Re: Speed of gravity?
- From: Ace0f_5pades <m4deep_@xxxxxxxxxxx>
- Date: Sat, 8 Aug 2009 16:49:58 -0700 (PDT)
On Aug 8, 10:58 am, Laurent <cyberd...@xxxxxxxxx> wrote:
On Aug 7, 6:04 pm, Sam Wormley <sworml...@xxxxxxxxx> wrote:
Laurent wrote:
We know the Earth spins around the Sun held in orbit by the Sun's
gravitational pull, right? Now, if the Sun were to suddenly disappear,
would the Earth go loose instantaneously or would it take eight
minutes before flying off orbit?
Our best theory of gravitation predicts that changes in gravity,
such as the one you posit propagate at c.
Physics FAQ: Does Gravity Travel at the Speed of Light?
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html
To begin with, the speed of gravity has not been measured directly in the laboratory--the
gravitational interaction is too weak, and such an experiment is beyond present
technological capabilities. The "speed of gravity" must therefore be deduced from
astronomical observations, and the answer depends on what model of gravity one uses to
describe those observations.
In the simple Newtonian model, gravity propagates instantaneously: the force exerted by a
massive object points directly toward that object's present position. For example, even
though the Sun is 500 light seconds from the Earth, Newtonian gravity describes a force on
Earth directed towards the Sun's position "now," not its position 500 seconds ago. Putting
a "light travel delay" (technically called "retardation") into Newtonian gravity would
make orbits unstable, leading to predictions that clearly contradict Solar System
observations.
In general relativity, on the other hand, gravity propagates at the speed of light; that
is, the motion of a massive object creates a distortion in the curvature of spacetime that
moves outward at light speed. This might seem to contradict the Solar System observations
described above, but remember that general relativity is conceptually very different from
Newtonian gravity, so a direct comparison is not so simple. Strictly speaking, gravity is
not a "force" in general relativity, and a description in terms of speed and direction can
be tricky. For weak fields, though, one can describe the theory in a sort of Newtonian
language. In that case, one finds that the "force" in GR is not quite central--it does
not point directly towards the source of the gravitational field--and that it depends on
velocity as well as position. The net result is that the effect of propagation delay is
almost exactly cancelled, and general relativity very nearly reproduces the Newtonian result.
This cancellation may seem less strange if one notes that a similar effect occurs in
electromagnetism. If a charged particle is moving at a constant velocity, it exerts a
force that points toward its present position, not its retarded position, even though
electromagnetic interactions certainly move at the speed of light. Here, as in general
relativity, subtleties in the nature of the interaction "conspire" to disguise the effect
of propagation delay. It should be emphasized that in both electromagnetism and general
relativity, this effect is not put in ad hoc but comes out of the equations. Also, the
cancellation is nearly exact only for constant velocities. If a charged particle or a
gravitating mass suddenly accelerates, the change in the electric or gravitational field
propagates outward at the speed of light.
Since this point can be confusing, it's worth exploring a little further, in a slightly
more technical manner. Consider two bodies--call them A and B--held in orbit by either
electrical or gravitational attraction. As long as the force on A points directly towards
B and vice versa, a stable orbit is possible. If the force on A points instead towards
the retarded (propagation-time-delayed) position of B, on the other hand, the effect is to
add a new component of force in the direction of A's motion, causing instability of the
orbit. This instability, in turn, leads to a change in the mechanical angular momentum of
the A-B system. But total angular momentum is conserved, so this change can only occur if
some of the angular momentum of the A-B system is carried away by electromagnetic or
gravitational radiation.
Now, in electrodynamics, a charge moving at a constant velocity does not radiate.
(Technically, the lowest order radiation is dipole radiation, which depends on the
acceleration.) So, to the extent that A's motion can be approximated as motion at a
constant velocity, A cannot lose angular momentum. For the theory to be consistent, there
must therefore be compensating terms that partially cancel the instability of the orbit
caused by retardation. This is exactly what happens; a calculation shows that the force
on A points not towards B's retarded position, but towards B's "linearly extrapolated"
retarded position. Similarly, in general relativity, a mass moving at a constant
acceleration does not radiate (the lowest order radiation is quadrupole), so for
consistency, an even more complete cancellation of the effect of retardation must occur.
This is exactly what one finds when one solves the equations of motion in general relativity.
While current observations do not yet provide a direct model-independent measurement of
the speed of gravity, a test within the framework of general relativity can be made by
observing the binary pulsar PSR 1913+16. The orbit of this binary system is gradually
decaying, and this behavior is attributed to the loss of energy due to escaping
gravitational radiation. But in any field theory, radiation is intimately related to the
finite velocity of field propagation, and the orbital changes due to gravitational
radiation can equivalently be viewed as damping caused by the finite propagation speed.
(In the discussion above, this damping represents a failure of the "retardation" and
"noncentral, velocity-dependent" effects to completely cancel.)
The rate of this damping can be computed, and one finds that it depends sensitively on the
speed of gravity. The fact that gravitational damping is measured at all is a strong
indication that the propagation speed of gravity is not infinite. If the calculational
framework of general relativity is accepted, the damping can be used to calculate the
speed, and the actual measurement confirms that the speed of gravity is equal to the speed
of light to within 1%. (Measurements of at least one other binary pulsar system, PSR
B1534+12, confirm this result, although so far with less precision.)
Are there future prospects for a direct measurement of the speed of gravity? One
possibility would involve detection of gravitational waves from a supernova. The
detection of gravitational radiation in the same time frame as a neutrino burst, followed
by a later visual identification of a supernova, would be considered strong experimental
evidence for the speed of gravity being equal to the speed of light. However, unless a
very nearby supernova occurs soon, it will be some time before gravitational wave
detectors are expected to be sensitive enough to perform such a test.
I suspect that what they are measuring are, in some cases, the speed
of matter waves, in other cases, the speed of light.
I suscribe to Space Flow theories and Process Physics.
Cosmic background radiation is what fills the observable Universe. It
is composed by many different particles, like photons or EMR, which
are considered particles, and ZPR, also considered particles but of a
very different nature. What I call Aether is before this material
space, it is what Einstein called the gravitational ether. The
gravitational field, as described by Einstein, is continuous, not
quantized as the CBR is. Reality is built on quantized structures,
matter is always quantized, but sits on the gravitational field, which
is continuous.
no Darren, "quantization along a continuum". in fact --all things can
be classed as such.
hence the sum of quanta (their telling lies in the rate of changes).
I first coined that phrase in Chem lectures, in defending my position
that I understood "quantization", since our lecture claimed it was
somewhat counter intuitive.
What I call space is not the same as what 19th century and early 20th
century physicists called space. Back then, there were no CBR, nor
Wheeler's quantum foam. Today, space is considered to be material, a
collection of small particles, some of which are called dark energy
other, zero point radiation. This is why modern physics now say space
is grainy.
Gravity causes space to flow as an electromotive force is created by a
body's rotation. As morphic fields surrounding rotating bodies cut
through the CBR, there is friction which creates matter waves, just as
EMR is created when you shake an electron. These matter waves, or
matter selective quanta (bosons, quarks?), are pulled in by an
electromagnetic force orthogonal to the direction of rotation of
bodies, inwardly pulling ZPR particles to the center of the system.
This process depends on the characteristics of the matter waves, which
in turn depend on the characteristics of the rotating body. This model
explains why some planets have greater concentrations of some elements
than others.
you make some assumptions here, which I disagree with.
Gravitation comes from a pressure differential in material space
caused by the constant radial flow of matter waves into bodies with
mass, as quantum matter condenses and crystallizes into its objective
state. Space particles are carried by matter-selective, inwardly
flowing quanta, in an electrogravitational current, just as electrons
are moved by electromotive forces.
I only partially agree with this statement.
Picture two bodies like the Earth and the Moon, now imagine space
flowing into the Earth and into the Moon at the same time, that causes
gravitation. Because there is flow going in opposite directions and a
decrease in material space density, there is a drop in pressure,
causing both objects to drift towards each other. That is how we get
tide movement; the Moon casts a shadow that blocks space flow, causing
gravitic pressure to drop between the two bodies, consequently causing
the sea level to rise where the shadow is being cast.
even less likely
.
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- Speed of gravity?
- From: Laurent
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- From: Sam Wormley
- Re: Speed of gravity?
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