[Neil Pariser]

Neil Pariser  asked the Naked Scientists:
   
I have always been fascinated by the fact that a beam of light can travel for millennia through space.  So, if I could trap a light beam in a mirror lined tube or box would it remain trapped for eternity? 

And, if so, were I to put a Radiometer in the box (one of those devices whose blades of white and black turn in the presence of light) would this be akin to a perpetual motion machine?

Thank you.

Neil Pariser
New City, New York

What do you think?

[lyner]

Mirrors absorb around 1% of the incident light. How much would you have left after 100 reflections?
0.99^100 = 36%
That would take, perhaps a few hundred nanoseconds.
After a few microseconds you'd not be able to measure anything, I'm afraid.

The best you could do would be to use a length of fibre optic with mirrors each end. At least the Total Internal Reflection as it goes along the  fibre is pretty good. You could get a delay of a few hundred microseconds, I would imagine. Or bouncing light signals off the Moon gives you about a second before the energy fizzles out.

f course, your radiometer would be absorbing energy all the time, too!

[syhprum]

If you used really expensive mirrors such are specified for laser use you might extend the operating time ten thousandfold but enclosing a radiometer would very rapidly adsorb all the light.
I do not think using a fibre optic cable would work very well as a light frequency resonant cavity, vacuum is what you want in the light path the longer the better as long as the mirrors produce little divergence.

See http://www.rp-photonics.com/supermirrors.html

[Don_1]

Assuming you could actually do this, in a sealed, mirrored box, how would you know whats going on in there, once it has been sealed?

[dentstudent]

Assuming you could actually do this, in a sealed, mirrored box, how would you know whats going on in there, once it has been sealed?

You could get Schrodinger's cat to keep notes....

[Don_1]

Oooo, that's cruel, shutting poor Schrodinger's cat in a box with no food or water, and if it's air tight, the poor ***** will splificate.

Ooer, I can't say 'p*ssy'

[Vern]

the poor ***** will splificate.

When the little red dotted lines appear under a word you type, I think it means that the editor does not know the word. []

[Bored chemist]

What's the point of making the mirors particularly good?
The radiometer absorbs light and converts it (not very efficiently) into movement. Since it's moving in a gas (at low pressure) it will be slowed down by viscous drag.
What will happen is that the radiometer will slow down and stop and it, the gas in the box and the air will warm up slightly.

[syhprum]

Sophiecentaur digressed into resonant cavities without any radiometer and was pointing out how rapidly the light would fade out with 99% reflectivity mirrors, I was making the point that we can do much better than that with the very best mirrors.
Perhaps the solution would be to drive an electrical generator from the rotation of the radiometer and use it's output to drive some lasing medium in the light path!

[irish del]

This might cause a quantum effect
If you where to try observe what was happening in the box ,your observing the effect would alter the outcome or would it?

[Vern]

This might cause a quantum effect
If you where to try observe what was happening in the box ,your observing the effect would alter the outcome or would it?

Sure; as soon as you open the box you spill all the light.

[Vern]

Neil Pariser  asked the Naked Scientists:
   
I have always been fascinated by the fact that a beam of light can travel for millennia through space.  So, if I could trap a light beam in a mirror lined tube or box would it remain trapped for eternity? 

And, if so, were I to put a Radiometer in the box (one of those devices whose blades of white and black turn in the presence of light) would this be akin to a perpetual motion machine?

Thank you.

Neil Pariser
New City, New York

What do you think?

Assuming a perfect reflector, which we know we can't get, and a perfect frictionless bearing, which we know we can't get, the Radiometer would keep turning. The problem is the same as with all perpetual motion machines. The loses quickly kill the motion. In this case, as already pointed out, the loses would be in the mirrors and in the bearing.

[Bored chemist]

"Assuming a perfect reflector, which we know we can't get, and a perfect frictionless bearing, which we know we can't get, the Radiometer would keep turning. "
No it wouldn't.
Didn't you understand the point I made about the gas in a radiometer being viscous?

[Vern]

Oops; yep you're right. I forgot about the gas inside. It would kill the action.