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> As a non-scientist Unfortunately you're missing the thrust of Hossenfelder's developing argument: in upgrading MOND to a full relativistic theory, you necess

> As a non-scientist Unfortunately you're missing the thrust of Hossenfelder's developing argument: in upgrading MOND to a full relativistic theory, you necessarily seriously blur the difference between "modified gravity" and "particle dark matter". There's some meat on this in the comments on that blog page, notably the point raised (and her subsequent reference to it) by "M_Malenfant". Essentially her point is that when you relativize some theories of MOND (detailed examples in Famaey & McGauch https://arxiv.org/abs/1112.3960 https://arxiv.org/abs/1112.3960 section 7) you are inevitably adding a field to the existing fields. Whether you put it on the right or the left of the Einstein Field Equations is a matter of personal taste. The thoretical requirements however are that the field couples with matter somehow (even just gravitationally), just like the particle dark matter case (where particles may interact weakly or even just gravitationally). Moreover, when you have a dynamical, relativistic field like this, you tend to think of how to quantize it, which typically means particles as the field content and particles mediating the interaction between that and the field content of the Standard Model. So, as she says, perhaps it's more a question of sociology than mathematics that splits particle dark matter theorists (solving cosmological problems like the details of the cosmic microwave background and its neutrino equivalent, or solving problems with the Standard Model like the QCD strong CP problem) or from modified gravity theorists (solving velocity curve problems). At some level, almost all of the still-viable proposals for these and related problems boil down to adding a field on curved spacetime. On the other hand -- and this is where Hossenfelder (among others) deserves some credit -- this wouldn't have been quite so obvious ten or so years ago, let alone in the early 1980s when Milgrom began developing MOND.

truantbuick 9y ago
I was specifically responding to the commonly held belief that modified gravities and observations of the bullet cluster are absolutely irreconcilable. I'm not necessarily drawing any line in the sand about whether I believe in modified gravities over particle dark matter, or whether that distinction is even important. In general, I'm suspicious of dark matter. It would make an interesting development if it ended up being the planet Vulcan of our time, where we doggedly postulate its existence despite mounting counter-evidence because it's unpleasant to question baser theories. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)
- raattgift 9y ago
  > commonly held belief Chandra data for the Bullet Cluster is from 2004; it was a crisis for then non-relativistic MOND, precisely because the Bullet Cluster is manifestly a relativistic, dynamical system. It is not now a crisis for appropriate relativistic theories of "modified gravity" where an additional field is added to General Relativity (again, see Famaey & McGauch 2011 -- section 8.3 is directly relevant) where in the non-relativistic and static limit (with some additional symmetries) the MOND interpolating function is recovered. This was tremendous progress, but leads ineluctably to the question: what is really the difference between adding a vector or scalar field on one side or the other of the EFEs? The result: convergence on the idea that there is some field content that drags on luminous matter. We already had that with particle dark matter, with the main disagreements being the nature of the field (WIMPs vs axions etc.). Now it's more a question of how to do an initial values formulation that concords with available evidence. For straightforward particle dark matter, that's really easy once a candidate is found in a lab experiment. For less straightforward particle dark matter -- and for "modified gravity" fields on curved spacetime -- it's not so obviously easy, but almost certainly easier than was believed in 2004! In all cases, structure evolution simulations are possible today that were undreamed of in the 1980s when MOND and CDM were first proposed. > suspicious of dark matter There can't be many people whose job -- or even calling -- it is to convince you personally of anything rather than, say, look for evidence to further constrain the CDM parameter space. However, I'm sure you could arrange something. You seem to like Bee, so perhaps http://backreaction.blogspot.com/p/talk-to-physicist_27.html http://backreaction.blogspot.com/p/talk-to-physicist_27.html is worth a shot.

truantbuick 9y ago

I was specifically responding to the commonly held belief that modified gravities and observations of the bullet cluster are absolutely irreconcilable. I'm not necessarily drawing any line in the sand about whether I believe in modified gravities over particle dark matter, or whether that distinction is even important. In general, I'm suspicious of dark matter. It would make an interesting development if it ended up being the planet Vulcan of our time, where we doggedly postulate its existence despite mounting counter-evidence because it's unpleasant to question baser theories. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet)

raattgift 9y ago
> commonly held belief Chandra data for the Bullet Cluster is from 2004; it was a crisis for then non-relativistic MOND, precisely because the Bullet Cluster is manifestly a relativistic, dynamical system. It is not now a crisis for appropriate relativistic theories of "modified gravity" where an additional field is added to General Relativity (again, see Famaey & McGauch 2011 -- section 8.3 is directly relevant) where in the non-relativistic and static limit (with some additional symmetries) the MOND interpolating function is recovered. This was tremendous progress, but leads ineluctably to the question: what is really the difference between adding a vector or scalar field on one side or the other of the EFEs? The result: convergence on the idea that there is some field content that drags on luminous matter. We already had that with particle dark matter, with the main disagreements being the nature of the field (WIMPs vs axions etc.). Now it's more a question of how to do an initial values formulation that concords with available evidence. For straightforward particle dark matter, that's really easy once a candidate is found in a lab experiment. For less straightforward particle dark matter -- and for "modified gravity" fields on curved spacetime -- it's not so obviously easy, but almost certainly easier than was believed in 2004! In all cases, structure evolution simulations are possible today that were undreamed of in the 1980s when MOND and CDM were first proposed. > suspicious of dark matter There can't be many people whose job -- or even calling -- it is to convince you personally of anything rather than, say, look for evidence to further constrain the CDM parameter space. However, I'm sure you could arrange something. You seem to like Bee, so perhaps http://backreaction.blogspot.com/p/talk-to-physicist_27.html http://backreaction.blogspot.com/p/talk-to-physicist_27.html is worth a shot.