The terms fast and slow are ways to try to explain the speed of something. That something could be anything from a photon to a snail, from a tortoise to a hare, from an ordinary automobile to a top fuel race car. OK, every one knows that a photon travels at the speed of light and nothing can move faster, right? But then came the tachyon. The supposed tachyon is a particle whose slowest speed matches the speed of light, but can go quite a bit faster!
They say that nothing can go faster than the speed of light. Most everyone agrees with that, but then why do they set up elaborate and expensive experiments (CERN) to find an allusive particle that has broken the "speed of light barrier"? What do they want to do, write it a speeding ticket? If such a thing does exist, could we develop a telephone that could send your voice message faster than the speed of light? That wouldn't work, your voice would get there before the phone could ring (grin). OK, my brain is starting to hurt, trying to understand and explain the tachyon, so it is time to end this posting. Now you all have a great day, you hear?