The Force Awakens

Scientists Detect a Vital Component of Gravitational Force
May 6, 2016 

Einstein was right. Again.

One hundred years after Einstein’s work predicted the existence of gravitational waves, physicists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced they have conclusively detected these waves, and further, they match Einstein’s prediction almost exactly.

The idea of gravitational waves comes from Einstein’s general theory of relativity, which describes how the force of gravity is controlled by heavy objects bending space-time. The LIGO discovery is the first to show proof of these waves and potentially opens the door to seeing new astronomical events as well as a vastly more complete understanding of gravitation.

The LIGO Project is sponsored by the National Science Foundation, Caltech and MIT and includes more than 1,000 scientists from around the world manning two gigantic gravitational wave detectors in Louisiana and Washington.

On September 14, 2015, while the detectors’ engineers were running a test of the equipment, both detectors picked up a strong wave almost simultaneously. LIGO Executive Director David Reitze said when scientists saw the data, they were flabbergasted at how quickly they were able to detect waves.

“It took us months of careful checking, rechecking and data analysis even to be sure we detected something real,” Reitze said.

Space-time warpSo what exactly is a gravitational wave? The idea of gravitational waves was born from Einstein’s vision of gravity.

Newton thought that massive objects are somehow able to attract other massive objects, and the force of that attraction was gravity. What Einstein thought was that massive objects can influence the fabric of the universe itself, called "space-time."

Think of space-time as a giant trampoline, and something massive, like a star, as a bowling ball. When you put the bowling ball on the trampoline, the trampoline sags, making a divot around the bowling ball. The bowling ball has altered the landscape around it, so now if you try to roll a marble past it, the marble will fall into the divot with the bowling ball.

This was Einstein’s vision of gravity. Really massive objects will bend space-time with their weight, and nearby objects will fall into the divots they create. 

Gravitational waves are not divots, but ripples in this space-time fabric. Thinking back to the trampoline metaphor, you'd have to be doing something pretty serious to get an honest to goodness ripple effect on a trampoline. The same is true for gravitational waves. Einstein’s equations predicted that only the highest energy events in the universe, like stars or black holes swirling around each other could create a sufficient disturbance to create a gravitational wave.

The LIGO observation actually documented two black holes spiraling towards each other and joining together 1.3 billion years ago. The energy released in the fusion of those black holes was roughly three times all the energy stored in our sun. See what that looks like in this video.

LIGO Data

So how can we see these gravitational waves? Lasers. LIGO’s first two letters stand for “laser interferometer,” which is a fancy way of saying “ruler made out of light.” The laser interferometer shines a beam of light down a vacuum-sealed tube almost two and a half miles long. At the other end, a mirror reflects the beam down the tube to a detector that reads how the beam and the reflection interact with each other. This allows the interferometer to measure the distance from one end of the tube to the other with extreme precision; the precision needed to detect gravitational waves.

Just like a ripple in water is an area where more water gets packed into a smaller space, a gravitational wave packs more space-time into a smaller space. So when a gravitational wave passes by Earth it shrinks the earth and everything on it for a split second and then flies away at the speed of light. The effect is so small, however, that you could never notice it. When the waves passed last fall, for example, everyone on Earth was about a millionth of a billionth of the width of a human hair thinner. You can see an exaggerated version of this effect in this video.

We cannnot see the waves, but the L-shaped laser tunnels of the LIGO stations can and doopening up a whole new way to record the universe.

“Up until now we’ve been deaf to gravitational waves but now we can hear them,” Reitze said. “What’s really exciting is what comes next.”

While this type of discovery may seem too subtle an experience here on Earth to be worth much, recording gravitational waves is actually a huge leap forward in the most basic understanding of how the universe works. 

All of the major forces in the universe have an associated wave that carries energy and helps communicate that force. They govern how the nuclei of our atoms are held together and we see them every day in electricity and light, but scientists had never seen how gravity’s waves work. By studying gravity’s waves, physicists can generate an idea of how all the universe’s forces work as one.

This project will continue as more LIGO installations are being built around the globe and in space to detect gravitational waves on more frequencies. And if gravitational waves are another language of the universe, now scientists can hear it and perhaps begin to translate it. 

 — Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.