If im on train travelling at the speed of light, and i turn on a torch light (which is obviously going at the speed of light (3x10^8))is that light going at 2 x the speed of light, or is light always constantly 3x10^8ms-1
Relative to the train it's travelling at the speed of light, relative to the ground it's travelling at 2x the speed of light/
A train with mass can not reach the speed of light. However, light is unlike shooting bullets forwards and backwards from a speeding (getaway?) car where the speed of the bullet is added to or subtracted from the speed of the car. If your train reached a relativistic speed (perhaps within 90% of the speed of light), light shining ahead from headlights or backwards from taillights would travel at the exact speed of light in either direction. As a practical matter many stars or galaxies are speeding away from earth but the light that we detect from them is measured to travel at the exact speed of light. The wavelengths of the light may be stretched causing a red shift, or a shift in the location of a known red line on a light spectrum from known elements in the star. Your speeding train would cause a red shift for observers behind it and a blue shift for observers ahead.
The light from the torch travels at the speed of light too, but since time slows down and would appear to stop (to the outside observer) when you reach the speed of light, you would not be able to switch the torch on anyway. If it was already on, the frequency of the light from the torch would be drastically changed. Say you were just below the speed of light, you would see it flash away at the speed of light, and because of the slowed time, it's frequency would look normal to you, according to Einstein
