  The superposition principle is one of the founding principles of quantum
theory. Spatial quantum superpositions have so far been tested only with small
systems, from photons and elementary particles to atoms and molecules. Such
superpositions for massive objects have been a long-standing sought-after goal.
This is important not only in order to confirm quantum theory in new regimes,
but also in order to probe the quantum-gravity interface. In addition, such an
experiment will enable to test exotic theories, and may even enable new
technology. Creating such superpositions is notoriously hard because of
environmental decoherence, whereby the large object couples strongly to the
environment which turns the delicate quantum state into a statistical mixture
(classical state). However, advances in the technology of isolation could in
future suppress such decoherence. Here we present a decoherence channel which
is not external but internal to the object, and consequently improved isolation
would not help. This channel originates from the phonons (sound waves) within
the object. We show that such phonons are excited as part of any splitting
process, and thus we establish a fundamental and universal limit on the
possibility of future spatial quantum superpositions with massive objects.
