Velocity of laser light pt. on moon > c ?

Click For Summary
SUMMARY

The discussion centers on the velocity of a laser light point on the Moon and its relationship to the speed of light (c). It establishes that while the laser's angular speed is 10 radians/sec, the time for light to travel from Earth to the Moon is 1.266 seconds. The calculated velocity of the laser point on the Moon is 3.8 * 10^9 meters/sec, but due to the light travel time, this does not exceed c. The conclusion emphasizes that the apparent speed of the laser spot does not violate the principles of relativity, as the light's travel time creates a lag that prevents exceeding the speed of light.

PREREQUISITES
  • Understanding of basic physics concepts, particularly relativity
  • Familiarity with angular velocity and linear velocity calculations
  • Knowledge of light travel time and its implications in physics
  • Awareness of the "Lighthouse paradox" in the context of light speed
NEXT STEPS
  • Research the "Lighthouse paradox" for deeper insights into light propagation and perception
  • Study the principles of special relativity and their implications on speed limits
  • Explore angular velocity and its relationship with linear distance in physics
  • Investigate the effects of light travel time on astronomical observations
USEFUL FOR

Physicists, students of physics, and anyone interested in the implications of light speed and relativity in astronomical contexts.

Point Conception
Gold Member
Messages
1,157
Reaction score
1,874
The Earth moon distance = 3.8 * 10^8 meters
Time for light to reach moon = 1.266 sec.
A laser at the origin on Earth pointed at moon rotates .001 radian
with angular speed w = 10 radians/sec.
The arc length distance described on moon ,s= (radians) (radius) =3.8*10^5 meters
The velocity of the point on the moon described by the change in direction of the laser=
v=(w)(radius) =3.8 *10^9 meters/sec
So the time it would take this point to travel on the moon = distance/rate = 1/10^4 sec as well as .001 radian/10 radians/sec at the origin, being the rotation of the laser.
Since the light takes 1.266 second to travel to moon then it does not seem possible for laser light point on moon to exceed c.
For example in this case it took 1/10^4 seconds for direction of laser point to describe the path on the moon but when the laser is in this final position after rotating .001 radian it will take the light 1.266 seconds to travel to moon.
To restate it seems that the actual velocity of the directed laser light point on the moon would not exceed c because of the time lag
from Earth to moon in light travel time .
 
Last edited:
Physics news on Phys.org
Sure, it took the light 1.266 seconds to travel to the moon, but the light that hit the "right" edge of moon arrived just 1/10^4 seconds after the light that hit the "left" edge (assuming you swept the laser from left to right). The travel time to the moon has nothing to do with the speed of the laser "spot" across the moon.
 
There is a fairly good discussion of this in the thread;

"Lighthouse paradox revisited"

It is three pages long, but most of the answers you seem to be looking for are on the first page.
 

Similar threads

  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 40 ·
2
Replies
40
Views
2K
  • · Replies 128 ·
5
Replies
128
Views
8K
  • · Replies 50 ·
2
Replies
50
Views
8K
  • · Replies 90 ·
4
Replies
90
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 10 ·
Replies
10
Views
4K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 42 ·
2
Replies
42
Views
3K