~From Dead Stars to Time Machines~

“Remember to look up at the stars and not down at your feet.

Try to make sense of what you see and wonder about what makes the universe

exist. Be curious.”

~ Prof. Stephen Hawking

Our universe has since the beginning been driven by mysterious objects and forces that we took a long time to understand and we have yet just begun to understand them. Modern-day physics depends on two primary theories: Theory of General Relativity and Quantum Physics. The invention of these theories led to the possibilities of many ongoing events in the universe of which some have been proved by either observation or experiment. Out of all these, General Relativity by Sir Albert Einstein predicted the existence of objects called Black Holes. But ironically, Einstein himself contradicted this possibility. But it is not only Relativity involved in the theory of black holes but also Quantum theories.

The first insight into Black Holes was offered by Sir Subrahmaniam Chandrashekhar. While traveling to Cambridge he did some calculations which showed that a star a few times bigger than our Sun in terms of mass will end its life with a collapse due to gravity. So how do they form at all?

We know that all stars end their lives at a certain point but they do so in different ways. During their entire lifespan they are indulged in a sort of tug of war between gravity and nuclear forces and in the end gravity wins. Stars are ultimately huge balls of hydrogen gas which diffuse hydrogen atoms into helium atoms throughout their lives and it is these nuclear reactions that help the star to emit heat and light. These reactions in turn also help the star to maintain its stability. To understand this let us consider an example by Stephen Hawking. He said that when a balloon is inflated two forces are acting on it: one from inside that keeps it stable from bursting and one from outside that acts on its outer face in a constant effort to deflate it. Same happens in stars. Where on one hand, nuclear reactions keep its shape, gravitational force acting on its outer surface tries to make it collapse. This collapse does take place when the star has no more fuel to burn and is left with a solid core. At first, its outer boundaries explode in a supernova sending matter out into the universe. Then what happens further depends upon the mass of the star. What will be left of a star like our Sun will be a white dwarf, a lot denser than the Sun although smaller in size because gravity squeezed all matter into a very small space. Now let us consider a star a few times bigger than our Sun. This size limit deduced by Chandrashekhar’s calculations is called the Chandrashekhar limit. A star whose mass is equal to the Chandrashekhar limit will continue the collapse until it is even thousand times denser than the white dwarves and a lot smaller in size. Such stars are called neutron stars. Now what about stars above the Chandrashekhar limit? These stars continue the collapse of their inner core even past the neutron star stage and form what we know as a black hole. This was to summarize our topic of discussion. Let us now dive deep into the depths of black holes and let time slow down.

To know about how stars live and reach their death you can look up my article on the lives of stars. Now let us examine the black hole scenario. After the final explosion, the cores of all stars collapse and a few become black holes for reasons discussed above. When this happens, gravitational forces which are acting on the outer surface of the star, squeeze all matter, atoms and even subatomic particles into very very tiny space. In fact, the largest stars can form the smallest black holes and also the densest. Now we know that all objects exist in the plane of space-time which is four dimensional as shown in General Relativity (space has three dimensions; length, width and height and time has one dimension). Every object bends this plane on account of its weight which allows for a gravitational field to exist around that body. The greater the curvature of space-time, the greater will be the force of gravity. Now when a star’s core collapses to form a black hole, it curves space-time to a very great extent that the point where this curvature ends, physicists have deduced a space-time singularity where, as of now, the laws of physics break down and also maybe time comes to a stop.

Curvature of space time

A black hole has many fascinating traits. One of which is that no object can escape from its gravitational pull- not even light! Here we have to understand something. If we want to escape the gravitational pull of an object then we have to move away from it with a particular velocity which helps us to free ourselves from the acceleration it offers on us in the downward direction. For earth, that speed is about 12 km/s. This velocity varies as per the object’s gravity. For black holes, this velocity is so high that even light at the speed of 300000 km/s cannot escape it. In Fact due to its gravity, time slows down around it. If you were orbiting a black hole all objects around you seem to be moving at a very high speed and you seem to be stuck. That is what I feel mostly while attending classes.

Now how do we detect these cosmic monsters? Since even light is sucked up by it, it doesn’t emit any light. The very first thing is gravity. In 1964 a star system was discovered where a star was orbiting an unseen object in the Cygnus constellation. This object was later deduced to be the first black hole known to us and is named Cygnus x-1. The second is the matter that orbits around it which makes a sort of ring around it which we know as the event horizon. Anything that enters the vicinity of the event horizon cannot escape. It contains all sorts of matter, particle pairs, photon particles, radiation etc. Yes, radiation! Now how can an object, from which light cannot escape, emit any radiation?

Due to the enormous tidal forces inside a black hole its surroundings heat up to millions of degrees thereby emitting x-rays or radio waves. These rays can be detected by telescopes on the earth’s surface. In this way we have as of now detected many black holes and also photographed one.

The first ever black hole to be photographed

Now over the years it has been a topic of discussion that what is beyond the black hole’s singularity. We don’t have the answer now either. But there have been many speculations. Some just accept that physics breaks down at singularities, so it is not possible for anything to exist beyond it. But it has been speculated these black holes can be portals to different points in space, thus different points in time, because space-time is inseparable. It has also been thought that they are actually wormholes to an alternate dimension; thus an alternate reality. There are a lot of possibilities associated with them but no one knows for sure which is true?

Prof Stephen Hawking (1942–2018)

Black holes- mysterious aren’t they? For years it was thought that there is no end to the lives of black holes. But then came Prof. Stephen Hawking who invented the theory of Hawking Radiation. We mentioned above about particle pairs orbiting black holes. The thing is that almost everywhere in our universe there are particle pairs consisting of a particle and its corresponding antiparticle which keep annihilating each other. But they behave differently around black holes. Suppose at the boundaries of a black hole such a pair exists then one of the particles will be sucked into the black hole and the other particle having no particle to annihilate with, will fly away into the universe in the form of radiation. In this way, over its lifetime, black holes send away such radiation into the universe and thus their mass decreases over time which results in them finally evaporating away. But this process takes a very long time. Maybe by now even the first black hole hasn’t evaporated and maybe it won’t until civilizations wipe out.

Black holes, even though they don’t emit any light, are stellar objects. They may hold the secrets to our universe’s past, present and future and thus studying them becomes very important. Their singularities may be where the underlying theory of the universe lies and who knows who will find it? We just know that it would be the sunrise for science and bear fruit for thinkers and scientists over the past millennia who have spent their lives discovering the secrets of our universe. Till then all we can do is wait and take a joyful ride through the pursuit of knowledge.

“Absence of evidence is not the evidence of absence” — Carl Sagan