5 ms·

Modified gravity theories don't fit all the evidence, such as the distribution of mass in the Bullet Cluster [1] or the large-scale structure of the CMB [2]. [

Modified gravity theories don't fit all the evidence, such as the distribution of mass in the Bullet Cluster [1] or the large-scale structure of the CMB [2]. [1] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster [2] https://en.wikipedia.org/wiki/Dark_matter#Cosmic_microwave_background https://en.wikipedia.org/wiki/Dark_matter#Cosmic_microwave_b...

truantbuick 9y ago
Some would disagree: http://backreaction.blogspot.com/2018/04/no-that-galaxy-without-dark-matter-has.html http://backreaction.blogspot.com/2018/04/no-that-galaxy-with... As a non-scientist, it seems like dark matter has been relentlessly adjusted and overfitted and still appears very inadequate to fit all the evidence.
- davidcuddeback 9y ago
  > Some would disagree Some do, but they're in the minority (which, to be clear, doesn't necessarily make them wrong). You can find scientists who refute climate change, too. That says nothing of the state of prevailing consensus, though.
  - zamalek 9y ago
    > You can find scientists who refute climate change, too. That's an appeal to nature as well as a false equivalency. It is difficult to refute climate change because we have incredible amounts of tactile observations that match predictions - the stuff from which the scientific method stems. At one point science (well, philosophy as the progenitor of science) said that the Earth was flat. It was a small number of rebellious scientists that corrected this misconception. Group-think and consensus do not mean that you are correct, it could mean that you are collectively wrong. We've never seen dark matter. The so-called "evidence" that is continuously regurgitated[1] is an image of the problem. It would be like making a map of the missing periodic table of elements and calling it all "Dark Elements," never bothering to figure out what the missing elements individually are. We didn't do that, which is why we can do so many useful things with heavier elements today. If axion decay is observed, I will have no choice but to agree with the WIMP theories. As it stands, there is zero credible and scientific evidence for the stuff. [1]: http://www.esa.int/spaceinimages/Images/2007/01/3D_map_of_dark_matter_as_seen_by_Hubble http://www.esa.int/spaceinimages/Images/2007/01/3D_map_of_da...
    - civilitty 9y ago
      > We didn't do that, which is why we can do so many useful things with heavier elements today. This article is literally about the development of new sensor technologies designed to make that kind of progress for dark matter. It seems you just have a chip on your shoulder and a complete misunderstanding of the concept of dark matter.
      - zamalek 9y ago
        > This article is literally about the development of new sensor technologies designed to make that kind of progress for dark matter. Which is exactly what the last paragraph in my comment addresses.
    - davidcuddeback 9y ago
      > It is difficult to refute climate change because we have incredible amounts of tactile observations that match predictions And yet people try. If this were a discussion on climate change, someone could link to a scientist trying to refute it and say "some disagree." We both agree that linking to a single blog post that refutes climate change would be unconvincing. Why hold this topic to a different standard? > The so-called "evidence" that is continuously regurgitated is an image of the problem. That's what evidence is, isn't it? Then scientists get to solve the mystery that's revealed by the evidence. Just like detectives solve a crime based on evidence at a crime scene. Evidence can precede answers. Having evidence doesn't mean we've figured out what dark matter is. It means we can put constraints on possible explanations. So far, scientists have ruled out several theories, because they don't fit the evidence. > As it stands, there is zero credible and scientific evidence for the stuff. If there were no evidence, there wouldn't be a mystery to solve.
  - boznz 9y ago
    Dark matter may be popular but only because it is currently the best theory that fits, it still has lots of assumptions and gaps that are not tested.
    - davidcuddeback 9y ago
      > it still has lots of assumptions and gaps that are not tested Agreed. I didn't mean to imply that dark matter is a solved problem. In fact, I'm pretty sure I implied the exact opposite when I said it "doesn't necessarily make them wrong." > Dark matter may be popular but only because it is currently the best theory that fits Exactly. That's a pretty good reason to favor one theory over another.
- raattgift 9y ago
  > 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)
- lulmerchant 9y ago
  >relentlessly adjusted and overfitted and still appears very inadequate to fit all the evidence Welcome to physics. The Copenhagen Interpretation is the most concise and robustly tested theory in all of physics. Yet it was dreamt up by a literal absurdist who thought that the universe itself has no objective state of existence. For all we know about the universe, we can still not answer many of the most basic questions about it. The lesson to be learnt from this is that our greatest theories are at best woefully incomplete, and that even our greatest scientists don't have the ability to talk about many things with any level of objective certainty.
  - dbasedweeb 9y ago
    Incomplete in very extreme conditions such as the interiors of black holes or the first instants after the Big Bang. I’m not really sure that’s “woeful” in a sense that matters to almost anyone other than a particle physicist or a cosmologist. For example the confidence in QED has been tested and found that the theory makes accurate predictions to within 10^-8. I’d also argue that you’re substituting “basic” for “fundamental” and that’s bordering on dishonesty. We don’t have answers to some of the. OST fundamental questions, assuming we’re even asking the right questions, but the basics are well covered.
    - lulmerchant 9y ago
      An argument between ‘basic’ and ‘fundamental’ is a semantic one, and bordering on pointless. QED works quite well in most cases, but only if you don’t look too closely, and only if you ignore one of the fundamental forces. QED offers no explaination for gravity, which even non-physicists know exists. What is gravity? What is a particle? What is space? These are all quite basic questions that were not even close to having complete answers for. QED provides a “good enough” explaination for how particles and space work, just as relativity gives a “good enough” explaination of gravity. But any physicist who’s being intellectually honest knows that we only have a rudimentary understanding of these concepts. The wave function is just an excellent tool we use to smooth out our lack of understanding. Your comment is a perfect example of the arrogance that pervades the scientific community, and the inability to acknowledge the limits of our own understanding. Which I think only inhibits the wider community’s ability to communicate effectively with the general public.
- dbasedweeb 9y ago
  Theories are tested and observations are made. If we’re lucky a positive result emerges, but more often what you get are results which just put constraints on the theory. For example MACHO theory was once seen as very plausible, but observations of gravitational lensing have placed strict limits on it so that it seems increasingly unlikely. Sometimes the process of constraining theory leads to a reformulation of the theory, sometimes to its abandonment. Other theories which more agreeably fit the constraints may come forward (i.e. WIMP theory on the case of DM), and they may in turn be abandoned or reformulated. That’s science working as intended, or as people here like to say, a feature and not a bug. The more a theory has to make assumptions or introduce things like new particles and fields to make predictions, the less popular it tends to be. Of course once you constrain the solution space to the point where the simplest theories can’t hold, you have to consider the more complex ones. The important thing to remember is that none of this happens in a vacuum, and while personal preferences play a role, they do so only insofar as the constraints placed on theory by observation allow for it. Incidentally that’s also why falsifiablility is such a big deal, because in its absence you can assert or believe any old thing.

truantbuick 9y ago

Some would disagree: http://backreaction.blogspot.com/2018/04/no-that-galaxy-without-dark-matter-has.html http://backreaction.blogspot.com/2018/04/no-that-galaxy-with... As a non-scientist, it seems like dark matter has been relentlessly adjusted and overfitted and still appears very inadequate to fit all the evidence.

davidcuddeback 9y ago
> Some would disagree Some do, but they're in the minority (which, to be clear, doesn't necessarily make them wrong). You can find scientists who refute climate change, too. That says nothing of the state of prevailing consensus, though.
- zamalek 9y ago
  > You can find scientists who refute climate change, too. That's an appeal to nature as well as a false equivalency. It is difficult to refute climate change because we have incredible amounts of tactile observations that match predictions - the stuff from which the scientific method stems. At one point science (well, philosophy as the progenitor of science) said that the Earth was flat. It was a small number of rebellious scientists that corrected this misconception. Group-think and consensus do not mean that you are correct, it could mean that you are collectively wrong. We've never seen dark matter. The so-called "evidence" that is continuously regurgitated[1] is an image of the problem. It would be like making a map of the missing periodic table of elements and calling it all "Dark Elements," never bothering to figure out what the missing elements individually are. We didn't do that, which is why we can do so many useful things with heavier elements today. If axion decay is observed, I will have no choice but to agree with the WIMP theories. As it stands, there is zero credible and scientific evidence for the stuff. [1]: http://www.esa.int/spaceinimages/Images/2007/01/3D_map_of_dark_matter_as_seen_by_Hubble http://www.esa.int/spaceinimages/Images/2007/01/3D_map_of_da...
  - civilitty 9y ago
    > We didn't do that, which is why we can do so many useful things with heavier elements today. This article is literally about the development of new sensor technologies designed to make that kind of progress for dark matter. It seems you just have a chip on your shoulder and a complete misunderstanding of the concept of dark matter.
    - zamalek 9y ago
      > This article is literally about the development of new sensor technologies designed to make that kind of progress for dark matter. Which is exactly what the last paragraph in my comment addresses.
  - davidcuddeback 9y ago
    > It is difficult to refute climate change because we have incredible amounts of tactile observations that match predictions And yet people try. If this were a discussion on climate change, someone could link to a scientist trying to refute it and say "some disagree." We both agree that linking to a single blog post that refutes climate change would be unconvincing. Why hold this topic to a different standard? > The so-called "evidence" that is continuously regurgitated is an image of the problem. That's what evidence is, isn't it? Then scientists get to solve the mystery that's revealed by the evidence. Just like detectives solve a crime based on evidence at a crime scene. Evidence can precede answers. Having evidence doesn't mean we've figured out what dark matter is. It means we can put constraints on possible explanations. So far, scientists have ruled out several theories, because they don't fit the evidence. > As it stands, there is zero credible and scientific evidence for the stuff. If there were no evidence, there wouldn't be a mystery to solve.
- boznz 9y ago
  Dark matter may be popular but only because it is currently the best theory that fits, it still has lots of assumptions and gaps that are not tested.
  - davidcuddeback 9y ago
    > it still has lots of assumptions and gaps that are not tested Agreed. I didn't mean to imply that dark matter is a solved problem. In fact, I'm pretty sure I implied the exact opposite when I said it "doesn't necessarily make them wrong." > Dark matter may be popular but only because it is currently the best theory that fits Exactly. That's a pretty good reason to favor one theory over another.
raattgift 9y ago
> 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)
lulmerchant 9y ago
>relentlessly adjusted and overfitted and still appears very inadequate to fit all the evidence Welcome to physics. The Copenhagen Interpretation is the most concise and robustly tested theory in all of physics. Yet it was dreamt up by a literal absurdist who thought that the universe itself has no objective state of existence. For all we know about the universe, we can still not answer many of the most basic questions about it. The lesson to be learnt from this is that our greatest theories are at best woefully incomplete, and that even our greatest scientists don't have the ability to talk about many things with any level of objective certainty.
- dbasedweeb 9y ago
  Incomplete in very extreme conditions such as the interiors of black holes or the first instants after the Big Bang. I’m not really sure that’s “woeful” in a sense that matters to almost anyone other than a particle physicist or a cosmologist. For example the confidence in QED has been tested and found that the theory makes accurate predictions to within 10^-8. I’d also argue that you’re substituting “basic” for “fundamental” and that’s bordering on dishonesty. We don’t have answers to some of the. OST fundamental questions, assuming we’re even asking the right questions, but the basics are well covered.
  - lulmerchant 9y ago
    An argument between ‘basic’ and ‘fundamental’ is a semantic one, and bordering on pointless. QED works quite well in most cases, but only if you don’t look too closely, and only if you ignore one of the fundamental forces. QED offers no explaination for gravity, which even non-physicists know exists. What is gravity? What is a particle? What is space? These are all quite basic questions that were not even close to having complete answers for. QED provides a “good enough” explaination for how particles and space work, just as relativity gives a “good enough” explaination of gravity. But any physicist who’s being intellectually honest knows that we only have a rudimentary understanding of these concepts. The wave function is just an excellent tool we use to smooth out our lack of understanding. Your comment is a perfect example of the arrogance that pervades the scientific community, and the inability to acknowledge the limits of our own understanding. Which I think only inhibits the wider community’s ability to communicate effectively with the general public.
dbasedweeb 9y ago
Theories are tested and observations are made. If we’re lucky a positive result emerges, but more often what you get are results which just put constraints on the theory. For example MACHO theory was once seen as very plausible, but observations of gravitational lensing have placed strict limits on it so that it seems increasingly unlikely. Sometimes the process of constraining theory leads to a reformulation of the theory, sometimes to its abandonment. Other theories which more agreeably fit the constraints may come forward (i.e. WIMP theory on the case of DM), and they may in turn be abandoned or reformulated. That’s science working as intended, or as people here like to say, a feature and not a bug. The more a theory has to make assumptions or introduce things like new particles and fields to make predictions, the less popular it tends to be. Of course once you constrain the solution space to the point where the simplest theories can’t hold, you have to consider the more complex ones. The important thing to remember is that none of this happens in a vacuum, and while personal preferences play a role, they do so only insofar as the constraints placed on theory by observation allow for it. Incidentally that’s also why falsifiablility is such a big deal, because in its absence you can assert or believe any old thing.