n0nsensical

You're hitting on another aspect of relativity, which is relative mass and energy. Indeed, mass and kinetic energy are also relative and subject to the same rules. You've heard the equation E=mc^2. This is the rest energy of matter. The total energy is the rest energy plus the relativistic kinetic energy. This energy is different depending on the velocity of the two frames of reference. As the velocity approaches the speed of light, the relativistic mass/energy approaches infinity. Another way to say that nothing with mass can move at the speed of light is that it would need to have infinite energy.

If you're mathematically inclined, look into the Lorentz Transformation for the mathematical explanation of special relativity. Some people can visualize equations better than words. From that and other equations, such as Maxwell's equations, and a whole lot of experimental data, is how physicists figured this out. Some of the consequences of relativity were observed before anyone had any idea what was going on. Other parts were beyond the accuracy of instruments of the time. (It's pretty hard to measure time dilation without an atomic clock) Most prominently Einstein, but he stood on the shoulders of other giants. Other physicists really made the important discoveries separately, Einstein just put all the pieces together into a few neat papers. Then he moved on to General Relativity which is a whole other can of worms that even fewer people understand.



Light really does slow down when it travels through matter, and it travels in a straight line (actually the precise motion of individual photons is a statistical distribution, but the whole distribution travels straight on average), but there isn't much of a why about it, it just does. I'm not sure it can be explained in macrophysical terms. It can be modelled in different ways by some complex quantum mechanics that I wouldn't have the slightest clue about. The simplest way I could explain it is that different media are differently susceptible to the application of electric and magnetic fields, and light is a moving electromagnetic field. (Electricity and magnetism are equivalent through relativity) It's similar to the way sound travels faster in water, because the speed of sound waves is dependent on the density of the medium. It's similar for EM waves, but somewhat more abstract, because they don't necessarily need a medium to travel at all whereas sound waves do.

So the speed depends on the properties of the medium in which it travels, sometimes expressed as electric permittivity and magnetic permeability. (They are really just one number but if you're not into tensors and things it's usually more convenient to express them separately) In a vacuum, these are known as the permittivity and permeability of free space and are simply some magical universal constants the square root of the inverse product of which is equal to c. The permittivity and permeability are different in various media, and even within a single medium can vary with the field strength and frequency.

The reason that visible light does not travel through usual matter is that the permittivity is approximately 0 at the appropriate frequencies; the energy is either reflected or absorbed. The permittivity of air is very close to that of free space and I think the permeability is equal. (There are some wave equations that can tell you what will happen to light at the boundary of media but I didn't do too well in that part of electromagnetics and ended up with a C-. Hehehe. I don't necessarily know what I'm talking about. =P) But glass has an unusually high permittivity. In (some?) nonmagnetic media, the permeability is exactly equal to the permeability of freespace and thus the speed of light is only dependent on the permittivity.

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