  Magnetic flux generated and intensified by the solar dynamo emerges into the
solar atmosphere, forming active regions (ARs) including sunspots. Existing
theories of flux emergence suggest that the magnetic flux can rise buoyantly
through the convection zone but is trapped at the photosphere, while its
further rising into the atmosphere resorts to the Parker buoyancy instability.
To trigger such an instability, the Lorentz force in the photosphere needs to
be as large as the gas pressure gradient to hold up an extra amount of mass
against gravity. This naturally results in a strongly non-force-free
photosphere, which is indeed shown in typical idealized numerical simulations
of flux tube buoyancy from below the photosphere into the corona. Here we
conduct a statistical study of the extents of normalized Lorentz forces and
torques in the emerging photospheric magnetic field with a substantially large
sample of SDO/HMI vector magnetograms. We found that the photospheric field has
a rather small Lorentz force and torque on average, and thus is very close to a
force-free state, which is not consistent with theories as well as idealized
simulations of flux emergence. Furthermore, the small extents of forces and
torques seem not to be influenced by the emerging AR's size, the emergence
rate, or the non-potentiality of the field. This result puts an important
constraint on future development of theories and simulations of flux emergence.
