Black Holes – Applying Theory of General Relativity

The Color of Black Holes

Wait. I just mentioned previously that black holes have so much of gravity that light can not escape it. How come then a black hole have color? A hint. Event horizon. Black holes have no hair nor color. But event-horizon, the only observable part of black hole is a very colorful and alive.

Step in theories of Stephen Hawkins of evaporating black holes.

Evaporating Black-Holes

The black holes can evaporate. A black hole emits particles at the event horizon. This is based on the uncertainty principle which says that at any given point, we can never be sure of both energy or location of an atomic/sub-atomic particle. This defines there is always a “zone” where the probability of find that particle. Thus, a space is never stark empty. Any given piece of space is part of this “zone” for a close by particle.

The vacuum in quantum field theory is not really empty; it’s filled with virtual pairs of particles and antiparticles that pop in and out of existence, with lifetimes determined by the Heisenberg uncertainty principle. (less than h/E, where h is Planck’s constant and E the energy).

Sometimes one member of a pair crosses the horizon, and can no longer recombine with its partner. The partner can then escape to infinity, and since it carries off positive energy. The negative energy falls into black hole and the energy (and thus the mass) of the black hole decrease. To an observer the decrease will be exactly same as the the particle that got freed and will deduce that the particle was emitted by the black hole and it lost the equivalent mass/energy.

In quantum field theory, modes with positive frequencies correspond to particles, and those with negative frequencies correspond to antiparticles.

Note that this doesn’t work in the other direction – you can’t have the positive-energy particle cross the horizon and leaves the negative-energy particle stranded outside, since a negative-energy particle can’t continue to exist outside the horizon for a time longer than h/E. So what ever falls into black hole will only be negative energy part of the pair. So an observer will only see a steady stream of particles only.

So the black hole can lose energy to vacuum fluctuations, but it can’t gain energy. That means, there is continuous drain in black holes energy. This is the gradual erosion of a black hole. And one day, when it’s gravity is so low that it can not hold itself, it will explode in form of a burst of gamma ray showers.

The average life expectancy of a black hole is 10 billion years.

To understand the colors, let us plunge into the black hole.

Falling into Black Hole

Say there is a scout mission to go inside a black hole and report back as much as possible before crushing under it’s gravity.

The Mothership’s view: They will see the scout ship plunge into the black hole and it would seem to them to take forever.

As the ships reaches the event horizon, the reflected light (which helps us see) finds it harder and harder to get out. So objects that fall into a black hole appear from the outside to freeze in time at the moment they cross the event horizon.

As the light struggles to get out we will observe a red-shift. The scout will not vanish. But will fade from white to red till invisible light only makes it out and soon, when no light is able to come out as he falls at huge speed which is approaching c. If we could see a clock in the scout probe, the clock would appear to us to slow to a halt.

As the probe and other bodies are absorbed into black hole and they go red-shifting, we can see a the whole range of colors just above the even horizon. Black hole, will appear to very colorful indeed.

The Scout’s view: Remember, Einstein said, all laws of physics are same when the speed of light is approached. For the scout, every thing will appear normal. The light is crossing the event horizon so he will be able to see the mother ship till the gravity stretches it and flattens on the black hole.

Image: NASA Images