Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Curious question about gravity

  1. Jun 25, 2016 #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??
     
  2. jcsd
  3. Jun 25, 2016 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    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".
     
  4. Jun 25, 2016 #3
    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??
     
  5. Jun 25, 2016 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    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>".
     
  6. Jun 25, 2016 #5
    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
     
  7. Jun 26, 2016 #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.
     
  8. Jun 26, 2016 #7
    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??
     
  9. Jun 26, 2016 #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.
     
  10. Jun 26, 2016 #9

    phinds

    User Avatar
    Gold Member
    2016 Award

    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.
     
  11. Jun 26, 2016 #10

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    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?
     
  12. Jun 26, 2016 #11

    phinds

    User Avatar
    Gold Member
    2016 Award

    @Umair Shariff, see this is the kind of usefully stated question that I am talking about.
     
  13. Jun 26, 2016 #12

    phinds

    User Avatar
    Gold Member
    2016 Award

    No, the Sun's gravitation field propagates to infinity so to limit it to the Ort Cloud is incorrect.

    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.
     
  14. Jun 26, 2016 #13
    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.
     
  15. Jun 26, 2016 #14

    phinds

    User Avatar
    Gold Member
    2016 Award

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Curious question about gravity
Loading...