Particle physics has a long history of noticing gaps in the experimental data that don’t match up with current theories and positing that maybe a new particle would solve the problem. This goes right back to things like the neutrino (which was posited to preserve momentum in certain particle interactions IIRC) and even the neutron.
Sometimes the suggestion pans out, sometimes it doesn’t. You can see this kind of thing in action - when there was that anomaly in the CERN data a year or two ago there were a flurry of papers suggesting possible particles, none of which panned out because the anomaly turned out to be statistical noise. The various particulate explanations for the observed discrepancy between the spiral galaxy rotation curves and the observed distribution of matter are very similar: Take a gap in the data & speculate about what kind of particle would fill the hole.
When you combine the fact that, historically, speculating about whether new particles could explain anomalies in the observed data has been very productive for C20th physics with the reality that physicists are also loath to toss aside general relativity (which has been spectacularly successful) just because of one observed anomaly that has yet to be explained, it’s entirely unsurprising that physicists are keen on new particles as explanations for the galactic rotation curves.
At the same time, every physicist will happily admit that there’s a great big hole in the standard model - i.e. that it doesn’t include gravity at all - and that any unifying theory might also explain away the galaxy rotation curves in a way that doesn’t require new particles. Indeed, if a unifying theory could pull off that trick it would be evidence that the theory has a good chance of being true. Sadly, to date, no one has managed to come up with such a unification, and we have no real idea what such a theory would look like beyond the obvious: that it should explain the observed facts & match QFT & GR in the regimes where those theories are accurate.
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 :( ).
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!)
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.
But I’ll chase up some of the ASG references. Thanks for those.
(yoctoNewtons! We live in amazing times...)
I recommend the introduction to
Class.Quant.Grav.27:245026,2010 DOI: 10.1088/0264-9381/27/24/245026
https://arxiv.org/abs/1008.3621
which is reasonably accessible and high-level in a way that's hard to find for asymptotically safe gravity (which is unsurprising given that renormalization group flow is a bit abstruse).
Annoyingly, experimental evidence can be hard to come by because any viable mathematical formalism has to reproduce the successes of GR, and it is shockingly easy to depart from GR in a serious way even in very weak gravity.
ASG's main anti-features are that it's not in itself a fundamental ToE; what it would accomplish (if it holds up) is getting the gravity part right to Planck scales and avoiding the incompatibility with QFT. Who knows what beyond-the-Standard-Model physics outside the gravitational sector will look like at those scales?
ASG is not alone in this, though.
String theory, on the other hand, would let gravity and matter emerge from something like a field of strings resembling dark energy, where cooling increases the apparent volume of the cosmological frame at a given scale factor. Slow thermalization in turn produces everything else, like dark matter, baryons, light, and so on.
There are other less popular theories from which matter and spacetime geometry emerge, too.
And of course it goes the other way around too, where matter theories can arise from geometry (Cartan torsion is still viable, and Poplawski still tries to get people to pick up Riemann-Cartan geometry; and quantum geometrodynamics remains viable too).
> We live in amazing times
The amazing thing is that collaboration, simulation, and publication is hugely improved upon even a couple of decades ago. If in the early 1900s people had WWW and message boards, email and message boards, LaTeX, ArXiv, and so forth, who knows what might have been different given the big brains around at the time !
In richer countries we have relative peace, antibiotics and sanitation. Feynman's wife died of TB while he was working on the Manhattan project; that's the trifecta there, just once removed from a theoretical physicist. Henry Moseley was shot through the head on the battlefield at Gallipoli. Many other scientists had their work seriously disrupted during both world wars and during cold war repressions (Sakharov on the one hand, Condon on the other; the former got it much worse of course).
That’s fair. I wasn’t aware that anyone had managed to make any potentially observable predictions at this point, so thanks for updating me on that.
(As an observer from the sidelines of physics I’ve always had a fondness for Loop Quantum Gravity / Spin Foams over String Theory(ies) personally, but not for any particularly deep reason that I can point to as justification.)
If two things that are observably exactly the same behave differently, then what should one conclude is the simplest explanation?
If you look in a telescope for a year, and notice that an apparently free-floating star out in the galaxy with no neighbors is taking a sharply curved path and everything else in the sky is going basically straight, you have two choices: 1) either something you can't see yet is there and acting on the star, 2) conservation of momentum, and the law of equal and opposite reactions need to be thrown out because they're wrong, and we need to start from scratch with physics.
Which of those two would Occam say is more likely?
The Higgs boson took a long time to be discovered, but people thought it would be there because the alternative was the Standard Model being wrong. And so they looked, and there it was. Perfectly compatible with the Standard Model.
I have always had the same inclination. It seems like a classic candidate for using Occam's razor. Would love to hear someone who knows more about physics weigh in with a more nuanced viewpoint.