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(I have a physics degree, but it’s very rusty at this point) Particle physics has a long history of noticing gaps in the experimental data that don’t match up
by pja 10y ago
(I have a physics degree, but it’s very rusty at this point)
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.
- pka 10y agoMy impression was that QFT includes gravity and "only" breaks down when things get extreme, i.e. black holes/big bang/etc.
- pja 10y agoNo, there is no quantum theory of gravity & QFT doesn’t include gravity at all. The best you can do is add gravitational effects as an after-thought, but the things you might analyse with the full QFT are generally far too small & short lived for gravitational effects to matter anyway.
- raattgift 10y ago> 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.phys.lsu.edu/faculty/pullin/talks/pire1.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pdf http://www.staff.science.uu.nl/~hooft101/lectures/erice02.pd... http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-talks/FrankSaueressig13.pdf http://einrichtungen.ph.tum.de/T31/seminars-past/seminar-tal... http://www.damtp.cam.ac.uk/research/gr/public/qg_ss.html 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.html 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 :( ).
- pja 10y agoI’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!)