The concept of the effective mass is ubiquitous in the study of
electronic conduction in solid state physics, allowing for a
semi-classical treatment of a particle's response to an external force.
However, in 1954 Pfirsch and Spenke  predicted that this description
breaks down on very short timescales, and that the initial response to a
force is in fact described by the bare mass. For typical solid state
systems, the fast timescales of these dynamics have thus far prohibited
observation of this phenomenon. Here, we report on the first
experimental observation of effective mass dynamics using a
Bose-Einstein condensate of Rubidium-87 atoms initially in the ground
band of an optical lattice. By studying the response of the particles
to an abruptly applied force, we show that, while the long timescale
dynamics are described by the effective mass, the initial response is
indifferent to the presence of the lattice.
 D. Pfirsch & E. Spenke, Die effektive masse eines kristallelektrons und das Ehrenfestsche theorem,
Z. Phys. 137, 309 (1954).