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Gravity and spacetime

  1. Aug 27, 2003 #1
    If I understand everything correctly, gravity affects particles with mass/energy. How can it possibly affect spacetime? Doesn't it have to "grab" something?
     
  2. jcsd
  3. Aug 27, 2003 #2
    Bodies with mass/energy produce gravity which is the effect of bending space-time. Anything around that body has to follow a path determined by the curvature of the space-time.
     
  4. Aug 27, 2003 #3
    Hadn't occured to me to think about it like that... gravity warps ONLY spacetime, it doesn't "pull" things.

    That right?
     
  5. Aug 27, 2003 #4

    pmb

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    Gravity *can* curve spacetime. I doesn't mean that in all cases that it does. And it can curve space as well as spacetime.

    But matter reacts with matter - an matter defines spacetime (loosely speaking)

    Pmb
     
  6. Aug 27, 2003 #5
    What about neutrinoes? Or other non realistic particles that matter!
     
  7. Aug 28, 2003 #6
    When doesn't it? Can you mention at least one case?
    Spacetime includes space.

    That I cannot agree with, even though I don't know much about this.
     
  8. Aug 28, 2003 #7
    Re: Re: Gravity and spacetime

    They have mass/energy.

    What non realistic particles are you talking about?
     
  9. Aug 28, 2003 #8
    There's a story about gravity and relativity that is not told as often as it should. It goes back to Newton and is quite revealing of how physicists think.

    You see, we are so used to think of gravity as a "force" that it doesn't occur to us there's something strange about it. What's strange about it is that it's somewhat "spooky", as it acts without physical contact between bodies. To us the concept seems ordinary, but it's well documented that Newton himself disliked the idea, and adopted it because he had no other option. In Newton's time electromagnetism was not known, and the idea of two bodies interacting across empty space was quite out of the ordinary.

    Another problem revealed itself as time went on. The force of gravity has the peculiar feature of being proportional to mass. For physicists, that's a sure sign that it's not a force at all, but rather what they call a pseudo-force, something that seems to be a force simply because we are looking from the wrong perspective.

    Imagine a closed truck with a box and a camera filming the box and the interior of the truck. The box is not attached to the floor while the camera is. If you are looking at the movie of the box, you will notice something interesting. When the truck is moving in a straight line, the box stays in place, but when the truck is turning the box appears to undergo acceleration in the opposite direction. From the perspective of the movie, it's as if the walls of the truck attract the box. From the perspective of someone watching the truck from the outside, it's clear that the box is just trying to follow its inertial path while the truck is being accelerated. The impression that the box is experiencing a force is an illusion.

    How do you know if something is really experiencing a force or if it's just the appearance of a force? Well, one sure sign that you're not dealing with a real force is when the force is proportional to the mass of the object being accelerated. That is just too convenient to be correct. Gravity happens to be such a force (proportional to mass). Together with the fact that it's an ad-hoc solution conceived by Newton to explain planetary motion, physicists have always suspected gravity to be a pseudo-force. With that in mind, they set out to find the correct perspective which would reveal that objects appearing to be accelerated by gravitational forces are not accelerating at all.

    Einstein's General Relativity achieved exactly that feat. In GR gravity is not a force but simply a feature of the geometry of the universe. Two objects moving in a "straight" line ("straight" from a GR perspective) are bound to collide, not because they are being accelerated, but because spacetime is not really "straight" the way we perceive it. To borrow from the truck analogy, it's as if we were the box while spacetime is the truck; we are constantly trying to move in a straight line but the truck is going around in circles and as such we experience acceleration.

    So to answer your question, gravity does not affect particles with mass/energy, it's spacetime that does by the fact of being "curved". Just like a crooked road "affects" the load of a truck.

    Hope this helps.
     
  10. Aug 28, 2003 #9

    pmb

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    Tale wrote
    I can give you three examples

    (1) A uniform gravitational field
    http://www.geocities.com/physics_world/uniform_field.htm

    (2) A straight cosmic string cosmic string
    http://www.geocities.com/physics_world/cosmic_string.htm

    (3) A vacuum domain wall.
    http://www.geocities.com/physics_world/domain_wall.htm

    Obviously. However I think you missed my point. It's possible for space to be flat and spacetime be curved. In fact that's exactly the case for a flat universe - I.e. in a flat universe space is flat = however spacetime is curved.

    Pete
     
  11. Aug 28, 2003 #10
    Re: Re: Re: Gravity and spacetime

    Hello DOC, I am thinking virtual particles.Yes neutrinoes have a little mass as they say.Condensed high frequency powerfull points of energy are always full of something.
     
  12. Aug 28, 2003 #11

    FZ+

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    They still have mass/energy.(Albeit temporarily) Hence Hawking Radiation.
     
  13. Aug 28, 2003 #12
    mass.

    FZ, That tells me that matter is either made by the spontainious episode or that matter is a result of,or virtual particles bring mass with them.From a mass origen.The question is ---What piece of anything does not have mass.The ether must be the origen of this substance.
     
  14. Aug 28, 2003 #13

    FZ+

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    Er... no... The existence of virtual particles is due to quantum uncertainty, where probability creates so called "quantum foam". The nature of this "foam" is very different from classical ether.
     
  15. Aug 28, 2003 #14
    lost it

    I replied to you,my post was lost due to pop ups sorry.Bottom line where do virtual particles come from!
     
  16. Aug 29, 2003 #15
    Field fluctuations or something like that.
     
  17. Aug 29, 2003 #16
    field fluctuations

    The field must condense in certain points to produce a particle,And the low energy level or stability must make it decay as fast as it poped into existance. For on to stay in our dimension it would seem it needs more ummph.More energy to stabalize it and to be taken care of by other photons of the same frequency nearby.
     
  18. Aug 29, 2003 #17
    Rubber Sheet Analogy

    I know you have all heard the rubber sheet analogy when it comes to general relativity. If not this is it in layman's terms: If you were to stretch a sheet of rubber and put a bowling ball in the center it would depress the rubber. Now any object (round) placed on the outer edge of the depression will roll towards the heavier object, i.e. gravitational pull. But there is one flaw in this....while the bowling ball makes a depression in the rubber because "gravity of earth" pulls it down...the depression in space-time IS gravity, not an effect of gravity. So you can't say that a planet's "gravity" causes a bend in space-time, because that bend in space-time IS gravity. Just making a point, goodnight and Godspeed.
     
  19. Aug 29, 2003 #18
    Two sides

    There is always two sides to every depression!The mass indenting spacetime causing more mass to fall or get pulled or close its orbit to collide with the bowling ball also has a indent into the spacetime it is denting--Does this make any sense!Almost a oposite reaction on the --other side.The push of spacetime reflects or is opposed to anti matter .Just looking on the other side! TWISTER
     
  20. Aug 30, 2003 #19

    LURCH

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    Science Advisor

    Re: Two sides

    I believe the situation you are describing reffers to negative energy, not antimatter. Nagative energy (and whatever negative matter might be created from it) is gravitationally repulsive. Though I've not seen experimental data, it has been side several times here in the Forums that antimatter is attracted by gravity.
     
  21. Aug 30, 2003 #20
    Re: Re: Two sides

    Sorry,New here! Should the anti-matter be attracted to ant-gravity?
     
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