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Light is a directional wave...it is a linear wave which propagates in a specific direction.

QuoteLight is a directional wave...it is a linear wave which propagates in a specific direction. Not strictly true. It is, essentially, a spherical wave. It travels in the same speed in all directions. All 'rays' cover a range of angles, however small. The wavefront is spherical because each part of it has traveled the same distance after a given time.

Quote from: sophiecentaur on 11/12/2007 17:51:11QuoteLight is a directional wave...it is a linear wave which propagates in a specific direction. Not strictly true. It is, essentially, a spherical wave. It travels in the same speed in all directions. All 'rays' cover a range of angles, however small. The wavefront is spherical because each part of it has traveled the same distance after a given time. Both are true. A light field can be written in terms of summing a bunch of plane waves (directional waves) or it can be written as the sum of a bunch of spherical waves emanating from each source point.

2. jpetruccelli: what is the application for your work? It sounds interesting.

Probabilities only come into play when we don't know enough about the initial conditions to predict what outcome will prevail. If I have two cups on a table and I put a coin under one while you are not looking, there is a 50% chance that the coin will be under the cup you choose. However, if you peak over your shoulder you know with absolute certainty where the coin is even though there are still two cups and only one coin.

1. Lightarrow: don't you just need to replace 'intensity' with 'probability' when you are dealing with small numbers? The same pattern will apply at a distance.

But all this is confusing the question. You want to look at the classical field direction emitted by the atom, which is an averaging over a lot of photons. In that case, if you assume the atom is a small dipole, you probably could direct the light by somehow forcing the atom's dipole moment to align in a particular direction. You would almost certainly get a pretty wide beam of light, since there's quantum fluctuations if you're dealing with a single atom, but you could probably end up relating the direction of the light to the polarization of the atom.

Ok, so light is a spherical probability wave (it has no direction but outwards)...it is spread out over a spherical surface and any detector on that surface has a probability of detecting that photon inversely proportional to the size of the surface. That is all well and good and easy to picture but what is it about light that makes it behave that way? What mechanism decides where the light should be absorbed?

Not strictly true. It is, essentially, a spherical wave. It travels in the same speed in all directions. All 'rays' cover a range of angles, however small. The wavefront is spherical because each part of it has traveled the same distance after a given time. As Soul Surfer says, atoms don't have a directional light pattern. You can use many sources, radiating in phase, as do the atoms in a laser, or a number of radiating dipoles with the same radio frequency signal to produce a directional beam. Alternatively you can use a pinhole, lens or tube to select or direct the energy to go in a particular direction. The wavefront is still spherical but its energy is restricted to a narrow range of directions.

Forgetting for the moment about the particulate perspective, how does the work from the relativistic perspective? According to relativity, an EM wave is nothing but a manifestation of a contraction of space happening perpendicular to the direction of motion of a charge - so in which direction is the charge moving? To have a spherical wavefront, would not the charge must somehow be moving in 3 dimensions at once? Is this feasible?

My argument is that, until the uncertainty of which target system gets the photon energy is resolved, all you can discuss is the probablity (as in all quantum problems). Once there is an outcome (i.e. one system gets the energy) , that is communicated to all of space and no other outcome is possible (i.e. no one else gets the energy). This idea is ok, I think, because it applies in many situations.

Whichever way you look at this, there are some difficult concepts. You can't get away with a simple, overall classical approach (of course) so you need to introduce, either some 'super-directivity' of the atom as an emitter of waves or the concept of quantum entanglement, so that only one of the possible target systems gets the photon energy.

To have a spherical wavefront, would not the charge must somehow be moving in 3 dimensions at once? Is this feasible?

What I mean by a 'super directive' radiator is ok, I assume, so you have a problem with the other idea(?)

can't separate a quantum system's properties from the quantum system which measures them.

Quote from: another_someone on 13/12/2007 10:46:34Forgetting for the moment about the particulate perspective, how does the work from the relativistic perspective? According to relativity, an EM wave is nothing but a manifestation of a contraction of space happening perpendicular to the direction of motion of a charge - so in which direction is the charge moving? To have a spherical wavefront, would not the charge must somehow be moving in 3 dimensions at once? Is this feasible?Can you explain this? (It's new to me!)

Quote from: lightarrow on 13/12/2007 12:33:23Quote from: another_someone on 13/12/2007 10:46:34Forgetting for the moment about the particulate perspective, how does the work from the relativistic perspective? According to relativity, an EM wave is nothing but a manifestation of a contraction of space happening perpendicular to the direction of motion of a charge - so in which direction is the charge moving? To have a spherical wavefront, would not the charge must somehow be moving in 3 dimensions at once? Is this feasible?Can you explain this? (It's new to me!)Sorry, I probably did not explain myself very well.Magnetism is a manifestation of special relativity acting on a moving electric field. Thus electromagnetism is also a manifestation of relativity acting upon an oscillating electric field.The point is that for special relativity to be applied, you must have something that has a velocity, which means it has a vector (one dimension, not three). A spherical wave front has no vector.

[...]btw, it seems that your link is restricted to subscribers.

I suppose what I am saying is that either there is a decision made at the source about the direction it sends the photon or in the choice between detectors as to which one detects it. An analogy would be a person having two (acoustic) speaking tubes, going to two destinations. He could choose, randomly, to speak down either tube( first option) OR, there could be an electronic circuit connecting (almost instantaneously) the two remote earpieces and controlling which of them was open at any one time (second option). The result would be, in both cases, that one, but not both of the listeners could hear the message - on a random basis. You would have no way of knowing which system was at work if you couldn't get inside the system and examine it more closely.I favour the second option because I am not happy with an option which requires extremely non-classical behaviour of a wave. The second has its problems, too, of course but it has precedents. Is that clearer - or do you just not agree with my distinction between the two?

That's why I made my subsequent remark that everything must be connected to everything else by this quantum-entangling net. Constructing the detectors would involve connecting to the net at the time of construction. Bringing a detector into existence would involve slotting it into this net; in fact 'existence' as such, would mean connection into the net.The decision/probability wave would, effectively, be spreading out from the source and the region where the photon would be most likely to be detected would be spreading outwards at what we call the speed of light. The entanglement net would be over / near the surface of this (distorted, because of interaction with matter) sphere.The more I think about this, the more I see it necessary to have something like this mechanism or else you have to decide that the actual size of a photon is much less than the system it interacts with. If it has larger dimensions, then you still have to explain what happens when it makes a choice between two adjacent atoms. When it has chosen one, what does it do to tell the others that it has chosen? Does it 'swerve' at the last minute towards the one it has chosen?

Spherical wave:E = E_{0}(r/r^{2})e^{i(k•r - ωt)}.

if a detector is subsequently put in that location D, that will be the detector which will detect the photon and no others.

Quote from: lightarrow on 15/12/2007 08:17:03Spherical wave:E = E_{0}(r/r^{2})e^{i(k•r - ωt)}.Sorry, I should have written:E = (E_{0}/r)e^{i(k•r - ωt)}where E_{0} is a vector perpendicular to k.I have corrected my previous post.

Quote from: lightarrow on 15/12/2007 18:26:26Quote from: lightarrow on 15/12/2007 08:17:03Spherical wave:E = E_{0}(r/r^{2})e^{i(k•r - ωt)}.Sorry, I should have written:E = (E_{0}/r)e^{i(k•r - ωt)}where E_{0} is a vector perpendicular to k.I have corrected my previous post.I think there's an error in that equation. Spherical waves should be spherically symmetric, and can't depend on a vector r, but only on the radial position, r=|r|. What's usually called a spherical wave should be written asE=E_{0}/r exp[i(kr-ωt)],(where I think E_{0} is perpendicular to the outward radial direction at every point, but I'd have to double check).What you've written is a plane wave traveling in direction k and with an amplitude decaying as 1/r.

Now you've sorted your vectors out, what do think of the entanglement net thing?

btw, what's wrong with having a vector r? It's pointing in the right direction and it's the right length, - what more do you want? The E vector would be normal to it. Or is my maths too sloppy?

If you're talking about the spherical/plane wave discussion above, the two cases arrive from different assumptions. Both solutions start from the Helmholtz equation (which is the wave equation for light of a single frequency). If you assume nothing in particular about the system and solve it in Cartesian coordinates, the basic components of your field will be plane waves, each of which have a given direction vector k and their polarization is perpendicular to that vector. You can make fields by adding a bunch of these together.If you assume your system is spherically symmetric, two of your three dimensions drop out (you can rotate up/down and left/right as much as you want, and nothing will change). Because of this symmetry, the fields being radiated out from the source are going to look identical in all directions, so they can only depend on the distance, r, you are from the source (a scalar quantity). The basic solution under this symmetry is the spherical wave that lightarrow explained above.

But how does one detector know that the photon is not available because another one has just taken all its energy? Th two detectors could be in different galaxies, in different directions but the same distance from the source. They must be, in some way, aware of each other, quasi-instantaneously.

The photon exists in a detector only.

QuoteThe photon exists in a detector only.Yes; I see your point.You can only be sure it WAS there when you saw the detector click.However, you can very much detect individual gamma ray photons.You could know that a gamma photon had been emitted by looking for an appropriate nuclear decay and detecting the beta or alpha emissions, without actually detecting the gamma photon. So I am not sure that you are correct for all cases.As for your 'single photon' scenario. Why would you need to know it was just one photon? You could be sure there was at least one photon.In any case, we mustn't fall into the trap of asking "what exactly is happening?" We know there is not a complete answer to this , nor will there ever be.

a|1>+b|2>+∑|states missing the detectors> → |1>

I'm now wondering what can we say about how a detector works in detail. I know that you have to tune its amplification so that it's not too low or too high; in the last case the detector will "click" even with the source switched off!

QuoteI'm now wondering what can we say about how a detector works in detail. I know that you have to tune its amplification so that it's not too low or too high; in the last case the detector will "click" even with the source switched off!You are implying a model which sounds a bit too much like an 'amplifier' for me - I look upon it as a more passive device with, as you say 'tuning' involved.

quantum fluctuation of the void