Hellfire, you just going to make chroot Whilst you queastion and reasonning are quite sound, the response by chroot is not so. So let me elaborate on this:In quantum field theory, particles (and thus also photons) are excitations of fields. The the position is not a dinamical variable as such anymore, but just a label for each of the field excitations in space. The QFT use's fields exitations, the old argument of 'when is a particle a wave?'..the answer in QM interpretations is whenever it is convienient. But here is my answer and it will bare fruits later on, so the question again:'when is a particle a wave?'...the answer is when a 3-Dimensional particle is collapsed into a 2-Dimensional field. A wave is not 3-Dimensional, it may propergate across a 3-Dimensional space. A 3-Dimensional 'particle' cannot propergate in lesser dimensions and remain 'intact' as a geometric 3-D quantity. Here you state:For example, how does light propagate in a Casimir vacuum? If different as in a usual vacuum, how does this fit with special relativity? Take the QFT explination you get the variable statement:You can determine c from two properties of the vacuum: the permeability and permittivity. I'm not sure what you mean by "Casimir vacuum." The 'not sure' part is spread Universally across QFT. Here is my answer, The initial set up of two 'perfect' plates which are brought to gether, expells 3-D matter from the surface's of the plates, this produces a surface tension, which is 2-Dimensional, and 'VACUUM' like conditions arise, (use of like is justified for simplicity). Now the interesting thing is that the grip 'force' holding the plates together, creates a 2-D Attractive force within the two co-joined plates , the energy is then transformed through the plates and around, out onto the 3-D surface, which becomes visible as Casimir 'Radiation'. This radiation you have to understand is blue, and has been accelerated along the Surface Tension. This is very similar to the ultra-cold B-E-C condensates, which shows the miesner effect for superconductivity(you know where the liquid defies gravity and runs up the side of containers). Now you seem to understand the Geometric conditions that are Dimensionally causing effects the at Quantum Scales, but lets consider the Macro scale. Our Galaxy is 3-D matter, it is surrounded by a 2-D space. The 2-Dimensional space(My correct term is Electromagnetic Vacuum) produces a Tension around our Galaxy, it envelopes the Milky way, whilst the 3-Dimensional matter contained in our Galxay offset this tension with a Photon Pressure (3-Dimensional outwrard pouring of light/particles), the only thing that can travel through the E-M-V is waves! The configuration of 3-Dimensional Particles are collapsed through the Vacuum 2-D, until it meets the outward Photon pressure of another Galaxy 3-D, and so the energy of Macro 3-Dimensional matter can be seen to be travelling through a un-observed medium(2-D Vacuum) which contracts the Matter into waves, and then as it enters any Galaxy is 'expanded' or transformed back into visible and full 3-D energie's, Particles. The corresponding transformations in SR, Lorentz Transformations..etc..etc are the Relative explinations. But wait I hear you say..where are the Extra-Dimensional domains in current theories?..D-branes..the fifth-dimension..Higher dimensions of stringtheory?.. The truth is the Universe only needs a 2-Dimensional field for waves, and a seperate 3-Dimensional Spacetime for Particles.