Notice that I said that the surface is accelerating upwards, not that it’s expanding (which it’s not). The fact that you can have the one without the other is a consequence of the curvature of space. I won’t try to appeal to your intuition on this one, because unless your intuition is a darn sight better than mine, it isn’t intuitive at all. But in the context of General Relativity, gravity is a fictitious force, just like the centrifugal force. This is not true, by the way, of (say) the electromagnetic force, because electromagnetism can act in different ways on objects of the same mass.
For the record, there are only 4 forces…all others are built from them… this should be physics 101…
1)Strong Nuclear force
2)Weak Nuclear force
3) Electromagnetic force
4) Gravitational force
Don’t believe me? Do a google search for… 4 fundamental forces see how many HUNDREDS of universities teach their physics students this…
As far as General Relativity, it covers Gravity… but sepearte from the other forces, as there was no clear reason why it existed… BUT it exists…
No other forces are known to exist PERIOD…
Oh. Uh… Huh. I see. OK.
Well, that’s all well and good then, but I seem to recall reading somewhere that Einstein said that gravity was due partly to the curvature of spacetime by mass and partly to an attractive force between masses. But I don’t have a cite, so maybe my memory is playing tricks on me. Still… what’s driving the acceleration, if not a (“real”) force?
EEMan, it’s true that forces like friction and the normal force, for example, are due to the fundamental forces acting on the particles that make objects up, much in the same sense that the objects we observe are really just collections of electrons, quarks, etc. But we find it useful to view things on a more macroscopic level, so we treat these things as real, even though they’re really just simplifying abstractions. So you could say that non-fundamental forces don’t actually exist in reality, but only in the sense that flowers don’t actuall exist in reality.
But centrifugal force only exists in a non-inertial reference frame, so it’s not just a “composite” force; it’s a fictitious force.
My comment was to respond to the Gravity doesn’t exist comment…
Infact Gravity is one of only 4 forces to exist…
As far as using composite forces, that is fine a dandy (I’m an electrical engineer… EE man get it…) but to say Gravity is fictious is wrong… it exist… in fact it is one of the few that does exist…