Many years ago, an obscure polish scientific journal carried an abstract ,of an experiement that aguy did with gravity. He essentiallt tested the gravitational constant (between two lead spheres) which were within a strong electric field. At a very high field strength (kilovolts per cm.) he found that the gravitational constant that he measured was weaker.
Has anybody ever heard of this?
And, assuming the work was true, what wouldit mean for general relativity? 
I think that a scientist named Dr. Irwin Saxl was doing something along those lines, but it was with the torsion pendulum.
Wouldn’t mean a damn thing, assuming he was working from a Newtonian model of gravity in his calculations of “the gravitational constant”. The electromagnetic field is another source of stress-energy, and so contributes to the “force” of gravity.
I think the issue here is that the torsion pendulum is a sensitive device made to measure a very small force, and you have to take great pains to isolate out other factors that could throw the measurement off, like breezes or static charges. But by applying a large electric field, you’re just asking for trouble, introducing something that forces many many orders of magnitude larger than the small one you’re trying to measure.
Electric fields do contain energy, and that energy does produce a gravitational field. But that gravitational field would be far too small to measure in any experimental setup, and as CurtC points out, fields that large are going to play havoc with most experiments, since just about everything in your lab will react in some way to the electromagnetic fields.
Let’s not go overboard here. “Any” isn’t the right term. “This particular” fits better.
Energy does not produce a gravitational field; only a mass produces a gravitational field. A field produced by electricity is an electric field with its own properties of attraction, which are very similar to the properties of attraction that belong to a gravitational field (they are inversely proportional to the distance between the two objects, they exert a force that pulls the object of lesser mass/ charge towards the large one, etc), but they are completely separate phenomena. Any modification to the “gravitational field” is really the fact that the net forces acting on the object that they are experimented on are changed. So if you have an upward force generated from a magnetic field, you would add that to the force that stems from gravity; in short, one would be denoted as a + and one as a -, so that when they were added, the net force would be less than the force of gravity outside of the electric field. 
Wrong. Einstein proved that mass and energy are equivalent per the famous equation e=mc[sup]2[/sup]. Since c[sup]2[/sup] is large, it only takes a little mass to produce a large quantity of energy, but it takes considerable energy to produce a small equivalent mass. It would take an enormous amount of energy to produce a measurable gravitational field, but the effect is definitely there.
I’m going to pretend you didn’t say that.
I thought the idea was that you could use a rotating superconducting disk to block gravitomagnetic oscillations in a fashion analagous to the blockade of electromagnetic oscillations (which his how you get magnets levitating over superconductors). If memory serves, this idea takes it on faith that in some “unified field theory” you can just treat the quadrupole waves of space analagously to the dipole waves of an EM field. When I first read this, I couldn’t understand why anyone would make such a leap. Aren’t gravitational and EM fields, y’know, kinda really really different?
Ok, maybe I got confused here; I know that gravity in the einsteinian model is a property of space, but that property is only observed when there are two masses present in that space, and the force can then be seen acting. Either way, gravity doesn’t occur without there being a mass to observe.
Ok, maybe I got confused here; I know that gravity in the einsteinian model is a property of space, but that property is only observed when there are two masses present in that space, and the force can then be seen acting. Either way, gravity doesn’t occur without there being a mass to observe.
Energy and momentum are the source for the gravitational field, not mass. In principle, you could make a black hole out of nothing but photons.
No, the Einsteinian model relates stress-energy (of which mass, energy, stress, momentum, angular momentum from any source are parts) to the curvature of the space-time manifold. There are plenty of “toy universes” worked out with only a single body, no “bodies” (say, an isotropically distributed gas throughout space), and even no stress-energy at all.
Would you prefer if I had said “any experimental setup forseeable with a level of technology we might hope to attain in several centuries”? The smallest masses from which we have detected a gravitational field in the laboratory are of order ten kilograms or so. In terms of energy, that’s orders of magnitude greater than any nuclear weapon ever designed, much less built, so I would imagine that experimenters would be very reluctant to put that much electromagnetic energy together in one place. And then, of course, there are limits on how strong you can make your electric field before you start polarizing the vacuum, so you electromagnetic “mass” is going to need to be much larger than the chunks of metal used in current gravitational experiments. And that, in turn, means that the gravitational field is going to be weaker (since you’re further from the center of mass), so you’re going to need more energy yet.
And you can’t actually observe gravity without a test mass, but that test mass can be arbitrarily small, so it doesn’t significantly affect the gravitational field, and it could be completely electrically neutral, so as to not feel any electromagnetic effects. In principle, you could model a universe containing nothing but an electromagnetic field and a single neutrino (particle with very small mass and apparently no electromagnetic moment whatsoever), and that neutrino’s path would be affected by the gravity of the EM field.
Not really.
a) you’re making assumptions about future technology, which may turn out to be correct, or may turn out to resemble those pre-Wright brothers statements about powered flight being impossible.
b) you’re precluding the possibility of measurements made outside the lab, e.g. of astronomical phenomena, say along the lines of the “speed of gravity” experiment last year.
The word “forseeable” took care of that nicely.
Yes, except we’re talking about measuring the strength of the gravitational effect of an energy field. This is going to be vanishingly small compared to gravitational mass in the vicinity of the energy source we’re looking at. I don’t see how we can tease such a relatively small effect out of the noise floor outside of a laboratory setting, but if you can suggest a method, I’m all ears.
Well, no, it didn’t. I even gave an example of well qualified, knowledgeable people not being able to correctly forsee things which, as it turned out, were just around the corner.
This isn’t about what you or I can think of. It’s about the denial without proof that the effect is measurable.
Just because those technological advancements occurred does not mean that they were foreseeable, and in fact, they weren’t foreseen. And any such effect in a natural setting (say, cosmological observations dating from the radiation-dominated stage of the Universe) would be an observation, not an experiment.
Regardless, I think that we can all agree that the technology needed for such an experiment is far beyond anything we have currently.