[alancalverd]

Nobody is "worried" about the source, but everyone knows it has to exist. We have no evidence of a gravitational field with no mass at its centre.

the gravitational field in a region of space does not tell you what created it.

The presence of a red bus in the middle of London does not tell you where it was built, but we know that someone, somewhere, made it, and drove it to the city. We have no evidence of the spontaneous appearance of buses without factories.

[yor_on]

Pete, when you write "There's no source object whose mass the gravitational field is proportional to." Do you think of it as gravity's reach is limitless? There being no way, practically, to define who it 'belongs too'. Or are you thinking of it as a result of what frame I am in, finding a field existing for me? It's interesting. And it always seem to come down to comparisons between frames of reference. But even in a black box scenario, you can find if you're in gravitational field, assuming tidal forces acting on you. But that's no real proof, is it? You could imagine a gravitational field without tidal forces, me in a 'free fall' inside it. It's like there is a 'tension' of some sort, related to mass, motion, and energy, creating 'gravity' when locally accelerating, possibly? Because that is the equivalence principle, isn't it? That earth is constantly uniformly 'accelerating' at about one Gravity.

And if it is correct? Then I might imagine it as properties of a 'field' preexisting, although not presenting itself, for my measurements, except in certain circumstances. Or maybe I'm bicycling in the great younder again

[Pmb (OP)]

Nobody is "worried" about the source, ..

That’s not true. It was you who were worried about it. That’s why you said

Gravitation requires the presence of a second object, and the gravitational force on a "test" object is proportional to the mass of the "source" object.

and I was saying that this is simply not true. You’re stuck in a Newtonian frame of mind where one object exerts a force on another object whereas in modern physics one body generates a field and it’s the field that interacts with objects to exert forces on them. So all we need to know, and all that needs to exist, is the field, not the source. The laws of physics don’t require the existence of gravitational objects for gravitational waves to exist.

And even when there  is a source the force isn't proportioal to the mass since there's a velocity dependance factor there, i.e. in GR the gravitational force is velocity dependant. See Eq. (14a) in http://home.comcast.net/~peter.m.brown/gr/grav_force.htm

And these are two very different conversations. You’re stuck with thinking that all fields require a source and I was talking about a totally different subject, i.e. the force on a particle due to the gravitational field.

..but everyone knows it has to exist. We have no evidence of a gravitational field with no mass at its centre.

And that assumption is just plain wrong in general since there need not be a source. For example; if you're in an inertial frame of reference in flat spacetime. A particle is in that frame and is at rest. Now you change to a uniformly accelerating frame of reference. Then in that frame of reference there is a uniform gravitational field and there is a gravitational force on that particle. Yet there is no source of gravity to be concerned with other than the distant stars.

Then again there are ways to generate a gravitational field with a finite amount of matter – See http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm

So because you know the field in a region of space it doesn’t mean that you know what the source of that field is.

Go out and look at the CMBR. That radiation came from the birth of the universe and was not created by other charges. Therefore the force that it exerts on charges today is not a result of the presence of a second object, i.e. the electromagnetic force on a "test" charge is not proportional to the charge of any source so if you tried such an argument with the electromagnetic force then that argument would also fail. There is nothing in the laws of electrodynamics which requires a non-zero charge density everywhere to create an electromagnetic wave so its not charge itself which generates EM waves. You can have electrically neutral systems, i.e. systems with the same amount of positive and negative charges, to create EM waves.

[yor_on]

I don't know JP. A charged particle moving in a circle, is that a geodesic? If it isn't, then it is a acceleration. And if it accelerates it must lose 'energy'. (Thinking of it as 'fields', also assuming light to not 'propagate', you should get shapes describing it instead of a motion.)
=

Heh, a 'flickering' universe, locally defined.

[Pmb (OP)]

I don't know JP. A charged particle moving in a circle, is that a geodesic? If it isn't, then it is a acceleration. And if it accelerates it must lose 'energy'. (Thinking of it as 'fields', also assuming light to not 'propagate', you should get shapes describing it instead of a motion.)

A charged particle moving in a circle in flat spacetime is not moving on a geodesic. However if that charge is orbiting the earth and in that sense its moving on a cirlce then yes, it's moving on a geodesic since in this case the circle is a geodesic.

In flat spacetime a charged particle moving in a circle emits what is known as synchrotron radiation.

[yor_on]

true Pete, but what it made me think about was actually electrons 'orbitals'.

[Pmb (OP)]

true Pete, but what it made me think about was actually electrons 'orbitals'.

Where did you mentionor indicate that you were thinking about orbitals? Or did you? You wrote

A charged particle moving in a circle, is that a geodesic? If it isn't, then it is a acceleration. And if it accelerates it must lose 'energy'.

Electrons in orbitals are not moving in circles and are not accelerating. Those are classical ideas which don't belong in the realm of quantum mechanics.

[lean bean]

The laws of physics don’t require the existence of gravitational objects for gravitational waves to exist.

Trying not to take your opening question too of course here, but...
Can you explain something of what you mean there.
I stuck with thinking of either mass or energy or momentum as having something to do with the origin of gravitational waves. Or are you saying not to worry about the origin of the waves?

[Pmb (OP)]

Or are you saying not to worry about the origin of the waves?

Yes. Anytime I've ever had to calculate the force exerted on a particle I never had to know the source of the field. All I had to know was the field.

[lean bean]

Or are you saying not to worry about the origin of the waves?

Yes. Anytime I've ever had to calculate the force exerted on a particle I never had to know the source of the field. All I had to know was the field.

Ok, now I know what you mean, but, I do think you could have worded that better... confused for a moment thinking what other laws, if not those of physics, do you require to make G waves.

[yor_on]

I know that Pete, on the other hand you have 'photographic evidence', although created by taking several images/snapshots, of something called a particle (electron) in that Swedish experiment (think it was Lund's university). So we have both in some weird way, or not so weird if propagation can be questioned, instead using the idea of a field, keeping a local 'constant' arrow for any observer, allowing this 'particle' to be depicted through repeated 'snapshots'. It's the way my mind works Pete Also called galloping senility.
=

(As well as thinking of it as if a 'acceleration' can be translated to something at a particle level, we do it with a planet, do we not? Earth 'accelerates', so the question I started to ask, and still ask, myself is if one could apply the same sort of view on a atom and its constituents. Accelerations versus uniform motion. )
=
And to come a little clearer Not talking about a acceleration as something 'spending a energy' for this. As far as I know Earth do not spend any 'energy' by 'constantly uniformly accelerating' at about one gravity
==

Maybe you can express it this way. If Earth is 'accelerating', then it is in no measurable direction for us. Instead we find a uniform motion that we can measure, through the universe. And that motion is a geodesic as I understands it, spending no 'energy', never mind how much matter, neutron stars, etc, I involve in Earths path.

That one gives me one headache, no 'friction', the other is what a 'acceleration' can be, if I define Earth as accelerating. And then we have particles, they should have a gravitational field too, shouldn't they? And they, just as Earth, should then 'accelerate'.
==

Maybe the question could be. Do a acceleration spend 'energy'? I sort of automatically have assumed it must do so, it seems the reasonable assumption to me. But what about the equivalence principle then, and Earth 'accelerating'?

[alancalverd]

one body generates a field and it’s the field that interacts with objects to exert forces on them. So all we need to know, and all that needs to exist, is the field, not the source. The laws of physics don’t require the existence of gravitational objects for gravitational waves to exist.

Spot the selfcontradiction.

If you can generate a gravitational field  without a source object, you can have all the Nobel prizes at once. But until then, I think anyone observing the effect of a gravitational field will (rightly) infer the existence of a massive object. That is, after all, how astronomers conduct their business, and to date, they have been right every time.

Not sure how the laws of physics determine the existence of gravitational waves. Scientific laws are discovered by observation, not imposed by Brussels.

In Einstein's theory of general relativity, gravity is treated as a phenomenon resulting from the curvature of spacetime. This curvature is caused by the presence of mass.

But what did Einstein know about relativity? My friend pmb says you don't need mass. So there.

[Pmb (OP)]

Ok, now I know what you mean, but, I do think you could have worded that better... confused for a moment thinking what other laws, if not those of physics, do you require to make G waves.

Well I can't be good at everything.    But could have worded what exactly, better?

[yor_on]

It's a really tricky one Alan. But think of 'energy' for it. If there is a equivalence between energy and mass, then 'energy' should be enough for making a defined 'container of a universe, even in the absence of matter, as long as we assume gravity to have a relation to 'mass'. Light can be referred to as having mass too. So you're right in that we need a relation, but what that relation need to be? Then there is the way you can translate away a 'gravity' by changing your reference frame. If I imagine a gravitational field as a defined color, then this statement is the same as saying that the color disappear just by me changing some parameter 'inside' it, as stopping my acceleration, free falling. And if we involve different frames of reference, then me on earth may define a satellite as 'free falling' inside a gravitational field, but for the satellite itself there is no gravitational field to be measured. Or if we define it as some sort of grid lines, then me accelerating either becomes a very local experience of 'gravity or, assuming gravity's infinite reach, those locally created 'field lines' reach forever, as long as I keep accelerating.

[alancalverd]

then me on earth may define a satellite as 'free falling' inside a gravitational field, but for the satellite itself there is no gravitational field to be measured

Oh yes there is! The observer on the satellite will note that he is accelerating towards a massive object, and deduce that he is therefore in a convergent gravitational field.

[Pmb (OP)]

then me on earth may define a satellite as 'free falling' inside a gravitational field, but for the satellite itself there is no gravitational field to be measured

Oh no there isn’t

Oh yes there is! yor_on was attempting to explain Einstein’s equivalence principle to you. That principle states that at any event in spacetime it’s possible to transform the gravitational field away. And likewise it’s possible to create a gravitational field at any point in spacetime. If you really want to learn Einstein’s general theory of relativity then you should pick up a GR textbook and read it rather than making all of these false assumptions about it attempting to prove it wrong. Start here The Foundation of the General Theory of Relativity by Albert Einstein (1916), Annalen der Physik, 49. See
http://hem.bredband.net/b153434/Works/Einstein.htm

It will be seen from these reflexions that in pursuing the general theory of relativity we shall be led to a theory of gravitation, since we are able to "produce" a gravitational field merely by changing the system of co-ordinates.

The observer on the satellite will note that he is accelerating towards a massive object, and deduce that he is therefore in a convergent gravitational field.

That’s a misinterpretation of what yor_on said. He didn’t say that it’s possible to transform the gravitational field away at all events. He actually meant that at even event you an transform it away. However in a curved spacetime like that around earth it can’t be transformed away everywhere. In this case the observer on the satellite will not be in a gravitational field if he stays close to the (small) satellite. However in this case it’s the earth that is in free fall in his frame of reference. I.e. the earth is in the gravitational field now, not the satellite.

[CPT ArkAngel]

If you are in a box or an elevator, maybe the walls are hiding the truth...

Nice article Pete!

[alancalverd]

So you are saying that according to GR, gravitational fields exist in the absence of mass.

Now here's a fine mess, because one principle of all non-newtonian physics is that it must approximate to the newtonian at the mesoscopic level, because that is what we observe and we don't like arbitrary discontinuities in our theories. So where does the mass of the bodies come from, in our carefully measured "Gm1m2/r^2" forces? 

[Pmb (OP)]

So you are saying that according to GR, gravitational fields exist in the absence of mass.

It's certainly allowed by the field equation, that's for sure.

Now here's a fine mess, because one principle of all non-newtonian physics is that it must approximate to the newtonian at the mesoscopic level, because that is what we observe and we don't like arbitrary discontinuities in our theories.

This is what I've been talking about. If you actually sat down and chose to learn general relativity then you'd no longer make mistakes like this.

In this case you're confusing the notion of incommensurate theories with the correspondence principle and then you thought you found a problem but the problem doesn't exist. The problem is with your poor knowledge of GR.

What you're trying to refer to is the correspondence principle in general relativity that is described here -
http://en.wikipedia.org/wiki/Physical_theories_modified_by_general_relativity

That principle demands that the weak field approximation of GR be what Newtonian gravity predicts. This is one of the things you'd learn correctly if you actually took our word for it and studied GR formally by picking up a good GR text and hitting it hard. Perhaps you don't have the math skills? Is that why you've been refusing to learn it? The math is something you learn as part of learning physics. It's not a roadblock to it but part of the education.

Or what is the reason that you refuse to learn GR before you continue to claim that it’s wrong?

However the correspondence principle doesn't apply to this. For example; in Newtonian gravity there are no gravitational waves so in this respect the correspondence principle doesn't apply. Then there's the fact that frames of reference in a freely falling frame are non-inertial frames in Newtonian mechanics but are inertial frames in GR. Also a place where the correspondence principle doesn't apply.

So where does the mass of the bodies come from, in our carefully measured
"Gm1m2/r^2" forces? 

That's the problem. You're wondering where the mass of the bodies comes from but
don't even ask yourself what body is it that you're referring to. In Newtonian gravity all
gravitational fields had massive bodies as sources. However that's Newtonian gravity,
not general relativity. Again you're confusing the two theories.

You also don't seem to be able to understand anything except the gravitational fields outside of planets and stars. This is how you're stuck in Newtonian gravity. If you learned GR then you'd learn the error in making such assumptions.

Are you familiar with Mach's principle? I don't believe so since if you did then you wouldn't have made such a statement. Please learn about it at http://en.wikipedia.org/wiki/Mach's_principle

It explains things like this. E.g. suppose you were in a frame of reference that's rotating relative to an inertial frame of reference. In the rotating frame there'd be gravitational fields. However this is only true in GR and not in Newtonian mechanics where those forces are referred to as "inertial forces" and go by names such as the Coriolis force and the centrifugal force. In GR those are gravitational forces. The source of those  gravitational forces is the matter from the distant stars. Consider how Einstein viewed such forces/fields. The following comes from an article that Einstein wrote which appeared in the February 17, 1921 issue of Nature

Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

You could learn more about this if you really wanted to learn about GR by looking at http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm
Search for the term inertial induction.

I've had you in my kill file for a very long time. Do you want to know why? If so then it's because of the fact that you refusing to learn GR means that you keep making mistakes and when I saw a mistake I felt compelled to correct it. Since there are so many mistakes and it takes a lot of work to correct you and you have no desire to go out and learn this for yourself it's a waste of my time.

Since this is getting to be way to much work again to keep correcting your mistakes I'm putting you back in mh kill file. Sorry but this is supposed to be a two way street, not just one way. You have to be willing to learn and you're not willing to read a good book on GR

[alancalverd]

Relax, man. I haven't questioned the validity of GR, nor would I bother to do so. I'm concentrating on the question of whether gravitation is an inertial force, that is, one that exists between two bodies.