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Author Topic: In what way is a reflected photon degraded?  (Read 22608 times)

Offline JP

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In what way is a reflected photon degraded?
« Reply #75 on: 18/11/2011 14:11:02 »
Symmetry breaking doesn't really have much to do with this, nor do collisions.  It all comes down to understanding thermodynamics, which is beyond this discussion.  Perhaps we can start another thread with a discussion of why sometimes energy can be used to do work and sometimes it can't.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #76 on: 18/11/2011 16:11:59 »
Yeah, it's vague. But it's a valid question.

"A reflected photon imparts pressure to the reflecting surface.
This would imply that work has been done.
The photon has not been absorbed but it has been degraded by the loss of energy in some form.
In what way is the photon degraded by the loss of energy?" By Mike.

Can easily be seen to cover that question. And if we think entropy and thermodynamics is a branch of physics then we can use it in a mixed environment too. If that's not possible then it's not a branch of physics at all, which would be weird.
« Last Edit: 18/11/2011 16:37:46 by yor_on »
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #77 on: 18/11/2011 16:36:01 »
Yeah, it's vague. But it's a valid question.

It is indeed a valid question, which is why many brilliant physicists spent years working out the mathematics of thermodynamics and statistical mechanics in order to answer it.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #78 on: 18/11/2011 16:38:23 »
Sorry JP :) We collided there.
(the postings I mean)
=

Maybe you could say that the 'energy' becomes indeterministic in that it 'tags down'?
Or?
« Last Edit: 18/11/2011 16:46:25 by yor_on »
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #79 on: 18/11/2011 19:08:20 »
Yor_on, you're asking for very broad answers to things which don't have them.  Thermodynamics and statistical mechanics talk about very precise cases in which energy or information gets lost in an irreversible way in a system.  Your broad question about things being "degraded" or "tagging down" need to be put in much more precise terms for anyone to answer them scientifically.

However, I can give you a very hand-waving answer despite all that.  Imagine making a movie of a collision process.  If you can play the movie backwards and it still seems like a likely collision to you, then information/energy wasn't irreversibly lost in the collision.    If you can definitely tell which way was forward, then something was lost. 

For example, two billiard balls colliding is pretty reversible.  But the break shot in a game of pool is a pretty irreversible process. 

In the case of a single photon reflecting elastically off a mirror, it is reversible.  It would be just as plausible for a mirror to be moving, a photon to strike it, and the reflected photon carry away the energy of the mirror as it is for a photon to strike a stationary mirror and cause it to start moving.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #80 on: 19/11/2011 00:57:39 »
In what way would that matter? Are you thinking that as we can't define the process as going 'one way' (time reversibility) there is no 'tagging down'? Or is there something else you mean? As for why we have to look at it as a 'system' I find it a very nice conceptual tool. The question though, is where the 'energy' is thought to go, as 'heat' or as ?

Then, on the other hand, this is assuming that whether you look at it from one 'direction', or the other, we still are discussing objects colliding, not 'systems'.
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #81 on: 19/11/2011 01:15:00 »
Two particles colliding elastically is a system that happens to be reversible, so nothing is lost.  You don't have to lose energy to heat in a collision.

I'm not sure what "tagging down" means.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #82 on: 20/11/2011 17:24:08 »
Ah, just meaning a 'interaction' :)

So let's see, we got three ways of a 'interaction' then?

1. Losing 'energy', measured as a 'photon'
2. Gaining 'energy', measured as a 'wave'

(Both of those passing a medium as glass, or water.)

3. Neither, a interaction without anything happening, the 'elastic collision' in where we still see 'something' changing its direction at the 'rebound'. And that measured as 'waves', and 'photons'?

How about a experiment in where you 'split a photon (or wave, pick your choice :). Sending half back, letting half pass through some 'mirror'. Don't we call that a entanglement? Meaning that they are the exact same, except in their spin/polarisations (opposite)?

Can they be the same if I use any of those three definitions above?
3. right?

But the 'photon/wave' passing that mirror then, it should relate to either 1 or 2, shouldn't it?
« Last Edit: 20/11/2011 17:38:27 by yor_on »
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #83 on: 20/11/2011 17:34:23 »
The question is actually one of me trying to see what a 'wave function' is. If the 'wave function' is a local phenomena, belonging to something 'propagating' then it becomes tricky for me. If a 'wave function' is a expression of the surrounding parameters defining its 'existence' then there's 'nothing there', except the conservation laws.

Maybe :)
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #84 on: 21/11/2011 01:12:40 »
Ah, just meaning a 'interaction' :)

So let's see, we got three ways of a 'interaction' then?

No, you have an unlimited number of ways of having an interaction, so long as energy and momentum are conserved for the entire system.

Energy in = energy out
Momentum in  = momentum out

You're free to have any kind of interaction you want, so long as those hold (and potentially other conservation laws such as angular momentum, charge, etc.)

An elastic collision is one special case in which all the energy remains as kinetic energy.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #85 on: 21/11/2011 01:47:13 »
That one you should expand on JP.

And you're treating it as a 'system'. It's somewhat of a paradigm shift from treating it as objects 'interacting', isn't it? Not that I see it as wrong, actually I'm thinking along similar lines myself in my totally unscientific way :) discussing all interactions/outcomes as definitions from where and how we measure.

It makes more sense to look at it as 'systems', and becomes easier to define (conservation laws). But it also binds into how to see a entanglement for me. And there my question (as above) still would be how 'identical' those photons would be, in that entanglement where one 'part' passes the mirror and the other gets reflected.
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #86 on: 21/11/2011 03:31:07 »
That one you should expand on JP.

And you're treating it as a 'system'. It's somewhat of a paradigm shift from treating it as objects 'interacting', isn't it? Not that I see it as wrong, actually I'm thinking along similar lines myself in my totally unscientific way :) discussing all interactions/outcomes as definitions from where and how we measure.

You're right.  A system is a set of particles that are interacting.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #87 on: 21/11/2011 22:41:02 »
Yes, but as with using the conservation laws to describe it you treat the whole 'system' as one entity, and that's my point there.
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #88 on: 21/11/2011 22:46:15 »
Conservation laws hold whether you know about the whole system or not.  They're just not terribly useful for computing anything if you can't keep track of where all the energy went.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #89 on: 22/11/2011 01:42:53 »
I lose you there? Either you are treating each object by itself, and then define a collision and 'energies etc' for each one, relative that collision. Or you define it as a 'system' in where the 'whole system' is unchanged.

If you mean that you do treat it as individual objects meeting, although following conservation laws, each one exchanging 'whatever' with each other, then I think what I wrote before stands about the types of interactions described. And in the example I'm speaking of photons/waves passing a mirror, and getting 'split'. I think it is right? And it doesn't add up for me. Maybe you can point out where I go wrong there?
« Last Edit: 22/11/2011 01:44:54 by yor_on »
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #90 on: 22/11/2011 02:40:22 »
I think we probably agree and it's just a language issue.  I'm using system to mean a collection of interacting particles.  Each individual collision within that system should obey a set of conservation laws.

I'm not sure I follow your point about a photon and a mirror.  If you assume energy is conserved in the sense that Original Energy of Photon = Final Energy of Photon + Final Kinetic Energy of Mirror, and the same with momenta, then the photon will redshift, losing energy, and the mirror will move after the collision.  This is consistent with the idea that the photon gives up a little energy and momentum to the mirror. 
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #91 on: 22/11/2011 13:28:54 »
Well, it all goes together for me :) I was looking at my understanding of entanglements JP, and I'm not sure if I understand it at all. The same goes for a 'wave collapse' and how/where we would/could expect it. In reality, whatever I now mean by that, 'photons' and 'waves' should be 'the same as usual' as I see it, whether 'entangled' or not. 

So I used the idea of a 'mirror' of some kind, EM or otherwise to describe a situation in where you 'entangle' radiation by letting half pass through, the other half to 'bounce' back. Then I looked at the descriptions of how that light passing through the mirror would behave in a measurement comparing it to the idea of a 'elastic collision' as one might assume the light 'bouncing'. Finally I asked if they the would be 'identical'?

I mean, the polarisation/spin is definitely 'opposite', I'm sure of that one, so they are entangled in that motto. But if I got it right they should give us a different 'energy/momentum', also depending on how you measure, still not being 'identical' if my assumptions is right.

Those things all go into each other, don't they?
 

Offline JP

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In what way is a reflected photon degraded?
« Reply #92 on: 22/11/2011 14:35:45 »
I think you're confusing the matter a lot by jumping to advanced quantum mechanics without fully understanding classical collisions, including classical waves.  You really do need to build up from basics before advanced QM. 

Anyway, you can understand the effects you're describing purely classically.  Consider a wall with a bunch of holes in it and balls you're throwing at it.  If the ball hits the wall and bounces off, it will lose some energy and the wall will recoil a bit.  If the ball passes through a hole in the wall, it will continue on with the same energy and momentum as it initially had and the wall won't be effected.

Now, consider the case of a classical light wave hitting a 50% reflective mirror.  50% of the reflects off the mirror, which transfers some energy and momentum to the mirror while the light loses energy and changes direction, while 50% of the light passes through without causing the mirror to gain energy or momentum anything to the mirror.

If those two cases make sense, then the jump to QM is pretty simple.  If you send 1 photon through the mirror, then you're 50% likely to measure a final state that is: photon reflects + mirror recoils and 50% likely to measure the effect photon transmitted + mirror does not recoil.
 

Offline yor_on

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In what way is a reflected photon degraded?
« Reply #93 on: 22/11/2011 17:32:19 »
Ah well JP.

I'll let it rest for now.
 

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In what way is a reflected photon degraded?
« Reply #93 on: 22/11/2011 17:32:19 »

 

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