Black Holes and Gravity: Answers to Your Questions

[The Cow King]

Hello, I've been wondering about a few things, I hope you can answer my questions.

Question A.

If you look at a black hole that has something behind it, say a nebula in our example, what does the black hole look like?

Would it be a completely black spot in the middle of the nebula because all the light coming from the nebula gets sucked in?

Or would there be some kind of displacement/distortion around the black hole, but you'd still be able to see the nebula behind it?

Question B.

If you're dropped from an airplane, trapped inside an elevator, would jumping inside the elevator just before impact nullify the impact force of the huge fall, assuming the elevator lands on the ground in a 90-degree angle?

I often think about this when I use elevators, in case it should fall down.

 

[Drakkith]

A: It would look like a distortion. The exact look I couldn't say.

B: You could only avoid getting splattered if the elevator fell from a very very small height. Otherwise it is falling faster than you can jump. Also, if it is actually falling you will not be on the floor anymore, but be free falling. So unless you pushed yourself off the top to the bottom, you'd have nothing to push off of anyways.

 

[davenn]

Agreed, there would be a gravitational lensing distortion. like seen from the Abell 315 galaxy cluster and the way it distorts the view of more distant galaxies behind it.

This is not the pic I wanted but still one of the lensing effects discovered
particularly the Very curved galaxy towards the right of the image

ok found the other one...

notice in this one the many many curved streaks of the light from the distant galaxies being lensed around the forground Abell1689 galaxy cluster

Dave

 

[davenn]

I find this stuff just totally amazing :) it never ceases to amaze me the weird things discovered "out there"
I guess one of the reasons I have been into astronomy since I was a kid


yeah and for your Q2 yup you and the box are both in freefall.

and as the ol' saying goes ... it aint the fall that hurts, its the sudden stop at the bottom" ;)
Dave

 

[Lsos]

If an elevator fell out of an airplane, it would probably be moving at a few hundred miles per hour by the time it hits the ground. By jumping or pushing yourself off of the elevator, you can only propel yourself at a few miles per hour at best.

A few hundred mph - a few mph is STILL a few hundred mph...which is how fast you will be moving when you hit the ground.

 

[NlCK]

... By jumping or pushing yourself off of the elevator, you can only propel yourself at a few miles per hour at best. ...

I was kind of thinking the same thing as Drakkith when I read the question:

... Also, if it is actually falling you will not be on the floor anymore, but be free falling. So unless you pushed yourself off the top to the bottom, you'd have nothing to push off of anyways.

I was also thinking that, if an elevator with a large surface area were falling inside of a small, enclosed shaft, the elevator might generate enough wind resistance/drag to partially affect the free fall environment experienced inside the elevator. I figured, even if that were the case, you'd only end-up accelerating slightly faster than the elevator -- so it'd be like barely having any gravity at all.

In either case, I figured, that, if you were able accomplish pushing on a surface shortly before impact, you might just bonk your head on the opposite end because you and the elevator are both free falling. :rofl:

I often think about this when I use elevators, in case it should fall down.

If you had found yourself in such a position, how would you have decided when to jump?

 

[ibysaiyan]

Part B of your question is how Einstein initially linked special relativity to his upcoming general relativity i.e law of equivalence.So I believe you'd be in the state of "free fall" so to jump would't make any sense.Someone correct me if I am mistaken.
Regards,
ibysaiyan

 

[bcrowell]

http://en.wikipedia.org/wiki/File:BH_LMC.png

 

[IsometricPion]

I was also thinking that, if an elevator with a large surface area were falling inside of a small, enclosed shaft, the elevator might generate enough wind resistance/drag to partially affect the free fall environment experienced inside the elevator. I figured, even if that were the case, you'd only end-up accelerating slightly faster than the elevator -- so it'd be like barely having any gravity at all.

In either case, I figured, that, if you were able accomplish pushing on a surface shortly before impact, you might just bonk your head on the opposite end because you and the elevator are both free falling.

The Mythbusters tested this scenario with results approximately as you suggest. http://www.youtube.com/watch?v=6zPx5Amyt7A" Needless to say, "jumping" didn't seem to help Buster very much.

 

[satya98]

Is their a zero gravity point in the universe.

Is the gravitational force zero at the center of the earth.

 

[DaveC426913]

Is their a zero gravity point in the universe.

There are surely places where the net potential balances to zero, but there is no place where gravity does not reach.

Is the gravitational force zero at the center of the earth.

The net potential is theoretically zero (As in: you will not go anywhere if you were floating in a cavity). I say "theoretically" because the Earth is uniform in neither shape nor internal density.

 

[satya98]

A
You can't even spot the presence of block hole at that point.
It seems to be like an empty space at that point in the nebula
U know that light cannot pass through the black hole, block whole even absorb the light passing near by it.
so
U can't see that part of the nebula and some part around it(behind the black hole).
U can compare it some what with the corona of the sun during eclipse.

 

[DaveC426913]

A
You can't even spot the presence of block hole at that point.
It seems to be like an empty space at that point in the nebula
U know that light cannot pass through the black hole, block whole even absorb the light passing near by it.
so
U can't see that part of the nebula and some part around it(behind the black hole).
U can compare it some what with the corona of the sun during eclipse.

While one may not be able see the black hole itself, they tend to be highly visible due to the disk of infalling gas and dust, which emits a lot of radiaton as it falls.

Black holes do not absorb light passing by if it is outside the event horizon (which is pretty small). They do bend light, but that actually makes the phenomenon more visible, rather than less visible.

And finally, please use full English words. It's actually PF rules:

In the interest of conveying ideas as clearly as possible, posts are required to show reasonable attention to written English communication standards. This includes the use of proper grammatical structure, punctuation, capitalization, and spelling. SMS messaging shorthand, such as using "u" for "you", is not acceptable.

 

[satya98]

I think their is no zero gravity point in the universe , because no body in the universe is stable and every body in the universe revolve around some other body weather it may be a small meteor to a large galaxy.

 

[DaveC426913]

I think their is no zero gravity point in the universe , because no body in the universe is stable and every body in the universe revolve around some other body weather it may be a small meteor to a large galaxy.

But more importantly, because the influence of gravity extends to infinity.

 

[satya98]

sorry to say but I did't get you clearly what your idea is.

 

[satya98]

Is the gravitational force zero at the center of the earth.

I think the Earth's gravitational force may become zero at that point. But the influence of sun, near by galaxies, universal gravitational constant will act on it. So the net gravitational force will not be zero at the center

 

[Fewmet]

The Mythbusters tested this scenario with results approximately as you suggest. http://www.youtube.com/watch?v=6zPx5Amyt7A" Needless to say, "jumping" didn't seem to help Buster very much.

I generally value what the Mythbusters do for critical thinking. This was one of the times, however, that they had poor modeling.

If I recall correctly they made Buster (the dummy) "jump" by attaching him to a spring welded to the floor. The physics of that are different from trying to jump in free fall. It seems to me that if you try to squat to jump, your center of mass will stay in the same place with respect to the floor. Rather than lowering your body to the floor so you could push off, your legs will rise and your head will lower.

 

[DaveC426913]

Is the gravitational force zero at the center of the earth.

I think the Earth's gravitational force may become zero at that point. But the influence of sun, near by galaxies, universal gravitational constant will act on it. So the net gravitational force will not be zero at the center

It is zero because it is in freefall. If it were not zero, there would be a net force and we would be accelerating in that direction. We're not.

 

[Lsos]

If I recall correctly they made Buster (the dummy) "jump" by attaching him to a spring welded to the floor. The physics of that are different from trying to jump in free fall. It seems to me that if you try to squat to jump, your center of mass will stay in the same place with respect to the floor. Rather than lowering your body to the floor so you could push off, your legs will rise and your head will lower.

You can always grab against the sides of the elevator, or bounce off the ceiling to force yourself into the floor. It might not be easy, and it might require inhumanly perfect timing, but it IS possible and I believe goes along with the spirit of the question.

It is zero because it is in freefall. If it were not zero, there would be a net force and we would be accelerating in that direction. We're not.

But we ARE in constant acceleration towards the direction of the sun, and center of the galaxy, are we not?

 

[Drakkith]

But we ARE in constant acceleration towards the direction of the sun, and center of the galaxy, are we not?

I think what he means is that the net gravitational force from the Earth is effectively zero at the center of the Earth and the influence from the Sun and Galaxy are so small as to be effectively nil for everything on our own scale.

 

[mjacobsca]

If you had found yourself in such a position, how would you have decided when to jump?

I assumed a glass elevator was implied. :)

 

[NlCK]

I assumed a glass elevator was implied. :)

I figured that something could be arranged for an experiment/hypothetical scenario, but I was responding to, and quoted, his closing statement:

I often think about this when I use elevators, in case it should fall down.

I never even thought of glass elevators, so don't let me rub you the wrong way. I just wanted to clarify.

The Mythbusters tested this scenario with results approximately as you suggest. http://www.youtube.com/watch?v=6zPx5Amyt7A" Needless to say, "jumping" didn't seem to help Buster very much.

I guess that I missed that one. Thanks for the link.

I generally value what the Mythbusters do for critical thinking. This was one of the times, however, that they had poor modeling.

If I recall correctly they made Buster (the dummy) "jump" by attaching him to a spring welded to the floor. The physics of that are different from trying to jump in free fall. It seems to me that if you try to squat to jump, your center of mass will stay in the same place with respect to the floor. Rather than lowering your body to the floor so you could push off, your legs will rise and your head will lower.

I still think that would depend on the drop height because the person and the elevator should have different drag coefficients. I also think that, even if the elevator's brakes were to fail, the elevator would generate a lot of friction due to torsional stress on the guide-rail caused by the elevator's instabilities/tendency to tumble, or rock, during a fall without a cable.


On the other hand, the difference could be fairly negligible. If that's so, I'll probably die in some defunct elevator while trying to avoid floating puddles of urine. I guess that I could use my arms and legs to teach the other passengers about Newton's third law of motion while continually announcing my status as 'king of the ceiling.' Don't look at me like that! Most of them would be in a state of panic anyway.