Re: The Elevator in Free Fall

On 12/20/24 12:51 AM, Luigi Fortunati wrote:

The cables break and the elevator goes into free fall.
Newton told us that the elevator accelerates and, therefore, there is a
force that makes it accelerate.
Then Einstein came along and told us that this is not true

This is one of your problems. Physics is not about "true or false"; we
make MODELS of the world we inhabit -- Newtonian mechanics and GR are
DIFFERENT MODELS. The problem is that you intermix nomenclature
willy-nilly between them.

and that
there is no force that accelerates the elevator in free fall.
But if there is no force that accelerates the elevator, it means that
the elevator does not accelerate.
And if it does not accelerate, then it moves with uniform speed.

This is another of your problems. You did not specify the coordinates
relative to which "speed" is measured. Relative to an elevator-fixed
LOCALLY inertial frame, it moves with uniform speed.

But speed is not absolute: it is relative.

This is another of your problems. Your words are HIGHLY ambiguous.

And so I ask: is there any reference system with respect to which its
speed is uniform?

Yes. Any elevator-fixed locally inertial frame.

This is for Newton's second law: force that accelerates mass.
Instead, for the first law, Einstein says that a body in the elevator
in free fall is at rest with respect to the elevator itself.
So, why does a body placed below the center of gravity of a
free-falling elevator accelerate downwards, and if it is above the
center of gravity, it accelerates upwards?

For this last to happen the elevator does not meet the criteria of a
locally inertial frame (see my recent post in this newsgroup). It is of
course due to tidal forces inside the elevator, the acceleration of
which exceeds one's measurement accuracy. Objects separated horizontally
will also accelerate towards each other (due to tidal forces that exceed
measurement accuracy).

I repeat: you need more precision in thought and words.

Tom Roberts