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

How does Gravity travel the speed of light

  1. Nov 5, 2006 #1
    How does Gravity travel the speed of light, if it accelerates at 9.8 m/s² on Earth? Shouldn't it travel at a constant c?
     
  2. jcsd
  3. Nov 5, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Don't mix these up:
    (1) The speed of gravity--the speed at which a change in gravitational field propagates. That speed, per general relativity, is c.
    (2) The acceleration due to gravity of an object near the Earth's surface. That's an acceleration of an object, not the speed of gravity.
     
  4. Nov 5, 2006 #3
    But, why are they different?
     
  5. Nov 5, 2006 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Why would you think they would be the same? You do realize that speed and acceleration are two different things--so that right off the bat, equating them makes no sense. (Kind of like aking why is 3 dollars different than 3 feet.) And further: one is the speed of gravity, while the other is the acceleration of an object. What's the connection?
     
  6. Nov 5, 2006 #5
    Speed of gravity: Let's say the sun was to disapear, it would take a little over 8 minutes (distance earth-sun sun at velocity C) for the earth to change gravitational course.
    Hence; the "effects of gravity" travel at C.

    Acceleration due to gravity: Corresponds to the gravitational force of a mass. On earth: 9.8 m/s²

    They are both very different things.
     
  7. Nov 5, 2006 #6

    LURCH

    User Avatar
    Science Advisor

    A ship with Solar sails is propelled by photons. The force propelling the ship travells out from the Sun to the ship at c, but the ship itself does not. It will be constantly accelerated by the force of the photons, but it will travel much slower than the force that is propelling it.

    True for EM force, true also for gravitational acceleration (not saying the two are similar, but the principle of things moving much more slowly than the force that is accelerating them is still relevant).
     
  8. Nov 5, 2006 #7

    Mk

    User Avatar

    I tend to think of it more like this (ignore unit troubles please): Objects do not fall at c. But gravity get's to the object that fast.
     
  9. Nov 5, 2006 #8
    Gravity permeates space at the speed of light. On earth, objects fall towards its surface at a speed of 9.8 meters per second. The first statement is gravity's own speed while the 9.8 is gravity's affects on other object's speed toward the Earth.
     
  10. Nov 5, 2006 #9
    I am a newb too but I somewhat get this (I hope)
    Here's how I think it works:
    Gravity accelerates 9.8m per second. So in 1 sec, it would be 9.8, in 2secs, 19.6m/s(9.8x2) and in 3secs, it would be 29.4(9.8x3). and so on but it would stop until it reaches c.

    Am I Right or Wrong?
     
  11. Nov 5, 2006 #10

    russ_watters

    User Avatar

    Staff: Mentor

    Completely and utterly wrong. As the Doc said, 3 dollars and 3 feet are not the same thing, are they?

    You do know that g (acceleration due to gravity) is different on every planet and also differs depending on how high above the earth you are, right? That 9.8 is not a universal constant(g isn't G), it is just about what g happens to be on earth, on average. It isn't even the same everywhere on earth!
     
  12. Nov 5, 2006 #11
    I meant that for earth only, not universally. Would I be right then?
     
  13. Nov 6, 2006 #12
    I don't mean to be disrespectful to anyone in this thread, but honest to god what kind of physics teachers do you all have?

    Raza, no you wouldn't be right. Think before you make any conclusions. I honestly don't know how to reply to make it clear to you (in other ways than have been previously stated).

    I'll try this:

    Let's say that each particle of earth emits gravitons which fly outward in all directions. Then, when some object, like yourself, is hit with a graviton, it is pulled towards the origin of the graviton. The average position of the origin of all the gravitons from earth is the center. So, on average, these gravitons pull you towards the center. They pull with a force proporitonal to your mass. The acceleration you experience is NOT acceleration of the gravitons, it's the acceleration of your physical human body towards the center of the spherical earth. The individual gravitons fly at the speed of light to meet and provide a force onto you.

    Would it make sense for gravity itself to begin at rest and accelerate on its way towards an object?
     
    Last edited: Nov 6, 2006
  14. Nov 6, 2006 #13

    russ_watters

    User Avatar

    Staff: Mentor

    You're being rediculous. Please stop.
     
  15. Nov 10, 2006 #14
    ok guys...lets make it clear....raza....i thing u must go and study the chapter onn gravitation....if u did then here how it goes....u stand on the earth....gravitation force...which is defined as (Gm1m2)/r^2 = mg..were m1 is mass of you and m2 is the mass of earth...r is radius of earth..ok?....if u are standing on the earth...the gravitational equation..which is universal must apply to the force that u learn...m1g....hence....in order to find g=(Gm2)/r^2 and this quantity is 9.8ms-1...get it....
     
  16. Nov 10, 2006 #15
    m1g..sorry
     
  17. Nov 10, 2006 #16

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Can you stop writing like a 4-year old?
     
  18. Nov 10, 2006 #17
    now suppose...for some reason...the sun was taken out of ur solar system....earth would only get deflect away from the former sun 8mins(approx.) since it takes 8mins for the INFLUENCE OF GRAVITY between the earth- sun system to dispappear...this influence of gravity...i think...which is the gravitational field....dun even think about gravitrons right now...
     
  19. Nov 10, 2006 #18
    sorry...i am onli 14 years old and i dun speak english as my first language....sorry for any inconveinece
     
  20. Nov 10, 2006 #19
    arildno...u got a masters in fluid mechanics...omg...can i ask u a question...does bournoullli's equation apply in real life?...is it an approximation of any kind?
     
  21. Nov 10, 2006 #20

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

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

Have something to add?