I What happens to an OAM light beam, with helical mode m>1, when accelerating under gravity?

1. Jan 17, 2017

calinvass

It is known that wavefronts of internal OAM photons travel slower than light but I wonder what happens if you accelerate such a beam. This should be possible under gravity.

2. Jan 17, 2017

Staff: Mentor

You can describe it as superposition of waves that do travel at the speed of light (but with different angles). and study how they propagate separately. Your beam will diverge, with each component following the spacetime curvature.

3. Jan 17, 2017

calinvass

Thanks.

Do you mean the radius of the helix, the wavefront speed and the frequency increase as it descends ?
I assume the helical mode maintains its integrity.

4. Jan 17, 2017

Staff: Mentor

Every finite beam of light diverges (at least after some distance). Orbital angular momentum will change the divergence a bit.

5. Jan 18, 2017

calinvass

If the OAM beam is made of separate beams, how is the wavefront speed reduced? Is it because the poynting vector follows a helix ?

6. Jan 18, 2017

Staff: Mentor

A classical analogy works quite well: Have multiple beams, all tilted a bit with respect to the main direction of motion, aligned in a twisting motion. That is not completely accurate, but it follows the same idea.