It is wrong to think of space as being completely devoid of anything. The noted Italian astronomer Paolo Maffei has suggested that the density of matter in the deepest regions of space between the galaxies is in the order of about one hydrogen atom per cubic centimeter. A proper appreciation of just how sparse the population of particles in inter-galactic space is can be realized by understanding that in a volume of hydrogen gas at the surface of the Earth there are about 37.728 x 10^5 hydrogen atoms on a line one centimeter long.

A pure vacuum does not exist, and the partial vacuum of space certainly does not hamper the radiation of electro-magnetic (EM) waves. In fact, we can draw some interesting conclusions relating the “speed of light” to the distance traveled by the wave front of an EM wave at one atmosphere of pressure versus the near-vacuum of intergalactic space for a given time frame.

Given that the rate of atomic interaction, which is a constant for a given frequency but variable over a wide range of frequencies, determines the rate of propagation of EM waves, then the distance traveled by an EM wave front for a given time frame is proportional to the number of particles in the medium which effect its transmission for that distance. We can do a logical calculation with respect to the actual distance that a light wave would travel through intergalactic space for a given time frame using data compiled for the values we have collected from experimentation at one atmosphere of pressure at 1G, or, on the surface of the earth. The medium of the atmosphere is much denser than the medium of intergalactic space. Experiments to determine the speed of light were performed in a breathable atmosphere. The statement that the speed of light in vacuum is 3×10^10 cm/sec is therefore an assumption, albeit not entirely incorrect, but it requires qualification.

The number given above representing the density of hydrogen atoms aligned on a line one cm long when multiplied by the rate of EM wave propagation in the atmosphere, which is 3×10^10 cm/sec, gives us the distance that light waves propagate in intergalactic space in one second. This is 1.132×10^12 km, at a rate which at 1 gravity and 1 atmosphere of pressure is measured as being 3×10^10 cm/sec. That’s 1.132 tera-km in intergalactic space!

Similarly, the distance traveled by an EM wave for a given time frame is greater for interstellar space as well, just as it can be said that the distance traveled by an EM wave for a given time frame is shorter through a dense medium like water. The French physicist Leon Foucault observed this in 1855 and concluded that the speed of light was slower in water than in air. This is essentially wrong because the rate of atomic interaction is for all intents and purposes a constant for the limited range of frequencies that represent the visible part of the optical spectrum and it is really the distance traveled by the EM wave which changes depending on the density of the medium, for a given time frame.

We can conclude that the behavior of EM wave propagation is the same for whatever medium wherein it propagates, whether water, atmosphere, or space. Also, the rate of atomic interaction actually depends on the frequency, and this is corroborated by experimental evidence. Thirdly, and most importantly, the distance traveled by the wave front of an EM wave for a given time frame is dependent on the density of the medium through which the wave propagates.