  In modern physics only relative quantities are considered to have physical
significance. For example, position assigned to a system depends on the choice
of coordinates, and only relative distances between different systems have
physical relevance. On the other hand, in quantum theory the scenario where one
system, A, is localised around certain position while another system B is in a
spatial superposition is considered to be physically different from the
scenario where A is in a superposition, while B is localised. Different
physical effects are anticipated in the two scenarios especially when the two
systems have widely different masses. Here we show that for any superposed
amplitudes that are related by a symmetry transformation, the above scenarios
are physically equivalent: probabilities for any measurement are the same
whether we assign the superposition state to system A or to system B. More
generally, linearity of quantum theory guarantees that if a theory is invariant
under some symmetry transformations it is necessarily also invariant under
their arbitrary `superpositions'. Thus the notion of a superposition turns out
to be relative to the choice of coordinates, once it is realised that relations
between coordinates do not need to be classical.
