# Absolute light speed or velocity or both?

1. May 22, 2010

"Absolute" light speed or velocity or both?

Consider an inverse of Einstein's light clock. On a cartesian plane in a vacuum, a beam of light emanates from the origin at 45 degrees with velocity c. Is it OK that the x and y component velocities are each less than c (i.e. c divided by sqrt 2) or must time adjust to make component velocities also c?

2. May 22, 2010

### Staff: Mentor

Re: "Absolute" light speed or velocity or both?

It's the speed of light that is an invariant, not the velocity. The components of velocity can certainly be less than c.

3. May 22, 2010

Re: "Absolute" light speed or velocity or both?

Then Einstein's light clock gets a bit confusing. Say light radiates up the y axis to a reflector and back (in the frame of a rocket traveling along the x axis with speed v.) An observer stationary wrt the Cartesian plane sees the light take a diagonal path and we impose c as its speed limit. Fine. But the folks in the rocket don't see that their vertical light speed component can "certainly be less than c". They get their time dilated, from the stationary observer's perspective. Thus, my original post.

4. May 22, 2010

### Staff: Mentor

Re: "Absolute" light speed or velocity or both?

In the frame of the rocket, the light only travels vertically, so its vertical component = c. The light travels diagonally only in the "stationary" frame, so in that frame the components are less than c.