Is the Speed of Gravity Equal to the Speed of Light?

[yogi]

AM - But even though the field does not represent energy per se until you place a second charge - it does exist as some sort of potential -

I would like to ponder your point further - but let me see if I understand it - are you saying that a moving charge or mass conveys its present position instantly to all parts of the universe - but It is not a propagation of energy - but it is a force field that is revealed only when a test particle is inserted. But what if the test particle(s) is/are already present - e.g., all the other masses in the universe for example. Are they acted upon instantly vis a vis the present location of the moving source? Also, even though the field is not energy, does it not convey information in contravention of SR's prohibition against FTL signaling?

 

[Andrew Mason]

I would like to ponder your point further - but let me see if I understand it - are you saying that a moving charge or mass conveys its present position instantly to all parts of the universe - but It is not a propagation of energy - but it is a force field that is revealed only when a test particle is inserted. But what if the test particle(s) is/are already present - e.g., all the other masses in the universe for example. Are they acted upon instantly vis a vis the present location of the moving source?

Yes, but the key is the meaning of the term 'instantly'. Since the situation of a moving charge and fixed observer is exactly equivalent to a rest charge and a moving observer, the moving observer will observe the field to correspond to the position of the charge that the observer measures at the same instant that he measures the field.

Also, even though the field is not energy, does it not convey information information in contravention of SR's prohibition against FTL signaling?

No. It only conveys information when the field changes.

AM

 

[yogi]

AM - Doesn't it convey information instantly when the source of the field changes by moving relative to the rest of the universe?

 

[Andrew Mason]

AM - Doesn't it convey information instantly when the source of the field changes by moving relative to the rest of the universe?

The answer must be 'no', of course. But just how to analyse this is difficult. Relativistic effects must be taken into account. There appears to be more than one 'correct' approach (ie. the use of advanced, retarded or classical instantaneous potentials seems to produce the same solution).

Consider an observer O at the origin in his frame of reference and moving at speed v relative to charge Q. O is measuring electric potential from Q continuously (using a test charge q<<Q). At time=0, O measures the position of Q to be d. I suggest that he measures the potential to be kQ/d.

Then, at time=t in his frame, O measures the potential of Q. But at a slightly earlier time = t1<t where t-t1<d/c as measured in O's frame, Q experienced a sudden acceleration in the direction of O's motion and never makes it as close to O as d-vt. What potential does Q measure at time=t? Is it E=kQ/(d-vt)? I think the answer is: 'yes'.

Feynman spent a great deal of time and effort on this kind of electrodynamic question, as did John Wheeler. I get the sense from reading Feynman that the 'correct answer' and physical explanation was still a matter of debate (at least it seems it was 40 years ago). Perhaps someone will be able to provide us with a more up to date perspective. My physics on this is 30 years out of date, I am afraid.

AM

 

[yogi]

AM - Good to find another out of date physicists - I also am of Feynman-Wheeler vintage - I particularly like their approach to problems - they always sought a physical analogy rather than abstraction. As far as the issue of the speed at which fields make their presence known, I don't think the experiments are conclusive one way or the other. Nor do I think it is good to blindly accept einstein's prejudice re the ultimate velocity at which information might be conveyed, although I would still regard Einstein as the greatest contributor to Science since Newton. My first wife could talk so fast I am sure she must have violated at least some prohibition against FTL communication.

Regards

Yogi

 

[Andrew Mason]

AM - Good to find another out of date physicists - I also am of Feynman-Wheeler vintage - I particularly like their approach to problems - they always sought a physical analogy rather than abstraction. As far as the issue of the speed at which fields make their presence known, I don't think the experiments are conclusive one way or the other.

The fact may be that one can get experimentally equivalent results for the field of a charge using theories based on retarded, advanced or "half retrarded/half advanced" (I think this means the instantaneous values) potentials, which seems to be what Feynman thought.

I have the highest regard for Feynman (who doesn't?). For me, his ability to take a different approach to something - which was always equivalent to the way others looked at it - was what distinguished him from most others. His fascination with the principle of 'least action' is a good example. It underlies his novel approach to quantum theory, QED (involving the sum over probabilities and decoherence). But it also is another way of looking at Newtonian mechanics and general relativity. I find that I rarely fully understand Feynman and I probably misunderstand a lot of what he says. But he is sure interesting to read, if only for the occasional glimmer.

Nor do I think it is good to blindly accept einstein's prejudice re the ultimate velocity at which information might be conveyed, although I would still regard Einstein as the greatest contributor to Science since Newton.

I don't think it is fair to say that Einstein had a prejudice re: c as the ultimate velocity. His belief was based on evidence. Prejudice usually refers to a conclusion one reaches without facts. If one agrees that the speed of light is independent of its source (which is based on evidence), there is no other conclusion that one can reach. So saying that Einstein had a prejudice re: the ultimate velocity (c) is equivalent to saying that he had a prejudice that the speed of light is independent of the motion of its source.

AM

 

[yogi]

AM - I would say that very few doubt that the speed of light is independent of the source - that is always the situation with wave phenomena - but Einstein took it further by asserting that the receiver (the observer in motion) would always measure light to have a velocity c as well - that was/is the bold step - and it was an assertion that was not required by the experiments - MMx had provided good evidence that the over and back velocity would be measured as c - but there are no experiments that have conclusively proved that the round trip velocity will be measured as c - it is taken as constant for one way measurements - in the derivation of the transforms - but it is not even to this day confired by experiment - although GPS provides some pretty solid data that it is - at least in the Earth centered reference frame.