The question is not so much testing which is the superior theory/model Ki or Physics because they are entirely different but acknowledging they are tools for understanding and using the right tool when required.
As a generalisation the historical scientific traditions in the East are based on observation, where as Western Science is deductive. Got a new phenomena to explain, just call it 'Ki' and the Eastern model is satisfied, or in the West design a reductive experiment to test only that phenomena to come up with wether its explainable or not.
The two traditions can collide (deductive vs. observational) when they meet and so Western science says - hey I can explain unbendable arm therefore Ki doesn't exist, whereas the Eastern tradition says OK but thats just unbendable arm but what about unraisable body, intent etc.... The physicist says you can't do that its a wandering argument, the Ki-ster says yeah but if you can't explain all of it then physics doesn't explain it all.
I think western science has had a pretty good crack at explaining most of the phenomena you are likely to see in Aikido, but there are gaps - cause this stuff is really hard to understand, explain or devise an experiment for. Reduction of complex systems means eliminating variables... but there are just so many damn variables and in complex psycho-physical interaction elimination is nigh on impossible
As an aside on complexity, just because we can explain chemistry through atomic level interactions doesn't mean its always convenient to do so. Ssee this link
for metabolic pathways in a cell - thats a whole lot of equations if the abstraction of chemistry isn't used, and still damn complicated when chemistry, as a model, is used ).
In a like (though perhaps not scientificTM) way in the dojo 'extend Ki' is a simplified abstraction that works well as a conceptual tool for a whole host of phenomena (momentum, collisions, biomechanics, intent, perception, conscious/sub conscious training, dual mental tasking etc...) wether or not it is acknowledged as a 'mysterious phenenoma' it works and can be useful.