What is LIGO?

Laser Interferometer Gravitational-Wave Observatory or LIGO is a global network of laboratories that can detect gravitational waves, which are tiny ripples in the fabric of space and time. LIGOs have detectors designed to look for these tiny cosmic ripples. For instance, the LIGO detectors can pick up a change of distance that is several times smaller than a proton.

So far, there are four such facilities in the world. There are two LIGOs in Washington State and Louisiana in the US. The one in Italy is called Virgo. The fourth one, named Kamioka Gravitational-Wave Detector (KAGRA), is in Japan.

What causes gravitational waves?

Gravitational waves are caused by some of the most violent events in the universe involving massive objects in motion, such as merging neutron stars or black holes, or exploding stars. Albert Einstein has first predicted the existence of gravitational waves soon after formulating the theory of General Relativity in 1915. Gravitational waves can help expand our understanding of the universe.

When LIGO made history

LIGO detected gravitational waves for the first time in 2015. Two black holes, 29 and 36 times the mass of the sun, merged 1.3 billion years ago to produce those waves. The scientists involved in the project — Rainer Weiss, Barry C Barish, and Kip S Thorne — won the Nobel Prize in Physics in 2017.

The detection confirmed Einstein’s theory that space and time are not distinct entities but are woven together in a fabric-like structure that curves, stretches, and even warps, because of gravity waves created by massive objects moving at high speeds. Since then, LIGO and its partner observatory Advanced Virgo (the facility in Italy) have detected more than 50 such signals.

Why have several gravitational wave detectors?

When a LIGO detector picks up a signal, scientists need to confirm that the source is indeed an event in space and not something on Earth, like an earthquake, traffic, or similar. One of the ways to rule out any errors is by looking for similar signals from all the detectors spread worldwide. With all the detectors working in tandem, scientists can nail down the sources of gravitational waves. India will add the fifth observatory to this elite network.

How the observatory works

The LIGO detectors comprise two 4-km-long L-shaped interferometers, which are essentially vacuum chambers about 4 ft in diameter with mirrors at the end. Light rays are released simultaneously in both chambers. The rays split into two beams traveling back and forth down the arms. The light should return at the same time in both chambers. However, if a gravitational wave passes through, there is a change in the length — smaller than one-ten-thousandth the diameter of a proton — which the LIGO detectors can pick up.

GRAVITATIONAL WAVE DETECTORS

Gravitational wave detectors work by using laser beams measured at a precise length reflected back by telescopes. Any changes in the length of the beam is caused by the passage of gravitational waves. However, there are a number of phenomena that can cause the detectors to detect the same signal as the passage of gravitational waves, including vibrations and electromagnetic contamination. Researchers are investigating if dark matter can also interfere with the signals.

INTERFEROMETERS

Interferometers are investigative tools used in many fields of science and engineering. They are called interferometers because they work by merging two or more sources of light to create an interference pattern, which can be measured and analyzed: hence 'Interfere-meter', or interferometer.

The interference patterns generated by interferometers contain information about the object or phenomenon being studied. They are often used to make very small measurements that are not achievable any other way. This is why they are so powerful for detecting gravitational waves--LIGO's interferometers are designed to measure a distance 1/10,000th the width of a proton! 

What does an Interferometer Look Like?

Because of their wide application, interferometers come in a variety of shapes and sizes. They are used to measure everything from the smallest variations on the surface of a microscopic organism, to the structure of enormous expanses of gas and dust in the distant Universe (using radio interferometry), and now to detect gravitational waves. Despite their different designs and the various ways in which they are used, all interferometers have one thing in common: they superimpose beams of light to generate an interference pattern. The basic configuration of a Michelson laser interferometer is shown at right. It consists of a laser, a laser beam-splitter, a series of mirrors, and a photodetector (the black dot) that records the interference pattern. 

Interference Pattern

To better understand how interferometers work, it helps to understand more about 'interference'. Anyone who has thrown stones into a flat, glassy pond or pool and watched what happened knows about interference. When the stones hit the water, they generate concentric waves that move away from the source. Where two or more of those concentric waves intersect, they interfere with each other. This interference can result in a larger wave, a smaller wave, or no wave at all. The visible pattern occurring where the waves intersect is an interference pattern. 

The changing black wave is the interference pattern created by the red and blue waves as they pass through/interact with each other. 

How do Gravitational Waves Affect LIGO's Interferometer?

Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is "Differential Arm" motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways.

As described above, as the lengths of the arms change, so too does the distance traveled by each laser beam. A beam in a shorter arm will return to the beam splitter before a beam in a longer arm--as the wave passes, each arm oscillates between being the shorter arm and the longer arm. When they arrive back at the beamsplitter (where they re-merge), the light waves no longer meet up nicely; they are out of phase. Instead, they shift in and  out of alignment for as long as the wave is passing. In simple terms, this shifting alignment results in a flicker of light emerging from the interferometer.

While in principle the idea seems almost simple, in practice, detecting that flicker is not. The change in arm length caused by a gravitational wave can be as small as 1/10,000^th the width of a proton (that's 10^-19 m)! Furthermore, finding a gravitational wave flicker amongst all the other flickers LIGO experiences (caused by anything that can shake the mirrors, like earthquakes or traffic on nearby roads) is another story. LIGO Technology describes in detail how LIGO filters out much of that "noise" in order to detect the telltale flicker of light caused by a gravitational wave.

PRACTICE QUESTION

Q. Consider the following statements:

1.Gravitational waves travel at the speed of light (186,000 miles per second).

2.When two big stars orbit each other, Gravitational waves are formed.

3.Interferometers work by merging two or more sources of light to create an interference pattern, which can be measured and analyzed.

4.Laser Interferometer Gravitational-Wave Observatory or LIGO is a global network of laboratories that can detect gravitational waves, which are tiny ripples in the fabric of space and time.

Which of the above statements is/are correct?

(a) Only 1 and 3

(b) Only 2 and 4

(c) 1, 3, and 4 only

(d) All of the above.

Correct Answer: (d) All of the above.