# Ideas about black holes, light, and time.

1. Jul 24, 2010

### Capt_Jet23

Alright, so I'm kinda new to posting here, sorry if I don't understand things fully at this time. As for questions, I have a few of them. Firstly, what is the force of gravity at the event horizon of a black hole. Since photons have no mass, what would be the force required to trap it?

Secondly, I hate the fact that some people see light speed as a realistic impossibility because Einstein said so. Did Einstein not mean that it was a PERCEPTUAL impossibility? Merely meaning that if there is something that can exceed the speed of light, we cannot perceive it?

Lastly, if gravity is infinite through all points in space, imagine this: you line up a point on the face of the earth with a black hole, or where the black hole SHOULD be. If you could line up an object in between the earth and the black hole, and the object were close enough to the black hole, wouldn't we perceive the object in slow-time?

2. Jul 24, 2010

### nicksauce

Force is not a well-defined concept in relativity, so I'm not convinced that these questions even make sense.

It is not because Einstein "said so", but because the mathematics of relativity forbids it.

3. Jul 24, 2010

### relativityfan

well, gravity does not depend on the mass of the object that is falling: it is an acceleration.

furthermore, photons must have a "relativistic mass" but no rest mass because they have energy, and energy=mass.

in the frame of reference, there is nothing with rest mass that can reach the speed of light but for example at the event horizon of a black hole, such relative light speed can be reached for an infalling object for t->infinite
relative faster than light speed should exist but inside of the black hole, so it cannot be observed

4. Jul 24, 2010

### JesseM

There is no "force" in general relativity, instead gravity is explained in terms of curved spacetime. You can't explain why nothing can escape a black hole using the classical notion of "escape velocity", since in classical physics escape velocity only applies to objects in free-fall that have no forces other than gravity acting on them, it would be possible to escape the Earth traveling at less than the escape velocity if you were using a rocket that was applying a constant non-gravitational force upwards, whereas with a black hole nothing can escape from inside regardless of how it's moving.

5. Jul 24, 2010

### djy

To reword it even more precisely: the limit of acceleration required to keep an object at a constant distance from the event horizon is infinite as the distance approaches zero.

6. Jul 24, 2010

### nicksauce

This doesn't make sense. Acceleration and velocity don't have the same units, so how are you comparing them?

7. Jul 24, 2010

### JesseM

That doesn't make sense either--how can an acceleration exceed a velocity? According to dimensional analysis you can only compare quantities that have the same "dimensions" (like distance/time). If you try to compare quantities with different dimensions, which is larger can depend on your choice of units! For example, an acceleration of 9.8 meters/second^2 is equal to an acceleration of 3528000 centimeters/minute^2, while a speed of 10 meters/second is equal to a speed of 60000 centimeters/minute. So in units of meters and seconds, the numerical value of the speed is larger than that of the acceleration (10 is larger than 9.8), but in units of centimeters and minutes, the numerical value of the acceleration is larger than that of the speed (3528000 is larger than 60000). So, there's no unit-independent way of deciding whether the acceleration or the speed is "larger", it's not a meaningful physical question.

One way to describe it is to note that according to the equivalence principle, any freefalling observer in curved spacetime can construct a "locally inertial frame" in a small region of spacetime around them where the laws of physics will be arbitrarily close to those of special relativity. For any freefalling observer in the immediate region of the event horizon, the event horizon will be moving outward at the speed of light in their locally inertial frame, so there's no way to catch up with it again once you've crossed it.
"Depression" is misleading, there's no "up" or "down" that would allow you to distinguish a depression from a raised bump (the 'rubber sheet analogy' is confusing in this sense). All that matters is the curvature of spacetime, and the fact that free-falling objects follow geodesics in curved spacetime.

8. Jul 25, 2010

### JesseM

Then it would be "the gravitational velocity is exceeding that of the velocity of light", but the term "gravitational velocity" doesn't have any meaning AFAIK.

9. Jul 25, 2010

### Staff: Mentor

What makes you think that's what he meant?

10. Jul 25, 2010

### JesseM

I'm not sure what that would mean. The velocity of a free-falling object in a gravitational field isn't uniquely determined by the strength of the field even in classical physics--for example, in a constant gravitational field (where the field strength is the same everywhere), the velocity of a falling object is constantly changing as it falls.

11. Jul 25, 2010

### JesseM

Definition of what? I already suggested one way of understanding why nothing can escape the event horizon:
Another way of understanding it would be to point out that for any event on or inside the event horizon, no part of the future light cone of that event lies outside the horizon.

Last edited by a moderator: Apr 25, 2017