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

Gravity class is amazing

  1. Sep 22, 2005 #1
    Hey how are ya? I'm new to physics and all that I took a class in school and it blew me away! A lot of questions are popping up that my teacher can't answer. My first one is,according to him, pretty deep. Here it is; what is gravity? I mean what is it ? How does it work. I understand that it is all around us, and all the basics but this one is bugging me. Any answeres out there?
     
  2. jcsd
  3. Sep 22, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The first step is to learn how gravity is described. Have you studied Newton's law of universal gravity? I'd start there. (When you've mastered that, then you can look into how General Relativity describes gravity.)
     
  4. Sep 22, 2005 #3
    yet another qravity question. Isn't it true that gravity is faster than light? If it is then wouldn't all fields be faster than waves considering light is a wave.
     
  5. Sep 22, 2005 #4
    I don't think gravity is faster than light, gravity doesn't travel, it causes objects to travel.
     
  6. Sep 22, 2005 #5

    Doc Al

    User Avatar

    Staff: Mentor

    No. According to General Relativity, gravity propagates at the speed of light.
     
  7. Sep 22, 2005 #6
    Well think about this scenario. What if you are out on a nice lake in the middle of the night with a nice full moon lighting up the night. Then out of nowhere an alian ship decides to play a prank and zap the moon from the sky. Wouldn't the water recede from it's tide about 2 3/4 seconds before you see the moon dissapear?
     
  8. Sep 22, 2005 #7

    Doc Al

    User Avatar

    Staff: Mentor

    What makes you say that?
     
  9. Sep 22, 2005 #8
    <B>If<B/> I am correct in saying that light travells at 650,000,000 mi/hr. and considering how far the moon is. Say that the moon effects our waters with an almost instantanous field of gravity. Light travels in waves so if the moon was suddenly zapped away out of existance the light the sun reflects of the moon would still be traviling to our eyes, but the field would, like i said, dissapear almost instantanously causing the waters to recede out of tide before the light could reach our eyes. Say the sun dissapeared, it would take 5 minuts before we realize it's gone but the planets would be, to my guess, already floating out of orbit. Of course things could be differant in the quantum level of matter.
     
  10. Sep 22, 2005 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Since gravity travels at the speed of light, if the moon were instantly zapped, its gravitational effect on us would disappear at the same time as its image.
     
  11. Sep 22, 2005 #10
    <Q>Since gravity travels at the speed of light, if the moon were instantly zapped, its gravitational effect on us would disappear at the same time as its image.<q/> I didn't say that. I said gravity was instantanous, in thoery, and light was slower, light bends to gravity.
     
  12. Sep 22, 2005 #11

    Doc Al

    User Avatar

    Staff: Mentor

    I know you didn't say that. That's why I did. Gravity is not instantaneous.
     
  13. Sep 22, 2005 #12
    What's your proff?
     
  14. Sep 22, 2005 #13
    sorry, proof.
     
  15. Sep 22, 2005 #14

    Doc Al

    User Avatar

    Staff: Mentor

    It's a consequence of general relativity.
     
  16. Sep 22, 2005 #15
    *stupid me* right, right. But not enough to change my mind.
     
  17. Sep 22, 2005 #16

    Doc Al

    User Avatar

    Staff: Mentor

    Please don't. Stick to established physics.
     
  18. Sep 22, 2005 #17
    There is no established physics as to what gravity is, does, or how it works. Speculation is all we have.
     
  19. Sep 22, 2005 #18
    I'm going to guess that zapped means removed from existence. If so - The gravitational effect would disappear instantaneously, because the gravitational field of the moon is every bit as much the moon as the rock itself, while the photon coming your way is no longer a part of the moon and must travel at C.
     
  20. Sep 22, 2005 #19

    G01

    User Avatar
    Homework Helper
    Gold Member

    some things we have to accept until we able to fully comprehend them. So whitedragon, I suggest you accept this fact until you have enough physics and math under you belt to proove this fact for yourself. (I know I hate just accepting stuff too but sometimes it has to be done.)
     
  21. Sep 22, 2005 #20

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    The answer to this question is the same as the answer to the question of "what would happen if a charge suddenly disappeared" - would the force suddenly disappear? If so, wouldn't this indicate that light travelled faster than light?

    The answer to both questions is that the question itself is no good, because mass cannot suddenly disappear without violating physical law, just as charge cannot disappear without violating physical law. Attempting to solve the resulting equations yields the result that the inital assumptions were invalid.

    One can realistically perturb a charge (or a mass), and ask how fast the change in the resulting field propagates. This gives a theoretical speed of 'c' both for gravity and for light. (Actually this is an upper bound, but under most conditions fields are weak enough that 'c' is the right answer.)

    There are currently no good measurements on the speed of gravity, just theory. There are of course numerous good measurements on the speed of light.

    There were some recent attempts to measure the speed of gravity, but when one carefully analyzes them they have all failed. This is not to say that the speed of gravity cannot be measured - when LIGO gets to the point where it can actually detect gravitational waves, we need only compare the arival of the gravity waves from a cosmic event that emits both gravity waves and light (such as a binary star inspiral).

    This is all mentioned in the sci.physics.faq on the speed of gravity

    http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

    The article on

    "What is gravity" might also be of some interest.

    http://math.ucr.edu/home/baez/physics/Relativity/GR/gravity.html

    though it's not very detailed, it simply talks a bit about the idea of gravity as geodesic deviation.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Gravity class is amazing
  1. Amazing physics! (Replies: 5)

  2. Amazing video (Replies: 3)

  3. Physics is Amazing (Replies: 5)

Loading...