  In the framework of special relativity, all particles are point-like or
string-like. This nature of particles has caused the divergence difficulties in
quantum field, string and superstring theories. In the framework of special
relativity, due to the non-uniformity of the m-space and phase space in the
usual inertial coordinate system, Boltzmann's hypothesis of the equality of the
probability of equal volume element is no longer appropriate. That makes it
very difficult to construct Lorentz-invariant statistical mechanics and
thermodynamics for many-particle systems. Besides, some observations on special
relativity itself and its experimental facts are also reported. The conclusions
from these observations are: Special relativity is not an ultimate theory; Some
modification is needed; Any modification must not violate the constancy of the
light speed and the local Lorentz invariance; It seems that we have to change
the assumption on local structures of gravity-free space and time in special
relativity.
