Gravity: Only a Theory


Skeptic magazine published this comic about climate change, gravity, and what pseudoskeptics get wrong when they complain that something is “only a theory.” Since I recently had a troll on the blog claiming to be well-versed in science and yet not understanding what “theory” means in the natural sciences, I thought I ought to share it.

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  • arcseconds

    The observed precession of the perihelion of Mercury is 573 seconds of arc (no relation) per century. The precession predicted by a Newtonian model is 534″. So the discrepancy is ~43″ per century. That’s hardly “wild”!

    Me, I’m pretty slap-dash, I would have just said “meh! who cares about such a tiny amount over such a long time period! Newton FTW!’

    It was also recognised fairly late, in 1859. And even at that point it wasn’t clear that it was insoluble.

    One reason for this is that Philosophæ Naturalis Principia Mathematica didn’t immediately establish a ‘complete theory’ in the sense of a definitive set of predictions or something like that, but rather principles for building kinematic models, which were refined over time. When you have incrementally refined models (which had, of course, involved incorporating hitherto unknowns like entirely new planets) it’s not necessarily clear at which point you give up and start looking for a new theory.

    • arcseconds

      I mention this because a lot of people appear to think that the orbit of Mercury was just a complete failure of Newtonian physics from the start, (and this comic tends to reinforce that impression). If they know that it’s about the precession, they often think Newtonian physics can’t account for the fact it precesses at all.

      Whereas in fact all the planetary orbits precess, and the major reason for this is the gravitational influence of the other planets, which Newtonian physics is quite adequate for. Mercury is no exception, it’s just that it has a significant (but still small) contribution from relativistic effects.

      Also, do people know what a second of arc is? it’s a 360th of a degree. 43 seconds of arc is a little over 0.1 degrees.

      Per century.

      Even being able to detect that is an impressive feat.

      • Nick G

        Also, do people know what a second of arc is? it’s a 360th of a degree.

        Given your nym, I’m surprised you made this elementary blunder. I’d think it was just a typo if it wasn’t, in effect, repeated. A second of arc is a 3600th of a degree. 43 seconds of arc is a little over 0.01 degrees.

        • arcseconds

          I thought when I was writing that that really didn’t sound right, but what can I say? I’m not very good at arithmetic!

          And I even ‘checked’ it twice, in the sense I asked myself ‘Is a 60th of a 60th’ really 360th? Yep, sure is! seems larger than I expected, oh well’.

  • ButILikeCaves

    We prefer to call it “Intelligent Falling”

  • Jon-Michael Ivey

    Gravity is the least well understood of the 4 fundamental forces.

  • TomS

    How do you know? Where you there?
    Up until the space age, we had no “direct observation” of gravity except near the surface of the Earth. We just had Newton’s guess that it applied everywhere in the universe, and then, by “circular reasoning”, if it were true, then planets would move in such-and-such a way. Remote science is not science, whether it is remote by space or by time.
    Yes, I believe in “micro-gravity”, but even today, “micro-gravity”, beyond the Solar System?

    (BTW, the speed of fall by gravity on Earth is an acceleration of 32 feet per second per second, not as measured by a constant velocity.)

    • arcseconds

      I think the comic knows that gravity is a constant force, which therefore manifests itself as acceleration. The girl corrects the senator about the constant velocity thing.

  • Phil Ledgerwood

    If this is true, why is there still gravity?