Black Holes Explained

Black Holes Explained

Black holes are curious bodies that occur in cosmological physics. They sound almost exotic, with their unreal properties and air of mystery. Scientists studying them learn a lot more about our universe, at the extreme scales.

Understanding Black Holes

Introduction

As experience on the Earth shows, most objects tossed up come back down. There however is a speed over which a tossed object does not return. This is what scientists call escape velocity. We know that this is the speed a rocket must reach to get away from the earth.

A fact about escape velocity: It increases when radius decreases, mass being constant. A basic fact of modern physics is that the speed of light is the ultimate speed limit – nothing can go faster.

But what happens when the escape velocity in a region is higher than the speed of light? The answer is simple: escape is impossible.

Black holes are such bodies.

Concept of a black holes
Concept of a black hole. (Source)

The Event Horizon

A black hole is a body of highly dense matter, with strong gravitational fields surrounding it. As we have seen, black holes have such high escape velocities that nothing, not even light, can escape them. They are one-way gates: once something gets in, it doesn’t get out.

But does this mean that black holes suck everything in around them?

This is a common misconception. Black holes merely have very strong gravitational fields, but objects can remain safe at great distances from them. They get “sucked in” only when they cross a certain boundary called the event horizon.

The radius of the event horizon is given by the Schwarzschild radius formula.

Schwarzschild radius formula

where,
M is the mass of the body
G is the universal gravitational constant
c is the speed of light.

Thus, there is no escape for objects that fall within the event horizon. This is because they need to go faster than the speed of light to escape – which is impossible. However, objects outside that radius orbit it without getting sucked in – provided they are not disturbed.

Detecting Black Holes

As we know, most of our astronomical observations come from light or other forms of electromagnetic radiation (radio waves, X-rays, etc.)

But since light from black holes cannot escape, how can we detect them?

An artist's visualization of a black hole, complete with an accretion disk.
An artist’s visualization of a black hole, complete with an accretion disk. (Source)

Scientists do this by playing to the strengths of a black hole: its gravitational field. Because of its extreme strength, the gravitational field of a black hole influences the motion of nearby bodies a lot. By studying their behaviour, scientists can predict the location and characteristics of a black hole.

Another, more recent way is to use gravitational waves. These waves are tiny disturbances of spacetime produced by the motion of bodies. Only massive objects like black holes produce gravitational waves that are strong enough to detect. This has become an active field of research since 2015 when the first gravitational waves were confirmed.

The first image of a black hole, by the Event Horizon Telescope collaboration. (Source)
The first image of a black hole, by the Event Horizon Telescope collaboration. (Source)

An interesting thing about black holes: while they cannot be “seen” directly, light from objects they influence is visible. This is often seen as an accretion disk, a flat disk of burning gases from stars who are pulled at by black holes. The light from accretion disks can help us see a black hole.

Scientists took the first photo of a black hole in April 2019, by studying light paths around a candidate black hole.

Origin

We have seen some properties of black holes. But what exactly are they, and how do they form?

A massive object compressed into a small space can produce a black hole. This is what happens in reality. Massive, burnt-out stars shrink under their own gravitational pull.

Those which are relatively light become white dwarves or neutron stars. If they are sufficiently heavy, beyond a certain limit, their collapse is unstoppable, and they end up forming what we call black holes. In other words, a black hole is commonly a gravitationally collapsed star. Their masses are in the range of huge stars.

There are other, much more massive black holes called supermassive black holes. These are postulated to form by fusion of many smaller black holes, or by the addition of matter to normal-sized black holes. Their masses are equal to that of millions or more stars. They are much less understood than stellar-mass black holes.

Occurrence

Scientists believe that the early universe had a large number of massive stars that are black hole candidates. These massive stars collapsed and formed the earliest black holes. They are called stellar-mass black holes. More recent star collapses give birth to younger black holes.

Scientists believe that the centers of galaxies contain supermassive black holes. They could often be associated with quasars – other mysterious celestial objects.

Properties

Due to their immense mass and gravitational pull, black holes display properties that sound exotic. For example, the very strong gravitational fields around them cause large time dilation, a consequence of general relativity. They also demonstrate gravitational lensing, in which light near them is warped by the strength of the field around them.

Another light-related property is the gravitational redshift. It means that light that reaches us from near the black hole is redshifted: it has a lower frequency than the original.

An artist's impression of black holes lensing light
An artist’s impression of black holes lensing light. (Source)

As black holes have very strong fields, these effects are prominently seen. A popular illustration of this can be seen when we consider a person (or object) falling towards a black hole. To an outside observer, the combined effects of time dilation and gravitational redshift have this effect. It seems that the falling person slows down immensely, while slowly vanishing. For the falling person, it feels like time flows normally, and they don’t notice this slowing down (though they would be torn apart by the tidal forces near the event horizon)

Singularity: Inside a Black Hole

No one knows for sure what exists within a black hole. After all, no information seems to get out of one.

However, scientists have some predictions. Certain terms of Einstein’s Field equations go to infinity inside a black hole. This is what physicists call a singularity.

An interesting property of a singularity is that anything sucked into a stationary black hole goes right to the centre (where the singularity is). This can be avoided for a rotating black hole, but it potentially can lead to what are called closed timelike curves – opening up the possibilities of time travel.

The singularity: the mystery at the centre of the black hole
The singularity: the mystery at the centre of the black hole. (Source)

Singularities are poorly understood and modeling them is not easy. It is hoped that modern developments of physics sheds light on them and solves the problem of singularities.

The Scope for Future

Since their theoretical discovery in the 1910s, black holes have provided a lot of material for theoretical and experimental physicists to study. Our understanding of them has also furthered general physics as well, with new concepts emerging from the study.

Their extreme conditions put two ends of physics together: general relativity and quantum physics. One deals with massive, large scale phenomena while the other deals with sub-microscopic phenomena.

It is hoped that a better understanding of these mysterious bodies is followed by a fusion of these two branches of physics. This could shed more light on the mystery of black holes, and their hidden interiors.

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