Stephen A. Lawrence wrote: > > > Harry Veeder wrote: >> Stephen A. Lawrence wrote: >> >>> When you are actually _in_ a rotating frame, such as a car going around >>> a corner, you naturally "think" about the situation from the POV of that >>> frame, and in that frame, the centrifugal force -- and the Coriolis >>> force -- are both quite real, even though they are /called/ "fictitious" >>> forces. >> >> In orbit about the Earth you don't feel a centrifugal force. > > Well, sure, but then it's balanced by gravity. And besides, you never > "feel" a centrifugal force; you just feel the /centripetal/ force, which > is the "real" one. > > In GR gravity is also considered to be an "inertial" force (or a > "fictitious" force). It has all the same properties as other > "fictitious" forces, including, most significantly, that it vanishes in > a locally inertial frame. Equally significant -- and necessary, if it > is to vanish in any frame -- is that gravity affects all materials > equally. If any divergence between inertial and gravitational mass is > ever found, however small it may be, it will be a an enormous blow to > the validity of GR, because it will imply that gravity is /not/ a > fictitious force, after all. >
There are probably many tacit assumptions in GR which can be tested. One that concerns me is the assumption that with increasing speed gravitational mass varies in lock step with inertial mass. In fact, I think gravitational mass decreases with increasing speed. Harry
