B Light Taking U-Turn: Transformation of Star to Black Hole

[shivakumar]

sir , i always taught what would happen to light traveling away from the mega sized star at the time of transformation of this star into a black hole? will it trace back its path after it taking a u - turn?

 

[Ibix]

The event horizon forms within the star as it undergoes the final stages of collapse, and expands out to its final size. Light that is outside the horizon escapes. Light that is at the horizon can, in theory, remain there forever. Light inside the horizon ends up in the singularity, but this does not involve a u-turn. The singularity inside a black hole is more like a moment in time than a place in space, and there isn't really an "inwards" and "outwards" direction - the topology of the space inside the black hole is not the same as it was before the black hole formed.

Black holes spacetimes can be surprisingly complicated even for fairly simple idealised situations.

 

[malawi_glenn]

sir

Did you just assume our genders?

 

[shivakumar]

I meant respect to more learned person, madam

 

[Dale]

sir , i always taught what would happen to light traveling away from the mega sized star at the time of transformation of this star into a black hole? will it trace back its path after it taking a u - turn?

I don’t think that there is any need to restrict yourself to the time of collapse. For a formed black hole light is trapped at the horizon, it orbits at the photon sphere, and it is deflected at larger distances. So a U turn is just a large deflection, but less than an orbit. That means that at some distance above the photon sphere you should be able to make a null U turn for a stable black hole

 

[Ibix]

So a U turn is just a large deflection, but less than an orbit.

I think we may need to ask @shivakumar for clarification of what "U turn" means in this context. You seem to be imagining light falling from infinity and doing a half orbit. I was imagining light coming up from the star, turning back and falling on to the star/black hole again. Your scenario is possible, mine isn't (hence my longer explanation).

 

[Dale]

You seem to be imagining light falling from infinity and doing a half orbit. I was imagining light coming up from the star, turning back and falling on to the star/black hole again. Your scenario is possible, mine isn't (hence my longer explanation).

Good point. Yes, that is what I was considering.

 

[vanhees71]

Did you just assume our genders?

Please not also on PF...