View Full Version : does a photon-sphere only exist once a black hole has formed?

2018-Aug-11, 07:31 AM
does it only exist once there is an BH, and not before, and expand outward ahead of the expanding event horizon as a star collapses?

2rS isn't it(the PS)?

2018-Aug-11, 11:03 PM
If I am not mistaken, the gravitational signature at 1.5 times the Schwarzschild radius (Rs is the same whether the body's radius is less than Rs or a bit greater. Thus, if I am correct, we could have a photon sphere without having black hole conditions anywhere in the body, at least in a thought experiment. I cannot say whether or not any real body such as a collapsing star can ever be in such a state for a brief amount of time.

2018-Aug-12, 03:29 AM
It was hypothesised that certain kinds of ultracompact neutron stars could form with R < 1.5 Rs. I think that they are not strongly favoured due to additional constraints on the neutron star equation of state that were found after their proposal. Presumably if quark stars are a thing then it is possible (too little is known about the equation of state for them) that you could see this effect with them.