How gravitational waves are the key to 'see inside' black holes

Ever since Einstein’s Theory of General Relativity predicted the existence of gravitational waves, and an infinitely dense region of space-time, which are now known as black holes, these celestial oddities have been an enigma.

It was not until 1971 that scientists first confirmed the existence of black holes through observing a blue supergiant star called Cygnus X-1 being sucked rapidly of its contents into seemingly empty space.

Black hole detection has always been a massive problem for scientists to solve. This is because they don’t emit light, at least not past their event horizon. Whatever light is emitted is from the hot swirling gasses that revolves around the black hole in an area is called its Accretion Disk.

The now-famous first-ever image obtained of a black hole is perhaps the greatest discovery of the astrophysics community in the current century.

But prior to the image, it was gravitational waves that took the physics community by storm. Scientists have always theorized that large galaxies must contain a supermassive black hole at its center. Furthermore, if two galaxies ever collided in the vast expanse of space, the resulting collision would also cause the merger of two such entities present at the center of each.

Again, it was Einstein and his General Theory of Relativity that predicted the existence of gravitational waves, a kind of ripple in the fabric of space-time, and it took almost 100 years to accurately confirm that prediction.

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Exploring gravitational waves and why they are important to study black holes

In 2015, when scientists at LIGO (Laser Interferometer Gravitational-Wave Observatory) detected gravitational waves, it was from the collision of two black holes that took place about 1.3 billion years ago. In the final 20 milliseconds during the merger, these cosmic bodies emitted 50x more energy than the light radiated by the stars in the observable universe.

These cosmic emissions are exactly what they sound like, waves of intense gravity generated by extreme sorts of celestial events like the mergers of black holes and neutron stars, and violent supernovae. These are analogous to ripples created in a water body due to, say, throwing rocks in it.

Here, the merging of black holes is akin to rocks, and them disrupting the space-time fabric and creating ripples in it is akin to waves in water.

To detect gravitational waves, long L-shaped arm-like structures are built, at least four kilometers long. Unsurprisingly, the process of detection is so intricate and complicated that building a spaceship looks like assembling Lego toys in comparison.

Gravitational waves create such a miniscule ripple in space-time that according to scientists, these shifts are no bigger than 1/1000th the width of a proton. These LIGO structures, however, are capable of measuring these waves with sensitivity as small as 1/10,000 the width of a proton.

Thus, the L-shaped structure is erected and a high-power focused laser is employed. Also, to rule out any interference from trapped gasses, a vacuum of one-trillionth of one atmospheric pressure unit is created inside.

Next, scientists can detect gravitational waves by splitting this extremely high-intensity laser and passing the split beam simultaneously through each part of the L-shaped structure.

If and when gravitational waves do pass through Earth, it will shorten and elongate the space-time fabric for the two arms differently. This way, the time taken for the laser photons to arrive back for detection will change, thus changing the intereference pattern ever so minutely.

To rule out any misreadings thanks to outside disturbances like earthquakes, traffic, electrical thunderstorms, and other phenomena, scientists operate at least two LIGO units simultaneously. Thus, if both detectors detect a reading at the same time, it can’t be due to the same local disturbance.

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Gravitational waves can explain black holes like never before

Previously in the scientific community, the idea of black holes colliding was considered far-fetched. It wasn’t considered possible for two such cosmic bodies to orbit each other, let alone collide and merge. But with the detection and analysis of gravitational wave data, it is abundantly clear that these events are much more common than originally thought.

The consensus in the scientific community is that every black hole is unique in terms of its spin, mass, and electric charge. Even so much as slight variations in any of these parameters render it truly unique and surprisingly different from one another.

This is where gravitational waves come into the picture. According to a research paper published in the journal arXiv by Elisa Maggio, it now seems possible to "glimpse inside" a black hole by analyzing the data from such mergers.

This is naturally a figure of speech since the inherent properties of black holes make it impossible to glimpse inside them. This is thanks to the curvature of space-time that takes place to such an extent that even light cannot come out of it but keeps falling towards the singularity infinitely.

With LIGO detection systems, it now seems possible to analyze pre-merger states of black holes with high accuracy while extensively studying various parameters. As such, this allows for a 'peek' inside the properties of such cosmic bodies like never before.

Many prevailing hypotheses try to explain black holes in their own way. The Quantum Gravity model predicts one without an event horizon, which does not have singularity at its core. Perhaps such black holes, if they do exist, may emit an entirely new kind of gravitational wave than the one already detected.

Thus, more sensitive LIGO detectors will enable detection of various kinds of waves, and subsequently, various kinds of black holes, possibly even solving the conundrum around the nature of such cosmic phenomena forever.

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We live in very interesting times. Not only was human civilization able to capture the very first image of a black hole, but we also discovered the beats that these celestial oddities 'dance' to.

It might take a while for scientists to solve the black hole dilemma and combine the Theory of Relativity with Quantum Mechanics. What is certain, however, is that exciting times are ahead for scientists and space enthusiasts alike.