Two crucial claims of relativity theory can be directly confirmed  Utilizing elementary particles in high-energy accelerators, we can demonstrate that at high velocities objects are subject to time dilation and an increase in mass  However, a third claim of the theory, the Lorentz contraction thesis, which is connected to the other two, is not directly confirmable
But the fact that the theory in general is supported by experimental results indirectly confirms the contraction thesis
The theses of a physical theory that cannot be confirmed by observable phenomena can only be confirmed indirectly.