Does Gravitation Start at Black Hole Event Horizon?

In summary, there is no magical disappearance of light and time at the black hole event horizon. The event horizon is simply a point at which signals from the other side cannot be received. Gravitational fields and time continue to exist inside the black hole, but measurements and communication from inside become impossible due to the strong gravitational pull. Clocks inside the black hole will continue to function normally until they are destroyed by the singularity.
  • #1
Shaw
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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.
 
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  • #2
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
 
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  • #3
Shaw said:
It seems to me that if light and time disappear at the event horizon
They don't, some popularised descriptions notwithstanding.
 
  • #4
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.
 
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  • #5
Shaw said:
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.
 
  • #6
Dale said:
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.
 
  • #7
Shaw said:
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.
 
  • #8
Shaw said:
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!
 
  • #9
Shaw said:
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.
 
  • #10
Thanks for clearing this up for me.
 
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FAQ: Does Gravitation Start at Black Hole Event Horizon?

1. What is a black hole event horizon?

The event horizon of a black hole is the boundary surrounding it beyond which nothing, not even light, can escape the gravitational pull of the black hole. It marks the point of no return; once an object crosses this threshold, it cannot escape the black hole's gravity.

2. Does gravity exist at the event horizon?

Yes, gravity exists at the event horizon. In fact, the gravitational pull at the event horizon is extremely strong. The event horizon itself is not a physical surface but rather a mathematical boundary. The gravitational effects of the black hole are felt even before reaching the event horizon.

3. How does gravity behave near a black hole's event horizon?

Near a black hole's event horizon, gravity becomes increasingly intense. As one approaches the event horizon, the gravitational force increases, leading to significant tidal forces that can stretch and compress objects. This phenomenon is known as "spaghettification," where objects are elongated in the direction of the black hole due to the differential gravitational forces.

4. Can we observe the effects of gravity from outside the event horizon?

Yes, we can observe the effects of gravity from outside the event horizon. The gravitational influence of a black hole affects nearby stars and gas, causing them to orbit around it. Additionally, the light from surrounding matter can be influenced by the black hole's gravity, leading to phenomena such as gravitational lensing.

5. What happens to gravity if an object crosses the event horizon?

Once an object crosses the event horizon, it cannot escape the gravitational pull of the black hole. From an outside observer's perspective, the object appears to slow down and fade away as it approaches the event horizon due to the extreme gravitational effects on time and light. However, from the perspective of the object itself, it continues to fall into the black hole without experiencing any sudden change at the event horizon.

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