3

Wikipedia says that

Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum. The oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave.

The page also includes this image:

electromagnetic wave

which shows that.

But I find that sometimes the wave is represented with B-field at is peak on the nodes, like here:

Alternate picture of wave with fields out of phase

taken from Wikimedia Commons, and it would make some sense, too, considering that it grows with acceleration and this is maximal there.

Can you please say if the second picture is wrong, and if those representations are both a mere pictorial, fictional, simplified, arbitrary representation of an EM wave?

Do you know if modern instruments are able to record with precision the oscillations of the electric and magnetic field when detecting photons (now we have lots of collimated photons in laser beams can you detect the fiels at the emittter or receiver) ?

David Z
  • 76,371
  • 1
    Voting to reopen, as the question in the last sentence is not addressed in the cited duplicate. – garyp Aug 22 '16 at 12:47
  • I'm going to create a new question focusing on the experimental evidence. – garyp Aug 22 '16 at 13:01
  • Just making a note in response to some now-deleted comments that failure to cite and quote copied material is never okay, regardless of the quality of the source. – David Z Aug 22 '16 at 14:21
  • 1
    @garyp Indeed the final sentence poses a non-duplicate question, but unfortunately the main problem here is that the post asks multiple questions. So, part of it is a duplicate and part is not. OP should edit the post down to a single focused question. – DanielSank Aug 28 '16 at 23:10
  • @DanielSank, why multiple question? is laser not light? the question is one, and then there is the request to support the answer with experimental evidence –  Aug 29 '16 at 04:39
  • @user104372 perhaps you could edit the post to make it clear that the final paragraph is asking for experimental verification that the electric and magnetic fields are in phase. – DanielSank Aug 29 '16 at 04:47
  • @DanielSank, isn't that obvious? what else can it mean? Do you agree that in the receiving antenna they are just meausiring not the E-field of the photon but the field produced (induced) by the photon? I did not edit because I know nobody will or can answer that because it's just another ingrained misconception –  Aug 29 '16 at 04:52
  • @user104372 Whether or not you think it's obvious is immaterial. Almost everyone is misinterpreting the question. Few people get to the last paragraph, and those that do think it's a separate question. You and I had a long back-and-forth before I understood that you do not doubt the theory. If you really want responses edit the question – garyp Aug 29 '16 at 10:55
  • @garyp, how many responses did you get to your late question? wasn't it clear? Rewiewers are more or less like you, either they do not really read or , when they can't answer, close the question –  Aug 29 '16 at 11:22
  • I don't know what late question you are referring to. Nonetheless, we are who we are. If you can't accept the reality, you'll never get an answer. Finally, I've read your question carefully. The questions you wrote have been answered, and if you don't think so, then either 1.) you haven't read the comments carefully or 2.) you did not express yourself well in the question. – garyp Aug 29 '16 at 11:56
  • @garyp, the fair-copy of this question you deleted –  Aug 29 '16 at 12:09

2 Answers2

0

Radiation from the sun follows a black body spectrum more or less, and is not coherent, i.e. the phases between different slices of sunlight are not defined. The photons come from innumerable incoherent de-excitations from the plasma of the sun's surface.

It can be simulated by plane waves impinging at all the frequencies of its black body spectrum, which is your first plot. Those functions describe plane waves.

Incoherent electromagnetic waves can be made coherent when passed through small openings, a slit for example, that is why interference fringes appear at single slits. The appearance of fringes validates experimentally the plane wave functions describing the electromagnetic wave.

Do you know if modern instruments are able to record with precision the oscillations of the electric and magnetic field when detecting a photon?

The photon is a quantum mechanical elementary particle, and classical beams and their electric and magnetic fields emerge from a superposition of innumerable photons.

Photons when detected individually are a single point on a screen , leaving energy h*nu where nu is the frequency of the classical beam that was built up by such photons, and at most one can detect in its interactions the spin it has, +/-h in its direction of motion. No electric or magnetic fields, because the information about them is carried in the wavefunction describing the photon which is a complex function and cannot be susceptible to measurement. Only in the confluence of innumerable photons one reaches the classical regime where electric and magnetic fields can be detected. Yes, there are antennas which detect and measure electric fields from the electromagnetic radiation.

anna v
  • 233,453
  • laser light is coherent http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lascon.html . I do not think people bother with the magnetic field of an incoming electromagnetic wave. The electric field is what is caught at the antennas and registered. – anna v Aug 21 '16 at 19:55
  • anna, the electric field caught at the antenna is not the field of the photons, (is it? ) but the field caused by them making the electron in the antenna oscillate. It is therefore obvious that that E-field is synchronous with the incoming B-field of the radiation. Don't you agreee they have it all mixed up? –  Aug 22 '16 at 04:34
  • It is not the electric field of the photons. The electric field of the photon is in a complex number form in the wavefuncion. The interactions of photons always happen with the square of the wavefunction, so the electric field of a photon is not a measurable quantity. The antena picks up the field built up by the zillions of photons in the wave. The coherence of the laser light is a coherence in the electric field waveform. This phase can will be mathematically found in the complex phases of the wavefunction but is not measurable. only the confluence/supeposition of photons is measurable – anna v Aug 22 '16 at 05:24
  • As far as photons go, the phases is seen only in the built up probability density distribution. This mathematically coincides with the energy distribution of the classical wave, because both the classical and the quantum form come from maxwell's equation, for photons used as operators on the wavefunction. https://arxiv.org/ftp/quant-ph/papers/0604/0604169.pdf – anna v Aug 22 '16 at 05:26
  • Am I missing something, or are you actually saying that the fields and consequently their phases have never and can never be detected, and we must believe in their syncronicity as an act of faith to Maxwell's theory? –  Aug 22 '16 at 08:56
  • FOR INDIVIDUAL PHOTONS , the quantum mechanical substructure. One does have faith that validated theories mathematically describe unreachable spaces. The classical wave has been validating maxwell's equations from their conception – anna v Aug 22 '16 at 11:49
  • Surely light coming from the sun has a non-zero correlation time. I wonder what it is. If starlight were entirely incoherent I think the Hanburry-Brown Twiss effect wouldn't work, but I'm not completely sure if that's true. – DanielSank Aug 28 '16 at 23:12
  • @DanielSank I suppose someone has measured the coherence time of sunlight/starlight and it is certainly not zero. But there is confusion among questions and answers here: are we talking about E-E correlations, or E-B correlations? I'm never quite sure who's addressing what. – garyp Aug 28 '16 at 23:27
  • @DanielSank http://hank.uoregon.edu/teaching-modules/Broadband-Interferometer/BBcoherence.pdf – anna v Aug 29 '16 at 03:15
  • @garyp see above link – anna v Aug 29 '16 at 03:15
  • seems to me if one breaks down the black body radiation into frequencies /plarizations then coherence lengths can be defined. But this is a different story imo – anna v Aug 29 '16 at 03:23
  • @annav A theoretical calculation of the coherence length. Thanks. I don't quite understand your comment about breaking into frequencies. Still nothing on E - B correlation. – garyp Aug 29 '16 at 10:51
  • @garyp The paper discusses frequencies, i.e. the study is on monochromatic slices of black body radiation. Radiation comes altogether jumbled frequencies at time t. imo E B correlation has no meaning for an individual photon, as the E and B are in the wavefunction in a complex number. For the emergent wave of course they are correlated, depending on the boundary conditions – anna v Aug 29 '16 at 11:02
-4

The shift of 90° between the maximum of the electric field component to the magnetic field component is a very natural view on how photons are propagating in free space. First this is the situation in the near field of an antenna radiation. An electric field induces a magnetic field induces again a magnetic field and so on. Second this shift conserves the energy content of the photon in any point of it's movement in space.

The derivation of the sin is a cos is a -sin is a -cos and perhaps it is possible to transform Maxwells equations in such a way, isn't it?

Perhaps it is possible to interprete in the far field of the radio waves as no shifted by 90° but my question about measurement results for such a interpretation does not get any source to this measurements.

HolgerFiedler
  • 10,334
  • 1
    I don't think this answers the question. Also, note that the phase shift has nothing to do with conservation of energy. Energy is conserved when there is a phase shift (near the antenna) and when there is no phase shift (far away from the antenna). Note also that using the language of photons and the language of fields at the same time leads to a confused picture. – garyp Aug 21 '16 at 11:59
  • 1
    @garyp It seems to be a mantra that in the far field of a radio wave there is no shift since it was never measured, see your own comment. And how we can learn something about photons if not from the behaviour of radio waves there a huge number of photons get emitted from accelerated electrons? The appearing fields are made from this photons and this is why we know that each photon has varying electric and magnetic field components. – HolgerFiedler Aug 21 '16 at 12:05
  • @garyp And about what are photons, what is EM radiation and what are radio waves I expressed myself very clear here – HolgerFiedler Aug 21 '16 at 12:09
  • The E and B fields of radio waves can be measured, and in the far field they are in phase. There is no difference between the behavior of optical waves and radio waves. Fields are not made of photons. Photons are excitations of fields. The fields exist whether or not there are any photons in them. – garyp Aug 21 '16 at 12:10
  • 1
    @garyp please give a source for the measurement of the far field components and it's shift or no-shift of radio waves. – HolgerFiedler Aug 21 '16 at 12:16
  • I'll see what I can find. But the theory is so clear, so unambiguous about this, and the theory is so successful, that it's not likely that anyone would have ever done the experiment. – garyp Aug 21 '16 at 14:07
  • By the way ... do you doubt that E and B are in phase? If so, why? Maxwell's equations are unambiguously clear on this. Do you doubt Maxwell? Or are you just curious to see if it's ever been measured? – garyp Aug 21 '16 at 18:28
  • @garyp Did you read my answer? Did you find any inconsistencies? It is not ok to ask about evidence from measurement? – HolgerFiedler Aug 21 '16 at 19:06
  • I read it, but I don't understand it. As I said earlier, energy is conserved, in either case, in phase or out of phase. I don't understand what you are getting with with your mention of sin and cos. You can ask for evidence and if I find it I'll post it, but this is so well established that I'm not optimistic about finding anything. You didn't answer my question: why do you doubt the answer given by Maxwell's equations? – garyp Aug 21 '16 at 20:30
  • @garyp I'm sceptic about the fullness of Maxwells equations because in a radio wave (in which zillions of photons bear the wave) an electric field bear an magnetic field bear an electric field an so on and this happens with the derivations of sin. I'm sure that it is possible to express this in modified Maxwell equations. BTW Ptolomeus earthcentristix view of the space was well established for approx. 1500 years :-) – HolgerFiedler Aug 22 '16 at 03:39