5 ms·

> "...crack pot theories like dark matter" Out of genuine interest, do you have any citations from reputable physicists who agree dark matter is a "crack pot t

by notfed 9y ago

> "...crack pot theories like dark matter" Out of genuine interest, do you have any citations from reputable physicists who agree dark matter is a "crack pot theory"?

dnautics 9y ago
I think he was being sarcastic. The point is the basic law of gravitation is being violated here, and it's like, zero point energy or something.
- trhway 9y ago
  not really. All the works postulating dark matter from the flat speed curve that i've seen so far are using basic orbital speed formula (sqrt(M/R)) which is only applicable to either spherically-simmetric masses and/or large (compare to the mass containing radius) distances between the interacting masses. Such simplification is just not valid for disk star orbital speed calculation because 1. disk galaxies are far from being spherically symmetric and 2. distance between a star and the galaxy center is of the same scale as the galaxy's radius. Very detailed Newton based calculation of those orbital speeds taking into account the shape of the galaxy mass distribution produces much flatter curve and, for example, for Milky Way to get a flat curve matching observations i needed to speculate only additional 20%-30% of galaxy mass to be present in the galaxy halo (like gas and dark dwarfs of all kinds which some works suggest there are and even that may be unnecessary - need to find and check some additional observational data for that) instead of additional 900% of dark matter that typical "dark matter" works speculate for.
  - libeclipse 9y ago
    What are your credentials, if you don't mind my asking?
    - phkahler 9y ago
      He can do the math himself.
      - CamperBob2 9y ago
        All the math?
      - coldtea 9y ago
        Whereas professional physicists can't and haven't taken those maths into account?
        phkahler 9y ago
        That would be correct. I wondered if the problem was me reading things like Wikipedia which says the expected galactic rotation curve should follow something based on Kepler's laws. Then someone got me a subscription to Nature... You know the prestigious "peer reviewed" journal where the serious stuff is. Don't get me wrong Nature had some really awesome content. But there was a paper about a galaxy found in the early formative stages and the authors measured a rough rotation curve and felt the need to compare it to something Kepler would predict. Ugh. If anyone can tell me why Kepler applies galaxies (vs 2-body systems with certain constraints), I'd be happy to show where the error in that analysis is. Kepler doesn't apply to galaxies. It's really that simple.
    - trhway 9y ago
      Credentials are irrelevant here as what I'm talking about is high school level integration of Newtonian forces.
      - SamBam 9y ago
        And you, personally, correctly calculated the actual expected rotation? I can't imagine the amount of superiority you must feel to assume that physicists don't know high school level integration of Newtonian forces.
        deleted 9y ago
        [deleted]
        davidmr 9y ago
        I used to work in a well-regarded Astronomy and Astrophysics department at a research university. Every few months, I would go through the department "crackpot folder" when I was especially bored. Every couple days the department chair or some well-known professor would get a letter (this was before most of the crackpots had email accounts, I guess) claiming that some theory--particularly general relativity--was wrong and all it took was a simple application of high school arithmetic to see it. One of our faculty happened to coin the phrase "dark energy" while I was there, and man did the letters start coming in after that. Of course, the only absolutely universal commonality among all of the letters was that they were all nonsense, like the above. In general, I felt sort of bad because you could tell that a lot of these people were really not well, but it's still hard for me to see things from someone's perspective who genuinely believes that the entire astrophysics community (save a tiny handful of contrarian actual physicists) has somehow engaged in a massive conspiracy to ignore introductory physics in favor of some other vastly more complicated theory for absolutely no possible benefit. (Especially when these professors are literally teaching introductory physics to students while they're pursuing their research.)
        trhway 9y ago
        I feel your pain :) Back then the most such fun we had were Fermat theorem proofs coming to the algebra department.
  - phkahler 9y ago
    Finally! Someone else made the same observation I did. All that "expected" rotation curve stuff is bullshit since it's based on Keplers laws, which don't apply to a flat disk. They do that based on a misapplication of the divergence theorem. They assume that all the mass inside an orbital radius can be treated as a point-mass which is true for uniform spherical distributions. They also assume the mass outside the radius has no net infuence - which is true for uniform spherical shells. Neither of these is valid for non-uniform distributions - a flat disk in not symmetric in 3-space. If they'd just do some electrostatics problems they'd know the different formulas for disks and rings. I was going to google a sample page but there's too many to sift through and I gotta run an errand. Welcome to the club - it's incredibly small...
    - trhway 9y ago
      Yet DM is so sexy and mysterious ... Even Sheldon couldn't resist the temptation.
    - michaelrccurtis 9y ago
      I really encourage you if you are actually interested in the arguments to progress beyond the entry level explanations. Binney & Tremaine (https://press.princeton.edu/titles/8697.html https://press.princeton.edu/titles/8697.html) is the standard work - I highly recommend it. At the very least, it will put to rest your notion that astrophysicists only consider spherically symmetric potentials or fail to consider the effects of a disks etc., which is just patently untrue.
      - idlewords 9y ago
        And here I thought Hacker News was going to get the Nobel Prize! I can't believe quickly deriving something from first principles also occurred to the people devoting their lives to this field.
      - phkahler 9y ago
        Thanks for the link. They have a .pdf of chapter one which I gave a look. Right there on page 14 when estimating the mass of our galaxy inside our suns radius we have: "The approximation that the mass distribution is spherical is reasonable for the dark halo, but not for the flat stellar disk. Better models suggest that this estimate is probably high by about 30%, since a disk requires less mass to produce a given centripetal acceleration" And there you have it. While he notes that a spherical distribution is different than a flat disk, he estimates the error at about 30%. The problem is that the error is not one of a constant factor, it's a different function. This unspecified better model also doesn't seem to care about the matter outside the solar radius - which also affects our velocity in a disk, but not in a sphere. There is a lot of math in chapter one, but also a lot of hand waving.
  - deleted 9y ago
    [deleted]
  - scythe 9y ago
    Of course you are completely wrong: http://iopscience.iop.org/article/10.1086/508162/meta http://iopscience.iop.org/article/10.1086/508162/meta There is a great deal of evidence for dark matter that does not depend on Kepler's laws, please stop spreading misinformation. Source: assistant editor of a physics journal, know several astrophysicists personally, currently dealing with a MOND paper this very week...
  - wallace_f 9y ago
    I don't mean to just be a pedant when I say this, but the use of the word crackpot is missplaced here. Crackpot theories are eccentric and not commonly believed. I am interested in crackpot theories because sometimes they end up turning out right, and I think within that dynamic there are important lessons for humanity that we end up learning in very hard ways. For example, the Great Depression paved the way for the acceptance of Keynesian economics--ideas previously written-off as crackpot theories. Another: NASA engineers warned of the exact dangers of a foam strike, under the exact conditions in which it took place, were at the time written-off by senior management. Social scientists that studied this described part of the problem as Groupthink.

by notfed 9y ago

> "...crack pot theories like dark matter" Out of genuine interest, do you have any citations from reputable physicists who agree dark matter is a "crack pot theory"?

dnautics 9y ago

I think he was being sarcastic. The point is the basic law of gravitation is being violated here, and it's like, zero point energy or something.

trhway 9y ago
not really. All the works postulating dark matter from the flat speed curve that i've seen so far are using basic orbital speed formula (sqrt(M/R)) which is only applicable to either spherically-simmetric masses and/or large (compare to the mass containing radius) distances between the interacting masses. Such simplification is just not valid for disk star orbital speed calculation because 1. disk galaxies are far from being spherically symmetric and 2. distance between a star and the galaxy center is of the same scale as the galaxy's radius. Very detailed Newton based calculation of those orbital speeds taking into account the shape of the galaxy mass distribution produces much flatter curve and, for example, for Milky Way to get a flat curve matching observations i needed to speculate only additional 20%-30% of galaxy mass to be present in the galaxy halo (like gas and dark dwarfs of all kinds which some works suggest there are and even that may be unnecessary - need to find and check some additional observational data for that) instead of additional 900% of dark matter that typical "dark matter" works speculate for.
- libeclipse 9y ago
  What are your credentials, if you don't mind my asking?
  - phkahler 9y ago
    He can do the math himself.
    - CamperBob2 9y ago
      All the math?
    - coldtea 9y ago
      Whereas professional physicists can't and haven't taken those maths into account?
      - phkahler 9y ago
        That would be correct. I wondered if the problem was me reading things like Wikipedia which says the expected galactic rotation curve should follow something based on Kepler's laws. Then someone got me a subscription to Nature... You know the prestigious "peer reviewed" journal where the serious stuff is. Don't get me wrong Nature had some really awesome content. But there was a paper about a galaxy found in the early formative stages and the authors measured a rough rotation curve and felt the need to compare it to something Kepler would predict. Ugh. If anyone can tell me why Kepler applies galaxies (vs 2-body systems with certain constraints), I'd be happy to show where the error in that analysis is. Kepler doesn't apply to galaxies. It's really that simple.
  - trhway 9y ago
    Credentials are irrelevant here as what I'm talking about is high school level integration of Newtonian forces.
    - SamBam 9y ago
      And you, personally, correctly calculated the actual expected rotation? I can't imagine the amount of superiority you must feel to assume that physicists don't know high school level integration of Newtonian forces.
      - deleted 9y ago
        [deleted]
      - davidmr 9y ago
        I used to work in a well-regarded Astronomy and Astrophysics department at a research university. Every few months, I would go through the department "crackpot folder" when I was especially bored. Every couple days the department chair or some well-known professor would get a letter (this was before most of the crackpots had email accounts, I guess) claiming that some theory--particularly general relativity--was wrong and all it took was a simple application of high school arithmetic to see it. One of our faculty happened to coin the phrase "dark energy" while I was there, and man did the letters start coming in after that. Of course, the only absolutely universal commonality among all of the letters was that they were all nonsense, like the above. In general, I felt sort of bad because you could tell that a lot of these people were really not well, but it's still hard for me to see things from someone's perspective who genuinely believes that the entire astrophysics community (save a tiny handful of contrarian actual physicists) has somehow engaged in a massive conspiracy to ignore introductory physics in favor of some other vastly more complicated theory for absolutely no possible benefit. (Especially when these professors are literally teaching introductory physics to students while they're pursuing their research.)
        trhway 9y ago
        I feel your pain :) Back then the most such fun we had were Fermat theorem proofs coming to the algebra department.
- phkahler 9y ago
  Finally! Someone else made the same observation I did. All that "expected" rotation curve stuff is bullshit since it's based on Keplers laws, which don't apply to a flat disk. They do that based on a misapplication of the divergence theorem. They assume that all the mass inside an orbital radius can be treated as a point-mass which is true for uniform spherical distributions. They also assume the mass outside the radius has no net infuence - which is true for uniform spherical shells. Neither of these is valid for non-uniform distributions - a flat disk in not symmetric in 3-space. If they'd just do some electrostatics problems they'd know the different formulas for disks and rings. I was going to google a sample page but there's too many to sift through and I gotta run an errand. Welcome to the club - it's incredibly small...
  - trhway 9y ago
    Yet DM is so sexy and mysterious ... Even Sheldon couldn't resist the temptation.
  - michaelrccurtis 9y ago
    I really encourage you if you are actually interested in the arguments to progress beyond the entry level explanations. Binney & Tremaine (https://press.princeton.edu/titles/8697.html https://press.princeton.edu/titles/8697.html) is the standard work - I highly recommend it. At the very least, it will put to rest your notion that astrophysicists only consider spherically symmetric potentials or fail to consider the effects of a disks etc., which is just patently untrue.
    - idlewords 9y ago
      And here I thought Hacker News was going to get the Nobel Prize! I can't believe quickly deriving something from first principles also occurred to the people devoting their lives to this field.
    - phkahler 9y ago
      Thanks for the link. They have a .pdf of chapter one which I gave a look. Right there on page 14 when estimating the mass of our galaxy inside our suns radius we have: "The approximation that the mass distribution is spherical is reasonable for the dark halo, but not for the flat stellar disk. Better models suggest that this estimate is probably high by about 30%, since a disk requires less mass to produce a given centripetal acceleration" And there you have it. While he notes that a spherical distribution is different than a flat disk, he estimates the error at about 30%. The problem is that the error is not one of a constant factor, it's a different function. This unspecified better model also doesn't seem to care about the matter outside the solar radius - which also affects our velocity in a disk, but not in a sphere. There is a lot of math in chapter one, but also a lot of hand waving.
- deleted 9y ago
  [deleted]
- scythe 9y ago
  Of course you are completely wrong: http://iopscience.iop.org/article/10.1086/508162/meta http://iopscience.iop.org/article/10.1086/508162/meta There is a great deal of evidence for dark matter that does not depend on Kepler's laws, please stop spreading misinformation. Source: assistant editor of a physics journal, know several astrophysicists personally, currently dealing with a MOND paper this very week...
- wallace_f 9y ago
  I don't mean to just be a pedant when I say this, but the use of the word crackpot is missplaced here. Crackpot theories are eccentric and not commonly believed. I am interested in crackpot theories because sometimes they end up turning out right, and I think within that dynamic there are important lessons for humanity that we end up learning in very hard ways. For example, the Great Depression paved the way for the acceptance of Keynesian economics--ideas previously written-off as crackpot theories. Another: NASA engineers warned of the exact dangers of a foam strike, under the exact conditions in which it took place, were at the time written-off by senior management. Social scientists that studied this described part of the problem as Groupthink.