  Hadron tomography has been investigated by three-dimensional structure
functions, such as generalized parton distributions (GPDs) and generalized
distribution amplitudes (GDAs). The GDAs are $s$-$t$ crossed quantities of the
GPDs, and both functions probe gravitational form factors for hadrons. We
determined the pion GDAs by analyzing Belle data on the differential cross
section for the two-photon process $\gamma^* \gamma \to \pi^0 \pi^0$. From the
determined GDAs, we calculated timelike gravitational form factors of the pion
and they were converted to the spacelike form factors by using the dispersion
relation. These gravitational form factors $\Theta_1$ and $\Theta_2$ indicate
mechanical (pressure, shear force) and gravitational-mass (or energy)
distributions, respectively. Then, gravitational radii are calculated for the
pion from the form factors, and they are compared with the pion charge radius.
We explain that the new field of gravitational physics can be developed in the
microscopic level of quarks and gluons.
