The dark matter dialogues – Day 1

NGC1300 imaged by NASA

Spiral Galaxy NGC1300. Galaxies like this one are believed to be surrounded by a halo of dark matter. Courtesy of NASA, ESA and The Hubble Heritage Team (StScI/AURA)

An email exchange with Dr Bob Rich, a psychologist and science fiction author with no training in astronomy, could help to illuminate the mysteries of dark matter. Here is day one of our discussions:


I’ve been thinking about dark matter since we talked about that. Looked it up in Wikipedia. One of the articles there said that lumps of it regularly pass through the sun and the earth, and we don’t even know it. But, if it is affected by gravity, won’t that change orbits if the lump is big enough, and be captured and add to mass if it is small and/or slow enough? This sounds to me like a testable thing. The total mass of the earth should be what we can sense, + what has been captured over millions of years.


Good point. The paths of the Earth and of the other planets are continously monitored to phenomenal accuracy and there is no deviation from the motion predicted by the gravity of the matter we can see in the Solar System. This indicates that there are no lumps of dark matter passing through. Dark matter is probably far more spread out with extremely low density in terms of kilograms per cubic meter or even per cubic light year. However, as space is so big even matter with a very tiny density will build up to large measurable masses in terms of galaxy-sized objects. Neutrinos are possible constituents of dark matter – these travel at the speed of light and have only recently been shown to have extremely small, but non-zero masses.


So, you don’t think there are stars and planets and things made up of dark matter?


Not small things like stars and planets but maybe large things equivalent in size to galaxies.

5 responses to “The dark matter dialogues – Day 1

  • Nick,
    Perhaps, I should have made it more clear. At my point 9) what I
    said was Newtonian Law and dynamics must be preserved by
    the motions of visible matter. But that is exactly what we do not
    observe in real life. The rotation curves are flat and non Newtonian.
    It is as if some how mass and gravity are both modified. Like I said,
    we need a new alternative.

    /Tissa Perera

  • No Tissa. No. No. No. If you make 90% of the matter in an N-body simulation invisible, the remaining 10% will continue to be affected by the invisible 90%. Any watching observer could only explain the motion of the 10% by taking into account the invisible 90%. Dark matter does matter.

  • A Dark Matter thought experiment.

    1) Dark Matter was invented to augment the observed motion of
    real visible matter, right? right!
    2) Dark Matter interacts with real matter via gravitational forces
    only, right? right!
    3) Dark Matter too interacts with Dark Matter gravitationally
    right? right!
    4) Therefore Dark Matter must take part in the orbital dynamics
    of the system, right? right!
    5) The only difference is that Dark Matter cannot be seen, other
    wise, it takes part in the gravitational dynamics of the system,
    which is purely Newtonian motion, right? right!

    Now that we agree, let us do a pure thought experiment.
    Let us do an N-Body simulation, where N will be in the billions.

    6) All N Bodies will therefore do a Newtonian Samba. Each of the
    N Bodies will clearly have rotational motion velocities defined
    by Newton’s laws. Only Newton’s Law accounts for their motions.
    7) Now we will use some magic, we will make 90% of the N bodies
    invisible, and call them Dark Matter!
    8)What do you think would happen to the 10% that are still visible?
    9)You guessed! they will keep moving as before and still obey
    Newtonian Law’s with no velocity discrepancies.
    10)Moral of the story is, Dark Matter don’t matter.
    11)There must be another way! I have a hypothesis for new
    alternative to Dark Matter. See cosmicdarkmatter.com

    /Tissa Perera

  • Well, I don’t think the lumps (or clumps, as they are also called — regions of higher density of dark matter) would have a density high enough to be noticed through local gravitation effects.

    But Bob is right about the accumulation of dark matter particles. Indeed, if the dark matter consists of weakly interacting massive particles, then some of them will interact and be gravitationally captured in bound orbits in the solar system. Through the lifetime of the sun, a portion of these particles will scatter multiple times and lose enough energy to sink to the core of the sun. Capture in the planets also occurs, but less efficiently. So it’s right, the total mass of the sun (or planets) includes some dark matter — only not enough to be obvious from the mass alone. If the density gets high enough the dark matter particles will annihilate with eachother, so there is a maximum, equilibrium density. The solar system is not old enough to have reached equilibrium yet, according to the best estimates.

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