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#55918 - 05/24/16 09:52 PM Are we confusing photons with stars?
Bill S. Offline
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One problem seems to arise because of the switch between treating light as waves, and treating it as particles.

Let’s start with a star. It is a sphere which emits light in every direction. Looked at, at any point in space or time, the light progresses as a sphere. This is a reasonable wave model of light.

Sometimes there seems to be a tendency to consider a photon as though it were actually a sphere and could be observed from any angle. Such cannot be the case. A single photon would not radiate light; in fact, a single photon could not be observed until it was absorbed, after which it would no longer be a photon, or even light.

To complicate the matter further we cannot say that any travelling light is in any way particulate. It travels as a wave. This seems to be the cause of some confusion in that it is easy to slip into the “spherical radiation” image as soon as waves become significant. However, where light is emitted directionally the spherical radiation is not an appropriate model, however many photons might eventually be observed.

Popular science authors do not help with this issue when they talk about emitting light “one photon at a time”, because this suggests that the light travels as a particle.

It is tempting to progress to other aspects of the wave/particle duality, but these are the rambling thoughts of a “hitch-hiker” trying, one faltering step at a time, to make sense of reality, whatever that might be.
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#55919 - 05/24/16 10:56 PM Re: Are we confusing photons with stars? [Re: Bill S.]
paul Offline
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a waveicle

a single photon passing through both slits.

a photon of light is a expanding sphere.

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#55920 - 05/25/16 03:44 AM Re: Are we confusing photons with stars? [Re: Bill S.]
Orac Offline
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Originally Posted By: Bill S.
It is tempting to progress to other aspects of the wave/particle duality, but these are the rambling thoughts of a “hitch-hiker” trying, one faltering step at a time, to make sense of reality, whatever that might be.

Your thoughts are fine the problem is you a dealing with a very complex thing and it won't be tamed into a 3D world as there is no effective way to deal with the angular momentum because it isn't really spinning in the classical sense.

Try to describe orbital angular momentum of spinning light in either of your schemes above. You will resort to ending up with a 3D corkscrew image that you know doesn't really match the reality but it's the best you can do. If you want to see how bad your image is, your corkscrew represents the momentum now try and draw the E/B field positions on the corkscrew. Worse than that your corkscrew has all the normal collapse and spread properties of light so now try to deal with that. Fun trying to draw a 3D spreading sphere wave corkscrewing in every direction.

Want a good laugh send the OAM light through a double slit and try and describe it smile

You can't work with any of the results without introducing a 4th dimension .. there is no way around it. Generally the easiest way to then represent it is turn time into a real dimension.

If you look when we try to describe light OAM
http://phys.org/news/2016-04-characterize.html
Quote:
"The conjugate variable of OAM, angle, is in many ways similar to phase, which is itself similar to time. So perhaps the lessons learned here can be applied, in other experiments, to systems where we need to measure time."

You can play around with descriptions like I did in extending classical physics before but there are some results you just can't explain without putting the angular momentum somewhere other than in our 3D world. This goes back to our start statement quantum spin is nothing like classical spin and so your 3D description fails.

So now we have moved from something I can make a cute Classical Physics fairytale on to something that just can't be described in classical physics.

Out of curiousity ask Prof Chris Baird to describe light OAM to you, I am interested how he deals with it Bill S. He basically gave you the same classical fairytale as me but I can't work out how to extend it to OAM other than throw my hands up. It's a like entanglement to me, I have to switch frameworks to QM to deal with it.

We are back to this problem .. how do I describe this without making time real or having a 4th dimension. I need an animation to describe it I can't draw it. That is spin 1 but a photon with OAM takes on some similar movements on the E/B fields.

You can create some cute effects by controlling the spin
https://www.sciencedaily.com/releases/2015/04/150415103314.htm
Quote:
In fact, the light speeds up and slows down as it travels, periodically switching from one mode to the other. Following its helical path through space, the helix appears to wind up very tightly as it accelerates, and winds down very loosely as it decelerates. It is intriguing that by "twisting the twist," nature provides an additional momentum to the field causing it to accelerate as it spins.


Edited by Orac (05/25/16 06:15 AM)
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#55922 - 05/26/16 12:24 PM Re: Are we confusing photons with stars? [Re: Orac]
Bill S. Offline
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Registered: 08/20/10
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There seems to be some discussion as to whether or not OAM is intrinsic to individual photons, or is a characteristic of a light beam which can then be applied to a photon.

http://physics.stackexchange.com/questions/99016/orbital-angular-momentum-of-photon

The question that arises in my mind is: if detection destroys a photon, how can we test any theory relating to its nature prior to detection?
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#55924 - 05/26/16 12:41 PM Re: Are we confusing photons with stars? [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
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Loc: Currently Illinois, USA
Some of the replies, as is prone on physics forums are wrong.

You can apply OAM to a single photon, and we do.

https://en.wikipedia.org/wiki/Orbital_angular_momentum_of_light

Quote:
This expression is generally nonvanishing when the wave is not cylindrically symmetric. In particular, in a quantum theory, individual photons may have the following values of the OAM:

Probably run a search "single photon OAM" for a list of various experimental results and tests.

The problem is single photon OAM can't be described in classical physics which is the same answer wikipedia is giving. As I stated I need another dimension to describe it mathematically or by language. The same problem as our collapse of Paul's wavicle you are going to have to make a cute fairytale and I can't think how to do it.

That is why I was wondering if Prof Chris Baird has a way of dealing with this for layman using classical physics.

For the record you can also entangle single photons carrying OAM, classical physics worst nightmare. Which answers your final question about how do we know, if detection destroys the photon. You measure the entangled partner (destroying it) but leaving the original to proceed as normal. The record for photon entanglement is with around 3000 atoms so you can get a number of measurements if you want. Remember destroying an entangled photon does nothing to the original partner you just don't share QM correlations anymore.

Now you should be able to understand how this experiment works. Your entangled photon you measure never sees the target but the entangled partner does so you can see it.
http://news.nationalgeographic.com/news/...photon-science/

So in answer to your question, well we are more than a little sure, by experiments. This is that thing that no other framework makes predictions that are correct. Infact most can't even explain it much less predict anything. Classical physics just stops and says what happens can't smile

Edit: LOL was seeing what was out there in search land and look different answer on same forum, same citation as me smile
http://physics.stackexchange.com/questio...-single-photons

Here is a pretty cute use of single photon OAM
http://www.extremetech.com/extreme/15583...uantum-internet


Edited by Orac (05/26/16 01:34 PM)
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