That’s entirely different. Thunder is caused when all that electrical force is channeled through air (which is actually a pretty good insulator), super heating it. What you hear, in effect, is the air expanding at a supersonic speed.

(The quantum version of this question is the problem of nonrenormalizability of gravitational theories, which as far I can tell from zillions of blog threads on the topic, LQG ignores completely.) - anon.

To answer the first point. You choose the classical theory of gravity which, when coupled to QFT, is based on verified facts and makes accurate predictions.

Regards the second point. Renormalization in gravitation will be a change in the effective value of gravitational charge, i.e., mass. Mass is supposed to be given by the Higgs mechanism, which must be gravitational charge because Einstein’s equivalence principle says gravitational mass is the same as inertial mass.

Renormalization of electric charge in QED is explained by the polarization of the vacuum around the bare core charge, which cancels part of the latter as observed from a distance. You can’t apparently polarize the vacuum to shield gravitational force as you can for electric force.

Polarization in an electric field works because virtual positive charges get attracted closer to the bare core negative charge of a particle than virtual positive charges, so the virtual charges give a net radial electric field which opposes and cancels part of the core charge.

Clearly this can’t occur in a gravitational field because all mass moves the same way; there are no opposite poles for mass and gravity, so no polarization or shielding occurs, at least directly. This makes it hard to see how any quantum theory of gravity can physically include renormalization of gravitational charge (mass).

However, the equivalence principle between gravitational and inertial mass in the context of quantum gravity has been attacked by Rabinowitz in http://arxiv.org/abs/physics/0601218 where it is argued:

“… a theory of quantum gravity may not be possible unless it is not based upon the equivalence principle, or if quantum mechanics can change its mass dependence. …”

In QED, both electric charge and electron mass are renormalized parameters and are scaled by similar factors. This seems to suggest that maybe the source of the electron’s mass is the mass-giving (’Higgs’) vacuum field outside the polarization region, if mass is associated with the electron by a coupling depending on the electric field of the electron. Thus, the polarization-shielded electric charge (not the core or bare electron charge) would be responsible for coupling external mass-giving Higgs bosons to the electron. So renormalization of the electric field automatically causes renormalization of the gravitationam charge (mass), because the shielded electron charge is responsible for the vacuum field effects which give mass to an electron.

In the Standard Model, all masses are given to particles by field which is separate to electric charge. The only way such a mass-viving field can couple to an electron core without mass is obviously by some kind of coupling to the electron’s electric field. So renormalization effects in quantum gravity are likely to be indirect, i.e., the effect of electric field renormalization (which does have a very simple, empirically confirmed physical mechanism; vacuum charge radial polarization).

yes, that is true. I don’t know - neither do I think anybody knows.

Hi Sean

Say I have a big lump of mass, whose wavefunction is half concentrated here, and half concentrated a mile away. What is its gravitational field?

The question that you are asking is how a quantum field is coupled to the field equations - if the metric isn’t an operator this must involve some kind of expectation value. Again, obviously, I can’t answer this question. I just don’t see why the answer necessarily has to imply a quantization of the metric. This depends crucially on whether we understand quantization correctly. E.g. to address your question: has anybody ever measured the gravitational field of an entangled state? Do we really ‘know’ the wave-function and not its abs value must play a role there? But even if, I’m not saying the gravitational part would be unmodified, just unquantized. I just think that we shouldn’t ignore the fact that our quantization procedure crucially builds up on flat space, plain waves, asymptotically free states (cluster decomposition) etcetc, and it seems to me that gravity might play a role in the very foundations of quantization (I believe this is hardly a new idea, but I can’t give you any references, sorry, it’s just a topic that’s currently circling in my head). cu,

B.

(The quantum version of this question is the problem of nonrenormalizability of gravitational theories, which as far I can tell from zillions of blog threads on the topic, LQG ignores completely.)

I doubt it, just based on the limited success so far. And even if classical modifications of GR resolved singularities, the requisite modifications would likely kick in near the Planck scale, where quantum gravity is going to be important anyway.

Sorry, I almost forgot I left that comment here. Thanks for the answer. What do you mean with ‘quantum gravity’ in the above context? I see it this way: gravity works perfectly fine also at the Planck scale. The only thing that goes wrong in GR are the singularities. What we don’t know is what to do with quantum field theory when curvature gets close to the Planckian regime, and we don’t know how to couple QFT to GR (okay, and even if we knew, backreaction effects would make the equations enormously ugly, but lets put that aside for a moment). If I look at it this way, it seems to me it’s rather background indep. quantization that we don’t know how to deal with instead of gravity. So, what good reason do we have to believe that the metric has to be subject to quantization as well?