Whatever Happened to Ning Li?

[SpitfireIX]

Her team was working on a proof-of-concept of this idea to manipulate gravity by altering EM fields as far as back as 2000, when this Popular Mechanics article was published:

http://www.highbeam.com/doc/1G1-65730414.html

Unfortunately, you have to have a highbeam account to read the whole article (which I do not), but it may jog some of your memories about when this article was originally published.

Here is the issue of PM, from Google Books. Article begins on p. 40.

 

[SpitfireIX]

I have no idea what happened to her. But her work is crap. Complete nonsense from top to bottom. If anyone at the DOD ever paid any attention to her work, it's because there are gullible suckers everywhere.

Undoubtedly the same guys who paid attention to

 

[Astrodude]

Anything with angular momentum - either due to quantum spin or simple rotation - produces a "gravitomagnetic field". But it's absolutely tiny for anything short of a rapidly rotating black hole (it took decades of effort and an extremely precise satellite experiment to detect for the earth's rotation), and it's far more tiny when it comes from atomic or molecular spin than when it comes from (say) a rotating bicycle wheel.

Right, but isn't "gravitomagnetism" is just gravity from another frame of reference? It's just a result of the affine connection between the observer's point and the other points. I'm talking about a theory that posited a far greater "force" where rotation of neutral matter would induce an electromagnetic field. Blackett and Schuster conjectured that the Earth's electromagnetic field could be explained by it's rotation.

Give the matter some charge, and rotating it certainly produces a magnetic field. Similarly, atoms and particles with spin also produce magnetic fields. Unfortunately that has nothing to do with antigravity.

If a gravitational field could be induced somehow by electromagnetism in a unified field theory, it would make sense that one could reverse the effects of gravity(under probably extreme conditions) just as one may reverse the polarity of a magnet by having it be a coil's core.

 

[theprestige]

Right, but isn't "gravitomagnetism" is just gravity from another frame of reference?

Ziggurat actually answers this question in the third post in this thread. Gravitomagnetism has nothing to do with magnetism.

 

[Ziggurat]

Right, but isn't "gravitomagnetism" is just gravity from another frame of reference? It's just a result of the affine connection between the observer's point and the other points. I'm talking about a theory that posited a far greater "force" where rotation of neutral matter would induce an electromagnetic field. Blackett and Schuster conjectured that the Earth's electromagnetic field could be explained by it's rotation.

I suspect you are correct about what they were thinking. But it's still nonsense. Rotation of neutral matter most definitely does not produce electromagnetic fields.

If a gravitational field could be induced somehow by electromagnetism in a unified field theory

General relativity already accounts for the gravitational effects of electromagnetism. There's nothing special about it. Electromagnetism is just another form of energy, and energy distorts space. We don't need a unified field theory to describe that. But given how little of an object's mass comes from electromagnetism, and more importantly the fact that this will still be attractive gravity, it can't lead to any sort of antigravity.

it would make sense that one could reverse the effects of gravity(under probably extreme conditions) just as one may reverse the polarity of a magnet by having it be a coil's core.

You can reverse gravitomagnetic effects by reversing the direction of rotation (which is what happens to an ordinary magnet which switches polarization too). But as pointed out above, those effects are very tiny. And gravity itself can't be reversed this way. It's monopolar, not dipolar, and there's only one kind of charge (ie, only positive mass, no negative mass). So no, gravity cannot be reversed.

The closest thing to antigravity which is possible is gravitational "swimming". That is, if you've got a mass which can change shape in a non-uniform gravitational field, you can exploit the variations in the field to exert a force on the object by rapidly changing shape in the right manner. In a sense, you "swim" in the gravitational field. But the thing is, in order to actually apply noticeable force using such methods, you need to have the different parts of your object moving at relativistic speeds relative to its center of mass. Furthermore, the energy required to do something like hover using this method becomes a significant fraction of the rest energy of the object. Both of these constraints are, from a practical perspective, impossible barriers to overcome. The theory works, but we could probably never actually do it, and it would be vastly inferior to conventional propulsion even if we could.

 

[Astrodude]

What about isolating the conditions theoretically?

Given an empty space, a mass, and physics as we know it, is there any reason to think that rotating the mass will produce an electromagnetic field? Or vice-versa?

It may depend on other conditions like the density of the mass.

Is there reason to believe it? I don't have one. That doesn't mean there aren't reasons to consider the possibility. The spiral motion of the galaxy is one thing that causes me to be intrigued by the idea that rotational motion is part of what would unify electromagnetism and gravity. Is this spiral motion following the geodesic in orthodox general relativity?

I mean, help me understand: You're talking about the possibility of an inherent property of mass, and/or electromagnetism, that has been there all along, that has played a part in every single observation we've ever made, but so far not even a hint of it has shaken out of any experiment yet performed in QM, SR, or GR?

Isn't that a bit like saying maybe phlogiston is real, but we can only detect under certain conditions that no one has ever been able to isolate?

Not quite because, as of right now, the same can be said for the Higgs boson. We have perfectly good explanations for combustion so phlogiston is unparsimonious, but we do not have a unified field theory.

 

[JoeTheJuggler]

I am not an engineer.

 

[sol invictus]

Right, but isn't "gravitomagnetism" is just gravity from another frame of reference?

Kind of, yes.

I'm talking about a theory that posited a far greater "force" where rotation of neutral matter would induce an electromagnetic field. Blackett and Schuster conjectured that the Earth's electromagnetic field could be explained by it's rotation.

Even if that were true, what does it have to do with the OP? The OP is about gravity, not E&M.

Moreover, we have a theory that tells us precisely what kind of electric, magnetic, and gravitational fields result when you rotate matter. The E&M part has been around for about 150 years. The gravity part has been around for almost 100 years. Both theories have passed every single experimental test ever done, including many that involve rotation and matter.

If a gravitational field could be induced somehow by electromagnetism in a unified field theory, it would make sense that one could reverse the effects of gravity(under probably extreme conditions) just as one may reverse the polarity of a magnet by having it be a coil's core.

But again, we already have a theory that tells us precisely how electromagnetic fields gravitate. There's no need for a "unified field theory" - we already know how to unify EM and GR, at least apart from (tiny) quantum effects. Because electromagnetic fields have positive energy, they gravitate more or less just like any other form of energy - they create attractive gravitational forces.

 

[Astrodude]

You can reverse gravitomagnetic effects by reversing the direction of rotation (which is what happens to an ordinary magnet which switches polarization too). But as pointed out above, those effects are very tiny. And gravity itself can't be reversed this way. It's monopolar, not dipolar, and there's only one kind of charge (ie, only positive mass, no negative mass). So no, gravity cannot be reversed.

If gravity is positive definite always, something sure is peculiar with the math of general relativity. Look at general relativity's equation describing the invariant differential metric:

[latex]ds^2={g^{\mu \nu}dx_{\mu}dx_\nu=ds'^2[/latex]

The metric is ds, but it's given as a quadratic. This quadratic must have two solutions. These two solutions are ds=[latex]\sqrt{{g^{\mu \nu}dx_{\mu}dx_\nu}}=ds'[/latex] and ds=[latex]-1*\sqrt{{g^{\mu \nu}dx_{\mu}dx_\nu}}=ds'[/latex]

The fact that there are two solutions to this metric suggest that gravity is in fact "negative" under some circumstances. It isn't accurate to call the negative solution "antigravity" because it would have to affect the entire universe if it were to operate, but it still could be kind of like anti-gravity, just operating everywhere in the universe.

 

[sol invictus]

If gravity is positive definite always, something sure is peculiar with the math of general relativity. Look at general relativity's equation describing the invariant differential metric:

[latex]ds^2={g^{\mu \nu}dx_{\mu}dx_\nu=ds'^2[/latex]

The metric is ds, but it's given as a quadratic. This quadratic must have two solutions. These two solutions are ds=[latex]\sqrt{{g^{\mu \nu}dx_{\mu}dx_\nu}}=ds'[/latex] and ds=[latex]-1*\sqrt{{g^{\mu \nu}dx_{\mu}dx_\nu}}=ds'[/latex]

That sign ambiguity doesn't affect the geodesics, and therefore doesn't affect the "force" of gravity.

The equation for ds is very closely related to Pythagoras' theorem C²=A²+B² for the length of the hypotenuse of a right triangle. What does the sign ambiguity mean for a triangle? If you think of "C" not as a length, but rather as a displacement along an axis parallel to the hypotenuse, then the sign simply relates to which direction you go along that axis - but of course it doesn't change its length, nor does it change the triangle itself.

ETA - to elucidate that a little further, imagine a trajectory where a fast-moving object (a baseball, say) passes by the sun and then escapes off to infinity. Because gravity is attractive, the trajectory of the baseball will bend towards the sun. If gravity were repulsive, the trajectory would bend away from the sun.

What does that have to do with the sign ambiguity Astrodude is worried about? Well, as I said that sign ambiguity corresponds to reversing the direction along the trajectory. In other words, by reversing a sign or two you can reverse the direction of motion of the baseball. But as you can see (draw a picture if you can't), reversing the direction doesn't alter the fact that the trajectory bends towards the sun. In other words, with the reversed sign, gravity is still attractive.

 

[theprestige]

Not quite because, as of right now, the same can be said for the Higgs boson.

But that's not really true, is it? The Higgs boson is a name we've given to something that theory and observation give us cause to expect. Hints of it, or something like it, have very much shaken out of our experiments and observations. That is, we have a theory, and we have observations that support that theory, and that theory calls for something like the Higgs boson.

Neither theory nor observation gives any hint of anything like the kind of anti-gravity effect you seem to be describing.

 

[Dancing David]

And if the Higgs is not found then the theories will move on, no collapse of the whole system.

Unlike magically coupling a weak electric force to gravity before unified field theory.

 

[Astrodude]

But that's not really true, is it? The Higgs boson is a name we've given to something that theory and observation give us cause to expect. Hints of it, or something like it, have very much shaken out of our experiments and observations. That is, we have a theory, and we have observations that support that theory, and that theory calls for something like the Higgs boson.

Neither theory nor observation gives any hint of anything like the kind of anti-gravity effect you seem to be describing.

It's probable that Schuster-Blackett hypothesis is incorrect and Blackett even said it was. However, intuition tells us that electromagnetism and gravity should be unified. There should be a way to induce gravity from electromagnetism. If this can be done, it's only natural to assume that gravitational effects can be diminished via electromagnetic induction.

At times, there appears to be gravitational repulsion between matter and antimatter.

 

[Astrodude]

That sign ambiguity doesn't affect the geodesics, and therefore doesn't affect the "force" of gravity.

The equation for ds is very closely related to Pythagoras' theorem C²=A²+B² for the length of the hypotenuse of a right triangle. What does the sign ambiguity mean for a triangle? If you think of "C" not as a length, but rather as a displacement along an axis parallel to the hypotenuse, then the sign simply relates to which direction you go along that axis - but of course it doesn't change its length, nor does it change the triangle itself.

The actual length isn't negative though. The location of the points on the line segments respecting a certain coordinate system is negative. Also, representing C as the modulus of a complex number might solve the sign ambiguity because a modulus cannot be negative.

ETA - to elucidate that a little further, imagine a trajectory where a fast-moving object (a baseball, say) passes by the sun and then escapes off to infinity. Because gravity is attractive, the trajectory of the baseball will bend towards the sun. If gravity were repulsive, the trajectory would bend away from the sun.

What does that have to do with the sign ambiguity Astrodude is worried about? Well, as I said that sign ambiguity corresponds to reversing the direction along the trajectory. In other words, by reversing a sign or two you can reverse the direction of motion of the baseball. But as you can see (draw a picture if you can't), reversing the direction doesn't alter the fact that the trajectory bends towards the sun. In other words, with the reversed sign, gravity is still attractive.

This is a good thought. I'll have to think about this.

 

[sol invictus]

The actual length isn't negative though. The location of the points on the line segments respecting a certain coordinate system is negative.

Right. Put another way, you can assign an orientation (a direction) to the line, and if you do that the two solutions (positive and negative) for C correspond to the two choices of orientation for the line, either of which can be used to complete the triangle.

Also, representing C as the modulus of a complex number might solve the sign ambiguity because a modulus cannot be negative.

Sure, there are lots of situations where that equation could arise but C is known to be non-negative. But in the case of geometry - both for plane triangles and more generally the metric in GR - the meaning of that sign is probably best thought of as I've explained.

 

[Farsight]

At times, there appears to be gravitational repulsion between matter and antimatter.

There isn't. Really. Gravity doesn't repel, ever. You achieve "antigravity" by creating a gradient in electromagnetic four-potential above your head. If it matches the earth's gravitational field, you don't fall down. If it surpasses the Earth's gravitational field, you fall up. But conservation of energy still applies. I can't lift you up to free space without supplying the energy equivalent to the kinetic energy you'd exhibit if you fell to earth. At 11km/s that's quite a lot of energy. When I don't supply that energy, you suffer adiabatic cooling and end up a popsicle. Ever seen the movie Mutant Chronicles?

 

[sol invictus]

At times, there appears to be gravitational repulsion between matter and antimatter.

I missed that. What are you referring to? Matter and anti-matter both have positive energy and mass, so they attract gravitationally. Moreover they have opposite charge, so they attract due to all other forces as well.

 

[theprestige]

However, intuition tells us that electromagnetism and gravity should be unified.

It does? What does intuition say, exactly?

There should be a way to induce gravity from electromagnetism.

There should? Why?

If this can be done, it's only natural to assume that gravitational effects can be diminished via electromagnetic induction.

It is? Why?

At times, there appears to be gravitational repulsion between matter and antimatter.

There does? At what times? What form does this appearance take?

 

[Astrodude]

It does? What does intuition say, exactly?

Intuition says that two apparently distinct concepts that are always mutually inclusive, i.e., there is always gravity where there is electromagnetism and vice versa, are inherently unified.

There should? Why?

Whenever two fields can be unified, one field can be explained in terms of the other. If a vector has two components and the dot and cross products are known as well as one of its component, one can mathematically determine the value of the other component. Note: I don't even think you need all of the information I gave to do that, but I'm listing superfluous information just to be certain and I don't feel like checking right now.

It is? Why?

Because magnetic effects can be diminished by electricity and vice versa. There is no charge without mass.

There does? At what times? What form does this appearance take?

Gravitational repulsion between matter and antimatter has been suggested by Massimo Villata in his peer-reviewed paper "CPT symmetry and antimatter gravity in general relativity" published in Europhysics Letters and submitted on March 25, 2011.

It's also possible that matter-antimatter pairs have repulsive gravity upon "annihilation." I suspect that Einstein's idea of unifying gravity and electromagnetism with a symmetric and anti-symmetric tensor for both gravity and electromagnetism respectively was the right idea and one of the quaternion factorizations to do this is correct.



Circumstances exist where a gravitational field from our frames of reference right now would be viewed as an electromagnetic field.

 

[Astrodude]

I missed that. What are you referring to? Matter and anti-matter both have positive energy and mass, so they attract gravitationally. Moreover they have opposite charge, so they attract due to all other forces as well.

Read Villata's journal entry. Matter and anti-matter both have positive mass, but they don't necessarily attract each other gravitationally.