Physics Blog

on Fundamental Theoretical Physics

Homepage GRB rhammond@email.unc.edu        


Dear Dr. Hammond,

Hey man, great book, it really got me thinking. My question is short, is negative mass real, has it ever been observed?

Jason

Jason,

Negative mass is a theoretical possibility, but it has never been observed. It is theoretically analyzed by letting m -> -m. It has very unusual properties, for example negative mass chases positive mass, and it has been called armor piercing because it will barrel its way through any material. Check out some new entries on my homepage.

Dear Professor Rich,

I have been reading about cloaking, and making an invisible man. Is this hype or true?

Big Al

Dear Big Al,

There is always hype, but cloaking is getting closer to being a possibility. You don’t make a person invisible per se, you cloak her. Imagine a cylinder three feet in diameter and 8 feet tall. It is like a giant test tube, but instead of glass it is made of metamaterials. Metamaterials are manmade, and the index of refraction is not constant but depends on the radius. So, to cloak Mary, have her step into the cylinder. If the index of refraction function is just right, Mary will be invisible to the outside world. There are problems, for example, it is very hard to make these metamaterials and no one has made one that big, yet. Worse, is the “dispersion” problem. This means, suppose the cloak is designed to work for red light, then it won’t be quite right for blue. Much work is being done to overcome these problems. The Army, Air Force, and Navy are all funding research in metamaterials, as well as other agencies.

Dear Professor,

I know torsion arises from intrinsic spin, but doesn’t the Dirac equation already describe particles with spin?

A confused grad student

Dear grad student,

The Dirac equation describes particles of spin 1/2 (it does not describe spin one or spin two particles, for example). But, to the heart of your query, consider the electron, which has spin and charge. In order to account for the electromagnetic field, we must generalize the derivative to the gauge covariant derivative by including the electromagnetic potential. In the same way, to account for the torsion field, we form a gauge invariant derivative with torsion terms. For all the gory details, see: Torsion gravity - Hammond, R.T. Rept.Prog.Phys. 65 (2002). Hope this helps Prof H,

I have been reading other blogs and forums but cannot get an answer to a simple question, what is the physical meaning of an antisymmetric metric tensor? I know Einstein tried to make the electromagnetic field, but that failed, as far as I know. Can you help?

Jan in the Pan

Jan,

After Einstein, the Canadian physicist John Moffat developed a theory where phi (I will use phi to describe the antisymmetric part of the metric tensor) was non-zero, but it was just another part of gravity. This means the origin was mass. Most recently, a new theory has been developed in which phi is the potential of a spin field. In other words, start with phi, take the antisymmetric derivative, and you have a field that arises from the intrinsic spin of elementary particles. This field is the torsion field and this formulation answers two other questions. It shows why the torsion is derived from a potential, and shows why phi cannot be zero (unless intrinsic spin is zero). This is a new result and needs more research, but it is very exciting. Technical details were given in arXiv:1207.5170v1 [gr-qc].



Dear Professor, According to Hawking's theorem, black holes radiate out there energy. Couldn't this energy cause the expansion to accelerate?
Cosmic Thinker

Cosmic Thinker, I like your name, but no, this cannot be. FIrst, large black holes (solar mass or bigger) have a very low (black hole thermodynamic) temperature and they emit less energy than a firefly on the Fourth. Also, if they did somehow, we'd observe such a huge amount of energy. But most diabolical is this. Bear in mind that energy is conserved, so the radiation gained is at the cost of the mass lost. It turns out your insights are good, the radiation pressure pushes a little more (cosmologically speaking) than inert black holes, but you still lose. Remember, you need some new kind of energy or field (or theory of gravity), and radiation just won't do :(


Why is this happening? No known form of energy can account for this, which is why we sometimes postulate the existence of "dark energy." Simply put, "dark energy" is any form of energy that can accelerate the expansion of the universe. But what exactly is dark energy? 

No one knows, for sure! Comments...Questions???