Our value for the Planck constant $h$ can be found on experiments on Earth, but how do we know that the Planck constant doesn't change throughout space, for instance it depends weakly upon the curvature of spacetime? As far as I know we have only done experiments to determine the value of $h$ close to the Earth's surface, so I don't think there's any direct evidence to suggest it should be a constant throughout all space, so I proposed the idea to my friends.
One called me a nut, and said that the value for Planck constant has to be the same throughout all space otherwise the conservation of energy would be violated. He put forward the following situation: Suppose that in one region of space the Planck constant is found to be $1$ in SI units (call this Space $A$), and you have a photon of frequency $1$ Hertz. This photon then traveled to a different region in space where the value for the Planck constant is now $3$ in SI units(call this Space $B$). Hence the photon has gone from having $1$ Joule of energy to $3$ Joules of energy (by the relation $E = hf$). In his own words, a twitch of a finger here can cause an explosion in a different region in space. To this I replied: So what? As far as I know the conservation of energy argument derives from the fact that space looks the same everywhere we look, and if the Planck constant was really different in different areas of space, then that argument shouldn't hold, and conservation of energy shouldn't hold either. Even if I did want to abuse this system by creating a feedback loop where the energy got larger and larger, the point was if I ever tried to extract energy from Space $B$ back to Space $A$ it would become a small amount of energy again since the value for $h$ decreases.
Another friend called me a nut, because he believed that in sending the photon from Space $A$ to Space $B$, I would violate the Second Law of Thermodynamics by decreasing the overall entropy. He didn't really expand on it, and (I believe) neither of us have the abilities to calculate the actual entropy change, so this point remains unresolved.
Finally, another friend called me a nut, because he believed that if the value for $h$ was different in different regions of space, then all our calculations ever done in physics dealing with the stars and their brightness and so on and so forth were all incorrect, and that would be troubling. To this I replied that firstly, the changes in the value for $h$ could possibly be very small for 'normal' regions of space, and secondly that there was nothing inherently WRONG about all these other calculations we've done being wrong, in fact we know they are wrong already because we can't account for either dark matter or dark energy! So in my mind this wasn't a really good argument as to why the Planck constant couldn't be variable over space.
With all that being said, I'm still fairly certain that the Planck constant is a constant over space, because it is called a constant. So is there an error in my previous arguments, or what is the conclusive reason that the Planck constant cannot be variable over space?