Black Holes and their Elusive Mystery

Amongst all the quirky space objects under the scrutiny of scientists, black holes seemingly sparked the most interest in both laymen and professionals alike. They even seeped their way into pop culture garnering quite a bit of interest – and with good reason.

Singularity – where the laws of physics crumble

To put it simply, black holes are dense entities in space, each containing a massive amount of matter condensed into a small form. This infinitesimally small region can be simply referred to as a singularity. One of the most fascinating aspects of a singularity is the infinite density it possesses, and the implications of such a possession. To quote Kip Thorne,

‘The singularity is a point where all laws of physics break down.’

Gravitational force – pulling it all in

While constantly theorising this facet of black holes, a singularity’s effect on the gravitational force – the attractive force between two entities, is quite prominent. Newton’s law of gravitation characterised a strong gravitational force as a large mass and a small distance. Fittingly, black holes are the epitome of such characteristics. The dense form of a black hole results in a very powerful gravitational force. Adding on to that, it is so powerful that it causes a distinct feature in black holes – its near invisibility.

Artist’s rendition of a dust-bound supermassive black hole with a jet of plasma emerging from the hidden accretion disk. [source]

Invisibility – an entity of blackness

Black holes trap everything that comes their way, from matter to electromagnetic radiation, including visible light. As a result of this, black holes are undetectable – for the most part. Although black holes cannot be studied directly, scientists can observe the effect they have on their surroundings and surrounding matter. This may require the detection and observation of certain phenomena. For example, stars orbiting an invisible entity, matter absorbed by an invisible entity (via an enormous release of radiation) or the formation of accretion disks – flat disks of dust and gas, around an invisible entity.

While it is true that all black holes trap electromagnetic radiation, the prominent astrophysicist, Stephen Hawking, theorised that black holes emit Hawking’s radiation. It is a form of electromagnetic radiation from outside the event horizon. Though Hawking’s radiation is too feeble for its existence for proving or disproving, it does bring forth intriguing queries regarding the blackness of the black hole, so to speak.

Spacetime – and the warping it undergoes

If we refer to Newton’s law of gravitation, mass plays an important role in gravitational force having an effect. Based on this, it is apparent that black holes have no trouble trapping matter, owing to their possession of mass. Nevertheless, the question of how black holes trap visible light remains, since electromagnetic radiation does not possess mass. Herein enters spacetime and Einstein’s contribution to it.

One of Einstein’s most exciting contributions is the formulation of E = mc2. It not only explained the relationship between mass and energy, but also theorised that the closer an object reached the speed of light, the heavier it became. Additionally, Einstein’s theory of special relativity created a link between space and time, and hence the conceptualisation of spacetime. Spacetime is simply the amalgamation of the three dimensions of space (x-axis, y-axis and z-axis) and the dimension of time into a four-dimensional space-time continuum.

To take this one step further, Einstein came up with the theory of general relativity. Subsequently, this led to the finding that massive objects have the ability to distort the spacetime fabric. A massive object possesses a strong gravitational field, which in turn warps the spacetime surrounding it. In other words, in the presence of a strong gravitational field, spacetime is no longer straight – it is curved. Light, follows a straight path, will have no choice but to curve according to the curvature of its path. Thus, it will inevitably get trapped by the curved spacetime of a black hole.

Artist’s rendition of spacetime. [source]

Event horizon – the point of no return

An event horizon can be pictured as a boundary in spacetime, wherein the gravitational pull is so strong, escape is impossible. Matter and radiation can pass through the event horizon into the singularity present within; they, however, cannot move outwards. Event horizons also possess a curious attribute contributing to their name – they function much like any other horizon, hence preventing a distant observer from discerning the occurrence or non-occurrence of an event within one.

Artist’s rendition of a supermassive black hole. [source]

Gravitational time dilation – where time is relative to you

Let’s create a hypothetical scenario where you’re a distant observer and you are observing an object approaching the event horizon of a black hole. While monitoring the object from a distance, you will notice that the object begins to slow down, becoming infinitely slow, until it finally fades away. While you might think the object fading away signifies its entry into the black hole, it is in fact due to the light fading out. This occurs as a result of gravitational redshift, wherein photons (particles that make up light) shift to a longer (and redder) wavelength, and a lower frequency.

However, if you were travelling along with the object you would notice that the object in question would pass through the event horizon in finite time, with no variations in the speed – and so would you. This phenomenon where time moves relative to the observer is the gravitational time dilation.

With the help of ticking clocks, one can visualise the concept of gravitational time dilation. To a distant observer, a clock near the event horizon would appear to slow down; in actuality, the clock would continue ticking normally.

Spaghettification – or a chance to escape

Let’s consider another scenario where an individual finds themselves passing through an event horizon. The consensus is that the individual will undergo spaghettification or the noodle effect, where they will be vertically stretched and horizontally compressed by tidal forces, and essentially torn apart, before ending up at the singularity, ready to be crushed. In order to comprehend the said individual’s journey through the rest of the black hole, it is essential to recognise that one way of classifying black holes is on the basis of their charge and spin. In the event of the black hole being a Schwarzschild black hole (charged and rotating), they would end up with the singularity as their destination, and they’d be crushed by its infinite density before adding to the mass of the black hole.

However, the fate of someone after they have travelled through an event horizon is not necessarily bleak. If it turned out to be a Reissner–Nordström black hole (charged) or a Kerr black hole (rotating), the individual might have hit the jackpot and could possibly exit the black hole into another spacetime via the elusive wormhole.

Artist’s rendition of a supermassive black hole surrounded by an accretion disk, while the remains of a star ripped apart by tidal forces resulting in a burst of light. [source]

Wormholes – where time is putty

A wormhole is like a spacetime conduit connecting two different timelines. Wormholes can theoretically function as a pathway between a black hole and a white hole (an entity that is the exact opposite of a black hole, where light and matter cannot enter the event horizon but can escape through it), thus allowing for time travel to take place. It has become a standard theme in science fiction, and while Einstein’s theory of general relativity allows for its existence, it is still an alluring mystery.

The existence of a wormhole, according to current predictions, would be one on the brink of collapse due to its instability and size. Scientists have offered exotic matter as a solution to this – matter with negative energy density and a large negative pressure. Wormholes are pretty exciting but the jury’s still out on their existence.  

The best is yet to come

Black holes are one of mankind’s most fascinating discoveries in the vast expanse of the universe. They continually gave room for the entirety of physics to be broken down and expanded upon, and inspired the melding of classical and quantum physics together. By bringing forth an onslaught of intriguing questions yet to be answered, they provoked the scientific community, as made evident by the historic picture taken of one, released on 10th April, 2019. As scientific discovery continues on its exciting pathway, one can rest assured that the best is yet to come.

First and groundbreaking image of the black hole found at the centre of the M87 galaxy. [source]

Written by Shweta Manoharan

Curiosity doesn't always kill the cat.

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