This week in physics...

[ComradeRed]

OK, I decided that since there is (at least) some people (i.e. one other person other than me) interested in quantum gravity, that it may be good to summarize some of the technical papers that came out this week at ArXiv.

Newtonian Quantum Gravity (gr-qc/0612025)

The idea in this paper is to quantize Newtonian gravity as an approximation to the weak field. The weak field is the gravitational field past the Schwarzschild radius of the object (and when you are inside the Schwarzschild radius, it is the strong field).

The idea is that if something is seen here, some quantum anamoly, then it should be predicted by quantum gravity. Remember that when people talk about quantum gravity they mean quantum general relativity

An interesting approach to someone who's bored is to take quantum newtonian gravity and relativize it (afterall, we know how to go from Newtonian gravity to General Relativity). This is easier said than done. The authors' motivation was that if Newtonian gravity couldn't be quantized, then that would demonstrate the impossibility to quantize general relativity...at least with the current paradigm of quantum theory.

There are some problems with this approach and some promises. For one thing, Newtonian gravity allows information to flow faster than the speed of light ("action at a distance"), and it is somewhat unclear whether it is this or the quantization of gravity that solves a number of problems.

But the problems it does solve are the black hole information paradox (basically, if we throw a system with a lot of information into a black hole, what happens to that information?), it explains a black hole without resorting to naked singularities (no more infinities come up), and it (logically) violates the equivalence principle of general relativity.

The equivalence principle can be demonstrated up as this: when you fall from a chair, do you feel your own weight and force? No, of course not. The reason is because of the equivalence principle. In other words, gravitational force in an inertial reference frame is equivalent to a pseudo-force in an accelerated reference frame.

What's really interesting about this approach that the authors followed is that they said "Well, the coulomb electrostatic force in a Hydrogen atom is mathematically identical to the gravitational attraction between two bodies" and followed regular quantization procedures.

Violation of the Equivalence Principle in Modified Theories of Gravity (gr-qc/0612002)

This I thought was fascinating since it demonstrates that all the alternatives to general relativity are not really "updates" of GR but rather patches of Newtonian gravity (as I interpreted it, since if it were updates of GR then it would have either the original or an updated version of the equivalence principle).

This paper was a short little thing (4 pages) but important nonetheless!

Stable dark energy stars: An alternative to black holes? (gr-qc/0612030)

This I thought was rather interesting, but there is no reason (for me anyways) to accept "Dark Energy".

It is novel to draw comparisons between the two, and the results are fascinating. But what about unstable dark energy stars? What would those be?

There's really not much I can say about this paper because of my undying hatred for dark matter/energy.

That's pretty much the noteworthy things that came out this week(ish) in quantum gravity.

[Vladislav]

haha don't think we'll be doing this at high school level, but this might come in handy.

Thanks ComradeRed