Franz Kafka Abducted by ET Alien Grays on a Flying Saucer
Franz Kafka, feeling like a cockroach under a magnifying glass, wakes up having been abducted by Gray cyborgs. He is in a closed room with no windows. He manages to undo the straps of the table bolted to the floor and he begins to float off the table. He holds on to one of the straps. Being a good physicist as well as story writer, Kafka knows instantly he is on a timelike geodesic out in space. Warp drive is the same experience the difference is that the Captain can control his ship's geodesic glide path to point in any direction and speed he likes without feeling any g-forces at all because no propellant is ejected in the maneuvers including sharp turns in space even instant U-turns. This is metric engineering the dark energy-matter envelope around the ship.
Kafka manages to find a port hole cover. He opens it. To his horror he sees that the ship is falling toward the Sun! That explains why it's been getting hotter. He sees a control panel on the side of the table he had been strapped to. He pulls on the strap to get closer to it. In desperation he starts playing with a joystick and suddenly he is pulled to the table. He feels weight returning. The ship's normal impulse swivel rocket engines have been switched on. He plays with the stick and some of the knobs and finally through his psychic intuition he finds a setting in which the ship stands still relative to the distant Sun that fortunately is still as far away as Venus. Indeed looking in a different direction he can see the planet Venus. The zero point energy powered rocket engines must continually fire to keep the ship hovering at a fixed distance from the Sun and from Venus. This hovering fixed point is a non-geodesic path through spacetime. Having wiped the sweat off his brow, he sees a spigot on the table side and is able to get some sweet-tasting water that instantly gives him a sense of well-being and relaxation in addition to a feeling of great muscular strength and youthful vitality. At that moment three Grays enter the room. One delicate one that seems more feminine than the other two says to him telepathically without sound "That was nicely done Professor Kafka. We apologize for putting you under stress, but you were in no real danger. Would you like us to explain to you how our ship works." Kafka answers - thinking telepathically "Is The Pope Catholic?"
The Equivalence Principle
What Einstein meant, and what everyone in physics still means today, by the equivalence principle in simplest form is:
"There is no local way to distinguish a real gravity "force" from an inertial "fictitious" g-force."
You have to carefully define "local" here, and what "fictitious" inertial forces are. Step hard on the accelerator of a powerful sports car - you feel the "fictitious" inertial g-force. Stand still on the floor. You feel your weight - gravity "force."
Inertial forces are caused by the acceleration of the non-inertial rest frame of reference of the object feeling the inertial force. Note the counter-intuitive way of using "inertial forces" in "non-inertial frames." Unfortunately that is what Murray Gell-Mann calls a contingent unfortunate "frozen accident of history." It's bad English. We should call them fictitious non-inertial forces in non-inertial rest frames, but physicists do not - perhaps to confuse the masses if one wants to get conspiratorial. Note when you sit in the rigid car you are at rest relative to the car. The car is your non-inertial rest frame. Note that the word "inertial" roughly translates to "timelike geodesic". Given two point events in a 4D curved space-time. Imagine all possible neighboring paths, or world lines, connecting the same two points. The geodesic is the longest one among them in a space-time. This leads to the the relative time dilation of two twins. The one who takes a non-geodesic between the two points will be younger than the one who takes the geodesic. You cannot move off a geodesic unless a non-gravity force operates and when you do the induced "g-force" felt as "weight" is a reaction to that non-gravity force. Therefore, in fact there is no such thing as a pure gravity force that could exist without a non-gravity force like an electric force on a charge.
Dual to the above, jump off a high place (in imagination unless you are a University of California Chancellor stealing money from the students like ENRON execs from stockholders and employees - then by all means be my guest ;-)) you will feel weightless - the gravity force disappears.
"Local" is defined in terms of idealized non rotating uncharged test particles. A test particle is acted upon but does not directly react back on the field it is detecting.
"Local" here means using only ONE test particle.
One litmus test of a real gravity field is NONLOCAL only done using a pair of test particles each in geodesic free fall close to each other. You measure their relative acceleration toward or away from each other. That tells you a component of their curvature tensor. The gravity field is real only if that relative acceleration is there and not zero for all possible directions at every point in a spacetime region.
There is no g-force in the above curvature measurement gedankenexperiment.
As mentioned above, the only way you feel a gravity force locally equivalent to an inertial g-force is if the test object is pushed off the free float geodesic path in the curved spacetime by some non-gravity force. This is done by electrical forces pushing on our feet when we stand in reaction to us pushing down on the ground. We are pushed off the natural geodesic path in the curved spacetime and that's why we feel weight - it's locally same as stomping on the accelerator of a Jaguar. The surface of the Earth is our local noninertial rest frame (LNIF).
Now there are a lot of deeper aspects of this having to do with the local gauging of the global symmetry of translations of source objects giving a compensating "connection" for parallel transport of oriented test particles that is essentially the inertial g-force in LNIFs. One way to tell if you are in a real gravity field is if you have to fire a rocket in space to keep at a fixed distance from the object. If you switch the rocket off you are weightless. That is, gravity fields emerge from the breakdown of the global homogeneity of the action of the source object, The gravity field in a sense restores the lost homogeneity symmetry by adding new geometrodynamic degrees of freedom to the action in addition to the degrees of freedom of the original source object.
All forms of mass-energy density, pressures and stressed induce a real gravity field with a non-vanishing invariant curvature pattern. I mean ALL FORMS including virtual zero point quantum inside the vacuum, i.e. both repulsive dark energy and attractive dark matter are virtual exotic vacuum phenomena as distinguished from ordinary real matter of atoms, magnetohydrodynamic plasma, radiation, neutrinos - possible exotic particles. When you put a slab of dark energy near a slab of dark matter with equal and opposite zero point energy pressure you have a weightless geodesic warp drive and that's basically how the "saucers" fly.