Expert: Jayendra Upadhye - 4/29/2008

Question
Why do accretion disks form around black holes? Why doesn`t the matter (from a star e.g.)doesn`t directly fall into the black hole through its event horizont, but forms a spiral instead? Could you please answer? Thank you.

Answer
Hi Aleksander,
I found this interesting question in the pool so i am answering it.

The answer lies in the reason why planets dont fall into their respective "suns", but instead, orbit their parent stars.

This is because, the universe is so random.
That under all situations, you always end up with sufficient tangential velocity for matter to describe a spiral and not a straight line.

Actually, even the slightest tangential component to the velocity of infall, will make the infalling matter describe a spiral.

Only a slight percentage of matter will "by chance" have a velocity vector directly pointing at the gravitating object in question.
That too may be transitory, as both objects (infalling matter & the hole) will continue to have their own proper motions, that need not be in total synchronism.

Once the matter is properly gravitationally bound, the spirals "even out" sharper infalls meet matter with less sharp infall speeds and overtakes it and merges, reulting in a new spiral for the cimbined clump and so on.

In the end one observes an "overall spiral" accretion disk that itself is a play of transients! moment to moment, new spiral paths would be seen "close to the average", as new matter enters the virtual accretion area which is only an area that happens to lie within the limits set by the "current" passing steady state.
(very much as a cloud defines the boundaries of "current transient" steady state between two temperature & presssure & dew point fronts.
individual moisture droplets appear when in the cloud area and disappear when out of it).

Accretion disks are like clouds in that respect, only "flattened versions", and with spiralling tendencies as compared to chatotic random motion in clouds.

Actually when a satellite "falls" to earth after loosing velocity to friction with earths outer atmosphere, it is actually describing the same spiral tendency latent in all infalling objects!!

Skylab did that, and the shuttle that met a fiery end, and so do meteorites, the degree of spirality differs with incoming speeds.
meteors travel at 20 to 40 km per second and their paths appear almost straight, but are still spital in nature. (parabolic if they hit before traversing 180 degrees!)  

Hope that suffices.
Please do rate the answer if you find it meaningful.
Jayen