Originally posted by lasersailor184
2.) Everyone claims that time is the fourth dimension. But given that it doesn't actually exist, I don't see how people can claim this. Why aren't actual physical properties the other dimensions, like weight, density (though those two may be one dimension), electrical charge, energy, chemical properties, momentum...
Why is it given that time does not exist? It is hard to define it, but there is a direction that sets the past apart from the future. Some physical processes are irreversible and therefore there is a clear "before" and "after".
When you talk about time as a forth dimention it is because it behaves similar to space dimentions in relativity (although not exactly the same). You have propagation in time and/or of space. There is no propagation in electric chanrge, it is a conserved quantity. Momentum transforms between different frames of reference similar to space, but seperately, and is mixed with energy, same as space is mixed with time. Momentum and position do affect each other in quantum mechanics. Special importance in quantum mechanics is given to the "phase space", which is composed of the 3 space dimentions and their corresponding 3 momentum components. In this 6D phase space the picture of the world is "pixeled" and instead of moving smoothly through it, it is divided into discrete cells with phase-space volume of the Plank constant to the power of 3.
