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Author Topic: Gravity and Gravity  (Read 4319 times)

Offline thebrain13

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Gravity and Gravity
« on: 11/02/2007 05:05:09 »
Gravity changes geometry, so wouldn't gravity change gravities path as well? Has gravitational paralax ever been observed? And secondly, if blackholes cause infinite curvature of spacetime how can gravity excape?


 

Offline daveshorts

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Gravity and Gravity
« Reply #1 on: 11/02/2007 11:13:38 »
I think to discover this we would really need to be able to detect gravitational waves, which we can't although there is talk of a couple of space missions to do so in a very crude way in about 10years. Basically gravity is immensely weak, and very fundamental to space itself, so fluctuations in it are ridiculously hard to detect let alone measure and work out where they are coming from.
 

Offline lightarrow

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« Reply #2 on: 11/02/2007 21:10:50 »
Gravity changes geometry, so wouldn't gravity change gravities path as well?
What do you mean with "gravities path"?
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Has gravitational paralax ever been observed? And secondly, if blackholes cause infinite curvature of spacetime how can gravity excape? 
What do you mean with "gravitational paralax"?
About the fact gravity couldn't excape from black holes, it doesn't need it!
As I said in the thread "Gravity Shield": <<all there is it's a space-time curvature here already present because of the black hole there; do you remember the warped elastic membrane analogy?>>. Out of the Event Horizon, all you need to know is the value of the space-time curvature where you are.
I don't know if this explained your question.
« Last Edit: 11/02/2007 21:19:59 by lightarrow »
 

Offline thebrain13

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« Reply #3 on: 12/02/2007 20:56:16 »
Propogation of gravity should be bent the same way light is, due to gravitational curvature, I just titled it gravitional paralax. And I dont understand your explantation for blackholes. Im saying gravity doesnt travel fast enough to escape a blackhole. Gravity has to obey every geometrical altercation just as light would have to.

And daveshorts you wouldnt need to see a gravity wave to test the subject, just the strength of gravity, and that can be measured very precisely.
 

Offline JP

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« Reply #4 on: 12/02/2007 23:47:17 »
Propogation of gravity should be bent the same way light is, due to gravitational curvature

I think the problem is that general relativity treats gravity as something special, not like the other forces that are "bent" by gravity.  Gravity is modeled by its effect on space--it bends it, as you said.  Light is something that travels around in this space and is thus subject to the bending.  Gravity itself is created by the geometry of the space and therefore it isn't bent the same way light it by itself.  One way to think of it is like a bathtub--light is a ping pong ball floating on the surface of the water--if you pull the plug it gets sucked to the drain (like light into a black hole).  Gravity is the water itself.  The shape of the water changes if you pull the plug,  but you can't really talk about it moving in the same way as the ping pong ball.

"Gravitational waves" do exist and do travel and could be a form of "traveling gravity."  The idea is that if a piece of matter is moving in space, it's going to generate waves in space (similar to how moving charges generate light waves, if you're familiar with that).  If we can detect these waves through something like http://www.ligo.caltech.edu/, this will be strong support that General Relativity is correct.

I'm not an expert in General Relativity, so I don't know how a big mass (or black hole) would affect gravitational waves.  I'd imagine they'd be changed somehow, but probably not in the same way light is--it's probably some wierd geometric interaction.

Finally, there's theories out there to describe gravity as a particle-like force (like light).  The big theory in this region is String Theory, but no one has tested it experimentally.  I know very little about it, so I don't know what it says about gravity particles being deflected by themselves.
 

Offline lightarrow

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« Reply #5 on: 13/02/2007 14:53:05 »
Propogation of gravity should be bent the same way light is, due to gravitational curvature, I just titled it gravitional paralax. And I dont understand your explantation for blackholes. Im saying gravity doesnt travel fast enough to escape a blackhole. Gravity has to obey every geometrical altercation just as light would have to.
Ok. Now I ask a question to you: you are at some distance from a black hole, outside the horizon. Why do you feel a gravitational force on you? Because of something inside the black hole? Not at all! You feel it because in the point where you are there is a space-time curvature. You don't even need to know that there is a black hole somwhere to make your computations, you can simply study the space-time properties locally.
You don't have to think that gravity "comes" from the black hole; it warped the point of space where you are even before it became a black hole, and it's this warping that you feel now, nothing else than this.
 

Offline thebrain13

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« Reply #6 on: 13/02/2007 19:02:36 »
Allright then, we know that gravity is emitted from stars, planets, any point of mass. So our measurement of where the gravity is coming from is where we view the objects position. Gravity alters our perception of geometry, thus altering our perception of where an object is (paralax) So if gravity didnt bend around massive objects, that would mean that we would perceive gravity as coming from a different place than the particle that emitted it. Have we ever measured that?

And also I dont see how we are able to conclude that gravity is a change in geometry or a force.

I mean, Einsteins equivalency principle states that gravity acts the same as a uniform force.
 

Offline mothsafterglow

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Gravity and Gravity
« Reply #7 on: 14/02/2007 02:36:05 »
Although I'm not sure I have an answer to your first question because I'm kind of fuzzy on what you're asking, I think I may be able to lend a (possible) answer to your question about black holes.  Gravity itself is a force, not matter.  Light itself can be a particle or a wave, both of which can be influenced by the four forces, but gravity being itself a forces, has nothing to "move".  Gravity is simply a bending of the fabric of space and time which influences all matter which has mass.  Because gravity is a force and not matter itself, it cannot be trapped by it's own forces in a black hole.  It merits clarification that gravity does not travel at the speed of light, it's impact does.  It's the same principle of a stone being dropped in a lake, the waves moving out are caused by it, and the water itself moves, but the force of the rock is simply exerted.  The force is exerted on the water, and the water moves, exerting its own responding force on the rest of the water.
 

Offline lightarrow

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« Reply #8 on: 14/02/2007 18:48:00 »
Allright then, we know that gravity is emitted from stars, planets, any point of mass. So our measurement of where the gravity is coming from is where we view the objects position. Gravity alters our perception of geometry, thus altering our perception of where an object is (paralax) So if gravity didnt bend around massive objects, that would mean that we would perceive gravity as coming from a different place than the particle that emitted it. Have we ever measured that?
Don't think that all questions about gravity or GR are already answered or that they are well known from physicists, however you have to understand that light moves IN the space-time, while gravity IS the space-time. Can you see the difference?
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And also I dont see how we are able to conclude that gravity is a change in geometry or a force.
I mean, Einsteins equivalency principle states that gravity acts the same as a uniform force.
Not exactly. It says that an inertial field can be substituted with a gravitational field and the other way round. Example: You are inside your house and all your house is taken and accelerated in the outer space at 1.47g with rocket engines put under the pavement. Inside, you can feel the effect of this acceleration, but, without looking out of the window, you cannot establish in any way if you have rockets and you are moving, or if you are still on a planet's surface with 1.47g gravity acceleration. (I think that, better than this, no one have ever told you).
 

Offline thebrain13

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Gravity and Gravity
« Reply #9 on: 19/02/2007 20:10:24 »
So the answer to this question is no. Therefore gravitational force is measured from a different place than where we view the light coming from, if its around gravity, thats what you all are saying right?
« Last Edit: 19/02/2007 22:47:17 by thebrain13 »
 

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