Death of a Black Hole

This is a very Frequently asked question...

How do black holes die?

Well, some people think they last forever, and they're very nearly right. Black holes do die, but it is on a timescale of billions of years. No one has ever seen a black hole actually die, but astrophysics Stephen Hawking has proposed a theory about black hole death.

Black holes aren't black at all. In fact, they constantly emit radiation in the form of small virtual particles that are born in pairs out of photons, i.e. particles and antiparticles. At the event horizon, there is a sort of gravity well, which seperates the innards of the black hole from the outside. At this point, the energy is sufficient enough for the photons, or energy packet (also called quanta) to 'condense' into matter. Since the law of conservation of mass and energy is maintained, the particles have to be each others' anti selves.

Mostly, these particles do not last more than a few millionths of a second, quickly annihilating each other to form photons again. This is the actual Hawking radiation, the emission of photons by such a physical reaction. Unfortunately, this radiation is much too faint to be detected by even the most powerful telescope on earth.

But sometimes, it doesn't work out so well for the particle. They do not get to meet with their other halves. The anti particle gets whisked away beyond the event horizon, falling down the intense gravity into the object's core. The other particle is left partner less, and wanders off alone. Since it has a positive mass, it has a positive momentum and it can escape the gravitational field of the black hole (mind you, the event horizon is the point of no return. Anything outside that can and will escape).

The antiparticle goes and annihilates with a positive particle at the black hole's core, thereby reducing the mass of the black hole by one particle.

So what happens then?

Billions of years later, the mass of the black hole drops sufficiently to fall beneath the Chandrasekhar limit of 1.44 Solar Masses. At this stage, Pauli's exclusion principle kicks in. The particles, that have been oppressed and crushed into this tiny little space for billions of years by their own gravity, explode outwards violently. The energy released in this explosion is phenomenal. It could blow away half of the galaxy in which the black hole resides. Let's just say that if the Black hole at the centre of a galaxy a few billion light years away exploded by this method, when the light eventually reached us, it would make our nights as bright as day. That is the awesome power of an exploding black hole.

Will we be able to see one any time soon?

No. Like I said, black holes don't live forever, but they come very very close. If the theory is correct (and so far, it has never been disproved, and all evidence seems to back it), it'll be a few billion years (few hundred billion years, actually) before one does die. And even then, depending on its distance from us, it'll take even longer for the light to reach us. But, as you know, Earth isn't gonna last more than 5 billion more years, thanks to our daddy, the Sun. It's gonna become an old star, expand its size till it engulfs all four terrestrial planets (Mercury, Venus, Earth and Mars) before shrinking into a tiny white dwarf that will eventually cool off to become a hunk of helium floating in space.

More on this cheerful topic of the Earth getting fried next time... Tune in to Young-Geniuses.

Black Holes continued...

Delving deeper into black holes after my last post, I think I'll need to add a few points here.

Black holes are formed not when the whole star has a mass of 1.44 solar masses. That is the mass of the collapsed core alone. The Chandrashekhar limit (1.44 Solar Masses) is just the limit beyond which the exclusion principle cannot support the core and it collapses.

Here's what actually happens.

If the mass of the star is 24 solar masses (henceforth, I'll refer to it as Msun) or less, about 7/8ths of it is blown away by the explosion. The core suffers the original gravity of the star made much worse by its existing radius. So it collapses. Now there are three possible endings for this sort of stellar death.

1) White dwarf:

This is a type of star that is extremely hot, and extremely dense. White dwarf stars have from 0.8 Msuns up to 1.44 Msuns. When its parents star is about to die, it expands into a Red Giant, blows off its outer layers, and collapses into this beauty. Like I said, extremely dense. A teaspoon of white dwarf material can weigh upto 5 tons on earth! Gravitational force on this star can crush you into a pancake in less than a second.
Unfortunately, they haven't got the pressure required to initiate nuclear fusion. So even though they may live for a long long time, they'll run out of heat eventually, and die.

2) Neutron star:

This is what results when a White Dwarf star has more than 1.44 Msuns, but less than 3 Msuns. These are born pretty much the same way. But because of the intense Gravitational force, even the atoms are squeezed together. The electrons combine with the protons in the nucleus to form neutrons. Now, the entire star is composed of just atomic nuclei composed almost entirely of neutrons. This is very dense, and by very, I mean very. A teaspoon of this stuff can weigh a million tons on earth. That's VERY VERY heavy.
Neutron stars live on almost forever. The curvature of space-time created by Neutron stars is second to only black holes.

3)Black Holes:

When a stellar core has a mass of more than 3 Msuns, it turns into the ultimate cosmic mystery; a black hole. Now I've explained about black holes already, so I won't go into much detail, but all in all, at the event horizon, the curvature is so strong that time actually stops. Even light cannot escape the gravity of this monster.
Needless to say, a teaspoon of this substance on Earth would literally swallow it up, along with half the Solar System. So don't try bringing a black hole to Earth.

Black holes don't live forever, but they come very very close. This is because of a type of radiation called 'Hawking Radiation'. More detail on this subject will be in the next post. But according to this theory, black holes constantly emit radiation. In the end, this radiation reduces that mass of the black hole to so low that it crossed the Chandrasekhar limit on the way down. Once it's less than 1.44 Msuns, it does not need to be contained in the centre, and explodes in a magnificent display. If Sagittarius A* (that's Milky Way's very own black hole) explodes, we won't need the Sun. It'll be so bright, it'll be 'day' even at night.

FAQs- Black holes

What exactly are Black holes?
I get that a question a lot. People think that black holes are something out of science fiction, a region of nothingness, often embodying a wormhole (a tunnel-like connection between two distant points in space), etc. etc.

The truth is far less glamorous. If anyone want the professional and expert definition of a black hole, look it up on wikipedia. However, be warned that some of it may fly far above your head. This post has a brief description for kiddies.

A black is not science fiction. It is perfectly true, and a surprisingly common phenomenon. It is the wormhole part which is sci-fi. There cannot be any 'quantum' connection of any sort between two points in the Universe. The term quantum itself is misleading, like when people use high-funda terms to make it look as if they know something when actually they know squat!

Anyways, back to the topic.

A black hole is sort of an afterlife for a big star having at least 1.44 solar masses (for non-rotating bodies), or 24 solar masses (otherwise). When a star finishes its life cycle, the outer layers expand in a very large cosmic explosion, called a supernova. The core, however, retains all of the original gravity of the star, and collapses in on itself. This collapse is very rapid, and squeezes all the mass into a point that is ver small in diameter. At this point, the density is very high, and so is the gravitational force of the body. The force is so great, that even light can't escape the event horizon, and gets trapped inside. This body appears invisible (no light reflected off its surface reaches us), and so is known as a black hole.

If a Black Hole is invisible, how can we see it?
Simply by observing how visible matter interacts with it. Around a black hole, matter from nearby bodies (or stars, as in a binary system) gets accreted around it. All this matter forms a sort of disc (similar to Saturn's rings) called the accretion disc rotating at high speed. Also, at the poles along a black hole's rotationary axis, two beams of high energy X-rays are emitted. These are called jet streams. As a black hole rotates, these jet streams swing around. So if we see an hourglass pattern of X-rays in space with nothing at the centre, we know it's a black hole.

Another interesting thing is that black holes often act as giant cosmic lenses. If there's a black hole in the foreground with a band of stars in the back, by observing the motion of the band of stars, we can see if there's a black hole. The light is refracted and bent along the event horizon, giving it the appearance of a lens.

Here's another funny method. You know how kids play ring-a-roses? They just keep revolving around nothing in particular. By observing similar behaviour in nebula clouds, or in planets, we can guess at the location of a black hole. This is easier than it seems because of the absence of light from a black hole.

What will happen if you fall in a black hole?
Interesting question. Astrophysicists get that a lot. The phenomenon of falling into a black hole can be described by a term - 'spaghettification'. As hilarious as it sounds, that is probably very accurate in describing the effect.

If you stand on any body with mass, your feet, which are closer to the ground, experience more gravity than your head, which is farther away. Since gravity is a very weak force, on large planets, you won't feel the difference. But on a black hole, the radius is so tiny, that the difference will be very large. So your body will begin to stretch along the middle as you get closer. The lower portion of your body will fall faster than the upper part of your body, stretching it even more. Eventually, you'll be ripped into two. This will continue with the rest of your body (2, then 4, then eight parts) untill your reduced to atoms, and sucked down the even horizon. Funnily enough, you won't feel a thing, because you'll be long dead by then, crushed by the gravitational force way high up in orbit. Not very comforting, so don't try sky diving into one anytime soon!

What are wormholes?
Ah, finally the topic of Wormholes!
Well, theoretically, wormholes are possible. But practically, it's a long shot. According to Einstein's equations, space time is a sort of cloth on which galaxies and stuff are embroidered. Now, you could fold this cloth back upon itself. If you pass a needle (with a thread) through the two layers of cloth, passing through one end, out through the other), then this would represent a wormhole. It is an inter-dimensional tunnel connecting two distant points to each other via a shortcut.

Here, all the laws of physics would break down. All particles would revert to sub-atomic state (proton, neutron, electron) and out of the other end it would reassemble. The reassembling is the tricky part.

According to equations, they're allowed. But you know how equations are very annoying? Take for example, while solving a quadratic equation for area, you get two values; one positive, one negative. The negative one is obviously wrong, so you convert it to modulus of its form, blah blah blah... Practically, negative area is crap!

Similarly here. Mathematics can get you all sorts of crazy stuff. We logicians are given the heavy duty of sifting what is possible from what is crap. Logically, creation of such a region is filled with paradoxes. And nature doesn't take paradoxes too well. Two solution? Get out! It's a bit like that with the Universe.

Since I don't really entertain the idea of an inter-dimensional passageway, I didn't bother to research this topic. But if you sincerely wanna know, I can look it up for you.

What are singularities?
Singularities are related to black holes, but quite distantly. Singularities are factors with the denominator zero, or very close to zero. This gives the quantity the potential to go very near infinite. It's sort of like zooming in on a point. You can go in forever and ever, but it'll never get larger, because it has no dimensions. It's a place where all physical lines meet, and go no further.

Black holes are not exactly singularities because their radius (in this case, the radius is the denominator) are not quite zero.

More Questions and Answers in the next blog...