# Revolving laser beam path

#### Loren

What is the trajectory in space of a rotating laser beacon?

#### mathman

Forum Staff
You need to describe the setup more precisely. What is rotating around what?

topsquark

#### Loren

Think of a continuous ray of photons, like a laser, emitted into space vertically from a spot on the equator of Earth as it turns.

To an earthbound observer, what path (a spiral, say) does the ray describe, e.g., when encountering bodies further and further away?

(This line of sight may be limited by the constant speed of light, redshift and dark energy.)

#### mathman

Forum Staff
Wouldn't the trajectory depend on coordinate system? Someone in a geosynchronous orbit would see a straight line.

topsquark

#### Loren

I was thinking of vantages from the Earth's axis of spin. Please disregard seasonal tilt. I believe you're right about a geosynchronous orbit, mathman.

#### mathman

Forum Staff
I was thinking of vantages from the Earth's axis of spin.
What does this mean?

#### topsquark

Math Team
I was thinking of vantages from the Earth's axis of spin. Please disregard seasonal tilt.
Then it would look like it's moving in a spiral pattern.

-Dan

#### Loren

Then it would look like it's moving in a spiral pattern.

-Dan
Perhaps classically, but how about considering a constant speed of light, redshift, dark matter and dark energy?

#### topsquark

Math Team
Perhaps classically, but how about considering a constant speed of light, redshift, dark matter and dark energy?
It works for photons. A spiral has a "radius" that is changing at a constant rate, which would follow from a constant speed of light. (The speed of the photon is still c and would be pointed along the direction tangent to the spiral. It will work, though the calculation of the radius would have to be done in SR. The red shift won't be a factor as the speed of a red photon is the same as the speed of a violet photon. (If we have to we can use GR instead of SR.)

I can't really speculate on a general problem where dark energy or dark mattter exist. All I can say is that it would look similar to a spiral but the radius wouldn't be likely to have a constant rate of change.

-Dan

Loren