as indicated by the equation of continuity of hydrodynamics, when the source term [math]\sigma[/math] is zero, the vanishing of the divergence is a necessary and sufficient condition that the fluid in becoming incompressible and also its divergence is equal to the density.
[math] \frac{1}{\rho}\frac{d\rho}{dt}+\nabla\cdot \mathbf{v} - \sigma = 0[/math]
but the equation is derivable based on the validity of the following 3D Cartesian total and partial derivatives:
[math] \frac{d\rho}{dt}=\frac{\partial \rho}{\partial x}\frac{\partial x}{\partial t} + \frac{\partial \rho}{\partial y}\frac{\partial y}{\partial t}+ \frac{\partial \rho}{\partial z}\frac{\partial z}{\partial t}+ \frac{\partial \rho}{\partial t} [/math]
the total time derivative of density is the sum of the changes at the point (x, y, z) and the rate at which the fluid at this point is flowing to other parts of the field where the density is different.
A vacuum glass chamber can be artificially created by the use of a vacuum pump. It can be demonstrated that light can pass thru the chamber from one side to the other showing that light or electromagnetic waves are not hindered by the existence of a vacuum. Experimentally, the speed of light inside the chamber is ~186,000 miles per second, while its speed outside the chamber of normal air pressure is less. This implies that the compressibility of any vacuum is indicated by speed of light in that particular vacuum.


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