From stmx3:
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.

It's called a thought experiment. I suspect a primary reason is
because Einstein was well aware that there's no such thing as a
"uniform gravitational field".

Gravity follows an inverse square decay (not uniform linear decay).
Gravity extends radially (not uniformly linear once again).

These are the extremely fine ways to distinguish gravity from uniform
linear acceleration (the "moving elevator"). To repeat the easy way:

Just look out the window.

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.'

Here are two possible explanations for the above italics:

- Einstein was misquoted.

- Einstein was mistaken.

Let's not fall into the trap that, "Einstein said it then it must be
true". I don't know of anyone who is(/was) infallible.

Of course, a third explanation of the highlighted quote is that the
position I've been backing here is in error and that somehow gravity
ceases to exist when an object is in freefall ("there exists...no
gravitational field" in the immediate surroundings).

Both positions appear to be very well defined. We are all free to
reject what strikes us as absurd and to accept that which seems
logical.

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.

If Albert isn't turning in his grave over the use (nay, the
popularity) of that term, I am certain that Isaac is.

(my opinion)


~ CT