We'll know that the base of special relativity theory is "the light has a constant speed in vacuum". Time dilation and Lorentz contraction are consequences of the constance of light speed. If we excite photons, for example, its frequence increases (and the wavelength could decrease), but it happens for light speed continues constant. It's also an consequence of constance of light speed. Everything seems to be a consequence of constance of light speed. But, the question is: WHY light has a constant speed (in vacuum)? What about tachyons? (http://en.wikipedia.org/wiki/Tachyon" [Broken]) Any discussions are welcome.