Ethan takes on Johannes Kepler

Can you believe that I had a fight today with someone who’s been dead for over 350 years, and I’m losing? — Ethan, yesterday

Of course you can believe it, when the man I’m fighting with is Johannes Kepler. I don’t get a chance to tell you about my research very often, mostly because it’s still a work in progress. But my latest paper was just submitted and is now out of the way, and so I’d like to tell you what I’m working on at the moment.

Well, there we are in the galaxy. We look up at the night sky, and we see our planets as well as all the stars that surround us. But you know what we don’t see? Dark Matter. We know it’s there, in a giant halo around our galaxy, the same way we find evidence for it in all galaxies. So I can pretty easily figure out how much dark matter is in the galaxy where our solar system is:

My problem is, if I just did that calculation, I’d get the wrong answer! Why? Because we’re not just lazing around in our galaxy; our Sun, with our four inner, rocky planets and our four gas giant outer planets have been zipping around through the galaxy for over four billion years. In that time, they’ve had plenty of opportunity to scoop up dark matter, like a vacuum cleaner. And that’s why I’m having a fight with this guy:

Turns out this problem of figuring out how much dark matter is in our Solar System now is actually pretty hard. But if I can figure out how the dark matter is moving, how the solar system is moving, and how gravity works to capture some of that dark matter, I win. How? Because I’ll be able to predict how much dark matter we should have surrounding our Sun, and I can compare it with observations to see if it matches!

But first, I have to figure out a way to beat this Kepler guy. My strategy?


Well, that’s not going to work. I’ll have to buckle down and solve it with a combination of physics and astronomy, using calculations and writing simulations. But that’s what I’m working on, and if I can solve this, then for the very first time, we’ll know how much dark matter we have in our own solar system! And that’s pretty damned cool!


  1. #1 dave
    May 2, 2008

    Good luck, Dark Matter Master!

  2. #2 Phildo
    May 2, 2008


    I think I fall in love with a little more each day.


  3. #3 benhead
    May 2, 2008

    That’s great. What does it have to do with Kepler?

  4. #4 Brian
    May 2, 2008

    I was wondering the same thing. How does the post tie in with Kepler, exactly?

  5. #5 ethan
    May 2, 2008

    All of this analysis, all of the calculations I have to do, are contingent on 2- and 3-body gravitational interactions between the sun, a planet, and a particle of dark matter. “Central force motion” is what physicists refer to as “the Kepler problem,” since Kepler solved it. That’s why he knew that the planets moved around the Sun in ellipses.

    And yet, I’m trying to get these dark matter particles to go from hyperbolic orbits (unbound) to elliptical orbits (bound), and I am fighting with some ungodly complicated stuff to do it. So it’s Ethan vs. Kepler, in a battle for the dark matter.


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