B Does Gravitation Start at Black Hole Event Horizon?

[Shaw]

Has anyone done this calculation? It seems to me that if light and time disappear at the event horizon, it's a gravitation limit as well.

 

[Dale]

No. In any sense that gravitation starts, it starts at the white hole's singularity. Or in a more realistic spacetime it starts with the material of the original star before it collapses or even forms in the first place

 

[Ibix]

It seems to me that if light and time disappear at the event horizon

They don't, some popularised descriptions notwithstanding.

 

[jbriggs444]

Imagine, if you will, a hypothetical plane sweeping across the universe at the speed of light. No signal that starts on the back side of this plane can ever reach the front side. Any signal originating on the front side that crosses the plane can ever be reflected back to be seen again on the front side.

This is what a black hole event horizon is like from the perspective of an observer hovering nearby on the outside. Nothing magical is happening there. But if you want to prevent the nearby horizon from passing you at the speed of light, you have to start accelerating away quite promptly and very strongly.

Even though the horizon is faster than you are, the magic of relativity means that velocity comparisons at a distance can be ambiguous. You can stay ahead indefinitely if you can accelerate strongly enough.

 

[A.T.]

It seems to me that if light and time disappear at the event horizon, it's a gravitation limit as well.

You seem to be confusing fields with disturbances of fields (waves):

- Neither electromagnetic nor gravitational waves can propagate through the horizon from the inside.

- The electric and gravitational fields of the stuff that fell into the horizon both remain effective outside of it.

 

[Shaw]

No. In any sense that gravitation starts, it starts at the white hole's singularity. Or in a more realistic spacetime it starts with the material of the original star before it collapses or even forms in the first place

Thank you. That's reasonable. Do we have any information about what happens to time inside the black hole since gravitation and time are bound together. At the event horizon, time dilation can't increase since it shares the light limit velocity. If gravitation is calculated from the centre, and the max. velocity in gravitational space is c, we seem to have a void of gravitation and time from the centre to the event horizon.

 

[jbriggs444]

At the event horizon, time dilation can't increase

"time dilation" is never something that happens at a location.

Time dilation involves a comparison of clocks that are remote from one another. Comparing clocks that stay on opposite sides of an event horizon from one another is impossible. Any reasoning about how black holes behave based on "time dilation" is going to be misleading at best and "not even wrong" at worst.

 

[PeroK]

Thank you. That's reasonable. Do we have any information about what happens to time inside the black hole since gravitation and time are bound together. At the event horizon, time dilation can't increase since it shares the light limit velocity. If gravitation is calculated from the centre, and the max. velocity in gravitational space is c, we seem to have a void of gravitation and time from the centre to the event horizon.

You've certainly picked up some very strange misconceptions about black holes!

 

[Dale]

Do we have any information about what happens to time inside the black hole since gravitation and time are bound together.

Well, we don't have any experimental information, but we do have plenty of theoretical information. Clocks continue to function normally until they get close enough to the singularity that the tidal forces break it. Depending on the size of the black hole and the delicateness of the clock this can happen inside or outside the horizon. The only thing is that clocks inside the black hole cannot communicate outside, but they can continue to function.

 

[Shaw]

Thanks for clearing this up for me.