Right. And that’s part of why I’m not a huge fan of the rubber sheet analogy. The sheet is supposed to reflect curved spacetime, but it doesn’t behave the right way. Objects moving on the sheet don’t follow geodesics, and the shape of the sheet doesn’t change based on the velocity of the object. Instead you get a weird mix of a deformed sheet and objects moving on non-geodesic trajectories (due to actual gravity).
I don’t think the demonstration is totally useless–it does work surprisingly well as a simulator, and I liked EdwinAmi’s video link above–I just don’t think it’s a great analogy for how curved spacetime actually works.
Maybe it’s possible to make it work in some other way, like with little magnetic cars that travel “forward” even on a curved surface. It’s still imperfect but it may better convey what it means for something to be a geodesic.
Probably not what the OP is looking for, but I’ve always enjoyed Feynman’s riff (paraphrased):
When the motion of planets was measured, theories were put forward to explain it. A popular one was that angels were pushing them along their elliptical orbits - it being obvious to many that without continuous pushing the planets would stop moving.
Then Newton came along to explain how motion can persist without continuous pushing, and how a force of gravity proportional to the product of the masses and the inverse square of their distance would yield elliptical orbits.
But we haven’t been able to figure out the mechanism of gravity. So, though we call them by another name it turns out we still believe angels are pushing - the only real change is that we believe the direction of their push is 90 degrees different to what was previously thought.
So far as we can tell, yes, they are mutually contradictory. And yet both give every indication of being true. In other words, we’re confused by it, too.
We’re probably not confused at the same point you are, though. If you go back to that (admittedly flawed) rubber sheet analogy, you can imagine excitations of that rubber sheet which propagate across the surface (particles, in other words): That far, at least, makes sense. The problem comes when you try to apply the mathematical structures used to describe other force-carrying particles to gravity, and find that many of them don’t work.
What we do know (or at least, think we know) about gravitons comes from gravitational waves, which can be described in terms of either curvature or of particles. So, for instance, the general-relativistic description of gravitational waves tells us what the polarization states of the waves must look like, and the particle description tells us that particles with those polarization states must be spin 2. And the GR description of gravitational waves tells us that they travel at c, and the particle description tells us that they must therefore be massless. But that’s about the limit of what we know.
Well, if you can go from Newton to Keppler, my guess is that you know more math than the OP does. I never denied the role that Newton’s laws play or said they’re not true. What I said is that, from the perspective of a layman having conceptual difficulties, pointing out Newton’s laws is not going to clarify the situation.
I think what I was trying to say (before I got excited by the phrase “mutually contradictory”) is that the geometric explanation of gravity seems to obviate the need for a particle description of gravity.
I know that everyone’s trying to unify relativity and quantum, but it doesn’t seem that these two statements say the same thing at all:
-“ripples in the fabric of spacetime pulls everything together”
-“the general curvature of spacetime pulls everything together”
That’s cool – I’d wondered about that, and the video answers it.
So, next question: why are most orbiting objects in a plane? Is that due to initial conditions, or like the orbit direction, is it a consequence? I suspect the latter; starting with a cloud of stuff orbiting the common center but whatever plane has the most mass ends up attracting all the rest of the stuff to it. Though, why they don’t oscillate in the north/south direction mystifies me … stars orbiting around the galaxy center do that.
When a number of galaxies collide/join, they tend to create an elliptic. Does an elliptic wind up as a spiral, or stay elliptic?
It’s more about angular momentum than the mass distribution. The gas/dust cloud that formed the solar system had some degree of initial rotation, and as everything collapses this angular momentum becomes more concentrated. So not only is everything in the same plane, but (mostly) everything is also rotating in the same direction.
It stays elliptic. Elliptic galaxies are basically what you get when you scramble a more structured galaxy (i.e., spiral, barred, etc.). There’s no tendency for the stars to start forming structures again, and they’ll just eat any incoming structure.
Depends on what you mean by “need”. For any situation which we can expect to produce in a laboratory, or which we expect we might actually find naturally occurring in the Universe, at least one of general relativity or quantum mechanics is adequate for describing it, since at most one of those two theories is relevant. But neither of those theories prohibits the existence of situations where they’re both relevant.
For instance, suppose you have a black hole. The theories we have now are sufficient to predict that they’ll produce Hawking radiation and evaporate (this does actually involve some mixing of GR and QM, but in a way that’s innocuous: You basically assume that the particles are quantum, but living in a non-quantum spacetime). Just wait, and the mass of the black hole will decrease as a result of this radiation. It’ll take an extremely long time for the sort of black holes we expect exist, so we don’t expect to ever actually observe it, but it’ll happen. Under the current theories, the smaller a black hole gets the faster it’ll evaporate, and it’ll produce more energetic and more massive particles in the process. Except the current theories don’t actually put any limit on this: Extrapolate them out, and you would eventually get a black hole that’s so small that it’s emitting particles more massive than itself.
This is crazy, so clearly our current theories are incomplete: The proper description must include some way of making emission of such particles non-crazy, or of modifying the behavior of a hole that small so that it doesn’t emit particles larger than itself, or of somehow preventing a black hole from ever reaching a size where it would do that. But any of these expansions to the theory would require a fundamental mixing of GR and QM: You have to have some sort of quantum behavior of spacetime itself in order to achieve any of these effects.
Now, just exactly what form that quantum behavior of spacetime would take, well, we’re pretty close to clueless on that. But it’s got to be something.
Does QM require gravity to be mediated by a particle? Or does QM only apply to things that are particles? Or is whatever QM calls a particle not necessarily what we tend to think of as a particle, so the designation isn’t really that significant, other than to attribute quanta?
Well, this one is certainly true. Much of quantum mechanics does not conform to how we tend to think.
On your first question, though, I would not so much say that QM requires that things be mediated by particles, but rather that it states that however anything is mediated, can be described in the same terms that are used to describe particles.