Relativity of Entanglement
  It has recently been suggested that various entanglement measures for bipartite mixed states do not in general give the same ordering even in the asymptotic cases [S. Virmani and M. B. Plenio, Phys. Lett. A {\bf 268}, 31 (2000)]. That is, for two certain mixed states, the order of the degree of entanglement depends on the measures. Therefore, incomparable pairs of mixed states which cannot be transformed to each other with unit efficiency by any combinations of local quantum operations and classical communications exist. We make an analogy of the relativity of the order of the degree of entanglement to the relativity of temporal orders in the special theory of relativity. 
W.Y. Hwang, J. Lee, D. Ahn, S.W. Hwang