# If such an object does exists

Today my friend told me a strange theory:when the angular kinetic energy of something is large enough,some of the energy can be converted to gravitational potential.Does such an object does exists?(I don't think so.)

p.s. A poor English learner from China wrote the post.Don't laugh at me :tongue2:

Of course, anything with energy/momentum/angular momentum will affect spacetime. But observable effects are technologically not achievable in near future since they are very weak.

Frame-dragging?

I meant that in near future we cannot build a (controllable) instrument that generates frame dragging. What we are observing now is frame dragging caused by massive astronomical objects.

EmperorNeo, welcome to the forum. Your English is first rate. The analysis of rotation is very interesting because the centripetal acceleration reminds one of gravity. And there is no doubt that rotation of a massive object causes 'curvature' of space-time beyond mere centripetal acceleration. Some people might equate space-time curvature with gravitational potential.

This what Timur has said, in more detail.

Perhaps your friend is thinking of this. See the Kerr metric, which is the general relativistic description of simple rotation.

HallsofIvy
Homework Helper
Today my friend told me a strange theory:when the angular kinetic energy of something is large enough,some of the energy can be converted to gravitational potential.Does such an object does exists?(I don't think so.)

p.s. A poor English learner from China wrote the post.Don't laugh at me :tongue2:

Your English is far better than my Chinese! (Or just about any other language.)

Thanks for your reply.I think if we can make such an object oneday,the antigravity aerocraft would no longer be a dream.That will be very easy for it's only a conversion between gravitational potential and kinetic energy.

Just like a Kerr Black Hole! Its rotation change the gravitational field surrounding it!

Thanks for your reply.I think if we can make such an object oneday,the antigravity aerocraft would no longer be a dream.That will be very easy for it's only a conversion between gravitational potential and kinetic energy.
Gravity always attracts according to theory, and we have no experimental evidence to the contrary. Centripetal acceleration might be thought of as repulsive, so if you can bottle that you're in business.

If gravity is propagated by spin-2 massless bosons, then the only way to shield the effects of gravity would be to make them acquire mass and so have a shorter range.

see arXiv:hep-th/0207254 v1 29 Jul 2002
Abstract
The quantum gravity is formulated based on gauge principle. The model
discussed in this paper has local gravitational gauge symmetry and gravita-
tional field is represented by gauge potential. A preliminary study on gravi-
tational gauge group is presented. Path integral quantization of the theory is
discussed in the paper. A strict proof on the renormalizability of the theory
is also given. In leading order approximation, the gravitational gauge field
theory gives out classical Newton’s theory of gravity. It can also give out an
Einstein-like field equation with cosmological term. The prediction for cos-
mological constant given by this model is well consistent with experimental
results. For classical tests, it gives out the same theoretical predictions as
those of general relativity. Combining cosmological principle with the field
equation of gravitational gauge field, we can also set up a cosmological model
which is consistent with recent observations.

pervect
Staff Emeritus
Some comments: a rotating black hole is more massive than a non-rotating one. For some formal justification, see for instance A wikipedia article about black hole thermodynamics. Note that dM includes a term that is equal to the angular momentum multiplied by the angular velocity - this is the formal justification for a rotating black hole having more mass than a rotating one.

A very rapidly rotating black hole, where most of the mass (or energy) of the black hole is due to rotation has a name: it is known as an extremal black hole.

Robert Forward proposed the idea of trying to create such an extremal black hole by rotating some form of atomic nucleus, I believe. Unfortunately, I don't have a specific reference for exactly what he proposed. No substance made out of atoms with chemical bonds has even a chance of being strong enough - you need nuclear bonding energies to have even a chance of being strong enough to hold together.

Such a very small black hole should, however, evaporate quickly (very quickly, one might even say explosively), which suggests that this approach shouldn't really work in the first place due to quantum effects.

I ran across recently a rather interesting article (which is rather technical) that points out that the failure mechanism of such a hypothetical rapidly rotating body can be understood in terms of quantum mechanical tunnelling:

http://www.iop.org/EJ/abstract/1126-6708/2005/05/014/

Thus, even nuclear material will probably not (if I am understanding this correctly, and if the article itself is correct) be strong enough to hold enough energy to form a black hole. Tunnelling to a lower energy state will doom the proposal of forming a small black hole by rotating a small body rapidly enough.

This doesn't have anything at all to do with anti-gravity that I can see.

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