# Postulate 2

1. Jun 18, 2009

### EV33

I am currently trying to understand SR and am confused on this postulate.

Lets say somone is traveling on a some kind of machine that emits photons. When that photon is emited will the person traveling on the machine measure the photon going c or will they measure it going c-v? My next question is will an outside observer view the photon as going c or c+v? I'm guessing that the person on the machine will measure it as c-v and the stationary observer will measure it as c but I am not sure.

2. Jun 18, 2009

### JesseM

The second postulate says all inertial frames measure all light wave to be moving at c, no inertial observer will ever measure a light wave to be moving at c+v or c-v. It might help to point out that addition of velocities doesn't work the same way in SR as it does in Newtonian physics. Also, keep in mind that each observer is using their own system of rulers and clocks to measure distance over time for a light wave, and each observer says that other observer's rulers are shrunk and that their clocks are slowed-down and out-of-sync (I gave a little numerical example of how these effects come together to ensure they each measure the same speed of c for a light wave in post #5 of this thread)

3. Jun 18, 2009

### EV33

Thank you for the help.