I'm not terribly knowledgeable about relativity, but I don't think that gravitational repulsion is a very meaningful concept in GR. I would appreciate being corrected on this matter if that is not true.

Well then prepare to stand corrected.

Inside a charged black hole there is a second horizon. Beyond this point the black hole is gravitationally repulsive. http://casa.colorado.edu/~ajsh/rn.html

So it's not explicitly ruled out by relativity. However, that link says:

> The Universe at large appears to be electrically neutral, or close to it. Thus real black holes are unlikely to be charged. If a black hole did somehow become charged, it would quickly neutralize itself by accreting charge of the opposite sign.

> It is not clear how a gravitationally repulsive, negative-mass singularity could form.

So it falls under the same sort of category as negative- or imaginary-mass 'exotic matter': not ruled out, but there's nothing suggesting that it actually exists.

I wasn't ever arguing that it occurs in nature, or even that we will one day engineer it. I was just answering your question that GR does indeed allow for such a thing.

I vaguely recall other weird edge cases where gravity is repulsive, but I can't find any links right now.

Of course, and I do appreciate your reply. I was purposefully vague in my original wording because there is that niggling difference between "unphysical" and "not proved impossible" that I didn't want to get on the wrong side of. You have enabled me to speak more precisely about the subject in the future -- many thanks. My last reply was merely trying to place this information in context; that is to say not unphysical, but in the same category as stable wormholes, Alcubierre drives, FTL/time travel, and other such theories. And ultimately it seems like either the guy talking about graviton beams was either talking about something extremely far-fetched, or a great way to destroy large parts of the planet, or both.

> a novelty device that weighs more than what it's mass would lead you to think it weighs

This is an interesting concept. As far as I'm aware, we have ways of measuring weight, but no way of measuring mass. How would you know whether something weighed more than it "should", based on its "mass"?

You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight. Or you throw it at something of known mass and measure the speed it imparts onto the known object. Or you hang the known mass and the unknown mass on strings and measure the force of gravity between them, which may seem hard but can be done with stuff from Home Depo.

https://www.fourmilab.ch/gravitation/foobar/

> You tie it to something of a known mass and spin the pair. The motion of these two bodies measures mass without the concept of gravity/weight.

I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

> Or you throw it at something of known mass and measure the speed it imparts onto the known object.

Blind application of the principle of conservation of momentum does indeed tell us that we can measure the mass of one object by colliding it at known velocity with another object of known mass and measuring the resulting velocities. But I tend to worry that the mechanism for transferring velocity from one object to another object in a collision is the force it exerts during the collision, and that that force might be determined by the object's weight (also a force) rather than mass (a platonic concept). But, I'm not sure here either.

This ties in to the "fun factoid" that physics has no explanation for inertial mass and gravitational mass being the same quantity. If they in fact aren't necessarily the same thing, momentum transfer, measuring inertial mass, would solve this problem. If there is a reason they coincide, this approach will be confounded by that reason.

> Or you hang the a known mass and the unknown mass on strings and measure the force of gravity between them

I'm absolutely certain this wouldn't work to distinguish the mass and weight of an object that has extra weight because it's emitting extra gravity. The measured force of gravity is going to include the extra gravity you're trying to ignore.

>>As far as I'm aware, we have ways of measuring weight, but no way of measuring mass.

Then we are speaking of different things. I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.

>>I have no intuition for this. Maybe it's valid, but your other two examples raise grave doubts about this one.

The motion of the more massive pair will describe a smaller circle than the lighter one. The ratios of the two circles/motions allows you to calculate the unknown mass from the known.

We should be speaking of "a novelty device that weighs more than its mass would lead you to predict", that is to say, an object the measured weight of which does not correspond to what a different object of the same mass would weigh in the same location.

Measuring the force of gravitation between two objects definitely doesn't measure the mass of those objects without resorting to weight; the weight is the quantity you're measuring. Similarly, the fact that the center of gravity for a two-objects-attached-by-a-string system will lie closer to the massier object relies on the massier object also being heavier. If the massier object weighs less, why do you believe the center of gravity would still be closer to it?

> I understand 'weight' as how heavy something is within particular gravity field (ie on a bathroom scale on earth) whereas mass is independent of local gravity. The schemes I suggest measure mass without resort to weight.

Yes, those are the definitions of weight and mass. We can measure weight directly, because it's a force and we have tools to measure those. All of our methods of determining the mass of something, as far as I know, rely on the assumption that if you know the gravitational field at a point, all objects with the same mass would, if located at that point, have the same weight. The most common method of determining an object's mass involves measuring the gravitational attraction between the object and the earth (colloquially known as the object's "weight"), and then imputing a mass to it based on that weight.

In the spinning example, I could say that two objects attached by a string and set spinning around each other will spin around a point that balances the torque from each object (this might not be, strictly speaking, correct, but it's close enough that I think it's suggestive). But torque is defined by force, not mass -- if one of the objects gets heavier without becoming massier, that should draw the center of rotation closer to that object, shouldn't it?

Or phrased yet another way: if one of the two attached objects is heavier than it "should be" according to its "true mass", then the two-objects-and-a-string system will have the center of mass and the center of gravity in different places. Those terms are currently synonymous, but if we had a novelty object such as undersuit described they would be distinct. Is there any reason to believe that the two-objects-and-a-string system would, if spinning, rotate around the center of mass rather than the center of gravity?

All you need is a switch to turn it on and off.

Tell someone to hold it. Turn it off. Watch them struggle with the sudden weight. Turn it back on.

With today's technology, for some compounds... you can count atoms by physical measurements and known structure, and figure mass by atomic weight.

http://www.nist.gov/pml/si-redef/kg_new_silicon.cfm

That sounds like a tractor beam to me.

> I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs

Which would make space travel a lot easier - no more worrying about bone density loss!

Could such a device be used to increase the reaction mass of your fuel when it exits your engine? A sort of way to cheat F=MA by artificially boosting M, but only after you are in orbit?

No, you're confusing mass and weight. Mass is the amount of matter in a thing. Weight measures gravity's pull on the thing.

This theoretical device could make things weight more than with just Earth's gravity... but it wouldn't help your spaceship. Your engine is still pushing out the same amount of matter, so thrust remains unchanged.

toy model:

I, the spaceship, would like to accelerate through space. I take up some fuel and hurl it in the opposite direction, which requires me to apply force to the fuel I'm ejecting. It goes off into space at some rate determined by the impulse I applied and the mass of fuel I applied it to.

Newton's third law means that when I hurl the fuel, it applies a symmetrical impulse to me, accelerating me in the opposite direction.

In this model, the acceleration I get from the fuel doesn't depend in any way on the mass of the fuel I eject, only on the force I apply to it. What's wrong with the model?

You could definitely boost an orbit by increasing the force of gravity while approaching periapsis.

No more so than any other means of adding energy to the fuel.