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The author of the article apparently shares your view about dark matter — could you explain what you find objectionable about it? It seems to me that there is

The author of the article apparently shares your view about dark matter — could you explain what you find objectionable about it? It seems to me that there is a class of concepts in theoretical physics that some people take an almost instinctive dislike to: dark matter, string theory, eternal inflation and the many worlds interpretation of quantum mechanics are prominent ones. I get the impression that there is a correlation; a person who is sceptical about dark matter seems to be much more likely to be sceptical about certain other ideas too, and to have a more favourable view of alternatives like Modified Newtonian Dynamics and Bohmian Mechanics. It's as though there is a sort of partisanship in opinion about theoretical physics: just as it is often possible to tell what somebody's view is likely to be about a whole slew of political issues if you know their opinion about one or two controversial topics, the same seems to apply to a large extent in theoretical physics too. I'm fascinated by this because in the early to mid 2000s I would have counted myself as both a dark matter and many worlds sceptic, but I take the opposite view of both now, and I find it difficult to put myself back into my old frame of mind. My sense is that the divide is somehow related to different attitudes to falsifiability, abstractness and explanatory parsimony.

jonplackett 6y ago
That’s a really interesting observation. Well, the main thing I didn’t like was the labelling of an unknown as if it was a thing that was known. I don’t have anything against the theory, just the way it was named as if it was more certain to exist than it really is.
- JProthero 6y ago
  Thanks for this; reading your explanation reminds me of the objections I had too.
  - richardw 6y ago
    Put me in that camp too! I know a small text box won’t get across what changed for you but I’d love to know what I’m probably missing. What’s the text box version? For me, it feels like we limit the search because we’ve found a pretty good answer. There’s a fashion and if you aren’t fashionable then you can’t get tenure. We should continue to explore the unfashionable edges. PS I love the idea that it’s similar to clusters of political preferences.
  - jonplackett 6y ago
    What was it that changed your mind then?
    - JProthero 6y ago
      The original observation about galaxy rotation curves never meant much to me. I didn't know enough astrophysics to have an intuitive appreciation of the significance of the disagreement between the theory and the measurements, and I felt that the universe was a sufficiently large and mysterious place that some unexpected observations — particularly at the scale of galaxies — were inevitable. It seemed arrogant to me to expect that vast, complex cosmic structures should perfectly obey the predictions of a classical theory that had been developed centuries before it was known that galaxies existed; before their scale, composition and distance from us were understood, and before it was known that we were inside one. I thought — and still think — that the universe is probably full of undiscovered phenomena, and that the discrepancy would gradually go away as more was learnt about the types of matter already known to physics; how much of it exists, how it behaves on large scales and how it is distributed. I think I felt that the most likely explanation for the observations was what are now referred to as MACHOs (Massive Compact Halo Objects); massive but dark objects, like black holes, dust clouds etc., and I presumed there must be a lot more of them than thought. The assumption that the anomaly must be due entirely to a new form of matter seemed to me like jumping to conclusions; an unjustified leap. One thing that began to change my mind was learning more about particle physics. I found it interesting that certain well-understood forms of matter were known to be oblivious to some of the fundamental forces of nature. Hundreds of billions of neutrinos pass through our bodies every second almost completely without trace because they do not feel either the electromagnetic force or the strong nuclear force. I did know about neutrinos when I first formed my views about dark matter, but I initially didn't connect the two. As I thought more about the ability of some kinds of matter to ignore certain forces, I wondered whether there might be as-yet undiscovered forms of matter even more reluctant to interact than neutrinos are (neutrinos do at least interact via the weak nuclear force, hence how they have been detected). Given that neutrinos don't feel the electromagnetic interaction that dominates our visible world, perhaps there could be undiscovered forces that the matter we are made of is insensitive to as well. This led me to wonder whether there might be a whole zoo of 'shy' particles, perhaps similar to the normal luminous matter we are familiar with, but with their own complex interactions, structures and forces; existing around us, in the same spacetime we inhabit, but unseen. The idea of mutually invisible and non-interacting worlds existing side-by-side was fascinating to me; this is usually the province of speculative fiction, but perhaps there was some analogue to it in the real universe. I found that this 'dark sector' physics was an active area of research for credible scientists. My understanding is that the latest work tends to indicate that this kind of self-interacting matter can't be the dominant component of dark matter, but the possibility of a role for unseen structure like this remains open, and the idea was so interesting to me that it prompted me to reconsider my views about dark matter. I knew that before its detection was first confirmed, antimatter had been hypothesised to exist based on consideration of mathematical symmetries in physics. Working through the consequences of the symmetries of nature turned out to be a very productive method for discovering new particles, and the approach culminated in the theory of supersymmetry. In the early to mid 2000s, supersymmetry was well-established among leading particle physicists, and was regarded as the likey future of the field (along with the Higgs boson, it was a central motivation for the construction of the Large Hadron Collider). Supersymmetry predicted that many more particles and forces should exist than predicted by the standard model of paticle physics, and at some point I learned that it was a feature of supersymmetric models that the lightest particle they predict has the properties needed to explain the abundance of dark matter in the universe. This is known as the 'WIMP Miracle' (WIMP standing for Weakly Interacting Massive Particle). That the leading particle physics theory of the time was predicting the existence of a particle that just happened to have the properties needed to explain the observations that had first led dark matter to be postulated struck me as unlikely to be a coincidence. I understand that supersymmetry has since fallen out of favour to some extent, partly because of disappointment that the LHC hasn't yet found the evidence for it that the particle physics community was hoping for, but I think it would be fair to say that it remains the most credible framework for future research at the moment. At around the time I first learned of the WIMP Miracle I was also coming to understand more about cosmological inflation. In addition to its predictions being borne-out by measurements of the cosmic microwave background radiation (the Boomerang and WMAP experiments had recently pubished their results), inflation had several other properties that appealed to me, and the theory also happened to predict a mass-energy density for the universe that corresponded very well with the observationally derived abundance of dark matter. Since then, more recent work to map the distribution of mass in the universe using gravitational lensing; computer simulations that strongly suggest dark matter is required to explain the observed large-scale structure formation in the universe; and more detailed studies of supposed astrophysical evidence like the Bullet Cluster, have all tended to reinforce the conclusion for me that dark matter probably does exist and is an as-yet undetected particle or class of particles. I'm still a bit heterodox in my view about it though in that I hold out the hope for a significant role for self-interacting dark matter and dark sector forces, because I still think that's potentially the most interesting thing about it.
- patentatt 6y ago
  I agree. As a casual observer of these things, you read about ‘dark matter’ and come away with the impression that it’s, you know, matter. But it’s really just a bookmark, a placeholder for “our calculations are wrong, so we’re going to make something up.” It’s basically the physicist version of Kevin Malone’s “A mistake plus keleven gets you home by seven."
auntienomen 6y ago
I find it strange that a skeptic would have more favorable views of MOND & Bohmian Mechanics. These theories are different from, but by no means better, than their competitors. I think what we're looking at here is more like an impulse towards iconoclasm. But one must generally hold up something as an alternative and it's easier to choose a pre-existing candidate than to create your own.
ncmncm 6y ago
"...physics that some people take an almost instinctive dislike to..." It is true. The common feature is, "Sounds like BS to me." The solid answer to them, as to the previous quantum, relativity, and even neutrino skeptics, now satisfied, is evidence, but that stuff has been raggedly thin lately. This, at the same time that we hear, loudly, "extraordinary claims demand extraordinary evidence". Thank you, just ordinary evidence will do. The more the better. I don't think dark matter skeptics are especially enamored with MOND or supercausality; I think they are just rationally suspicious of a one-theory field that prefers to sweep contrary evidence under the rug, while parading the dirty laundry of challengers; and want to see things kept honest. I have no dog in this fight, but I have seen more than enough echo chambers operating in what is supposed to be science. Speaking of which: This might be the only useful result, now and forever, of all the $billions spent chasing Tokamak: a generation of physicists not afraid of plasma fluid dynamics. God knows there will never be even one solitary erg of competitive commercial energy production from it. And, while I'm here: "magnetohydrodynamics" is an extremely limited sub-field of plasma fluid dynamics, involving just the trivial parts. Astrophysicists seem to hate the expression "plasma fluid dynamics", and say "MHD" whenever they feel like they can get away with it. I have interpreted it as shame that they cannot cope with the mathematics that doing real PFD involves anywhere outside of the wholly artificial circumstances where MHD maths work; but I am open to alternatives.
- bollu 6y ago
  What's interesting textbooks andectures one can read on plasma fluid dynamics?
trenchgun 6y ago
Dark matter and string theory suck. Many worlds interpretation rules. Eternal inflation is also a cool idea.
DebtDeflation 6y ago
>could you explain what you find objectionable about it Not OP, but I think for most educated laymen if you said "15% of the total matter in the universe is unaccounted for and must be something exotic that we so far have been unable to detect" they would respond with "that's fascinating, I'm super excited for the physics community to discover what it is". However, when you flip it around and say, "all the matter that we know about only accounts for 15% of the total matter in the universe, the remaining 85% must be something exotic but we've never been able to detect it" the response tends to be "have you double checked your math?"
credit_guy 6y ago
> different attitudes to falsifiability Wait a second there. There are different attitudes about falsifiability? Isn't there a unanimous consensus that something not falisifiable is not science?
- JProthero 6y ago
  > Wait a second there. There are different attitudes about falsifiability? Yes. My understanding is that Karl Popper first proposed falsifiability as a criterion for a scientific theory in part because he found that some supposedly scientific ideas (Freudian psychoanalysis was a particular target of his) failed to make predictions that could be definitively tested: it was a feature of these 'theories' that they could be interpreted in such a way as to accommodate any conceivable observation. I think you are right that there is a strong consensus among scientists that for a theory to be considered scientific, it must make predictions which can be tested empirically, so that those predictions can be either verified or falsified. The difficulty some scientists have with a strict attitude towards falsifiability though is that many theories that make falsifiable predictions in regimes where the theory can be tested, also make consequential predictions about phenomena that are not readily observable, and might not even be observable in principle. For instance, observations suggest that the universe is expanding at an accelerating rate; that the pace of expansion increases with distance; and that the expansion is unconstrained by the speed of light. This means that there are vast numbers of galaxies which were once receding from the Earth slowly enough that signals from them (e.g. light that can be imaged by telescopes) could be detected on Earth, but which are now receding from Earth faster than light. Without instruments capable of probing the universe with signals that can exceed the speed of light (something that is currently thought to be impossible), the future of these distant galaxies that have crossed the 'particle horizon' [1] will be forever unknowable to observers on Earth. Predictions about the future evolution, or even existence, of those galaxies are therefore not strictly falsifiable, because it would be impossible to make the necessary observations. Despite this, astronomers have no reason to believe that those galaxies cease to exist, or that the known laws of physics cease to apply to them, once they can no longer be observed by scientific instruments on Earth. In other words, apparently credible scientific predictions could be made about a large and ever-increasing volume of the universe, but those predictions could never be tested observationally: they would be unfalsifiable. Currently accepted models suggest that, in the distant future, the entire universe except our local group of galaxies will recede into this unobservable domain, and will therefore be inaccessible to any astronomers around at the time. String theory similarly makes predictions that might never in principle be testable (and thus falsifiable) because doing so is thought to require experiments that would necessarily collapse into black holes were they ever to be attempted. Proponents of the Many Worlds interpretation of Quantum Mechanics arguably have a similar problem in that it is typically thought to be impossible to interact with the hypothesised other worlds in a way that could confirm or refute their existence. The cosmologist Sean Carroll (who is a proponent of the Many Worlds interpretation) has written a paper on this subject titled 'Beyond Falsifiability' [2], which is discussed on his website [3]. The physicist David Deutsch, who has played an important role in establishing the theoretical basis of quantum computation, has argued that a quantum computer could effectively demonstrate the existence of the other worlds of the Many Worlds interpretation [4], so falsifiability is perhaps less of a problem from that perspective. [1] https://en.wikipedia.org/wiki/Particle_horizon https://en.wikipedia.org/wiki/Particle_horizon [2] https://arxiv.org/abs/1801.05016 https://arxiv.org/abs/1801.05016 [3] https://www.preposterousuniverse.com/blog/2018/01/17/beyond-falsifiability/ https://www.preposterousuniverse.com/blog/2018/01/17/beyond-... [4] https://thereader.mitpress.mit.edu/the-many-worlds-theory/ https://thereader.mitpress.mit.edu/the-many-worlds-theory/

jonplackett 6y ago

That’s a really interesting observation. Well, the main thing I didn’t like was the labelling of an unknown as if it was a thing that was known. I don’t have anything against the theory, just the way it was named as if it was more certain to exist than it really is.

JProthero 6y ago
Thanks for this; reading your explanation reminds me of the objections I had too.
- richardw 6y ago
  Put me in that camp too! I know a small text box won’t get across what changed for you but I’d love to know what I’m probably missing. What’s the text box version? For me, it feels like we limit the search because we’ve found a pretty good answer. There’s a fashion and if you aren’t fashionable then you can’t get tenure. We should continue to explore the unfashionable edges. PS I love the idea that it’s similar to clusters of political preferences.
- jonplackett 6y ago
  What was it that changed your mind then?
  - JProthero 6y ago
    The original observation about galaxy rotation curves never meant much to me. I didn't know enough astrophysics to have an intuitive appreciation of the significance of the disagreement between the theory and the measurements, and I felt that the universe was a sufficiently large and mysterious place that some unexpected observations — particularly at the scale of galaxies — were inevitable. It seemed arrogant to me to expect that vast, complex cosmic structures should perfectly obey the predictions of a classical theory that had been developed centuries before it was known that galaxies existed; before their scale, composition and distance from us were understood, and before it was known that we were inside one. I thought — and still think — that the universe is probably full of undiscovered phenomena, and that the discrepancy would gradually go away as more was learnt about the types of matter already known to physics; how much of it exists, how it behaves on large scales and how it is distributed. I think I felt that the most likely explanation for the observations was what are now referred to as MACHOs (Massive Compact Halo Objects); massive but dark objects, like black holes, dust clouds etc., and I presumed there must be a lot more of them than thought. The assumption that the anomaly must be due entirely to a new form of matter seemed to me like jumping to conclusions; an unjustified leap. One thing that began to change my mind was learning more about particle physics. I found it interesting that certain well-understood forms of matter were known to be oblivious to some of the fundamental forces of nature. Hundreds of billions of neutrinos pass through our bodies every second almost completely without trace because they do not feel either the electromagnetic force or the strong nuclear force. I did know about neutrinos when I first formed my views about dark matter, but I initially didn't connect the two. As I thought more about the ability of some kinds of matter to ignore certain forces, I wondered whether there might be as-yet undiscovered forms of matter even more reluctant to interact than neutrinos are (neutrinos do at least interact via the weak nuclear force, hence how they have been detected). Given that neutrinos don't feel the electromagnetic interaction that dominates our visible world, perhaps there could be undiscovered forces that the matter we are made of is insensitive to as well. This led me to wonder whether there might be a whole zoo of 'shy' particles, perhaps similar to the normal luminous matter we are familiar with, but with their own complex interactions, structures and forces; existing around us, in the same spacetime we inhabit, but unseen. The idea of mutually invisible and non-interacting worlds existing side-by-side was fascinating to me; this is usually the province of speculative fiction, but perhaps there was some analogue to it in the real universe. I found that this 'dark sector' physics was an active area of research for credible scientists. My understanding is that the latest work tends to indicate that this kind of self-interacting matter can't be the dominant component of dark matter, but the possibility of a role for unseen structure like this remains open, and the idea was so interesting to me that it prompted me to reconsider my views about dark matter. I knew that before its detection was first confirmed, antimatter had been hypothesised to exist based on consideration of mathematical symmetries in physics. Working through the consequences of the symmetries of nature turned out to be a very productive method for discovering new particles, and the approach culminated in the theory of supersymmetry. In the early to mid 2000s, supersymmetry was well-established among leading particle physicists, and was regarded as the likey future of the field (along with the Higgs boson, it was a central motivation for the construction of the Large Hadron Collider). Supersymmetry predicted that many more particles and forces should exist than predicted by the standard model of paticle physics, and at some point I learned that it was a feature of supersymmetric models that the lightest particle they predict has the properties needed to explain the abundance of dark matter in the universe. This is known as the 'WIMP Miracle' (WIMP standing for Weakly Interacting Massive Particle). That the leading particle physics theory of the time was predicting the existence of a particle that just happened to have the properties needed to explain the observations that had first led dark matter to be postulated struck me as unlikely to be a coincidence. I understand that supersymmetry has since fallen out of favour to some extent, partly because of disappointment that the LHC hasn't yet found the evidence for it that the particle physics community was hoping for, but I think it would be fair to say that it remains the most credible framework for future research at the moment. At around the time I first learned of the WIMP Miracle I was also coming to understand more about cosmological inflation. In addition to its predictions being borne-out by measurements of the cosmic microwave background radiation (the Boomerang and WMAP experiments had recently pubished their results), inflation had several other properties that appealed to me, and the theory also happened to predict a mass-energy density for the universe that corresponded very well with the observationally derived abundance of dark matter. Since then, more recent work to map the distribution of mass in the universe using gravitational lensing; computer simulations that strongly suggest dark matter is required to explain the observed large-scale structure formation in the universe; and more detailed studies of supposed astrophysical evidence like the Bullet Cluster, have all tended to reinforce the conclusion for me that dark matter probably does exist and is an as-yet undetected particle or class of particles. I'm still a bit heterodox in my view about it though in that I hold out the hope for a significant role for self-interacting dark matter and dark sector forces, because I still think that's potentially the most interesting thing about it.
patentatt 6y ago
I agree. As a casual observer of these things, you read about ‘dark matter’ and come away with the impression that it’s, you know, matter. But it’s really just a bookmark, a placeholder for “our calculations are wrong, so we’re going to make something up.” It’s basically the physicist version of Kevin Malone’s “A mistake plus keleven gets you home by seven."

auntienomen 6y ago

I find it strange that a skeptic would have more favorable views of MOND & Bohmian Mechanics. These theories are different from, but by no means better, than their competitors. I think what we're looking at here is more like an impulse towards iconoclasm. But one must generally hold up something as an alternative and it's easier to choose a pre-existing candidate than to create your own.

ncmncm 6y ago

"...physics that some people take an almost instinctive dislike to..." It is true. The common feature is, "Sounds like BS to me." The solid answer to them, as to the previous quantum, relativity, and even neutrino skeptics, now satisfied, is evidence, but that stuff has been raggedly thin lately. This, at the same time that we hear, loudly, "extraordinary claims demand extraordinary evidence". Thank you, just ordinary evidence will do. The more the better. I don't think dark matter skeptics are especially enamored with MOND or supercausality; I think they are just rationally suspicious of a one-theory field that prefers to sweep contrary evidence under the rug, while parading the dirty laundry of challengers; and want to see things kept honest. I have no dog in this fight, but I have seen more than enough echo chambers operating in what is supposed to be science. Speaking of which: This might be the only useful result, now and forever, of all the $billions spent chasing Tokamak: a generation of physicists not afraid of plasma fluid dynamics. God knows there will never be even one solitary erg of competitive commercial energy production from it. And, while I'm here: "magnetohydrodynamics" is an extremely limited sub-field of plasma fluid dynamics, involving just the trivial parts. Astrophysicists seem to hate the expression "plasma fluid dynamics", and say "MHD" whenever they feel like they can get away with it. I have interpreted it as shame that they cannot cope with the mathematics that doing real PFD involves anywhere outside of the wholly artificial circumstances where MHD maths work; but I am open to alternatives.

bollu 6y ago
What's interesting textbooks andectures one can read on plasma fluid dynamics?

trenchgun 6y ago

Dark matter and string theory suck. Many worlds interpretation rules. Eternal inflation is also a cool idea.

DebtDeflation 6y ago

>could you explain what you find objectionable about it Not OP, but I think for most educated laymen if you said "15% of the total matter in the universe is unaccounted for and must be something exotic that we so far have been unable to detect" they would respond with "that's fascinating, I'm super excited for the physics community to discover what it is". However, when you flip it around and say, "all the matter that we know about only accounts for 15% of the total matter in the universe, the remaining 85% must be something exotic but we've never been able to detect it" the response tends to be "have you double checked your math?"

credit_guy 6y ago

> different attitudes to falsifiability Wait a second there. There are different attitudes about falsifiability? Isn't there a unanimous consensus that something not falisifiable is not science?

JProthero 6y ago
> Wait a second there. There are different attitudes about falsifiability? Yes. My understanding is that Karl Popper first proposed falsifiability as a criterion for a scientific theory in part because he found that some supposedly scientific ideas (Freudian psychoanalysis was a particular target of his) failed to make predictions that could be definitively tested: it was a feature of these 'theories' that they could be interpreted in such a way as to accommodate any conceivable observation. I think you are right that there is a strong consensus among scientists that for a theory to be considered scientific, it must make predictions which can be tested empirically, so that those predictions can be either verified or falsified. The difficulty some scientists have with a strict attitude towards falsifiability though is that many theories that make falsifiable predictions in regimes where the theory can be tested, also make consequential predictions about phenomena that are not readily observable, and might not even be observable in principle. For instance, observations suggest that the universe is expanding at an accelerating rate; that the pace of expansion increases with distance; and that the expansion is unconstrained by the speed of light. This means that there are vast numbers of galaxies which were once receding from the Earth slowly enough that signals from them (e.g. light that can be imaged by telescopes) could be detected on Earth, but which are now receding from Earth faster than light. Without instruments capable of probing the universe with signals that can exceed the speed of light (something that is currently thought to be impossible), the future of these distant galaxies that have crossed the 'particle horizon' [1] will be forever unknowable to observers on Earth. Predictions about the future evolution, or even existence, of those galaxies are therefore not strictly falsifiable, because it would be impossible to make the necessary observations. Despite this, astronomers have no reason to believe that those galaxies cease to exist, or that the known laws of physics cease to apply to them, once they can no longer be observed by scientific instruments on Earth. In other words, apparently credible scientific predictions could be made about a large and ever-increasing volume of the universe, but those predictions could never be tested observationally: they would be unfalsifiable. Currently accepted models suggest that, in the distant future, the entire universe except our local group of galaxies will recede into this unobservable domain, and will therefore be inaccessible to any astronomers around at the time. String theory similarly makes predictions that might never in principle be testable (and thus falsifiable) because doing so is thought to require experiments that would necessarily collapse into black holes were they ever to be attempted. Proponents of the Many Worlds interpretation of Quantum Mechanics arguably have a similar problem in that it is typically thought to be impossible to interact with the hypothesised other worlds in a way that could confirm or refute their existence. The cosmologist Sean Carroll (who is a proponent of the Many Worlds interpretation) has written a paper on this subject titled 'Beyond Falsifiability' [2], which is discussed on his website [3]. The physicist David Deutsch, who has played an important role in establishing the theoretical basis of quantum computation, has argued that a quantum computer could effectively demonstrate the existence of the other worlds of the Many Worlds interpretation [4], so falsifiability is perhaps less of a problem from that perspective. [1] https://en.wikipedia.org/wiki/Particle_horizon https://en.wikipedia.org/wiki/Particle_horizon [2] https://arxiv.org/abs/1801.05016 https://arxiv.org/abs/1801.05016 [3] https://www.preposterousuniverse.com/blog/2018/01/17/beyond-falsifiability/ https://www.preposterousuniverse.com/blog/2018/01/17/beyond-... [4] https://thereader.mitpress.mit.edu/the-many-worlds-theory/ https://thereader.mitpress.mit.edu/the-many-worlds-theory/