  Quantum theory is a mathematical formalism to compute probabilities for
outcomes happenning in physical experiments. These outcomes constitute events
happening in space-time. One of these events represents the fact that a system
located in the region of space where is situated a physical device has a
certain value of a physical observable at the time when the device fires the
outcome corresponding to that value of the observable. The causal structure of
these events is customarily assumed fixed in an absolute way. In this paper we
show that this assumption cannot be substantiated on operational grounds
proving that two observers looking at the same quantum experiment can calculate
the probabilities of the experiment assuming a different causal structure for
the space-time events constituted by the outcomes. We will thus say that in
quantum theory we have relativity of causal structure.
