Microgravity parable

Stuf4 wrote:
[snip]


"NASA scientists call this microgravity... The term is apt since
Albert Einstein said that acceleration caused by gravity is equivalent
to any other push."

The principle is about _mass_ equivalence, not acceleration
equivalence.

[snip]

That is incorrect. It was the happiest moment in Einstein's life when
he realized that an accelerated reference frame was equivalent to a
frame in a uniform gravitational field. From this basis, Einstein could
later show the equivalence of intertial and gravitational mass. But the
first preceded the second.

In the following, Einstein discusses how he came to believe there should
be *no* preferred reference frame for the description of physical phenomena:

"Then there occurred to me the ... happiest though of my life, in the
following form. The gravitational field has only a relative existence
in a way similar to the electric field generated by magnetoelectric
induction. *Because for an observer falling freely from the roof of a
house there exists--at least in his immediate surroundings--no
gravitational field* [his emphasis in italics]. Indded, if the observer
drops some bodies then these remain relative to him in a state of rest
or of uniform motion, independent of their particular chemical or
physical nature (in this consideration the air resistance is, of course,
ignored). The observer therefore has the right to interpret his state
as 'at rest.'

Because of this idea, the uncommonly peculiar experimental law that in
the gravitational field all bodies fall with the same acceleration
attained at once a deep physical meaning. Namely, if there were to
exist just one single object that falls in the gravitational field in a
way different from all others, then with its help the observer could
realize that he is in a gravitational field and is falling in it. If
such an object does not exist, however--as experience has shown with
great accuracy--then the observer lacks any objective means of
perceiving himself as falling in a gravitational field. Rather he has
the right to consider his state as one of rest and his environment as
field-free relative to gravitation.

The experimentally known matter independence of the acceleration of fall
is therefore a powerful argument for the fact that the relativity
postulate has to be extended to coordinate systems which, relative to
each other, are in non-uniform motion."

(Pais, A. (1982). 'Subtle is the Lord...': The Science and the Life of
Albert Einsteing. Oxford: Oxford University Press. p. 178)

So, Einstein would say that an ISS crewmember has the right to say
he/she is in zero gravity.