B Free fall floating in a closed spaceship near a gravity source

[roineust]

Floating inside a closed spaceship, no windows, not knowing if I'm free falling near a gravity source or far away from any gravity source, are there any experiments that can be made, to determine if there is a gravity source near and if there is such an experiment, to determine the gravity source magnitude and direction?

 

[PeroK]

Floating inside a closed spaceship, no windows, not knowing if I'm free falling near a gravity source or far away from any gravity source, are there any experiments that can be made, to determine if there is a gravity source near and if there is such an experiment, to determine the gravity source magnitude and direction?

Essentially, no. This is one aspect of the so-called equivalence principle.

That said, there may be tiny effects of tidal gravity that a sufficiently sensitive experiment could detect over a sufficiently short period of time.

 

[roineust]

The hypothetical particle of the graviton, is there a physical law, known already today, that even hypothetically, prevents it from carrying information about these tiny effects of tidal gravity?

 

[PeroK]

The hypothetical particle of the graviton, is there a physical law, known already today, that even hypothetically, prevents it from carrying information about these tiny effects of tidal gravity?

If that question makes sense, then the answer is no. The graviton would simply be part of the mechanism by which gravity is calculated quantum mechnically. Those calculations would be expected to reproduce the effects of tidal gravity.

 

[Ibix]

The answer is no in general, because you are only sampling curvature at one event, or along one worldline if you repeat the experiment. It's like me telling you that I'm thinking of a function $f(x)$ for which $f(0)=3$ and asking you to deduce $f$. Can't be done, because there are infinitely many lines that pass through $(0,3)$.

If you impose additional constraints, such as "I am in a spacetime dominated by a single spherically symmetric mass" it's possible to estimate the mass and location from the tidal effects @PeroK mentions and how they change over time. In a loose sense, that's how we discovered the outer planets - by looking at the orbits of planets we could see and estimating where an extra planet would have to be to account for the difference between the modeled and actual positions of the planets. Note that it took careful analysis of years of observational records to do it. It's a kind of closed box, but it was very large in the timelike direction.