Shining a Torch at the Moon: Does Light Travel Faster than c?

[Bruninho]

If I shine a torch at the moon the light will after a time be seen on the surface. If I then swing the torch so as to move across the moon does the spot of light on the surface travel faster than c?

 

[Doc Al]

Sure. But realize that this does not entail any physical object traveling at that speed.

 

[Gamish]

yes!

photons can travel faster than c, I am gana post a new thread on it right not.

 

[gonzo]

Gamish, the spot is not a thing to move. The spot is just individual photons hitting the moon at a certain place. When you "move the spot" it is just different photons hitting the moon somewhere else. Nothing is moving except the source of the photons.

You are just talking about two different photons hitting two different places very close to each other in time. Nothing moved.

 

[gonzo]

I was thinking a bit more about this, and can even the spot move faster than C? Wouldn't that imply that information can travel faster than C?

Think about a more extreme example. You shine a light 10 light years away in one direction, then flip around to shine on an object 10 light years away in the opposite direction. By the original example the "spot" has moved 20 lightyears in a very short time, but this neglects the travel time for the photons from the flashlight to the object.

I would assume there would be a similar issue on the moon. Or am I misunderstanding something?

 

[Doc Al]

I was thinking a bit more about this, and can even the spot move faster than C? Wouldn't that imply that information can travel faster than C?

Yes, the spot can move faster than c. But as the spot moves from one side of the moon to the other, no information travels between those two points. (Astronauts on opposite sides of the moon cannot use the spot to communicate; the only potential for information transfer is between the light source and the moon, and that information would of course travel at speed c.)

Think about a more extreme example. You shine a light 10 light years away in one direction, then flip around to shine on an object 10 light years away in the opposite direction. By the original example the "spot" has moved 20 lightyears in a very short time, but this neglects the travel time for the photons from the flashlight to the object.

The travel time only introduces a delay in the time it takes the "spot" to reach the object; but that delay is a constant 10 years, so it doesn't affect the "sweep rate" of the spot.

 

[jtbell]

I was thinking a bit more about this, and can even the spot move faster than C? Wouldn't that imply that information can travel faster than C?

As far as I know,there's no way you can use that spot to send a message from point A to point B on the moon, at a speed faster than c. If you've nevertheless figured out a way to do it, or read about a way to do it, please tell us about it.