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Gravitational Waves: Hear the Sounds of Space

by Ahana Banerjee

On the left, an optical image from the Digitized Sky Survey shows Cygnus X-1, outlined in a red box. Cygnus X-1 is located near large active regions of star formation in the Milky Way, as seen in this image that spans some 700 light years across. An artist's illustration on the right depicts what astronomers think is happening within the Cygnus X-1 system. Cygnus X-1 is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star. New studies with data from Chandra and several other telescopes have determined the black hole's spin, mass, and distance with unprecedented accuracy.

Up until a few weeks ago, the term “gravitational waves” was one well established in the academic world; having been predicted a century ago by Albert Einstein in his theory of general relativity. However, thanks to recent data received by LIGO (laser interferometer gravitational wave observatory), the existence of gravitational waves has been confirmed, and it has now become a widely coined household term thanks to the wide news coverage it has received in these past few weeks.

Most of us have a rough idea of what gravity is – it is the force that keeps us all from flying off the ground. However, to take this idea and apply it in the form of a wave may be confusing – to say the very least. Just as a wave ripples through an ocean, gravitational waves ripple through space. In an ocean, the medium through which the wave passes is water. In space, the medium through which the gravitational wave propagates is the fabric of spacetime itself. Spacetime is a complex concept, but its movements when affected by a huge mass like a star can be analogised to a ball on a taut bedsheet. Accelerating or moving that ball around creates curves in the bedsheet. If you were to bounce the ball up and down in the same spot, it would send out ripples across the sheet. Int the same way, huge masses in space curve spacetime and thus create ripples that stretch and compress space itself.

The recently detected gravitational waves resulted from the collision of two black holes (both roughly thirty times as large as our sun). As the black holes merged, they formed a mass approximately sixty times the mass of our sun, and shockwaves, similar to those in an earthquake, were released through space. As these black holes were so incredibly far away from our planet, it took 1.3 billion years for the waves to reach us (to put it in perspective, that’s over 200 times as long as the human race has existed). These ancient waves have been passing through the universe for an unfathomably large amount of time. As they are the result of something so old and so vast, you may be wondering how it is even possible for us to detect them? Afterall, it’s not like we were able to feel the gravitational waves ourselves…

Earlier, I briefly mentioned LIGO; the laser interferometer gravitational wave observatory. An interferometer is an extremely sensitive device that can measure the most minuscule shifts in space. It operates by having two perpendicular vacuum tunnels, equal in length and each with mirrors at the end. Light is reflected down these two tunnels, off the mirrors, and arrives at a sensor. When the light waves reunite after being reflected off the mirrors, they interfere and create a distinct pattern (much like ripples in water making patterns). If space stretches or compresses due to the gravitational waves, the beams of light will travel for different amounts of time (they will have different distances to travel according to the movement of space), when the light rays arrive at the sensor, they will not be perfectly in-phase. Observing the phase of the light waves allows us to detect even the tiniest movements of space, and this is exactly what the scientists at LIGO did to make this groundbreaking discovery.

Now, perhaps the most important question… why do gravitational waves matter? Well, the waves we observed outdate human history by almost 1.2 billion years. They provide us with insight into a time so far in the past, it is almost incomprehensible to the human mind. Black holes are still relatively mysterious cosmic objects; besides their existence, little is known about them. Detecting the waves generated by black holes helps us understand how they work, especially in terms of their mass, movement, and behaviour. We know that gravitational waves also form when supernovae explode, and massive neutron stars wobble. So, detecting these waves opens doors to our understanding and knowledge of the other cosmic events that cause the gravitational waves too. Gravitational waves are also a crucial part of Einstein’s general theory of relativity. Finding them not only proves that theory – it also helps us figure out where it goes astray, which could lead to a more accurate, more all-encompassing model, and perhaps point the way toward a theory of everything. As curious beings, knowing how the universe works is simply… cool (apologies, for the lack of better word choice). Spacetime is our landscape and being able to learn its ways teaches us about everything around us.
Whilst on the topic of “cool” things, do you want to hear what a gravitational wave sounds like? Unfortunately, gravitational waves do not generate sound themselves (as they are not sound waves), however, scientists at LIGO took the waveform of the chirp pattern generated by the actual gravitational waves, and converted it into a sound waveform; take a listen here!

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