3 ms·

I'm a long time out of the game (PhD in solid state physics in the early 2000s), but the key phrase here is "phase transition". She's making an argument along t

by adw 5y ago

I'm a long time out of the game (PhD in solid state physics in the early 2000s), but the key phrase here is "phase transition". She's making an argument along the lines of: * MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations; * Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly * because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales) * this can be analogized to a phase transition in solid state physics * the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap) therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools. I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.

raattgift 5y ago
> the people who understand phase transitions aren't working on astrophysics I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars. https://duckduckgo.com/?q=phase+transition+neutron+star https://duckduckgo.com/?q=phase+transition+neutron+star > wave-particle duality I don't see how that has anything to do with it. The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism). This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it. But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory. - -- [1] https://www.nobelprize.org/prizes/physics/1982/press-release/ https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgiving-5/ http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]

by adw 5y ago

raattgift 5y ago

> the people who understand phase transitions aren't working on astrophysics I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars. https://duckduckgo.com/?q=phase+transition+neutron+star https://duckduckgo.com/?q=phase+transition+neutron+star > wave-particle duality I don't see how that has anything to do with it. The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism). This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it. But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory. - -- [1] https://www.nobelprize.org/prizes/physics/1982/press-release/ https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgiving-5/ http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]