Could gravity be deemed to be invariant in a similar way that light is?

not in velocity but in effect........

What aspect of gravity?

Could gravity be deemed to be invariant in a similar way that light is?
Einstein spent *years* struggling with how to deal with gravity with the principle of relativity.
I'm currently working my way through Black Holes and Time Warps by Kip Thorne - I highly recommend it to all enquiring minds.

I guess what I am thinking about JamesR is that regardless of mass the value of gravity remains proportional in all frames.
In that the strength or intensity of gravity will be experienced the same by all that experience it at the same space time co-ordinate.....sheesh! it is a bit hard to explain......

QQ, I was thinking about posting something about this myself.

There are some good explanations of the different terms used regarding mass in Wikipedia here:
Inertial vs Gravitational Mass (http://en.wikipedia.org/wiki/Mass)
Relativistic Mass (http://en.wikipedia.org/wiki/Relativistic_mass)

The rest mass of an object is the true invariant mass of the object. That is, all observers in inertial reference frames will agree on what the invariant mass is. Which is precisely why it is a good thing to talk about. The relativistic mass, on the other hand, is observer dependent. For an object traveling with a velocity v relative to some inertial observer, the relativistic mass is given by
http://en.wikipedia.org/math/cd9c2f9484fc85846b1c0f5ba6da5110.png
where m is the invariant (rest) mass and c denotes the speed of light in a vacuum.

I know James R has stated velocity does not increase the gravitational attraction of a mass, but I haven't found anything definative to confirm or deny this. My understanding is that per the above, velocity increases mass, and that mass then has a higher gravitational attraction. From the point of view of an observer traveling on this mass, there is no increase in mass, its the other (stationary) objects which he is zipping past, which has increased mass. I'd like to get a better explanation of this, since (I believe) James disagrees, but would prefer to see references.

Regardless of the above issue, mass DOES increase gravitational force. Take binding energy for example. The binding energy of an atom is dependant on its atomic number, and that energy has an equivalent mass which increases weight. Here's a graph of binding energy versus nucleons in an atomic nucleus:

http://csep10.phys.utk.edu/astr162/lect/energy/bindE.gif
Ref: BindingEnergy (http://csep10.phys.utk.edu/astr162/lect/energy/bindingE.html)

Note the peak in the curve is for Iron.

Take another example, heat. Per the same reasoning as the above binding energy, heat (which is energy) adds to an object's mass and therefore its gravitational attraction. In other words, an object will weigh more in a gravitational field when it is hot as opposed to cold. The thermal energy adds to the mass per E=m*c^2.

Similarly, even potential energy for example, of a spring, increases mass, though this one I'm just slightly less sure of. Still I believe it is true that a compressed spring, which has more potential energy than the same spring extended, will have a greater mass and thus a greater gravitational attraction.

Could gravity be deemed to be invariant in a similar way that light is?

Not sure what you mean by this. Gravity is explained as a geometrical effect but is not strictly invariant under transforms the same way. Scratches head, there is no quantity in GR that is invariant under a transform, I think.

Recent work seems to support the idea that the speed of gravity is the same as light. Questions are being asked if G, the gravitational constant, is time dependant.

I know James R has stated velocity does not increase the gravitational attraction of a mass, but I haven't found anything definative to confirm or deny this.
I *think* that the Einstein Field Equations (http://scienceworld.wolfram.com/physics/EinsteinFieldEquations.html) might be what you're looking for.

by Q Goest:

I know James R has stated velocity does not increase the gravitational attraction of a mass, but I haven't found anything definative to confirm or deny this. My understanding is that per the above, velocity increases mass, and that mass then has a higher gravitational attraction. From the point of view of an observer traveling on this mass, there is no increase in mass, its the other (stationary) objects which he is zipping past, which has increased mass."
================================================== ==============

JamesR is supporting Relativity's swapping of frames of reference. The observer
'traveling' on this mass cannot view himself as traveling, since in relativity all inertial
frames are equal. The 'traveler' assumes the position of rest and the universe is
zipping by him. He can feel no changes due to his relative velocity, so the increase
in gravity, if one, is transferred to the 'moving' universe. The usual Relativity flip-flop.
Physicists will avoid your question because they know silly it shows Relativity to be.

the idea i have running around in my head is a little hard to explain as I lack the terminology but:

scenario

you have a gravitational mass that is emmitting light.

a star perhaps.

at 100,000,000 kms away you experience twoi things: 1] the gravitational pull and 2] the light.

As you move closer both the Gravitational and light intensity gets stronger.

Ok ...nothing startling in that hey?

If I was a person who had never heard that light had velocity what would I conclude from these two observations?

"that light and gravity are possibly one and the same pheno."

or "that light and gravity are intrinsic to each other by some means."

so no matter where I move in this gravity well the two are in direct relationship.....hmmmmmm.....

2inquisitive:

JamesR is supporting Relativity's swapping of frames of reference. The observer
'traveling' on this mass cannot view himself as traveling, since in relativity all inertial
frames are equal. The 'traveler' assumes the position of rest and the universe is
zipping by him. He can feel no changes due to his relative velocity, so the increase
in gravity, if one, is transferred to the 'moving' universe. The usual Relativity flip-flop.
Physicists will avoid your question because they know silly it shows Relativity to be.

You obviously didn't understand my previous explanation, so you chose to ignore it.

What I said was that curvature of spacetime in general relativity depends on the stress-energy tensor. One component of that tensor is the mass density of the object creating the curvature. But the tensor also has 15 other components. When you go to a different reference frame, the tensor transforms according to a generalised Lorentz transformation (operating on a tensor rather than a vector). The outcome of this is that although the mass density increases in a moving frame, the other components of the stress-energy tensor are changed in such a way that the spacetime curvature is constant.

To revert to terms you can understand: going faster doesn't increase gravitational attraction.

... The outcome of this is that although the mass density increases in a moving frame, the other components of the stress-energy tensor are changed in such a way that the spacetime curvature is constant.

To revert to terms you can understand: going faster doesn't increase gravitational attraction.

Does this mean that spacetime curvature is invariant across reference frames?

I think it must be, since in my understanding all frames are embedded in an absolute spacetime.

Also, spacetime curvature determines tidal forces, which should (I think) be frame invariant... it wouldn't do to have an object shredded by tidal forces in one frame, but stay intact in another.

(Disclaimer - I really don't quite know what I'm talking about in this domain!)

Pete

I think there is a counter-example to your case.

Consider the gravitational time dilation of a test mass falling into a black hole. As it approaches the event horizon an observor outside the gravitational field will see time dilation become effectively infinite. So the mass is never seen to pass through the event horizon. Yet it does in fact pass through the event horizon.

In other words, it is shredded (spaghettified) in one frame and not another. Ain't life wierd.

But in the external frame, it never gets to the spacetime location at which it is spaghettified...

What about a smaller black hole, with large tidal forces outside the event horizon?
Does the external observer see the spaghettification occur at the same location as the unfortunate infalling observer? They surely must, if the GR model is consistent?

But in the external frame, it never gets to the spacetime location at which it is spaghettified...

What about a smaller black hole, with large tidal forces outside the event horizon?
Does the external observer see the spaghettification occur at the same location as the unfortunate infalling observer? They surely must, if the GR model is consistent?

Never thought about that, nor read anything. I haven't a clue at the moment. My gut reaction is no, the smaller hole will shred you much faster than a larger one. But only because the tidal forces are stronger.

My texts are currently all in boxes due to room redecoration going on.

the smaller hole will shred you much faster than a larger one.
Yes, that's a given.

The question is whether the external observer sees the spaghettification occur at the same location as the unfortunate infalling observer.

Does this mean that spacetime curvature is invariant across reference frames?

As I understand it, yes.

Does this mean that spacetime curvature is invariant across reference frames?

This is an interesting thought......hmmmmm
It would have to be wouldn't it afterall light is invariant. If space time wasn't then light would be variant...I guess

What does space being invariant across reference frames mean? To someone traveling at close to light speed, space appears contracted in the direction of motion.

Pete: thanks for the link on tensors. Although if I understood tensors, I would no doubt be able to resolve the issue of mass/velocity/gravity, I don't, and there's nothing I've found even after searching the net for it that indicates velocity of a mass has no affect on gravitational force. Thats not to say you or James is incorrect, just that I've not seen any corroboration.

Hi Q_Goest,
Don't worry, I have no idea on that topic myself...

It just makes more sense to me that relative velocity would not affect gravity.
A change in velocity could be a different story, though...

Imagine a marble bouncing around in a box at high speed.
I think that that box+marble system would have greater gravity than a box + slow bouncing marble... but only because the high speed marble has ever changing momentum...

Unfortunately, I have zero knowledge of the GR field equations, so my idea is empty. I have no way of evaluating it.