You are here:

# Relativity/speed of light

Question
1. You have a laser rotating at 100rps
2. A light sensor is located 296.477 miles from the rotating laser beam. The laser beam at this point whizzes by at 186,282mps.
3. You should sense 100 pulses per second at this distance.
Questions:
1. If you increase the distance to the sensor to greater than 296.477 miles, what happens at the sensor?
2. Does the sensor still read 100pps (meaning the angular velocity of the photons in the laser beam are traveling greater than the speed of light)?
3. Does the pulse width get smeared out (because the angular velocity can't be greater than C)?
4. Is there a constant reading at the sensor (no pulse but just a continuous reading) because
the pulse is smeared out?
P.S.
this is not a homework question - just a 62-year-old's curiosity.

Victor, I am glad you identified this as not being a homework question!

Your question turns on the difference between linear velocity and angular velocity.

No matter what angular velocity you choose, the photons from your laser go in straight lines at speed c. No matter if the light spot on a distant screen travels at speed greater than c, no mass or energy is moving faster than c.  The light spot motion is an illusion. No photon is moving transversely at any time.

If you replace the light beam with a long stick, then you have mass moving transversely, and the stick tip cannot move with speed greater than c.  What will happen is that the stick will bend, if it does not break first.

Relativity

Volunteer

#### Uncle Ben

##### Expertise

I can answer questions regarding Einstein's Theory of Relativity, particularly in Special Relativity. I will not answer homework questions or mathematical problems that require special symbols.

##### Experience

I have taught physics at the college level, undergraduate and graduate, for many years including Special Relativity. I have taught at Johns Hopkins, Case-Western, and MIT. I have also served as a staff member of the Commission on College Physics, which was supported by the National Science Foundation to recommend improvements in the curriculum of college physics departments in the US. I am also the author of a textbook titled Vector Calculus, which was used at MIT in the teaching of electromagnetic theory and relativity. My research interests were mainly in solid state physics, especially the properties of metals at low temperatures. I am listed in the publication known as American Men of Science.

Publications
I have dozens of papers published in the Physical Review and in the American Journal of Physics.

Education/Credentials
I hold a Ph.D. degree in physics from the Johns Hopkins University.

Past/Present Clients
Johns Hopkins University, Case-Western Reserve University, Massachusetts Institute of Technology, Empire State College, Georgetown University, Commission on College Physics, and UNESCO.