Equivalence principle

the surface of a planet.

So, I ask: what stops us from measuring the presence (or absence) of
tidal forces? If tidal forces are there, then we are stationary on the
surface of a planet, if they are not there, we are experiencing a
non-gravitational acceleration.

Luigi Fortunati

[Moderator's remark: One has to keep in mind that the equivalence
principle is a local concept, i.e., the equivalence between the
observations in a gravitational field and in an accelerated frame of
reference in free space refers only to very small space-time regions. A
"true gravitational field" is of course never entirely equivalent to an
accelerated frame in flat Minkowski space, because according to GR the
gravitational field leads to space-time curvature, i.e., a non-vanishing
Riemann tensor, while Minkowski space is flat, which are
coordinate-independent notions, and only such notions are physically
interpretable.

Of course tidal forces are well observable, cf. the tides on Earth,
where the name "tidal force" refers to.

HvH.]