# Is gravity absolute?

-The speed of light changes depending on the medium
-The speed of sound changes depending on the medium
1)Is gravity's speed affected by anything?

-Magnetic fields affect metals
-Gravity affects light
2)Does anything affect gravity? (besides mass, since for every force there's an equivalent opposite force)

:-)
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Commented:
If you are really interested in the current understanding of gravity, you must learn at least something about the general theory of relativity which says that the presence of a mass results in a warping of space rather than the production of a field. For most, but not all, practical puroses, the field concept works fine. (The GPS system does require that application of the general theory however.)
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1. The infulence of mass (gravity) does travel at the speed of light.
That is, if the sun were to suddenly disappear, it would be 8 minutes before the orbit of the earth would be affected. Nothing slows this speed down (or speeds it up).
2.  mass only (your statement of the third law has nothing to do with the answer)
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Commented:
andreba,

1) Gravity doesn't have speed (unless you want to talk about gravitational waves).  It is a force (field) and you can measure its strength.

2) No.  Gravity is a property of the objects' masses.  You can introduce some new complicating object into the picture that changes the gravity, but that new object has mass.

By the way, what you said about equivalent opposite force is true of electric and magnetic force fields, but not of gravitational fields.  Gravity is always attractive, never repulsive.

mathbiol
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Commented:
Gravity is affected by distance from the center of the mass.

If you could weigh yourself at sea-level, 10,000 ft and 20,000 fett you would notice that you weighed less at the higher altitudes.

mlmcc
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Commented:
"since for every force there's an equivalent opposite force)"
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"what you said about equivalent opposite force is true of electric and magnetic force fields, but not of gravitational fields.  Gravity is always attractive, never repulsive."
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The first quote refers to the result of a field, not the sign of the field itself.
It is Newton's 3rd law and does not refer to the sign of the field or the source of the field. Gravity is indeed always attractive but the two forces referred to in the first quote act on different bodies.
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Author Commented:
Is there anything else that, like gravity, does NOT change speed?

:-)
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Commented:
Is there anything else that, like gravity, does NOT change speed?
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Not that I know of. HOWEVER I suspect that the nuclear force also is independant of a medium. It is however so short range that the question does not usually come up.
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Author Commented:

Since gravity seems to be absolute: is gravity relative? How can we say that relativity applies to gravity too? If it does at all..

:-)
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Commented:
is gravity relative?
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What do you mean by relative?
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Author Commented:
If it is not affected by anything, then the value of gravity is an absolute figure, regardless of the frame of reference..

Speed is relative to where the observer is..

:-)

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Commented:
If it is not affected by anything, then the value of gravity is an absolute figure, regardless of the frame of reference..
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Fine. Then gravity is absolute.
But gravity is not a number, It is the effect on space time caused by a mass.
Einstein in his special and general theories of relativity did not mean (or say) that nothing is absolute, that every thing is relative. Quite the opposite. He said that the laws of physics are independant of the  frame of reference.
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Commented:
The speed of gravity is difficult to measure physically, and experiments have not been totally conclusive,
but the best accepted results so far have been consistent with the best accepted theories that say that the gravitational effects propagate at the speed of light.

All massless particles will travel at the speed of light.
Photons, Gluons and Gravitons are thought to be massless.
Neutrinos are no longer thought to be massless.

If gravitons propagate through space-time, they should be affected by distortions in space-time, just as photons are.

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Author Commented:
Are they, ozo?

:-)
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Commented:
If a graviton has zero mass and zero energy, then i find it hard to imagine what exactly it is - surely a particle with zero mass simply doesn't exist?

Confused :-)
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Commented:
surely a particle with zero mass simply doesn't exist?

Photons are almost universaly thought to have zero rest mass and certainly exist.
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Commented:
deighton,

>If a graviton has zero mass and zero energy, then i find it hard to imagine what exactly it is.
Good thinking.  I don't know much about gravitons, but in general I think we can assume that a particle had better have mass or energy (or both).

>All massless particles will travel at the speed of light.
"Zero mass" is shorthand for "zero rest mass."  If we look at a photon as an outside observer, we will find that it has mass and energy (where its mass is given by E = m c^2).  But if we imagine ourselves riding the photon, we find that it has zero mass.  In other words a photon has zero "rest mass", meaning we're measuring from the photon's point of view, i.e. when the photon is at rest with respect to us, the photon riders.

mathbiol
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Commented:
Photons, gravitons and gluons are thought to have zero rest mass and positive "relativistic mass" aka "energy"
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Commented:
if a graviton has energy then are bodies with mass 'radiating' gravitons and thus losing energy?
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Commented:
at wikipedia it states

'Gravitons are postulated simply because quantum theory has been so successful in other fields. '

Sounds like an act of faith rather than science.
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Author Commented:
Highly interesting, I'd like an answer to the energy loss due to gravitons.

:-)
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Commented:
What has not been satisfactorily explained is "Action at a Distance". The force appearing on a particle in a field - electrostatic, magnetic or gravitational - is due to some other particle (or particles) at some distance away. The actual force is also proportional to the distance squared. This tends to suggest some sort of enamination which would dinimish in strength with distance. Interestingly the force is attractive.

Now Richard Feynman suggested the idea of "particles" enaminating from the "center" which interact with particles "in the field" - hence gravitons - rather like light shining out from the sun on the planets. The only problem is that normal particle interaction is repulsive and involves a certain amount of energy. Continual photonic emmission will result in a loss of mass, but it appears that graviton emmission does not. There are other properties of gravitons which would be necessary for their existance which we don't find with the particles we know.

I suspect that the process is somewhat more complex. Why should there be a limit to the speed at which energy can travel through space? Is not the limit something to do with space itself?
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Author Commented:
Interesting BigRat: how about a limit to energy's 'speed' because of 'traffic'? (air, water, ether...)

:-)
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