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Old 06-07-2015, 03:03 PM
 
Location: where you sip the tea of the breasts of the spinsters of Utica
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What happens when electomagnetic radiation such as light or microwaves is "trapped" in a container of perfect reflectors for that frequency?

Let's say that a microwave oven had an inner box of perfect reflectors - there IS some sort of reflective material in ordinary ones to keep the microwaves inside so they don't heat outside - but what if it were perfectly reflective, so that there were no escape either through absorption or allowing them to pass? Would the microwaves keep bouncing around inside forever even after the power was shut off?

Same thing for light .... if you had a room with walls of perfectly reflecting mirrors, with a perfect vacuum in it, and could briefly shine a light through a one-way mirror into the room, would it stay lit inside for millenia?
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Old 06-10-2015, 10:50 AM
 
Location: Somewhere in northern Alabama
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No, and no. First, the air in the container would convert the energy to sensible heat, and second - by definition - there is no "perfect" reflector. Whatever is being reflected has to impinge on a surface that reverses the direction. That reversal requires a manipulation of the energy.

The simpler version of your question is the infinitely bouncing ball idea, which is also false.
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Old 06-10-2015, 04:56 PM
 
Location: Heart of Dixie
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Quote:
Originally Posted by harry chickpea View Post
No, and no. First, the air in the container would convert the energy to sensible heat, and second - by definition - there is no "perfect" reflector. Whatever is being reflected has to impinge on a surface that reverses the direction. That reversal requires a manipulation of the energy...
I stay out of these discussions because they inevitably lead to "But, let's say it's in a perfect vacuum, and the surfaces ARE perfect reflectors, and no energy is transferred/lost to the walls." This leads to a physics question with the laws of physics being denied.
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Old 06-10-2015, 05:03 PM
 
Location: where you sip the tea of the breasts of the spinsters of Utica
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Thanks, and thanks!
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Old 06-10-2015, 09:03 PM
 
Location: Somewhere in northern Alabama
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Originally Posted by Dirt Grinder View Post
I stay out of these discussions because they inevitably lead to "But, let's say it's in a perfect vacuum, and the surfaces ARE perfect reflectors, and no energy is transferred/lost to the walls." This leads to a physics question with the laws of physics being denied.
I'm just a glutton for punishment. At the point you describe, it would no longer be a physics question but a koan or philosophical conundrum or even moral exercise. Those have a different type of value, even though - as you correctly note - it denies the laws of physics. If a sentient brain is kept alive for an infinite time in a perfectly reflective box, is it experiencing time and space, or only itself? Schrödinger's cat is a similar crossover. As a factual experiment of physics, it would fail miserably on a number of levels. As a metaphoric lesson it can be helpful.
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Old 06-13-2015, 09:24 AM
 
Location: Westwood, MA
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This is a much less complicated question then everyone seems to think. The behavior of the system can be described by the reflectivity of the walls. The number of reflections before absorption approaches infinity as the reflectivity approaches one.

The fact that there aren't actually any perfect reflectors only prevents you from doing the actual experiment, not the thought experiment. This kind of exercise is extremely common both in teaching physics and in practical applications. If you could only solve problems at their full level of complexity very little would get done. Understanding which assumptions are responsible for which behavior is also useful to both the student and the working physicist.

This problem actually has practical implications. It's essentially a "high Q optical resonator"

https://www.vahala.caltech.edu/Research/HighQ
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Old 06-13-2015, 11:02 PM
 
Location: Heart of Dixie
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Originally Posted by jayrandom View Post
this is a much less complicated question then everyone seems to think.
than!!!!!! Words matter.
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Old 06-13-2015, 11:15 PM
 
Location: where you sip the tea of the breasts of the spinsters of Utica
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So what does a high Q optical resonator do? Is it used in fiber optics?
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Old 06-15-2015, 07:00 AM
 
Location: Westwood, MA
3,502 posts, read 4,379,683 times
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Quote:
Originally Posted by Dirt Grinder View Post
than!!!!!! Words matter.
You caught me! You win three grammar police points. I was typing this on my tiny phone (which makes editing for errors nearly impossible). Even if it weren't I generally don't take the time to revise my posts. Why not? Because this is an informal setting and it isn't worth the effort. Words matter, but context matters, too.

You've contributed absolutely nothing of merit to this thread and by your own account are (not our) staying out of it. The idea that relaxing assumptions means you are denying the laws of physics indicates a significant misunderstanding of how scientists tackle real problems. If you're (not your) going to stay out of this thread on the technical side please don't think that your (not you're) proofreading skills are required, either. It's just extremely rude.

Last edited by jayrandom; 06-15-2015 at 07:33 AM..
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Old 06-15-2015, 07:31 AM
 
Location: Westwood, MA
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Originally Posted by Woof View Post
So what does a high Q optical resonator do? Is it used in fiber optics?
They have a variety of applications but are usually based on 1-dimensional confinement. Most lasers are based on these sorts of optical resonators. In fact, an optical fiber is a good example of something that does have nearly perfect reflection, albeit confined in only two dimensions and not three. An optical ring resonator confines the light in the third dimension by making a fiber "loop". They can be used as filters, as chemical- and bio-sensors, and even for detecting acceleration.

Your original question is related more to what is called a photonic crystal. These systems are generally periodic and can confine light in two or three dimensions. These aren't quite a box with reflective walls but the concept is quite similar.
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