From Alan Baker:
snip
And when astronauts came back from space, they would be crystal clear
that while they floated around, with their bodies having no relative
acceleration in relation to their spacecraft, gravity never came
anywhere close to zero at any point in their trip. They *never*
experienced zero gravity. They experienced zero acceleration.

They didn't even experience that. They were accelerating; it's just that
everything around them was accelerating at the same rate.

I took Joe's rebuttal as a correction. But your rebuttal, Alan, is
helping me to see accuracy in the original statement...

The word -experience- carries subjectivity. What you experience
hinges upon your -frame of reference-. From the astronauts point of
view, the point of view of their non-inertial reference frame of the
spacecraft that is affected by gravitational acceleration, the
experience is zero relative acceleration.


So let's say, for the sake of argument here, that we are agreed that
it is accurate to say that they _experienced_ zero acceleration. An
obviously salient question follows...

Could we not, by the principle of equivalence (gravitational mass
being equivalent to inertial mass) therefore conclude that this
experience of zero relative acceleration be equivalent to a statement
that:

They experienced zero gravity?

The answer might be yes, except for one showstopper. Unlike
Einstein's famous "elevator" thought experiment, it is quite possible
to determine that the spacecraft is in the gravitational field of the
planet (and Sun and Moon, etc). The most simple way to do this is to
look out the window.

(Einstein's principle here was addressed on an earlier thread a few
days ago.)


~ CT