- #1
- 8,520
- 16
From some previous discussions in this forum I had gotten the idea that under the equivalence principle, observations inside a small room sitting on the surface of a planet should be seen as equivalent, not just to any ol' accelerating room in flat spacetime, but to a room undergoing Born rigid acceleration; I thought, for example, that the assumption of Born rigid acceleration was needed to explain why clocks at the top and bottom of the room would have the same difference in ticking rates for both the room on the planet and the accelerating room. I've read that an object experiencing Born rigid acceleration actually measures G-forces at different points along it, so I had the idea that when people talked about "uniform" gravitational pseudo-fields in flat spacetime in the context of the equivalence principle, "uniform" just meant it would be experienced by an observer undergoing uniform acceleration in the Born rigid sense, not that the G-forces measured by accelerometers would actually be the same everywhere. But someone pointed out to me that in the Equivalence Principle Analysis on the Twin Paradox page, it says:
So, I guess I was probably wrong in some aspect of my understanding above, unless talk of a "uniform gravitational (pseudo) field" can have multiple meanings in different contexts...if I did go wrong somewhere though, can someone point out where?The Equivalence Principle analysis of the twin paradox simply views the scenario from the frame in which Stella is at rest the whole time. This is not an inertial frame; it's accelerated, so the mathematics is harder. But it can certainly be done. When the mathematics is described fully, what results is that we can treat a uniformly accelerated frame as if it were an inertial frame with the addition of a "uniform pseudo gravitational field". By a "pseudo gravitational field", we mean an apparent field (not a real gravitational field) that acts on all objects proportionately to their mass; by "uniform" we mean that the force felt by each object is independent of its position. This is the basic content of the Equivalence Principle.