# The speed of light - question

1. Feb 6, 2009

### iPhysicsi

Sorry to be one of those posters that doesn't follow suit, but I have a question that has been wandering the back of my mind.

A while ago now, I remember my physics teacher telling me something weird. 1 + 1 = 1 in some circumstances. He said that if a beam of light passes another beam of light traveling in parallel in the opposite direction, they will pass each other at the speed of light. Is this true?

Math tells us that they will pass each other at 2c But my teacher said they would pass at c. This is a weird concept for me to grasp if it's true, so can someone verify?

2. Feb 6, 2009

### malawi_glenn

Well math does not tells us that, it is addition of velocities in galilean dynamcs which tells you that the relative velocities should be 2c.

But galilean dynamics is not relevant when it comes to velocities close to light speed. Albert Einstein 'invented' a new dynamical framework in the early 1900's - the speed of light is constant to all observers! This is the first postulate of the theory of special relativity. He found that this is a plausible postulate, since it preserves the nice properties of maxwell's equations of electromagnetism.

From that postulate, one can derive formulas how addition of velocities work and so on, the result is: