"cannot be observed" is a correct and usable phrase in written English.
For example: The effects of climate change on certain species of fish cannot be observed within a short period of time.
Percepts of others manifestly cannot be observed, though their properties can be inferred from observable behaviour (verbal and nonverbal).
These cannot be observed directly and can only be inferred from related observations.
3. It is not open to testing, observation or quantification; it is based upon data that cannot be observed.
Experimental data are always subject to an observational threshold (the experimental detection limit): an infinitely small response cannot be observed.
These interactions cannot be observed in univariate experiments.
In particular, the processes cannot be observed directly.
This phenomenon cannot be observed for the 60° conic spinneret.
Note that thoughts/beliefs and feelings/emotions cannot be observed directly.
Black holes cannot be observed directly; their gravity is so strong that nothing, not even light, can escape their clutches.
Many Long Island and Connecticut harbors have submerged rocks, shoals or other hazards that cannot be observed from the surface.
Clinical reasoning in patient care is a skill that cannot be observed directly.
