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Unfolding Nature Shop: Unfolding Nature: Being in the Implicate Order

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Re: Speed of light travel [Re: outerwave]
    #5359459 -

if a new planet were to just pop into existence in the solar system, 1 light year away from earth (for example) you would think, since _nothing_ is supposed to go faster than light, it would take one year for the gravitational pull to reach to earth and effect us. when actually, that 'field' or 'beam' of gravitaional pull INSTANTLY starts pulling on its surroundings, be they 10 feet away or 10 light years away

This is not so.

Classical (Newtonian) physics incorrectly presumes instantaneous propagation of gravity, but General Relativity says otherwise. In fact, one of the things that lead to General Relativity was a discrepancy in the Newtonian calculation for the precession of the orbit of Mercury. The discrepancy was a mystery until Einstein's penetrating insight into the nature of gravity and the mathematical discovery that it DOES NOT propagate instantly.

If a disturbance in a gravity field traveled faster than C, it would invalidate General Relativity, violate causality, and summarily throw all of modern physics out the window.

That gravitation propagates at C has even been confirmed experimentally (though it was no surprise) a few years ago by astro-physicists with a little help from Jupiter's great mass.


--------------------
Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

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Re: Speed of light travel [Re: Diploid]
    #5359520 -

Diploid said:
if a new planet were to just pop into existence in the solar system, 1 light year away from earth (for example) you would think, since _nothing_ is supposed to go faster than light, it would take one year for the gravitational pull to reach to earth and effect us. when actually, that 'field' or 'beam' of gravitaional pull INSTANTLY starts pulling on its surroundings, be they 10 feet away or 10 light years away

This is not so.

Classical (Newtonian) physics incorrectly presumes instantaneous propagation of gravity, but General Relativity says otherwise. In fact, one of the things that lead to General Relativity was a discrepancy in the Newtonian calculation for the precession of the orbit of Mercury. The discrepancy was a mystery until Einstein's penetrating insight into the nature of gravity and the mathematical discovery that it DOES NOT propagate instantly.

If a disturbance in a gravity field traveled faster than C, it would invalidate General Relativity, violate causality, and summarily throw all of modern physics out the window.

That gravitation propagates at C has even been confirmed experimentally (though it was no surprise) a few years ago by astro-physicists with a little help from Jupiter's great mass.



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Re: Speed of light travel [Re: Diploid]
    #5360049 -

Gravitation, unlike electromagnetic forces, is a pure geometric effect of curved space-time, not a force of nature that propagates. there are more theories of relativity than einsteins. most notably lorentzian relativity which has proved favorable over the old methods.

theories such as causality are there to be improved upon. even then they don't need to be invalidated, just an occasional change in perspective. if you don't view gravitational radiation as a propagating force, there is no conflict. (now if i can change perspective in such a way that will pay my rent, i'm all set) the world ended up not being flat, the sun didn't orbit the earth, there is more to atoms that earth wind fire and water. the whole driving force behind unified theory is that conflicting things are true. it is this same 'contradiction' of facts that gave rise to general relativity in the first place; frames of reference, constant c, etc...

do you have the year and/or figures behind this experiment? my last course of study in the field was around 2000 (living down the block from fermilab helps too). but it is just that curved space predictions are the foundation of modern physics, string/M theory, etc... that these are still going strong leads me to believe that this experiment didn't have the profound impact i'd expect.


--------------------
take care,

outerwave

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Re: Speed of light travel [Re: DieCommie]
    #5360103 -

welp, did some quick research and here is what i've found so far. in 2001 S. Kopeikin proposed the experiment you mentioned, the latest update came on 1/8/2003. since then pretty much all of his work has been invalidated or disproven. the major grief of academia was that there was so much hype around his papers that several fundamental flaws were overlooked by the public. since then six experiments have confirmed that gravitational effects 'travel' possible a _billion_ times faster than light.

in this experiment Kopeikin stated "? a moving gravitating body deflects light not instantaneously but with retardation caused by the finite speed of gravity propagating from the body to the light ray. ? We calculated this correction for Jupiter by making use of the post-Minkowskian approximation based on the retarded Lienard-Wiechert solutions of the Einstein equations. ? Speed of gravity cg must enter the left side of the Einstein equations ? This will lead to the wave operator depending explicitly on the speed of gravity cg."
None of these statements is correct even in General Relativity, provided only that "the speed of gravity" retains its classical meaning for the past two centuries of force propagation speed. The Einstein equations require the potential field of all bodies to act from the body's instantaneous direction, not its retarded direction, because they set propagation delay for the gradient to zero. But Kopeikin adopts the Sun acting from its instantaneous position and Jupiter acting from its retarded position, which is inconsistent. In fact, although the Sun moves 1000 times more slowly than Jupiter, it is 1000 times more massive, making any hypothetical retardation effects comparably important. The Lienard-Wiechert equations consider retardation in mutual distance, but not in direction ? the latter being a much larger effect of propagation delay. And the parameter on the left side of the Einstein equations is c2, and therefore has nothing to do with the speed of gravity, as we noted above. This does not prevent Kopeikin from calling it "cg" and solving for this parameter as if it were the speed of gravity, which is what he has done.

Kopeikin here ignores both the existence of a long-standing controversy about the speed of gravity (defined as the propagation speed of gravitational force) and the aforementioned arguments raised against his original interpretation by others. Kopeikin used the notion that this experiment might determine "the speed of gravity" to aggrandize the experiment, and perhaps also to justify funding for doing it. Yet the cg parameter measured is more closely related to the speed of light per se than anything else.


anyway, thats in a current journalof modern physics. and the names discrediting his findings read like a who's who in quantum physics and cosmology. basically he cheated to find funding is what people are saying, which is actaully rather common in research fields...


--------------------
take care,

outerwave

Edited by outerwave (03/03/06 12:04 AM)

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Re: Speed of light travel [Re: Diploid]
    #5360667 -

Diploid said:
Classical (Newtonian) physics incorrectly presumes instantaneous propagation of gravity, but General Relativity says otherwise. In fact, one of the things that lead to General Relativity was a discrepancy in the Newtonian calculation for the precession of the orbit of Mercury. The discrepancy was a mystery until Einstein's penetrating insight into the nature of gravity and the mathematical discovery that it DOES NOT propagate instantly.



As far as I know, no experiment has ever been conducted to directly measure the speed of gravity. The validity of Kopeikin's results has been under debate for a few years and I'm not sure that was ever satisfactorily resolved. Experiments have been designed and are being conducted to measure the speed of gravitational waves, which are generally assumed and believed to travel at speed c, but if you calculate, for example, the orbits of a binary pulsar system using general relativity and the assumption that the gravitational force "propagates" at speed c, you get weird effects like a net increase in the angular momentum of the system over time. Cosmology isn't my field but last I heard, this was the subject of an active debate.

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Re: Speed of light travel [Re: ChuangTzu]
    #5361690 -

the speed of gravitational waves, which are generally assumed and believed to travel at speed c

Waves is what I mean by "a disturbance in a gravity field".


--------------------
Republican Values:

1) You can't get married to your spouse who is the same sex as you.
2) You can't have an abortion no matter how much you don't want a child.
3) You can't have a certain plant in your possession or you'll get locked up with a rapist and a murderer.

4) We need a smaller, less-intrusive government.

Extras: Filter Print Post Top
Re: Speed of light travel [Re: Diploid]
    #5362192 -

Unlike with electromagnetism for example, where all field interactions are mediated by quantized units, general relativity has no such units for gravity. Thus, a conceptual difference exists between the speed of propagation of the force itself due to mass and the speed of propagation of gravitational waves. Think of electrostatic fields vs. EM radiation. One is produced by charge, the other, by accelerating charge. The gravitational analogues are mass and accelerating mass.

It turns out that I was wrong about the angular momentum bullshit I was spouting before. GR incorporates some clever machinery to make it appear like gravity is propagating instantaneously when actually it is a velocity-dependent, non-central force (think electrodynamics)[ 1].

Still, nobody has yet directly measured the speed of either gravity itself or gravitational waves and GR is definitely not the final answer. So maybe it will turn out that what is accomplished by mathematical trickery in GR is physical reality in another model if/when such a measurement is done.

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Unfolding Nature Shop: Unfolding Nature: Being in the Implicate Order

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