# The Naked Scientists Forum

### Author Topic: Light faster than light - sideways?  (Read 4589 times)

#### Smeggit

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##### Light faster than light - sideways?
« on: 16/02/2004 09:24:54 »
I saw a documentary on time travel and one section got me thinking.  Imagine a photon bouncing back and forth between 2 mirrors.  If you move the mirrors sideways the photon will keep following the same path relative to the mirrors, i.e it will move sideways with the mirrors.
Am I right in thinking that a laser does not bend with movement?  If you move a laser around does it "drag" like the water out of a hose or will it keep a straight line?
My thinking is that if it does keep a straight line then what happens if you spin a powerful laser at high RPM?  Like a spinning arm the outer section moves faster than the inner section.  With an arm of photons stretching out hundreds or thousands of miles and spinning hundreds of times a minute there has to be a point where the photons move sideways faster than they move outwards.

#### Quantumcat

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##### Re: Light faster than light - sideways?
« Reply #1 on: 16/02/2004 15:31:30 »
"If you move a laser around does it "drag" like the water out of a hose or will it keep a straight line?" well if you think about it, a laser is ejecting photons like the hose is ejecting water molecules, so the answer would be yes ... only they move so fast you can't see the drag. I think. Unless you wiggled the laser super fast and there was heaps of dust in the air :)

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#### tweener

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##### Re: Light faster than light - sideways?
« Reply #2 on: 16/02/2004 17:13:03 »
Absolutely right QC!    And, welcome to the forum Smeggit!

A photon bouncing between mirrors will not move or "drag" with the mirrors. It reflects from the mirror surface and is completely independent of their motion.  If the mirrors are moving along the path of the light, the reflection will impart a Doppler shift to the light.  But there will not be any sideways motion of the photons.

The photons from a spinning laser are not moving sideways.  The water from a hose is not moving sideways either, but there is an optical illusion that it is.  Each drop of water is moving away from the end of the hose in a straight line in the horizontal plane.  In the vertical plane, the water is moving in a hyperbola due to the influence of gravity.

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John

#### Quantumcat

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##### Re: Light faster than light - sideways?
« Reply #3 on: 17/02/2004 07:03:26 »
Yeah, when two forces interact you get parabolas or the other thing that's the name for the shape that planets make in their orbits. The water looks like it's moving sideways but it's just each individual part heading out of the hose in a straight line, then the hose not being pointed that way anymore but the water keeps going in that direction, so it looks like it's moving sideways but it's not. I didn't really understand what he meant with the spinning mirrors, can you explain tweener?

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#### tweener

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##### Re: Light faster than light - sideways?
« Reply #4 on: 22/02/2004 01:34:09 »
I think he meant that if the mirrors were spinning they would be like rotating the water hose and make the photons go sideways (which they won't).  The planets make an ellipse in their orbits.  That is the shape that happens when the hyperbola is closed into a loop because gravity is strong enough to bring the planet back rather than let it go off into space.

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John

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##### Re: Light faster than light - sideways?
« Reply #4 on: 22/02/2004 01:34:09 »