waves, larger and more sensitive interferometers are being built. One, called the Kamioka Gravitational Wave Detector (KAGRA) in Japan, has already been constructed and is due to start operating next year. Another detector, LIGO India, is expected to start operating in 2024. There are even plans for a space-based interferometer, a trio of probes called LISA, which is slated for 2034. If all goes well, scientists can look forward to gravitational waves revealing more secrets of our Universe.