A Physicist Who Denies That Dark Matter Exists

I doubt she thinks MOND is a candidate for a family of fundamental theories (MOND itself is not relativistic; fixes for that -- at least where the result is a metric theory -- tend to cause geometry problems at solar system and cosmological length scales, and relic radiation is also extremely sensitive to geometry; and why would an extended metric theory have a better answer to perturbative non-renormalizability?).

However, there's lots of scope for taking an unphysical and even incomplete theory seriously [this supergravity researcher puts it well: https://arstechnica.com/science/2013/05/earning-a-phd-by-stu... ]

Relativistic approaches to MOND might eventually raise interesting questions about the EFT, and those are what a QG phenomenologist lives for. So it's easy to see why she'd take MOND seriously without taking it too seriously.

(As an example, Verlinde's recent work is partly provoked by MOND, and it's also nice to see a string theorist step outside of the AdS comfort zone and confront what we see in our sky).

> No, there is no quantum theory of gravity

Here's a couple of examples of quantum theories of gravity.

http://www.phys.lsu.edu/faculty/pullin/talks/pire1.pdf

http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd...

http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal...

http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html

The extent to which these are complete, consistent, natural (in the fine-tuning sense), and so forth is debatable but these are certainly existing examples of quantum theories of gravity, and indeed the first is a perfectly reasonable Effective Field Theory that people work in regularly.

> QFT doesn’t include gravity at all

I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity).

> the things you might analyse with the full QFT

Atoms and molecules have gravitational fields; when you send one through a double slit, which way does their gravitational field go?

http://www.nature.com/nnano/journal/v7/n5/abs/nnano.2012.34....

A quantum theory of gravity is needed to answer that.

Assemble a huge number of particles in superposition with 1/(sqrt 2) (|M @ a> + |M @ b>), with a & b separated. A quantum theory of gravity is needed to describe the gravitational influence of M on a small test object (General Relativity's answer is just wrong :( ).

Ok. Here's some rough explanations for some of the things in your reply.

Wherever you have a quantum field, you have particles, at least for some set of observers in a relativistic quantum field theory. You were right that there are lots of subtleties in a complicated QFT like an extension of the Standard Model that includes CDM, but this generic property of relativistic QFTs is behind the term "particle dark matter".

CDM is "cold dark matter". Matter because it's a source of gravitation; cold because it moves very slowly compared to the speed of light (otherwise it would run away from galaxies rather than hanging around keeping them heavy); dark because it doesn't feel electromagnetism.

From the perspective of a relativist, CDM need not be particle dark matter. However, we can treat General Relativity as a field theory and can even quantize it (with some caveats) and the result gives us particles (gravitons in perturbative quantum gravity). New degrees of freedom that are "gravitational" are probably only "gravitational" in terms of the strength of interactions with other particles (i.e., very weak) but will still be representable as e.g. a gauge boson.

In a quantum field theory like the Standard Model, the fields' local contents are invariant under a set of transformations. When a configuration (like a proton) looks the same under various transformations, there is a symmetry at work preserving that invariant "look", much like when you rotate a circle around its centre in the Euclidean plane, it looks the same whatever angle you turn it.

The Standard Model has a number of parameters whose values are only known empirically, mostly by experiment, with little clue about why the values are the way they are. A common approach to reducing the number of such parameters involves expanding the symmetry group. Supersymmetry is one such approach, and creates a bunch of "superpartner" particles -- sparticles -- for each of the particles in the Standard Model. In minimalist supersymmetry theories, the lowest-mass sparticle tends to have the characteristics of Cold Dark Matter listed above. Other sparticles appear and can (via a new symmetry) explain apparent problems in the Standard Model.

The Ricci scalar is a value at each point in spacetime that describes the curviness of a shell of all points at the same distance from that point or equivalently the internal volume of such a shell. When the scalar increases at a point, the surface of the ball is locally flatter or equivalently the internal volume is smaller. If you move a ball through a region of spacetime where the Ricci scalar takes on various values, an observer at the centre of the shell will think the shell is more or less cramped as the value changes up and down, while a small observer standing on the outside of the shell will see the "ground" curve away (or the horizon move closer, equivalently) as the Ricci scalar decreases. If you seriously increase the Ricci scalar inside the Earth you would still measure a diameter of about 12 700 km, but from eye-level you could see to the other side of your continent. Likewise, if you seriously decrease the Ricci curvature below zero, you'd measure the same diameter of 12 700 km, but from eye-level your next door neighbour's house would be over the horizon.

The Ricci scalar appears on the gravity side of the Einstein Field Equations along with other quantities describing how lengths, durations and angles are calculated at each point.

I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t.

But perhaps that’s me being picky :)

> I think you mean "The Standard Model of Particle Physics", which is a quantum field theory (as is e.g. perturbative quantum gravity).

Sure: I was just quoting the parent comment & using the term informally to stand in for the mouthful that is TSMoPP.

(I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive!)

> I’m not sure I believe that a mathematical formalism that is unable to make useful real world predictions deserves the “theory” moniker. Hence your list of quantum theories of gravity aren’t.

On the contrary, the ones I listed are all completely in accord with General Relativity up to strong gravity and absent superposed sources, which is found from studying the renormalization group flow of perturbative quantum gravity and is four loops of gravitons in a 3+1 dimensional spacetime. Strong gravity can only be found very close to the singularity of black holes (and well inside event horizons, except at the final evaporation), or in the very early universe. So we're good for neutron stars, and have no problems studying things around the event horizons of astrophysical black holes.

The only new mathematical formalism in perturbative quantum gravity is renormalization, and that goes back to the 1980s. Perturbative quantum gravity itself comes from the 1990s.

Sean Carroll has a good explanation of renormalization and effective field theory here:

http://www.preposterousuniverse.com/blog/2013/06/20/how-quan...

Asymptotically safe gravity posits an ultraviolet fixed point at which one can take a finite number of measurements, producing a strong gravity completion that perturbative renormalzation cannot; this is prompted by asymptotic safety in QCD. Below that limit, ASG completely matches perturbative quantum gravity, and so in the EFT limit it's the same as General Relativity.

There are five or six of other viable families of quantum theories of gravity, where viability means they accord exactly with perturbatively quantized General Relativity in its effective field limit, and thus agree completely with GR in the classical limit and weak gravity, and additionally are candidates as fundamental theories because they do not rely on perturbative renormalization by power counting and thus are expected to be useful to arbitrarily high energies.

Additionally it is not wildly irresponsible to think that mathematical research (perhaps not driven by physics!) will produce a tractable renormalization that does not require nature to select a convenient effect to suppress the explosion of parameters at high energies.

> I’d love to see an experimental setup that was capable of detecting the gravitational field of a single molecule: that would be impressive!

Everyone would. We're down below milligrams and yoctoNewtons:

https://arxiv.org/abs/1602.07539

http://newscenter.lbl.gov/2014/06/26/smallest-force-ever-mea...

I'm not as au fait about how the other side of the tunnel is approaching the ultimate meeting point, but it's not unreasonable to think of nanogram masses in superposition. Experiments were only at thousands of atomic mass units a few years ago, though: https://arxiv.org/abs/1310.8343

Unfortunately General Relativity can only have the whole gravitational influence of these molecules go through one or the other slits. However all of the quantum theories in my previous message have the distribution of the gravitational influence follow distribution of the matter, as one would expect.