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Author Topic: does light 'bend' to a magnetic field?  (Read 16449 times)

Offline yor_on

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Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #1 on: 11/09/2014 12:59:32 »
I read it, and was left scratching my head. Changing phase isn't anything special, but I didn't see how that's a magnetic field for light. Light is an electromagnetic wave, we make electrons (and positrons) out of light in pair production, and a magnetic field is a "curl" or "rotor" field. Have a look at some images showing electron motion in a magnetic field. The article doesn't seem to be much like that, or anything like Faraday rotation where light is rotated in a magnetic field. I am however reminded of Taming Light at the Nanoscale:

"Look around, and you will probably see numerous electronic and optical gadgets, such as mobile phones, personal digital assistants, laptops, TVs and digital cameras. These may all do different things but they have one thing in common: in the electronic circuits that drive these devices, charged particles flow through components and impart power via what is known as the conduction current. But is the motion of charged particles the only current we have available? Those with a good memory for Maxwell’s equations of electromagnetism will remember that in addition to the familiar electric field, E, there is also a displacement field, D..."

Light is essentially alternating displacement current, and you can contrive circuitry for displacement current in a similar fashion to the way you can devise circuitry for conduction current. The article you referred to is obviously something on those lines, but I didn't "get it". Maybe the work is reported elsewhere in a different fashion that makes things clearer?
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #2 on: 11/09/2014 13:49:13 »
Yep, confused me too. The heading of the article is "Defying physics, engineers prove a magnetic field for light." but that isn't what the article seems to be discussing? "Until now. Lipson, a leader in the emerging field of silicon photonics – sending light through waveguides instead of currents through wires – and colleagues have shown that an equivalent field for light does exist."

equivalent? as if optics bending light?

"When a light wave travels under normal conditions, its phase is proportional to how far it traveled, but it is unaffected by what path it has taken. Just like a magnetic field causes a current to switch direction, the researchers showed that by modulating the light with their device, ...... they could make the phase of the light change not only as a function of distance traveled, but also by the shape of its path....."

would that be what they mean, that you change phase by the path taken? And then imagining this equivalent to?? A electrons path inside a magnetic field?

I'm still confused :)
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #3 on: 11/09/2014 14:23:56 »
Me too. There's things out there like light bends itself round corners and tying light in knots and how to turn light into matter. But this doesn't seem to be much like that. We need to find another report about this, or the original paper. 
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #4 on: 11/09/2014 15:26:17 »
Here's the paper on arXiv: http://arxiv.org/abs/1309.5269

"Photons are neutral particles that do not interact directly with a magnetic field. However, recent theoretical work has shown that an effective magnetic field for photons can exist if the phase of light would change with its propagating direction. This direction-dependent phase indicates the presence of an effective magnetic field as shown for electrons experimentally in the Aharonov-Bohm experiment. Here we replicate this experiment using photons. In order to create this effective magnetic field, we construct an on-chip silicon-based Ramsey-type interferometer. This interferometer has been traditionally used to probe the phase of atomic states, and here we apply it to probe the phase of photonic states. We experimentally observe a phase change, i.e. an effective magnetic field flux from 0 to 2pi. In an Aharonov-Bohm configuration for electrons, considering the device geometry, this flux corresponds to an effective magnetic field of 0.2 Gauss."

I still don't get it. The Aharonov-Bohm experiment demonstrates four-potential. There's no electric or magnetic field outside the solenoid, but the electron wave is shifted. It was first predicted by Ehrenberg and Siday in their semi-classical paper. Here's a picture from it. If you've got a wall-mounted hosepipe, watch it rotate a little when you turn the water on. It's the electromagnetic equivalent of that. Hmmn. A magnetic field is a "rotor" field, maybe that's the connection.
 

Offline JP

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Re: does light 'bend' to a magnetic field?
« Reply #5 on: 11/09/2014 18:23:54 »
Not to mention that "direction-dependent phase" is well known in optics (anisotropy of the index of refraction).  The press release is poorly written and misleading. 

What they appear to be doing is using the fact that matter can interact with a magnetic field, and that light waves passing through that matter will depend on the matter's properties.  Therefore, by changing the magnetic field, you can indirectly influence light (by acting on whatever matter its passing through).  It seems a bit overblown to say this is demonstration of the Aharanov-Bohm effect for light, but its cool that the same mathematics describes the light waves in this experiment and it may be useful in designing photonic circuits.

The original paper that did this experiment, which is much clearer on what's going on, is here:
http://www.nature.com/ncomms/2014/140130/ncomms4225/pdf/ncomms4225.pdf
« Last Edit: 11/09/2014 18:26:40 by JP »
 

Offline jeffreyH

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Re: does light 'bend' to a magnetic field?
« Reply #6 on: 11/09/2014 18:46:44 »
So what exactly is the magnetic flux from 0 to 2pi referring to?
 

Offline evan_au

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Re: does light 'bend' to a magnetic field?
« Reply #7 on: 11/09/2014 22:35:23 »
Quote
So what exactly is the magnetic flux from 0 to 2pi referring to?

When you are measuring phase and/or angles, you can use degrees, where there is 360 degrees in a circle. This was invented by the Babylonians (if not earlier), and is taught to students at school.

Mathematicians, physicists and engineers often prefer to measure phase and/or angles in units called "radians", which is shown by the abbreviation "RAD" on a scientific calculator. In this scheme, there are 2pi radians in a circle. Radians have some useful mathematical properties when you combine them with imaginary numbers - for example there is a close relationship between the ex function and sin(x)+cos(x).

This results in one of the neatest results in mathematics eπi+1=0, which combines 5 of the fundamental constants of mathematics. (I think it was discovered by Euler?)
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #8 on: 12/09/2014 11:27:13 »
Ok, downloading pdf:s here :) Hopefully they will make sense to me.
==

Found a really interesting effect from the Aharanov-Bohm effect for light, also explaining it somewhat. http://phys.org/news/2014-05-breakthrough-paper-aharonov-bohm-effect-published.html Further down there's a link to "Aharonov–Bohm effect in the tunnelling of a quantum rotor in a linear Paul trap", which may fit your ideas of how the news release interpreted it John?

Or maybe not?
It's the same, just got stuck on this Quantum rotor, rereading you :)
« Last Edit: 12/09/2014 11:42:45 by yor_on »
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #9 on: 12/09/2014 11:38:00 »
Not to mention that "direction-dependent phase" is well known in optics (anisotropy of the index of refraction).  The press release is poorly written and misleading. 

What they appear to be doing is using the fact that matter can interact with a magnetic field, and that light waves passing through that matter will depend on the matter's properties.  Therefore, by changing the magnetic field, you can indirectly influence light (by acting on whatever matter its passing through).  It seems a bit overblown to say this is demonstration of the Aharanov-Bohm effect for light, but its cool that the same mathematics describes the light waves in this experiment and it may be useful in designing photonic circuits.

The original paper that did this experiment, which is much clearer on what's going on, is here:
http://www.nature.com/ncomms/2014/140130/ncomms4225/pdf/ncomms4225.pdf
Thanks for that, JP.

You know, I think this might be more important than the authors realise, in that in gamma-gamma pair production there's  photon-photon interaction. Not a photon-phonon, interaction, a photon-photon interaction. One for another day though.
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #10 on: 12/09/2014 11:52:49 »
Wouldn't it be better to call it a localized field, set under certain conditions, that will produce photons acting a specific way? Gives me a headache thinking of it as a, constantly existing, photon propagating, interacting with a phonon.
=

Maybe one could call it a 'constantly existing photon potential' propagating inside that field instead :)
Luv that
« Last Edit: 12/09/2014 11:57:58 by yor_on »
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #11 on: 12/09/2014 12:42:13 »
No, field is the wrong word. This is "an Aharonov-Bohm effect for light", and the Aharonov-Bohm effect is where there's no detectable E or B field, but there is detectable effect. Have a look at the Aharonov-Bohm effect on Wikipedia:

"Thus the Aharonov–Bohm effect validates the view that forces are an incomplete way to formulate physics, and potential energies must be used instead. In fact Richard Feynman complained[citation needed] that he had been taught electromagnetism from the perspective of electromagnetic fields, and he wished later in life he had been taught to think in terms of the electromagnetic potential instead, as this would be more fundamental."

Potential is more fundamental than field. Have a look at the Wiki  derivation from electromagnetic theory: "the curl operator on one side of these equations results in first-order spacial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first order in time". It's like four-potential A is the height of the hill, the electromagnetic field Fμv is the outline of it, the electric field E is the slope of it, and magnetic field B is the rate of change of slope. But in the Aharonov-Bohm effect there's a plateau all round the solenoid, only it's twisted around. Does that make sense? JP can you offer a better analogy to deliver some understanding?

I've got to dash, but I've just seen your update, and noticed this in the article:

"This purely quantum effect, verified in a multitude of experimental set-ups, is at the heart of much of modern physics".

It isn't a purely quantum effect. It was predicted by Ehrenberg and Siday in their 1949 paper "The Refractive Index in Electron Optics and the Principles of Dynamics". The Aharonov-Bohm effect was predicted in an optics paper, and here we are sixty five years later with an Aharonov-Bohm effect for light.
 

Offline jeffreyH

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Re: does light 'bend' to a magnetic field?
« Reply #12 on: 12/09/2014 18:04:44 »
Quote
So what exactly is the magnetic flux from 0 to 2pi referring to?

When you are measuring phase and/or angles, you can use degrees, where there is 360 degrees in a circle. This was invented by the Babylonians (if not earlier), and is taught to students at school.

Mathematicians, physicists and engineers often prefer to measure phase and/or angles in units called "radians", which is shown by the abbreviation "RAD" on a scientific calculator. In this scheme, there are 2pi radians in a circle. Radians have some useful mathematical properties when you combine them with imaginary numbers - for example there is a close relationship between the ex function and sin(x)+cos(x).

This results in one of the neatest results in mathematics eπi+1=0, which combines 5 of the fundamental constants of mathematics. (I think it was discovered by Euler?)

That was not what I meant although very interesting answer. I meant are they saying the wave of the photon can be shown to exhibit magnetism when non-symmetrical.
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #13 on: 16/09/2014 15:56:41 »
Yeah, you're right John. I also think so, but it's the first time I've seen someone define it here :) potentials are expressions that we later, after a outcome, can define as a product of a presumed 'field'. But if we lift forward somethings potential we also seem to be leaving a materialistic reality, in where stuff as EM exists. A potential is no boson. Anyone seeing a cave here :)
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #14 on: 16/09/2014 19:37:30 »
There's a lot of problems with the way electromagnetism is taught and understood, yor_on. I quite like this paper myself, wherein the photon is a "pulse" of four-potential. It's something like a lemon, and the space and time derivatives of potential gives you the archetypal sinusoidal waveforms. By the way, we maybe referred to the wrong paper above, see this: 

http://www.nature.com/nphoton/journal/v8/n9/full/nphoton.2014.177.html
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #15 on: 17/09/2014 00:48:00 »
Don't know , defining a point like particle as some sort of 'wave-train' containing a number of oscillations/waves seems a tricky assumption to me. Doing so a photon can't be point-like (as well as dimension less) anymore as I think of it? Then again, I need to read the whole paper, don't I :) then there is  the other link that speaks of "The photon and the laser pulse interfere and either reinforce or cancel one another out, depending on their shapes. "

Not sure how I should read that, waves do cancel and reinforce each other, but a light quanta (photon) doing it? We can send singular photons so it should be possible to test if you can get two photons to cancel each other? But if we could it seems to me that a photon can't be a final state, aka a discrete energy quanta any more. It also say that "Single photons and other quantum light states can also be generated in a variety of complex shapes." without defining how we can know this?
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #16 on: 17/09/2014 02:31:11 »
It may just be the way it is expressed through words (nomenclature). you have https://en.wikipedia.org/wiki/Wave_packet

There it states

"Quantum mechanics ascribes a special significance to the wave packet: it is interpreted as a "probability wave", describing the probability that a particle or particles in a particular state will be measured to have a given position and momentum.

It is in this way related to the wave function.

Through application of the Schrödinger equation in quantum mechanics, it is possible to deduce the time evolution of a system, similar to the process of the Hamiltonian formalism in classical mechanics.

The wave packet is thus a mathematical solution to the Schrödinger equation. The area under the absolute square of the wave packet solution is interpreted as the probability of finding the particle in a given region. The dispersive character of solutions of the Schrödinger equation has played an important role in rejecting Schrödinger's original interpretation, and accepting the Born rule."

I like that one better, than me imagining a photon as consisting of 'physical waves' making up a 'discrete' energy quanta.
=

Heh, finally reading the paper through it seems as the guys writing it agrees with me ::))
Just got stuck at the beginning there. and yes, I think it's a nice paper.
« Last Edit: 17/09/2014 02:44:11 by yor_on »
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #17 on: 17/09/2014 09:19:08 »
Don't pay too much attention to all that probability stuff. It's something of a popscience myth associated with the Copenhagen Interpretation and "quantum mysticism". It just doesn't feature in classical electromagnetism, or QED, or optics. The photon isn't some point particle, instead E=hf applies, because it has a wave nature. Long wave radio-wave photons can have a wavelength of kilometers. Have a look into weak measurement on physicsworld. The photon is no more a point particle than a seismic wave. See The secret lives of photons revealed and look at this picture:


3D plot of a single photon showing wave-like behaviour
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #18 on: 17/09/2014 23:12:50 »
I don't know John. I enjoyed it when the pdf you linked here wrote "The single photon must not necessarily be a wave train containing intrinsic oscillations, but it is rather a single pulse and it may be viewed as such one."

That is my view too. I don't see how it becomes a wave, behaving 'photon like'. What I can accept so far is the duality, that combined with the setup will give you one or the other. and maybe ? A field that uses both? But that field is one he* of a mystery to me.

 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #19 on: 17/09/2014 23:24:01 »
to see why consider it such as both time dilation and length contractions are true statements. That's what I think they are at least :) The world becoming observer dependent. Now create a system, let's call all objects in it moving 'observers' of each other, inside some common space. Motion exist and is easy to prove logically, so they are indeed moving. Each one of them will define this 'common space' differently. and all of them will be correct. there exist no (common) gold standard experimentally. Now try to put up a unitary field that take all this into consideration, giving us one practical description of that space, I will be pleased if you succeed. You cannot use Lorentz transformations for it, because they are simply logic, knitting together my description of a reality with yours. The space in where a Lorentz transformation exist is purely mathematical, but the universe you meet exist, and will always be locally true for you.
=

you might say that a Lorentz transformation, in itself, is neither here nor there. It's a link between two observers, giving a necessary logic and causality to a universe. because that is what physics are, a logic, or several depending on how you look at it. Same with everything else, all sciences assume logic to exist.
=

you could look at it this way. what do you need to construct a 'common space'?
Causality should be there, if you want it to be understandable.
Causality is a primary logic you need. Break it and the common space we define, our 'universe' becomes something else. What causality need to exist is a arrow. That arrow need a direction. Each one of us have a locally true, four dimensional reality, with a locally true arrow pointing us from our birth to our death. Then we have causality that knits together my local reality with yours, into that 'common space' or universe, as described through a Lorentz transformation.
==

The 'gold standard' that do exist, and create our physics are all local definitions, as 'c' and other constants. We use those for all sciences, and when we assume the universe to have the same physics, laws etc we go out from such local definitions, expecting them to be true wherever we go to measure them. So locally no ambiguities exist, there we can agree on that my time is yours too :) If it wasn't we wouldn't have 'repeatable experiments' and a computer would consist of 'black magic', as would the universe, or 'white' for those preferring that.

So relativity could be seen as a 'wrong statement', in a sense, as everything isn't relative, not when locally defined. But observing that common space it becomes relative, observer dependent.
« Last Edit: 18/09/2014 00:12:44 by yor_on »
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #20 on: 18/09/2014 12:12:43 »
If you would treat the universe as some matrix consisting of points, 'observers', of each other. Then each point have a adjustable slide, consisting of mass, relative motion and acceleration, versus all other points. All points exchangeable with each other, meaning that locally you never will experience any 'time dilation' or 'Lorenz Fitzgerald contraction'. Those present themselves only when comparing between frames of reference, and when you compare you presume your local reality to be the gold standard from where all other frames are defined.

Doing so, also using a logic resting on local measurements we automatically get a gold standard valid for us all. That's a primary reason why your clock won't 'stop' locally, or 'change its pace', you hovering over some event horizon.

It's easy to see why Einstein spent a lot of time looking for that 'fifth dimension' in where you could join observer dependencies, into one whole description of a 'common space' containing us all. And a 'field' needs that description too I think.

A added complexity to a description of that common space should be that for each observer (ideally) agreeing on observing a same frame of reference (point) no one will agree on the others description of a time or distance. If that was a illusion it wouldn't be as hard to digest. But it's no illusion, as I see it, at relativistic speeds your speed is not enough to explain how a universe can shrink for you.

So each observer have its own, very real, description of a common space as I think. If we now use QM to define it, presuming this to be real effects, which they are (NIST) then it has no end that I can define. And that is a question about discreteness too, to me. Is there a discreteness to the universe? Can I use some matrix with 'points', locally equivalent? The logic we use presume a local equivalence (as repeatable experiments) so? If there is a 'point' existing, how does it relate to those other points?

And that is decoherence, I suspect :)

and now I will stop mumbling in my beard ::))
 

Offline JohnDuffield

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Re: does light 'bend' to a magnetic field?
« Reply #21 on: 18/09/2014 12:46:22 »
Relativity isn't wrong, yor_on. What's wrong is the way people misunderstand it and make it complicated.

Have a read of The Other Meaning of Special Relativity by Robert Close. The crucial thing to throw into the mix is the wave nature of matter. The universe isn't some matrix consisting of points. The universe consists of waves. Or fields and waves if you prefer. But it doesn't much matter because there's not much difference between a field-variation and a wave, or between a standing wave and a field. That's why quantum field theory works.
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #22 on: 18/09/2014 12:55:23 »
The questions above is not answered John, the equivalence we define physics from need to be answered in a 'wave universe' too.
 

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Re: does light 'bend' to a magnetic field?
« Reply #23 on: 18/09/2014 12:59:21 »
Can you rephrase that please?

And do read that paper. It explains why you always measure the speed of light to be the same.
 

Offline yor_on

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Re: does light 'bend' to a magnetic field?
« Reply #24 on: 18/09/2014 13:35:11 »
The equivalence creating physics, repeatable experiments. To get to it you need to make all 'points' equivalent relative constants. As for Robert Close, he's new to me. had a look and read.

"A Dirac-like equation is derived by factoring the one-dimensional wave equation and then generalizing with rotations of velocity and polarization. The general vector wave equation can then be derived from the resulting 'bispinor' equation.  The general solution of the one-dimensional wave equation is a superposition of forward- and backward-propagating waves. These two independent states are separated in space by a 180 degree rotation and therefore form a spin 1/2 system. Complex spinors (Dirac spinors or bispinors) are required in order to rotate the wave velocities in three dimensions. Waves can propagate along curved paths (e.g. spirals) with mass  proportional to the velocity rotation rate."

He seems to use some sort of geometric definition of why mass (matter) exist, or would that be dimensional definition? It's a lot of theory to digest there actually :) As for the paper I found it confusing. How does he define relative motion? How does he define the boundaries of this wave universe?
 

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Re: does light 'bend' to a magnetic field?
« Reply #24 on: 18/09/2014 13:35:11 »

 

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