M3L3: The 5-mile long gravity detector Laser Interferometer Gravitational Observatory

What is Space? Stephen Hawkings explains us like this.

Imagine a ball falling on a trampoline. The trampoline bows downward first, stretching the fabric vertically and shortening the sides. Then as the ball bounces upward again, the horizontal movement of the fabric expands again. The trampoline gets stretched and squeezed in cycles itself.

Now imagine the universe as a stretched fabric. So, what happens when a giant star explodes? When a star explodes or collides, it suddenly redistributes its mass and causes an abrupt change in the gravitational field around itself. This distorts that fabric of space and time. This is something that Einstein had already hypothesized in his theory of relativity. He believed that such explosions should create powerful waves that should generate distortions in the normal fabric of space, and he called them as gravitational waves. These waves or propagation of disturbance should create outward ripples into the universe at the speed of light, stretching and compressing the space around any objects they happen to pass through. This also includes our planet Earth.

Astronomers believe that if we could detect these waves, they would illuminate much about the universe that is now hidden. However, since gravity is the weakest force in the universe, gravitational waves are extraordinarily difficult to detect. Scientists also believe that gravitational waves may also give them, a means to look back to the earliest moments of cosmic evolution, when the universe was still small and dense. And listen to the pulses in the universe, just as we use stethoscope to listen the interior of the body.

Newtonian physics doesn’t come close to explaining gravitation from black holes or other regions of strong gravity. Most of our current knowledge of the Universe comes from the observation of photons. They may take any of these forms, visible light, radio waves, X-rays, or any other type of electromagnetic radiation. They have a disadvantage since they cannot pass through the hot, dense regions which form the core of the stars. For example, the photons we observe from the Sun come from its photosphere, far removed from the hydrogen-fusing core. It can take a million years for a photon to work its way from the core of our sun to the surface, for example. And once photons leave the surface, they may be further altered or blocked by gas and dust in space before ever arriving at a detector on Earth. Therefore, we cannot investigate the properties of these regions by direct observation, but only by indirect inference. In order to observe the inner workings of the astrophysical objects and to obtain a description of the Universe over a larger range of energies, we need a probe which is electrically neutral, so that its trajectory will not be affected by magnetic fields, stable so that it will reach us from distant sources, and weakly interacting so that it will penetrate regions which are opaque to photons. The only candidate currently known to exist is the Neutrino. Other option, in the form of the wave can be gravitational wave.

To do this, physicists around the world have teamed with engineers to build technologically ingenious detectors to seek evidence of gravitational waves. The result of their efforts was LIGO. LIGO had to tackle two key problems: how to measure that minute distortion and how to reject any other noise coming from terrestrial sources that might mimic or mask the distortion. LIGO’ working is based on the concept that, since the gravitational wave stretches and squeezes the space around itself, it should momentarily change the lengths of LIGO’s two arms.

Some important features of LIGO from UPSC point of view

1.    LIGO consists of two “arms” that are about 2.5 miles long. The detectors are located in Livingston and another in Hanford. They act like a pair of ears listening simultaneously for the same bursts of gravitational waves.

2.    The detectors are separated by about 2,000 miles so the findings can be compared.

3.    The arms are actually vacuum pipes that measure about 4 feet in diameter and form an L shape.

4.    It is used to detect gravitational waves that hint at massive collisions in the deep universe.

LIGO isn’t the only game in town when it comes to hunting for gravitational waves. Here are a few other ongoing and future projects.

Ground-based interferometers

A couple of other detectors similar to LIGO are in Europe. The Virgo detector, near Pisa, Italy, is being upgraded and will team up with LIGO later this year. GEO600, near Hannover, Germany, has been the only interferometer running for the past several years while Virgo and LIGO underwent renovations. A third LIGO detector, this one in India, is scheduled to join the search in 2019.

Space-based interferometers  

In space no one can you hear you scream. Neither do you have to deal with pesky Earth-based phenomena like seismic tremors. Researchers have been lobbying the European Space Agency to put a LIGO-like detector in space — the Evolved Laser Interferometer Space Antenna — sometime in the 2030s. In anticipation of eLISA, ESA recently launched the LISA Pathfinder, a mission to test technologies needed for the full-fledged space-based gravitational wave detector.

Pulsar timing arrays

To pick up the relatively low-frequency hum of colliding supermassive black holes, researchers are turning to pulsars. These rapidly spinning neutron stars (the cores left behind after a massive star explodes) send out steady pulses of radio waves.

Cosmic microwave background polarization

Gravitational waves released in the wake of the Big Bang would have left a mark on the cosmic microwave background, or CMB. This radiation fills the universe and is a relic from the moment light could first travel freely through the cosmos, about 380,000 years after its birth. The CMB preserved how space stretched and squeezed following a phenomenal expansion a trillionth of a trillionth of a trillionth of a second after the Big Bang. Many telescopes are searching for this signature by looking for specific patterns in how the CMB light waves align with one another. It’s not easy though; the BICEP2 project already mistook dust in the Milky Way for its cosmic quarry.

Do you know? The ¥16.4-billion (US$148-million) observatory — Japan’s Kamioka Gravitational Wave Detector (KAGRA) — will work on the same principle as the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States and the Virgo solo machine in Italy. In the past few years, these machines have begun to detect gravitational waves — long-sought ripples in the fabric of space-time created by cataclysmic cosmic events such as the merging of two black holes or the collision of two neutron stars. With the addition of KAGRA, the growing global network of detectors will enable astrophysicists to locate the position of these feeble cosmic signals in the sky with greatly increased precision. They will be able to dissect the waves’ properties, such as how they are oriented in space, better than ever before, ultimately allowing them to learn more about the elusive cosmic objects that produce them. Should India also join this project? Which of the following advantages will India reap, by executing this project?