 Compatibility between the Conformal and Projective  Structures. That the conformal and projective structures are compatible is suggested by high energy experiments, according to  Ehlers, Pirani and Schild: “A massive particle \((m \gt 0)\),  though always slower than a photon, can be made to chase a photon  arbitrarily closely.” Ehlers, Pirani and Schild  therefore assume an axiom of compatibility between the conformal and  projective structures, and this leads to a Weyl space: If  the projective and conformal structures are compatible, then the  intersection