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.