Science Forums: Duda Jarek - Viewing Profile - Science Forums

Noether theorem says that with symmetries come conservation laws and so because of time, translation and rotation symmetry, EM field itself guards energy, momentum and angular momentum conservation.
While atom deexcitation there clearly appears energy and (orbital) angular momentum difference, so there should be created EM field configuration carrying this difference.
Photon's angular momentum is usually imagined as something only "quantum-transcendental", but in fact it is very real angular momentum.
For example Richard Beth in 1936 has measured the tiny reaction torque due to the change in polarization of light on passage through a quartz wave plate: http://prola.aps.org...R/v50/i2/p115_1
Here is nice video of rotating macroscopic object using circularly polarized light - at about 20 second the polarization was switched to opposite one:


Is optical photon something more than just EM wave carrying energy, momentum and angular momentum?
If not - what more? Other than EM interactions? Some electric/magnetic moments?
Is it just a "twist-like wave"? - like behind marine propeller, but this time in viscosity-free environment and so does not dissipate - can travel undeformed for years (is soliton) from a concrete single atom to anther one ...
One would say that because of spin conservation, it has also "spin 1" - e.g. due to electron changing spin from -1/2 to +1/2 ... but isn't it just 180deg rotation - twist again? Especially that in opposite to other particles with spin, photon doesn't have magnetic dipole moment...
Another question: why it has momentum? Is it that it was just required for this kind of waves or maybe there is some momentum change required for atom deexcitation itself?

There are getting popularity great Couder's experiments about classical objects having wave-particle duality: oil droplets on vertically vibrating liquid surface - constantly creating periodic waves around - interaction with these waves allows to observe 'quantum effects':interference pattern in double-slit experiment, tunneling depending on practically random hidden parameters or orbit quatization condition - that particle has to 'find a resonance' with field perturbations it creates - after one orbit, its internal phase has to return to the initial state.
It's difficult to find good intuition about these experiments from only static pictures - the first time I had occasion to see videos was on recent congress on emergent quantum mechanics where Couder had the opening lecture and most of speakers were excited about these experiments. Fortunately I've recently found youtube video of these experiments:


The main qualitative difference with physics is that while Couder uses external clock, particles should rather have internal one - such understanding of wave-particle duality was started by de Broglie in his doctoral thesis:
that with particle's energy: E = mc^2
comes some internal periodic process: E = hf
It is reminded in very interesting Hestenes paper, in which there is also described recent experimental confirmation of this effect (called e.g. zitterbewegung): http://fqxi.org/data...n_time_essa.pdf
Such internal periodic motion creates periodic wave-like perturbations of surrounding field - giving localized entity also wave nature ... localized constructions of the field are called soltions, so it suggests to search for particles solitons models, which often have such internal periodic motion, like breathers.

What do you think about these experiments? About such understanding of wave-particle duality?
Have particles both natures simultaneously, or maybe only one of them at the time?
In such case when and how it is switched? What about Afshar experiment?

I was just asked this looking trivial question ... and honestly it doesn't look so simple.
Atom is just a bunch of particles hold mainly by electromagnetic interactions - doesn't pure Coulomb repulsion tell that some electrons should immediately run away?
Would e.g. F- or Cl- atom be stable while just flying in empty vacuum? How stable?

My first answer is the magnetic attraction between oppositely directed magnets - electron couples?
But the distance between them seems to be too large for such attraction (1/r^4) ... however there are suggestions that magnetic conjugation still works on larger distance, like for Cooper pairs or that neutral positronium scatters like a charged particle: http://physicsworld....icle/news/44265

In quantum eraser experiments, getting information about one entangled photon decides if the second photon behaves classically or quantum (interfere). Optical lengths for these photons chooses time order of these events, so we can delay the "decision" to happen after what it decides about. But in "standard version" of such delayed choice quantum erasure this decision is made randomly by physics.
I've just found much stronger version - in which we can control this decision affecting earlier events.
Here is a decade old Phys. Rev. A paper about its successful realization and here is simple explanation:

We produce two entangled photons - first spin up, second spin down or oppositely.
Photon s comes through double slit on which there are installed two different quarter wave plates changing polarization to circular in two different ways.
Finally there are two possibilities:
u d R L
d u L R
where columns are: linear polarization of p, initial linear polarization of s, circular polarization of s after going through slit 1, circular polarization of s after going through slit 2.
So if we know only the final circular polarization of s, we still don't know which slit was chosen, so we should get interference. But if we additionally know if p is up or down, we would know which slit was chosen and so interference pattern would disappear.
So let us add polarizer on p path - depending on its rotation we can or cannot get required information - rotating it we choose between classical and interfering behavior of s ... but depending on optical lengths, this choice can be made later ...

Why we cannot send information back in time this way?
For example placing s detector in the first interference minimum - while brightness of laser is constant, rotating p polarizer should affect the average number of counts of s detector.
What for? For example to construct computer with time loop using many such single bit channels - immediately solving NP hard problems like finding satisfying cryptokey (used to decrypt doesn't produce noise):

Physics from QFT to GRT is Lagrangian mechanics - finds action optimizing history of field configuration - e.g. closing hypothetical causal time-loops, like solving the problem we gave it.
Ok, the problem is when there is no satisfying input - time paradoxes, so physics would have to lie to break a weakest link of such reason-result loop.
Could it lie? I think it could - there is plenty of thermodynamical degrees of freedom which seems random for us, but if we could create additional constrains like causal time loops, physics could use these degrees of freedom to break a weakest link of such loop.

What is wrong with this picture?

We are used to stationary Schrödinger equation. Slowly varying potential makes it more complicated. Physics should smoothen rapidly varying potential ... but let us discuss what's happening while theoretical rapid change of potential (no adiabatic approximation).
For example imagine that potential has one minimum before the switch moment and a different one after (e.g. capacitor charged in one way then in opposite one) - like in this picture:



In minus infinity electron should be in the ground state of one potential and in plus infinity in ground state of the other - the question is how the transition of wavefunction would look like?
The main problem is that quantum mechanics is time symmetric - such transition shouldn't be instant, so this symmetry suggests that the middle of this transition is the switch moment ... but it means that the wavefunction has started evolving before the switch???

I have to admit I don't understand the situation from perspective of quantum mechanics.
The above picture used Maximal Entropy Random Walk instead (page 48 of http://arxiv.org/abs/1111.2253 ) - corrected Brownian motion to finally become thermodynamical model - not only approximate maximum uncertainty principle, but really maximizing entropy. Thanks of it, it doesn't longer disagree with thermodynamical predictions of quantum mechanics - the equilibrium dynamical state has probability density being exactly the squares of the lowest energy eigenfunction of Schrödinger's Hamiltonian.
So this model agrees with the ground state in plus/minus infinities, but also naturally explains the transition - it indeed starts before the switch, but this time there is nothing strange about it: this model is thermodynamical - not fundamental but effective: we already know the history of potential and it allows us to estimate the best probability distribution of the particle. For example knowing that later it will be in another potential well allows us to tell that earlier it should be nearby.
There is another strange thing about above picture - its Ehrenfest Newton's equation has opposite sign - the particle accelerates uphill then decelerates downhill ... but it's just required e.g. to transport density between these minimums ...

But what's going on here in standard quantum mechanics?
Would the wavefunction start transforming in the moment of change or before?