Luigi Fortunati

2024-06-08 17:40:15 UTC

[Moderator's note: In the moderation process for this posting, I've cut

by accident the first lines of the message; so here's there complete

posting again. HvH]

In the video at minute 6:56

it states that there is no measurement that can be made to distinguish

whether youâre being accelerated or whether you are sitting still on 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.]

by accident the first lines of the message; so here's there complete

posting again. HvH]

In the video at minute 6:56

it states that there is no measurement that can be made to distinguish

whether youâre being accelerated or whether you are sitting still on 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.]