Curious question about gravity

In summary, the conversation discusses the relationship between black holes, gravity, and the propagation of gravitational fields. It is noted that all mass has the ability to bend light, but black holes have a stronger effect due to their concentrated mass. The concept of gravity being a "captive" of the black hole is also addressed and it is explained that gravity is a field that is set when the black hole forms and does not propagate. The conversation also delves into the differences between pre-relativistic and relativistic physics and how relativity affects the understanding of gravity within the solar system. Finally, a question is posed about the force experienced due to the gravitational attraction of the moon on Earth and whether it peaks when the moon is directly overhead, taking
  • #1
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??
 
Physics news on Phys.org
  • #2
Umair Shariff said:
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".
 
  • #3
phinds said:
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??
 
  • #4
Umair Shariff said:
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>".
 
  • #5
phinds said:
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 can't and that physics is universal. I just used the law of gravity to explain a question
 
  • #6
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.
 
  • #7
Let'sthink said:
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??
 
  • #8
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.
 
  • #9
Umair Shariff said:
It wasent an attempt on my part to break physics, I know I can't 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.
 
  • #10
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. Let's 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?
 
  • #11
CWatters said:
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. Let's 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.
 
  • #12
Umair Shariff said:
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 spatial point of origin.
 
  • #13
phinds said:
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.
 
  • #14
Tazerfish said:
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.
 
  • Like
Likes OmCheeto and davenn

What is gravity?

Gravity is a natural phenomenon by which all objects with mass are brought towards each other. It is a fundamental force that governs the motion of celestial bodies, as well as the everyday objects on Earth.

Why do objects fall towards the ground?

This is due to the force of gravity pulling them downwards. The Earth's mass creates a gravitational pull that attracts all objects towards its center.

How does gravity work?

Gravity is caused by the curvature of spacetime around massive objects. This curvature is created by the presence of mass, and the larger the mass, the stronger the gravitational force.

Can gravity be turned off?

No, gravity is a natural force that cannot be turned off or controlled. However, we can manipulate objects to counteract the effects of gravity, such as using rockets to escape Earth's gravitational pull.

Is gravity the same everywhere in the universe?

No, the strength of gravity depends on the mass of the objects and the distance between them. Therefore, gravity can vary in different locations in the universe, depending on the distribution of mass.

Suggested for: Curious question about gravity

Replies
7
Views
2K
Replies
28
Views
3K
Replies
2
Views
2K
Replies
2
Views
1K
Replies
36
Views
4K
Replies
4
Views
1K
Back
Top