Gravity, is it instantaneous or limited to light speed

[themos]

Hold on, "phonons" are excitations of the lattice, the arrangement of charges (nuclei and electrons) in a material. They are not excitations of the electromagnetic field itself. At least, that's what I remember from my physics course.

[toml42]

Hold on, "phonons" are excitations of the lattice, the arrangement of charges (nuclei and electrons) in a material. They are not excitations of the electromagnetic field itself. At least, that's what I remember from my physics course.

correct, the photon is absorbed by the structure of the materiel and its energy/momentum transmitted as a wave in the structure.

As you rightly point out, the lattice is an arrangement of charges, so it makes sense that an oscillating EM field can lead to an oscillation in charge elements, and vice versa.

It's not the individual atoms absorbing and re-emitting, it's the structure of the material, sorry if i confused matters earlier.

this guy explains it a lot better than me http://www.physicsforums.com/showpost.php?p=899393&postcount=4

[themos]

I suppose one can say that our digital camera sensors pick up the "phonon" excitations (that is, charge movements) in the crystal induced by the incoming photon.

[RikM]

This whole chain of photon --> phonon --> photon interactions is extremely cool. I did not know that and it is always good to learn new things. Thanks.

Rik

[tomys]

Maybe imagine a futuristic man in a big planet that he owns and he also has a time machine. He moves himself and the time machine and the big planet back in time to the age of the morden world. We would not see him at first because he is further than pluto but far heavier than any planet in the solar system. Because gravity from the planets and the sun are already spread all over the solar system, him and the planet will first start moving toward the center of the solar system. Then when we are able to 'see' him, we then start to feel a gravitational pull toward him.

Its just weird. Maybe really just leave it as a good theory but avoid confusing yourself

Hope this made you understand

[Shibby]

After all, light covers a broad range of wavelengths and if all wavelengths of light traveled at the same speed we wouldn't get rainbows.

I'm afraid that's not true. When we talk about the speed of light we actually mean the speed of all electromagnetic radiation, so that includes all visible light, radio, microwaves, infra red, x-rays etc. While the wavelengths do vary, so do the frequencies. c = freq * wavelength.

The reason we get rainbows is that the different wavelengths of EM radiation refract at different angles.

[Glider]

I just googled for refraction and got, "refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one wave to another at an angle".

If you add that to your last sentence doesn't it equal what I originally said?

[George Jones]

I just googled for refraction and got, "refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one wave to another at an angle".

If you add that to your last sentence doesn't it equal what I originally said?

At a microscopic level, light, that is a photon, always travels at the same speed. Macroscopically, light travels at a different effective speed when it passes through an optical medium. This results in refraction. Usually, effective speed depends on wavelength, which results in rainbow-like effects (for example, chromatic aberration).

For an overly simplified account, see

http://stargazerslounge.com/physics-space-science-theories/104098-do-we-observe-orignial-photon.html#post1434843.

A much better account is given at the link posted by toml42 near the bottom of post #27,

http://stargazerslounge.com/physics-space-science-theories/108597-gravity-instantaneous-limited-light-speed-2.html#post1477632.

[omneferuss]

Nothing is perfect.. concepts are vaguaries.. the water stays where it is (in relative terms) but the wave passes....... and all things are elastic....

[omneferuss]

I just googled for refraction and got, "refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one wave to another at an angle".

If you add that to your last sentence doesn't it equal what I originally said?

a change in angle (refraction) is a change in path, hence distance travelled, hence duration of travel..... the 'speed' remains the same... a different frequency takes a different 'journey'...

and, this all occurs only at the interface of different 'media' of transmission of differing refractive index..........

[Glider]

George Jones, you're right, the link at the bottom of #27 is a clincher.

I'm still thinking in terms of point sources, ray tracing, wavefronts, reflection and refraction and stuff form my O level Physics(many, many years ago!)

[George Jones]

I meant to reply to this earlier, but I was "on vacation" for a couple of weeks at my in-laws' place, where I find it hard to think.

A friend asked me "if gravity appears to act instantaneously over any distance does that mean the field effects propagate at faster than the speed of light?"

The key word is "appears".

According to general relativity the field effects propagate at light speed but appear static relative to moving masses and this somehow gives the appearance of instantaneous action over any distance.

Newtonian gravity predicts closed circular and elliptical orbits. This prediction depends on the fact that Newtonian gravitational force is directed along the line joining the instantaneous positions of objects, like the Earth and the Sun. If Newtonian gravitational force weren't directed along this line, orbits wouldn't be closed.

As the Earth orbits the Sun., the position of the Sun, relative to Earth, changes. If gravity propagates at the speed of light, shouldn't the Earth feel (gravitationally) where the Sun was (according to the Earth) eight minutes ago, that is, shouldn't gravitational force be directed along the line that joins where the Earth is now to where the Sun was eight minutes ago? And if this is true, then, according to the previous paragraph, how can the Earth's orbit be a closed ellipse?

To answer these questions, I am going to talk briefly about the main equation of Einstein's theory of gravity, general relativity, G = T. Here, G is a geometrical quantity that depends on the curvature of spacetime, and T is a physical quantity that depends on the distribution and flows of mass and energy in the universve.

In Einstein's theory, gravity is a manifestation of spacetime curvature. If T depends not only on position, but also on flow of matter, then (by the equals sign) G, spacetime curvature, and (thus) gravity are affected by the velocities of objects. This feature is not present in Newtonian gravity.

As an example, consider a uniformly dense planet. According to Newton, the gravitational field of the planet is independent of the spin of the planet. According to Einstein, however, a planet's gravitational field is not independent of its spin. Spin puts the matter of the planet in motion, so different spins give different gravitational fields. To test this for the Earth, a satellite carrying gyroscopes has been put into orbit.

Back to the Earth and Sun. Form the point of view of the Earth, the mass of the Sun moves, and so, according to Einstein, this motion contributes to the gravitational field of the Sun. The field of the Sun depends on where the Sun is, and on how the Sun moves.

Answer to original question: these two contribution's to the Sun's gravitational field, position and velocity, add to produce an "effective force" that appears to point towards where the Sun is now, not where it was eight minutes ago.

What happens if the Sun magically disappears? The Earth will continue on in its orbit for another eight minute under the influence of an "apparent force" directed towards where the Sun would have been. After eight minutes, the Earth realizes that the Sun isn't there, and stops orbiting the missing Sun.

This was the suject of a technical research paper published in 2000 by Steve Carlip (pdf file up and right of abstract),

[gr-qc/9909087] Aberration and the Speed of Gravity.

[RikM]

Many thanks,

Rik

[Steve922]

That's not the way the coyote falls off the cliff, though.

LOL. I guess the need to be funny isn't written into gravity's contract. :-)

Steve