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I don't understand why people think dark matter is "hairy." It's the simplest explanation imaginable - only one new particle. We used to discover particles all

I don't understand why people think dark matter is "hairy." It's the simplest explanation imaginable - only one new particle. We used to discover particles all the time, and this idea that we somehow must have already found them all is a recent product of the end of accelerator growth.

denton-scratch 5y ago
I think it's "hairy" because it's not an explanation - the proposed particle is defined (more-or-less) as the error-factor in an equation. Show me the particle, I'll reconsider my views.
- whatshisface 5y ago
  I know it sounds like a fudge factor that can be made to do anything, but the truth is that particle models of weakly interacting dark matter can only be made to do a few things. If any distribution of matter imaginable could be created through dark matter, you're right, that would be an empirical fudge factor, but only a few distributions of matter can be created this way, and it's noteworthy that one of them matches the one in real life. An example of a limitless fudge factor is epicycles. They could be made to do anything - now through our understanding of the Fourier transform, we can make them draw elephants. That's not what CDM is like because it can't be made to draw elephants.
  - denton-scratch 5y ago
    Thank you. I didn't know that epicycles had the creative expressiveness to generate elephants, I'm impressed, and I'll have to dig into that - sounds interesting. I thought that Ptolemy's epicyle ideas were defeated precisely because they were not able to explain observations.
    - whatshisface 5y ago
      This isn't an elephant but it demonstrates the principle: https://www.youtube.com/watch?v=QVuU2YCwHjw https://www.youtube.com/watch?v=QVuU2YCwHjw Epicycles were originally considered a good idea because it takes exactly two circles to make an ellipse, and Kepler's laws were satisfied by such an ellipse. They were "falsified,", in the two-cycle form, when closer observations revealed that Kepler's laws were not obeyed exactly. At that point, the choice was to either add more epicycles (working your way up to an elephant), or for Newton to discover universal gravitation.
    - stordoff 5y ago
      Drawing an elephant: https://i.stack.imgur.com/tOU5r.gif https://i.stack.imgur.com/tOU5r.gif Taken from this StackExchange post: https://mathematica.stackexchange.com/questions/171755/how-can-i-draw-a-homer-with-epicycloids https://mathematica.stackexchange.com/questions/171755/how-c...
      - denton-scratch 5y ago
        That's magnificent! Thanks! So it looks as if you can draw anything with epicycles. But is that appearance correct? I mean, has anyone ever looked into it? Can it be proved that you can draw anything with epicycles? Relatedly: Is that elephant-drawing machine capable of drawing anything? And relatedly-relatedly, is the idea of a Turing-complete spirograph totally off-the-wall? (Don't worry, I already take loads of medication!)
        vecter 5y ago
        Fourier Series [0] (infinite sums of sines and cosines) can be made to equal any "well-behaved" function. This is the same idea in more dimensions. [0] https://en.wikipedia.org/wiki/Fourier_series https://en.wikipedia.org/wiki/Fourier_series
        denton-scratch 5y ago
        OK, I get it. Thanks. [Edit] Hang on - so why can't you use epicycles to draw the actual orbits of planets? (modulo the n-body problem) If epicyles is equivalent to fourier, and can model any "ordinary" function, then I'm surprised someone hansn't made an epicycle model that's equivalent to the Newtonian model. Just wondering.
        tene 5y ago
        You can use epicycles to draw the actual orbits of planets. That's what "epicycles" are, or were: a fairly accurate model of the observed orbits of planets relative to Earth built through a composition of circular motions. One way to think about it is that you start by modeling the motion as a simple circular orbit. You then look at how the observed data differs from your circular model and add an epicycle whose parameters minimize the difference between your new model and your observations. Repeat this process until you get as close as you want to get. The success of heliocentric elliptic-orbit model isn't that it's able to represent something impossible with epicycles, but that it's dramatically simpler. Kepler's three laws are: 1) The orbit of a planet is an ellipse with the Sun at one of the two foci. 2) A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3) The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit. In contrast, the epicycle models had 40-80 cycles with parameters tuned to match the observed data. I really like this video on how Fourier series work: https://www.youtube.com/watch?v=r6sGWTCMz2k https://www.youtube.com/watch?v=r6sGWTCMz2k
- canjobear 5y ago
  What would you prefer? For scientists to throw up their hands and say there’s a discrepancy whose cause is unknown but it sure looks like a weakly interacting particle? Oh wait, that’s the definition of dark matter
  - Rury 5y ago
    Perhaps the problem with some people is in semantics. If that's the definition of dark matter, why not use a better term that means "something that influences things on galactic scales that we can't yet explain". Calling it 'dark matter', implies or assumes that it must be only be what constitutes as 'matter' that's behind the cause. It closes many minds to considering the possibility that something that has no mass or volume could be behind the cause. You know something 'non-mattery'? And that begs the question, if has 'non-mattery' attributes in behavior, perhaps calling it 'matter' in definition is a bit misleading?
  - whatshisface 5y ago
    They'd prefer a bunch of particle experiments be constructed to approach the problem from a laboratory perspective, which frankly no reasonable person would mind. :)
    - canjobear 5y ago
      Such experiments have been going on for decades.
- mr_mitm 5y ago
  The neutrino was discovered the same way. Conservation of momentum appeared to be violated, so we invented a new particle to save the conservation law. Ironically, neutrinos are one type of dark matter. Hot dark matter. I'm not saying that it's not a problem that we haven't seen cold dark matter in the lab yet. But the fact that dark matter sceptics are almost always not astrophysicists should tell you something.
  - denton-scratch 5y ago
    Hmmm. <scurries off to get white lab-coat> Ah. No white lab-coat in the wardrobe. And I'm not an astrophysicist, might as well just flag me. Look, I'm not a "sceptic"; that's like climate sceptic, i.e. non-believer zealot. I don't have a conviction about DM; I'm just waiting to be convinced.
    - Tagbert 5y ago
      I think most of us are waiting to be convinced. Current models don’t explain everything but the standard model and DM explain more than any competing theory so far. That’s why we keep investigating and theorizing to see if we can come up with something better. Eventually the current models are likely to be replaced by something else. We just don’t know yet if that something else is an extension of the current models or something out of the blue.
lend000 5y ago
I think it's because we aren't talking about some exotic, rare, or weakly interacting particle. We are talking about either an extremely common or extremely strongly interacting (or both) variant of matter which we somehow can't observe or produce in a lab despite observing much less common phenomena. It reeks of failures in other models.
- whatshisface 5y ago
  Non-interacting means non-interacting with detection equipment, but it also means non-interacting with production equipment. Quantum mechanics is time-reversible, so "can't detect (absorb) it" = "can't produce (emit, i.e. absorption in reverse) it." It's a very parsimonious thing.
  - lend000 5y ago
    Yeah, but if dark matter exists, it clearly is NOT non-interacting. It should certainly be easier to detect than a massless neutrino (which we can detect convincingly now).
    - whatshisface 5y ago
      In particle physics, interaction with forces make roughly independent parameters. For example in a world with only electromagnetism, we would think neutrons were undetectable. The idea that there is a particle that only interactions gravitationally is hardly at odds with all of the other neutral particles we know.
      - adrian_b 5y ago
        Your point is correct, but the example is not good, because neutrons interact electromagnetically, due to having a non-null magnetic moment. While the total electric charge of the quarks composing the neutron sums to 0, their movements are such that the magnetic effects do not sum to 0. The particles that we know do not have a random collection of properties, they all belong to a consistent system. For example in the set of 8 particles containing the 6 kinds of u and d quarks, the electron and the neutrino, the sum over the 8 particles of all the kinds of quantum numbers, e.g. spin, electric charge, color charge is null. If some new particle interacting only gravitationally will be discovered, it will not fit in any way between the known particles so it must belong to some class of particles totally different from anything we know about until now. Moreover, while it seems natural to assume that the dark matter if it exists must be composed of particles, like the matter we know, there is no evidence for this, so the dark matter might be some sort of continuous gravitational ether, for all we know.

denton-scratch 5y ago

I think it's "hairy" because it's not an explanation - the proposed particle is defined (more-or-less) as the error-factor in an equation. Show me the particle, I'll reconsider my views.

whatshisface 5y ago
I know it sounds like a fudge factor that can be made to do anything, but the truth is that particle models of weakly interacting dark matter can only be made to do a few things. If any distribution of matter imaginable could be created through dark matter, you're right, that would be an empirical fudge factor, but only a few distributions of matter can be created this way, and it's noteworthy that one of them matches the one in real life. An example of a limitless fudge factor is epicycles. They could be made to do anything - now through our understanding of the Fourier transform, we can make them draw elephants. That's not what CDM is like because it can't be made to draw elephants.
- denton-scratch 5y ago
  Thank you. I didn't know that epicycles had the creative expressiveness to generate elephants, I'm impressed, and I'll have to dig into that - sounds interesting. I thought that Ptolemy's epicyle ideas were defeated precisely because they were not able to explain observations.
  - whatshisface 5y ago
    This isn't an elephant but it demonstrates the principle: https://www.youtube.com/watch?v=QVuU2YCwHjw https://www.youtube.com/watch?v=QVuU2YCwHjw Epicycles were originally considered a good idea because it takes exactly two circles to make an ellipse, and Kepler's laws were satisfied by such an ellipse. They were "falsified,", in the two-cycle form, when closer observations revealed that Kepler's laws were not obeyed exactly. At that point, the choice was to either add more epicycles (working your way up to an elephant), or for Newton to discover universal gravitation.
  - stordoff 5y ago
    Drawing an elephant: https://i.stack.imgur.com/tOU5r.gif https://i.stack.imgur.com/tOU5r.gif Taken from this StackExchange post: https://mathematica.stackexchange.com/questions/171755/how-can-i-draw-a-homer-with-epicycloids https://mathematica.stackexchange.com/questions/171755/how-c...
    - denton-scratch 5y ago
      That's magnificent! Thanks! So it looks as if you can draw anything with epicycles. But is that appearance correct? I mean, has anyone ever looked into it? Can it be proved that you can draw anything with epicycles? Relatedly: Is that elephant-drawing machine capable of drawing anything? And relatedly-relatedly, is the idea of a Turing-complete spirograph totally off-the-wall? (Don't worry, I already take loads of medication!)
      - vecter 5y ago
        Fourier Series [0] (infinite sums of sines and cosines) can be made to equal any "well-behaved" function. This is the same idea in more dimensions. [0] https://en.wikipedia.org/wiki/Fourier_series https://en.wikipedia.org/wiki/Fourier_series
        denton-scratch 5y ago
        OK, I get it. Thanks. [Edit] Hang on - so why can't you use epicycles to draw the actual orbits of planets? (modulo the n-body problem) If epicyles is equivalent to fourier, and can model any "ordinary" function, then I'm surprised someone hansn't made an epicycle model that's equivalent to the Newtonian model. Just wondering.
        tene 5y ago
        You can use epicycles to draw the actual orbits of planets. That's what "epicycles" are, or were: a fairly accurate model of the observed orbits of planets relative to Earth built through a composition of circular motions. One way to think about it is that you start by modeling the motion as a simple circular orbit. You then look at how the observed data differs from your circular model and add an epicycle whose parameters minimize the difference between your new model and your observations. Repeat this process until you get as close as you want to get. The success of heliocentric elliptic-orbit model isn't that it's able to represent something impossible with epicycles, but that it's dramatically simpler. Kepler's three laws are: 1) The orbit of a planet is an ellipse with the Sun at one of the two foci. 2) A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. 3) The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit. In contrast, the epicycle models had 40-80 cycles with parameters tuned to match the observed data. I really like this video on how Fourier series work: https://www.youtube.com/watch?v=r6sGWTCMz2k https://www.youtube.com/watch?v=r6sGWTCMz2k
canjobear 5y ago
What would you prefer? For scientists to throw up their hands and say there’s a discrepancy whose cause is unknown but it sure looks like a weakly interacting particle? Oh wait, that’s the definition of dark matter
- Rury 5y ago
  Perhaps the problem with some people is in semantics. If that's the definition of dark matter, why not use a better term that means "something that influences things on galactic scales that we can't yet explain". Calling it 'dark matter', implies or assumes that it must be only be what constitutes as 'matter' that's behind the cause. It closes many minds to considering the possibility that something that has no mass or volume could be behind the cause. You know something 'non-mattery'? And that begs the question, if has 'non-mattery' attributes in behavior, perhaps calling it 'matter' in definition is a bit misleading?
- whatshisface 5y ago
  They'd prefer a bunch of particle experiments be constructed to approach the problem from a laboratory perspective, which frankly no reasonable person would mind. :)
  - canjobear 5y ago
    Such experiments have been going on for decades.
mr_mitm 5y ago
The neutrino was discovered the same way. Conservation of momentum appeared to be violated, so we invented a new particle to save the conservation law. Ironically, neutrinos are one type of dark matter. Hot dark matter. I'm not saying that it's not a problem that we haven't seen cold dark matter in the lab yet. But the fact that dark matter sceptics are almost always not astrophysicists should tell you something.
- denton-scratch 5y ago
  Hmmm. <scurries off to get white lab-coat> Ah. No white lab-coat in the wardrobe. And I'm not an astrophysicist, might as well just flag me. Look, I'm not a "sceptic"; that's like climate sceptic, i.e. non-believer zealot. I don't have a conviction about DM; I'm just waiting to be convinced.
  - Tagbert 5y ago
    I think most of us are waiting to be convinced. Current models don’t explain everything but the standard model and DM explain more than any competing theory so far. That’s why we keep investigating and theorizing to see if we can come up with something better. Eventually the current models are likely to be replaced by something else. We just don’t know yet if that something else is an extension of the current models or something out of the blue.

lend000 5y ago

I think it's because we aren't talking about some exotic, rare, or weakly interacting particle. We are talking about either an extremely common or extremely strongly interacting (or both) variant of matter which we somehow can't observe or produce in a lab despite observing much less common phenomena. It reeks of failures in other models.

whatshisface 5y ago
Non-interacting means non-interacting with detection equipment, but it also means non-interacting with production equipment. Quantum mechanics is time-reversible, so "can't detect (absorb) it" = "can't produce (emit, i.e. absorption in reverse) it." It's a very parsimonious thing.
- lend000 5y ago
  Yeah, but if dark matter exists, it clearly is NOT non-interacting. It should certainly be easier to detect than a massless neutrino (which we can detect convincingly now).
  - whatshisface 5y ago
    In particle physics, interaction with forces make roughly independent parameters. For example in a world with only electromagnetism, we would think neutrons were undetectable. The idea that there is a particle that only interactions gravitationally is hardly at odds with all of the other neutral particles we know.
    - adrian_b 5y ago
      Your point is correct, but the example is not good, because neutrons interact electromagnetically, due to having a non-null magnetic moment. While the total electric charge of the quarks composing the neutron sums to 0, their movements are such that the magnetic effects do not sum to 0. The particles that we know do not have a random collection of properties, they all belong to a consistent system. For example in the set of 8 particles containing the 6 kinds of u and d quarks, the electron and the neutrino, the sum over the 8 particles of all the kinds of quantum numbers, e.g. spin, electric charge, color charge is null. If some new particle interacting only gravitationally will be discovered, it will not fit in any way between the known particles so it must belong to some class of particles totally different from anything we know about until now. Moreover, while it seems natural to assume that the dark matter if it exists must be composed of particles, like the matter we know, there is no evidence for this, so the dark matter might be some sort of continuous gravitational ether, for all we know.