In a paper I was solving, there was a question stated as such:

A narrow but tall cabin is falling freely near the Earth's surface. Inside the cabin, two small stones A and B are released from rest(relative to the cabin). Initially A is much above the center of mass and B much below the center of mass of the cabin. A close observation of the motion of A and B will reveal that-

Now, as far as i understand, according to GR a freely falling frame in a gravitational field is an inertial frame of reference and thus bodies released from rest w.r.t. such a frame will tend to stay at rest relative to the cabin as no external forces are acting. This is one of the options given but the correct answer according to them is-

A moves slowly upwards and B moves slowly downwards relative to the cabin.

So have i misunderstood something or is my reasoning correct?


A free falling object in a curved spacetime follows a geodesic, so in principle no acceleration is experienced. However that is applicable to a test particle, i.e. a body of limited dimensions compared to the curvature.

Instead a macroscopic body measures also the tidal force, that is a radial higher acceleration in the part closer to the gravitating mass and a transverse acceleration in the side parts directed to the center of the body. That is called "Spaghettification".

Spaghettification explains why A and B are moving away from each other, A upwards and B downwards. As the objects are stones, being in the earth gravitational field, they do not suffer either the radial stretching or the transverse compression.


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