I've never really understood this visualization. It's a cheat in so many ways (wrong dimensionality, reliance on actual gravity to work, etc.). While such "gee-whiz" demonstrations are great at getting people excited about science, I worry that they create false expectations. They often produce the <i>feeling</i> of understanding without conveying <i>actual</i> understanding (and the limitations thereof).<p>The unfortunate truth is that many subjects in physics, including general relativity, are impossible to visualize in the conventional sense. I learned GR from Kip Thorne at Caltech and wrote my Ph.D. dissertation on the dynamics of rotating (Kerr) black holes, and my intuition for the subject is still fuzzy and inchoate. The best you can do is have a cobbled-together half-picture that kind-of, sort-of works, some of the time. Human brains simply are not designed to understand, at a deep intuitive level, things like the geometry of four-dimensional pseudo-Riemannian manifolds.
Mr. Burns the best science teacher ever. I remember he introduced the concept of Hertz to a half a sleep class by introducing the Avis and proceeding to casually talk about frequencies of Avis. After about 30 seconds he could tell which of us had read the chapter assigned the night before and which hadn't based on the laughter... His classes were the best.
This visualization always bothered me. The marbles 'stick' to the surface, but only because of the gravity pulling them from underneath. In the absence of gravity, marbles coming in from the outside could just continue in a straight line parallel to the ground, unaffected by the mass in the middle.<p>The only way I've been able to reconcile this in my mind is to think that this is a visualization of gravity warping spacetime in a <i>2D</i> universe, and that is why the masses are stuck to the surface, but I've never seen that said in any of the explanations.<p>Am I right in thinking about it that way?
What an amazing way to explain this concept. I'm a visual thinker, and I always preferred these kind of explanations of concepts. I just wish I had teachers who taught more like this in my science subjects during Secondary School.
This is definitely a nice, intuitive way to introduce people to the theory of gravity. However, I was a little confused about how he tried to model dark energy.