Heyy guys, this just came to my thought, if black holes have enough mass to generate gravity strong enough to bend light and (I m not sure about this but remember reading this somewhere) if gravity travels at the speed of light, does it not make gravity a captive of the black hole??

phinds
Gold Member
Heyy guys, this just came to my thought, if black holes have enough mass to generate gravity strong enough to bend light and (I m not sure about this but remember reading this somewhere) if gravity travels at the speed of light, does it not make gravity a captive of the black hole??
That's a bit of a confused question but I'll see what I can do with it. First of all, ALL mass "bends light", black holes just have more mass in a smaller area so have a larger local effect. Google "Einstein rings".

Gravity is a field. It forms as the black hole forms and when the black hole has formed, the gravitational field is set and does not propagate at all. It's exactly like the field from a magnet which also just "is" and does not propagate. So no, gravity is not a "captive of the black hole".

That's a bit of a confused question but I'll see what I can do with it. First of all, ALL mass "bends light", black holes just have more mass in a smaller area so have a larger local effect. Google "Einstein rings".

Gravity is a field. It forms as the black hole forms and when the black hole has formed, the gravitational field is set and does not propagate at all. It's exactly like the field from a magnet which also just "is" and does not propagate. So no, gravity is not a "captive of the black hole".

Interesting, although I want to know something and please bear with me, since this can get a little complex.

Lets take the law of gravity, gravity equals mass of two bodes divided by the distance between them squared (I hope that's right, been a long time since I studied physics) so, given a specific distance and assuming that two bodies of mass pop into existence spontaneously (or have formed simultaneously) will there be a time lag for the bodies to experience a gravitational pull from the other, or will it happen instanteously??

phinds
Gold Member
Interesting, although I want to know something and please bear with me, since this can get a little complex.

Lets take the law of gravity, gravity equals mass of two bodes divided by the distance between them squared (I hope that's right, been a long time since I studied physics) so, given a specific distance and assuming that two bodies of mass pop into existence spontaneously (or have formed simultaneously) will there be a time lag for the bodies to experience a gravitational pull from the other, or will it happen instanteously??
There would be a time lag if such magic could happen. The Sun's gravitational effect on the Earth has an 8 minute lag, so that's really a much better example since it doesn't break the laws of physics and it's never a good idea to ask a question (as you have) of the form "if the laws of physics don't apply, what do the laws of physics say about <insert nonsense of your choice>".

There would be a time lag if such magic could happen. The Sun's gravitational effect on the Earth has an 8 minute lag, so that's really a much better example since it doesn't break the laws of physics and it's never a good idea to ask a question (as you have) of the form "if the laws of physics don't apply, what do the laws of physics say about <insert nonsense of your choice>".

It wasent an attempt on my part to break physics, I know I cant and that physics is universal. I just used the law of gravity to explain a question

That is the fundamental difference between pre-relativistic(non-relativistic) and relativistic Physics. Prior to advent of relativity, we just presumed the validity of 'action at a distance' principle and time and time interval as absolute. what was simultaneous for one observer was so for for all moving observers as well. for a stationary objects field stays but for a moving object field propagates.

That is the fundamental difference between pre-relativistic(non-relativistic) and relativistic Physics. Prior to advent of relativity, we just presumed the validity of 'action at a distance' principle and time and time interval as absolute. what was simultaneous for one observer was so for for all moving observers as well. for a stationary objects field stays but for a moving object field propagates.

So, within the solar system, the sun simply has its gravitational field (which is exerted on the bodies within the Ort cloud), while the earth's gravitational field advances??

In relativistic approach you cannot make any absolute statements. 'With respect to what" is important and is to be stated. the quantum mechanics says that the observer influences the experiment and in relativity observer is the part of the experiment.

phinds
Gold Member
It wasent an attempt on my part to break physics, I know I cant and that physics is universal. I just used the law of gravity to explain a question
I understand that you think that's what you did and it's fairly important for you to understand that that's NOT what you did. You wanted the laws of physics to answer a question about magic and that's never a good idea. Best to formulate questions in a way that stays within the bounds of physics so as to be sure that the answer is actually meaningful.

CWatters
Homework Helper
Gold Member
Perhaps I can formulate a question...

You are standing on planet earth at the equator and experience a force due to the gravitational attraction of the moon. Lets assume the moons orbit is circular so the maximum force you experience occurs when the moon is directly overhead. Since light takes 1.3 seconds to travel from the moon to the earth does the force peak when the moon is overhead or 1.3 seconds later?

phinds
Gold Member
Perhaps I can formulate a question...

You are standing on planet earth at the equator and experience a force due to the gravitational attraction of the moon. Lets assume the moons orbit is circular so the maximum force you experience occurs when the moon is directly overhead. Since light takes 1.3 seconds to travel from the moon to the earth does the force peak when the moon is overhead or 1.3 seconds later?

@Umair Shariff, see this is the kind of usefully stated question that I am talking about.

phinds
Gold Member
So, within the solar system, the sun simply has its gravitational field (which is exerted on the bodies within the Ort cloud)
No, the Sun's gravitation field propagates to infinity so to limit it to the Ort Cloud is incorrect.

while the earth's gravitational field advances??
not sure what you mean by this. The Earth's gravitational field is no different that the Sun's gravitational field, except in magnitude and spacial point of origin.

I understand that you think that's what you did and it's fairly important for you to understand that that's NOT what you did. You wanted the laws of physics to answer a question about magic and that's never a good idea. Best to formulate questions in a way that stays within the bounds of physics so as to be sure that the answer is actually meaningful.
Don't you think that is a little harsh ?
Ridiculous thought experiments can often make you understand something in an easier way.
The "popping into existence" is much simpler to imagine than just a lag in the "action at a distance".
And these kinds of questions can give some insight.

phinds
Gold Member
Don't you think that is a little harsh ?
Ridiculous thought experiments can often make you understand something in an easier way.
The "popping into existence" is much simpler to imagine than just a lag in the "action at a distance".
And these kinds of questions can give some insight.
No. If I had wanted to be harsh, I would have stated it harshly.

In math I have often found that taking something to an extreme like that can be helpful but in physics it can lead to misunderstanding. I get what you mean, but I think your point of view about it might change as you study more advanced science. Learning to formulate physically meaningful questions is part of the process.

OmCheeto and davenn