How moving objects cause a ripple in spacetime - courtesy Clear Science

In 1916, Albert Einstein predicted that any event that disturbs spacetime will produce ripples that spread throughout the entire Universe. He called these ripples gravitational waves. This week, one hundred years after Einstein’s prediction, scientists revealed they had finally detected gravitational waves. The discovery being called the greatest scientific advance this century.  Here’s how they did it and why the discovery is such a big deal.

What’s all this mumbo-jumbo about gravitational waves and spacetime?

A ripple in spacetime - image courtesy Motion AddictsIn scientific geek-speak, gravitational waves are disturbances in the fabric of spacetime. If you drag your finger through a bowl of water, you will notice waves follow the path of your finger and ripple outward towards the edge of the bowl. Einstein predicted that the same thing happens when a heavy object moves through spacetime. It produces vibrations or waves in spacetime called gravitational waves.

How can spacetime have waves in it? Remember, spacetime is a four-dimensional (think height, width, depth plus time) “fabric” that can be shaped as objects move through it. Imagine you and your friends holding a sheet by four corners, pulling it tight. If you place a basketball in the center of the sheet, the fabric will sag and the basketball will settle in the center. If you place a baseball on an outer edge of the sheet, it will fall towards the basketball. The same thing happens with stars and planets and other objects floating around in the universe in spacetime. A star (like our Sun) exerts a pull on planets in the same manner.

Scientists thought gravitational waves existed existed but they had never been able to detect one. In 2002, they started searching for gravitational waves using an experiment called the Laser Interferometer Gravitational-Wave Observatory or LIGO.

The Laser Interferometer Gravitational-Wave Observatory or LIGO gravitational wave detector

The northern leg of LIGO interferometer in Hanford, WashingtonScientists from California Institute of Technology and the Massachusetts Institute of Technology (MIT) designed and built LIGO (it was paid for by the US National Science Foundation – Go Science!). LIGO is a huge gravitational wave detector consisting of two 2 ½ mile vacuum-sealed “pipes” joined at their ends to form an “L” shape (see illustration below). A mirror is placed at the end of each arm and two more mirrors placed where the arms join. A beam of laser light is split (using a beam splitter) and allowed to bounce back and forth between the mirrors inside the pipes. When the light returns to its source, it is measured using a photodetector. Ordinarily, when the two split beams are measured, they will be equal in length and cancel each other out but this is where things begin to get really, really weird.

Using a measurement of the laser beam’s length, scientists can tell if the length of the 2 ½ mile long pipe changes, even the slightest. If a gravitational wave passes through the earth, the length of everything on the planet (even your body) changes very slightly as the wave ripples through it. This of course, includes LIGO. Scientists can detect the change in LIGOs length when the length of the laser beams is not equal. This change in length is very, very small – much smaller than even the size of an atom (actually, about 1/1,000,000,000th the width of an atom). Thus, it is very difficult for scientists to measure and this is why gravitational waves are so hard to detect.

By 2010, LIGO had found no proof of gravitational waves but since scientists are persistent (they don’t give up easily), the experiment was reworked with better equipment to make it more accurate. In 2015, two new LIGO detectors (called Advanced LIGO) opened, one in Livingston, Louisiana and another in Hanford, Washington, nearly 2,000 miles apart. The new gravitational wave detectors were four-times more powerful than the first one. Having two detectors running at different locations allowed scientists to detect if a gravitational wave passed through Earth and the direction it was travelling.

Diagram showing how LIGO works

The discovery of gravitational waves

Aerial view of the LIGO experiment in Livingston, LouisianaOn September 14, 2015, the LIGO detector at Livingston saw something odd. It appeared to be a wave resembling the chirp of a bird (a rising low frequency that was clipped at the end). Scientists had never seen a signal like this before and since the equipment had just come back online, they wondered if it could be a glitch. They called Hanford to see if they had seen anything on their detector. They had. The signal had passed through the Hanford detector about 7 milliseconds later. LIGO had discovered its first gravitational wave!

Scientists later determined that the gravitational wave was the result of two black holes colliding. The black holes had spiraled into one another until they merged into a huge, massive gravitational sink in spacetime that weighed about as much as 62 suns. The collision had taken place over 1.3 billion light years from earth, a tremendous distance away – and this is why the discovery of gravitational waves is so important to science.

The discovery of gravitational waves and its importance to science

Scientists learn about the universe around us by observation. Until now, the means to observe the universe were limited to sight (telescopes), X-Rays and radio waves. With the discovery of gravitational waves, we now have another method to observe the universe – a method that allows very fine measurements of objects and events taking place billions of light years away. This will surely lead to a plethora of amazing discoveries over the course of the next few decades.

So what’s next?

Scientists are hoping to increase the sensitivity of Advanced LIGO even more – possibly as much as 10 times greater than the first LIGO detector. Now that they know LIGO works, they are much more likely to get the money needed to make such an important improvement to their equipment.

Inside one of the LIGO tubesThe discovery is widely considered to be one of the greatest scientific advancements in more than 100 years. It’s up there with some of the biggest discoveries like the discovery of the Higgs boson in 2012 and Hubble’s 1929 realization that the universe is expanding. It will almost certainly lead to a Nobel prize as well as opening up an entirely new field of gravitational-wave astronomy in which scientists will use LIGO’s cosmic microphone to listen intently to waves and learn more about the mysterious far-away objects that are producing them. It is the very real dawn of a new era.

Additional information

Listen to a gravitational wave produced by two black holes colliding

Wonder what a gravitational wave sounds like? Scientists provided the clip below which is a recording of the gravitational wave that passed through the Louisiana and Washington LIGO detectors. Weird!

Sources: Motion Addicts, Clear Science, California Institute of Technology, Massachusetts Institute of Technology, Laser Interferometer Gravitational-Wave Observatory (LIGO)
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