Gravity Can Be Neither Classical nor Quantized by Sabine Hossenfelder

(sorry for the link formatting; I forgot the slash!)

Hi Don,

It makes me happy to hear that my blogging has encouraged you to write down your thoughts. I'll have a look at your essay. Best,

B.

Dear Georgina,

I'm glad to hear you find it interesting. The video... was fun. I guess I'm overcompensating for my conservative colleagues ;o) Best,

B.

Hi Patrick,

Planck's constant is dimensionful, so if you want to speak of it varying, you should strictly speaking normalize it to make a dimensionless constant first. I did mention this in the arxiv paper - the normalization that suggests itself is the measured value of the Planck constant (at low temperatures). I'm not entirely sure what you mean with "variation". Do you mean a spatial variation or a temporal one? In the case I discussed there shouldn't be a spatial variation of Planck's constant unless you are in strong curvature regimes, ie towards black holes or towards the Big Bang singularity. (Think of particle masses, ie the higgs vev, it doesn't vary either.)

What might be a suitable potential, well, it has to be one that leads to a symmetry breaking at high temperatures and at the same time have quantum corrections that allow the convergence of the perturbative expansion, as I explained towards the end of the text. I don't know if such a potential exists. To begin with it would depend on the particle content of the theory. I don't think there's an easy answer to that, but all I wanted to say is that it does not seem impossible, and I believe it's a possibility worth investigating.

Best,

B.

Hi Jim,

I don't know what you mean when you say "The effects of gravity simply do not seem to appear in the interactions of quantum particles." They appear if you put in a gravitational interaction, and you can do that in a perturbative quantization coupled to the rest of the standard model. The problem is just that this theory only makes sense as an effective theory, not as a fundamental one. Best,

B.

Hi Ben,

These are all very good questions that I don't have quick answers to. As to 1: I can only say I certainly hope it does, it would be more elegant. As to 2. I think this might be similar to a signature change. It might be very interesting to think about further. And regarding 3. I haven't been envisioning this one way or the other. I'd think this is a question that would have to be addressed by experimental constraints, for example, as you say, inflationary imprints in the CMB because it affects what it means to have a quantum fluctuation to begin with. In fact, this seems to me the most fruitful direction to make contact with observation. Best,

B.

Dear Sabine Hossenfelder,

If the universe is with matters in continuum, gravitation is considered as the tensor product on deformation of matters, in that, matters are eigen-rotational strings. To describe this gravity of the universe with an infinite sum of string-lengths, quantization is inevitable in that a different framework of quantization to be adapted that is non-perturbative and conformal.

With best wishes,

Jayakar

Hi Bee,

A quick hypothetical question.

In a toy cosmos wherein all masses were quantized, would gravitational interactions (even if intrinsically classical) be quantized by default?

I have asked this question in several discussion forums, but I don't think I have ever gotten a straight answer. Is the question flawed in some way?

Rob O

I am a big fan of arguing discreteness is an emergent property, however isn't discreteness effectively a two state proposition, either its there or it isn't. Since planks constant has arbitrary units, it seems unnatural to me to talk about it evolving in time. I am thinking you mean that some fundamental ratio is evolving with time, but I am not sure if that is a correct characterization.

Hi Sabine,

You write: "Classical general relativity predicts the formation of singularities under quite general circumstances. Such singularities are unphysical and should not occur in a fundamentally meaningful theory." That is a powerful statement.

As I understand it, the theoretical physics community proposes to get past the problem of gravitational singularities by a clever definition in quantum gravity; instead of investigating the possibility that gravity is wrongly defined.

An alternate ansatz to describe gravity, which will not lead to singularities, you will find in my essay "Rethinking Geometry and Experiece", I really would appreciate your time and a feedback.

Regards

Anton

B.,

Actually, shouldn't a quantum gravitational effect be manifested more specifically as an effective attraction interaction among particles - proportional to their mass?

Has any such interaction among particles been observed?

Jim

Hi Sabine. You are correct that gravity is not fundamentally described and understood. That is a clear fact.

You do agree that true/real/theoretical quantum gravity requires grvitational and electromagnetic equivalency and balancing and gravity and inertia in FUNDAMENTAL equilibrium and balance? It also clearly requires balanced attraction and repulsion and FUNDAMENTAL instantaneity, correct?

If we want to fundamentally express F=ma, in conjunction with the fact that gravity cannot be shielded, everything listed in this post is necessary for true/real quantum gravity. (As you know, light is known to be quantum mechanical in nature.)

We really need to use the term "quantum gravity" in conjunction with the above terms.

Indeed, true/real quantum gravity FUNDAMENTALLY demonstrates F=ma. TRUE/REAL QUANTUM GRAVITY IS FUNDAMENTAL FORCE/ENERGY.

I would appreciate your thoughts. You are a very bright thinker, but outer space is a sinkhole. It is not fundamentally comprehensible or understandable.

My essay should be posted shortly. It will give you serious "food for thought". I would appreciate it if you would look at it and rate it.

Hello again Bee

Thanks for your reply.

When I wrote of Planck's constant "varying", I wasn't thinking carefully about the circumstances. Variation with either space or time would be worth understanding. As to "normalizing", perhaps I had better read your arxiv paper before writing more.

About choice of potential: you suggest two criteria.

The first, causing symmetry breaking, sounds worthwhile, but if there is any symmetry breaking potential then there will probably be very many such. It would be good to have a criterion which constrains the form of the potential much more. The second, convergence of the perturbative expansion, sounds rather ad hoc. It seems based on a model of calculation which excludes every conceivable model which is not just a simple perturbation of some basic state.

These are not criticisms of your essay. As far as I can tell, the entire main stream of current physics theory is developed around models described by such perturbations. The problem of convergence haunts them all. Even more serious: this approach excludes the possibility of models which are not such perturbations. The form of model suggested in my submission avoids this issue, which is one reason why I thought it worth writing. As things stand in current theory, the entire structure of most theories proposed these days is constrained by the very specific syntax which is available for describing Lagrangians in the notation inherited from a century-old form of maths. This aspect of theoretical physics has some catching up to do.

Best wishes

(Patrick) Alan H.

Thanks for the answers! I didn't realize until I checked your thread that it was you who posted as "Bee" over on my thread... I wrote some remarks there too. Anyway, you have some very good ideas... things I had never thought of before. Take care,

Ben

What do you mean with "masses are quantized"?

Hi Harlan,

As I already wrote in a comment above, in principle you are correct that since Planck's constant is dimensionful, one should take a ratio and then talk about a dimensionless quantity. The most natural thing to do seems to divide it by the measured (low energy) value of Planck's constant (\hbar_0). However, this would make the notation less intuitive, so I haven't done that for the sake of readability. I don't know what your referral to discreteness means. Best,

B.

Bee,

Thanks for the response.

The idea of discreteness comes from using planck's constant in hilbert space. The purpose of the constant is to transform an otherwise continuous spectrum into discrete spectrum. For instance

[math]e^{iHT}[/math]

which contains two continous quantities is modified into

[math]e^{(i/h)HT}[/math]

which now allows us to talk about integer components with respect to h (everything to the left of the decimal point e.g. x.yyyy where the integers are to the left of the decimal). When talking of infinties, the specific value of h isn't important, its that we now have discretized the product space HT. Since T is usually unbounded, HT is unbounded, so talking in infinities makes sense. Cantor showed that there is a definite distinction between discrete and continuous infinities.

This is the basis of the remark, the value of h isn't particularly important, so it isn't useful to discuss it changing with respect to time.

A very interesting question!

Say in this toy cosmos there are only the commonly known particles and atoms composed of them. The atoms have quantized masses in acordance with QM.

If the masses of all objects in this toy cosmos are quantized (not continuously variable masses, but definite masses, and in the case of atoms multiples of a unit mass), then are the gravitational interactions between and among them quantized by default, i.e., the interactions cannot be other than quantized even though GR is classical?

I do not see how to ask the question in a simpler or more straightforward way. For those seeking to quantize gravitation, this would seem like one of the most obvious first questions to ask of nature.

This is interesting, thanks Bee (BTW a prior 2nd-place winner.) The welcome defying of typical dichotomies reminds of my own insistence, the previous contest, that reality was neither digital nor analog (i.e., not fully representable by any kind of actual math.) Considering the big troubles that trying to conjoin GR and QM have provided, why not try a whole new attitude?

BTW I submitted a new essay for this contest, late Friday night so it hasn't shown up yet. The title gives a hint, it's about quantum measurement: "Can repeated interactions show more about a photon than current theory allows?"

PS: Is there a general contest discussion thread, like there was last time? I can't find one. tx. Cheers and good "luck" to all.