Credit: NASA

Gravitational-waves, the disturbances in space-time

Many events in the universe are not detectable or unknown to us as they don’t release EM radiation, but now we can study a new phenomenon through the gravity-waves various objects produce.

Moving towards the door, you are very cautious to not make a sound. You open it carefully but accidentally end up making a sharp noise. Your mom wakes up and catches you with the ice-cream red-handed! It might be a bad experience for you, but this story leads us to something exciting. When objects interact, a disturbance is generated, which travels as sound or any other form of energy depending on the situation. When you opened that door, your force caused the door to move and generated a sound.

Similarly, every object in the universe produces waves in the space-time medium by acceleration; thus, we get gravitational-waves (GWs). I know you must be thinking, where did the concept of space-time evolve from? And what does space-time even mean? Plus, how can gravity have waves? Well, space-time can be treated as a medium composed of 3 space and one dimension of time. This is one of the most challenging concepts to grasp, so don’t worry. Take your time to understand this idea! Gravitational-waves, on the other hand, are generated in the space-time medium by accelerating objects.  

How exactly are gravitational-waves produced? 

When an object like you pushes a door, gravitational-waves are generated in space-time like ripples in water. About gravitational-waves, we got gravity wrong before. Gravity isn’t a force but a bend in space-time caused by an object with a mass. When two objects interact, the bend they cause affects both, and the lighter object seems to be attracted towards the heavier one, but it’s the lighter one falling in the bend of the heavier ones. The more massive object is also affected by the lighter one, but the effect is lesser. As small entities living on a small planet like earth, we don’t observe any gravitational-waves, and space-time isn’t even thinkable, but all that changed when Einstein presented his famous relativity theory introducing us to the idea. LIGO (Laser Interferometer Gravitational Waves Observatory) proved experimentally in 2016 that gravitational-waves are indeed real after it observed GWs for the first time.  

Illustration of how mass bends space.
Illustration of how mass bends space. Credit: NASA

Types and sources of gravity-waves 

There are 3 known types of gravity-waves, Continuous, Compact Binary Inspiral, and Stochastic. If an object like a single neutron star spins at a constant rate, it will produce continuous gravity-waves. The amplitude and frequency of such a wave remain the same. When two compact binaries like two black holes, two neutron stars, or a neutron star and a black-hole spiral around each other, they release gravity waves as they move closer. These waves are called Compact Binary Inspiral gravity-waves.

All the detections by LIGO so far are from Compact Binary Inspirals. The heavier the objects, the shorter is the span of the release of these waves. The first signal that LIGO detected was from two merging black holes, and it was around 2/10th of a second. The detection from two neutron-stars in 2017 was about 100 seconds long. Finally, stochastic gravity-waves are mixed and feeble waves produced by random objects in the universe. These are very hard to detect, but it’s possible to study a part of these waves.  

Detecting gravitational-waves through ‘LIGO’

As the name suggests, “LIGO” uses Laser Interferometry in which interference of two laser beams creates a pattern that helps in studying an object. A laser beam is split into two waves, which hit two mirrors and these two waves hit back at the detector, making a pattern. As the wavelength of a laser is too small, any disturbance caused in the beams’ path is detected with accuracy. LIGO can detect gravitational-waves by the disruptions caused in the laser signal, which could be as small as 1/10000th the proton’s width. Just to make things clear, the radius of a proton, recently calculated, was around 0.84 x10-15 m. This shows the power of this great detector.

An aerial photo of the LIGO observatory in Hanford, Washington. Gravitational waves
LIGO is made up of two observatories: one in Louisiana and one in Washington (above). Credit: Caltech/MIT/LIGO Lab

Why is gravitational-wave detection rare? 

Gravitation-waves are produced by every small and big object in the universe, but the issue lies within these waves’ weakness. First, detecting GWs from such a huge crowd of electromagnetic-radiation (EM), seismic-noise, atmospheric-noise, etc., is very hard. Why is noise such a big issue? Is an essential question. You see, GW’s cover a broad frequency range, and yes, they have a wave spectrum just like EM waves do, but the thing is the frequency is too low, so sensitivity is not something to compromise on. The wavelength of gravitational-waves ranges from a few kilometers to the size of the universe! Yes, you heard it right. Detecting such waves thus becomes a challenge.  

Exploring gravity-waves through LISA 

Even though LIGO uses vacuum tubes to protect itself from external noise, the bumping is still there. To avoid this, bumping and hearing the call of GW’s “LISA” (Laser Interferometer Space Antenna) is coming to our rescue. It will consist of a network of 3 space-crafts at about a 2.5 million km distance. Being space-based will be the real advantage as external noise like experienced on earth won’t interfere, and data collection would become more accessible.  

LISA’s frequency range goes to as low as 0.1 mHz to 100 mHz compared to 10Hz to 100Hz for LIGO, which means it is more sensitive without all the noisiness. LISA is a project by the European Space Agency (ESA) and will be released in 2034.

Why are gravitational-waves important? 

Many events in the universe are not detectable or unknown to us as they don’t release EM radiation, but now we can study a new phenomenon through the gravity-waves various objects produce. The inside of a black-holes and the big bang are just two of the many things which can now be understood much further. Understanding the beginning of our universe and its possible end is starting to unfold. With a new tool that we are beginning to explore, we might one day have better answers about the universe’s birth and its fate. All we need to do is listen and wait! 

References 

https://www.livescience.com/space-time.html

https://www.ligo.org/detections/GW150914/

https://www.ligo.caltech.edu/page/what-is

https://www.scientificamerican.com/article/just-how-small-is-the-proton/

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html 

https://lisa.nasa.gov/

https://www.elisascience.org/articles/lisa-mission/lisa-mission-gravitational-universe

https://www.ligo.caltech.edu/page/gw-sources

Also, read: Mystery object observed in a collision with a black hole by LIGO and Virgo

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