I Wave-particle duality: nature of wave-particle as it travels

1. Mar 25, 2016

Peter99

Hi,

I'm trying to conceptualize the life of a particle as it travels through free space. I wish to start simple and then build from there.

Speaking about the wave-particle duality that we observe in fundamental particles.

So a photon is emitted from a source (perhaps an electron moving from a higher state to a lower state), it then travels in all directions (spherically?) as a wave, then it encounters some form of matter that can absorb it and it gets absorbed as a photon.

So an electromagnetic emission starts life as a photon, travels as a wave through free space, then ends life as a photon.

1) Would this be a satisfactory conceptual explanation for what appears to happen?

2) If so, how is the wave propagating? I mean, I can visualize how gravity waves propagate. I can visualize how, say, ocean waves propagate, or shock waves propagate through a material. But I have trouble visualizing how the energy of a photon is propagating through free space as a wave. I understand that EM radiation is an oscillating electromagnetic field and that a photon does indeed have momentum, but how does it move?

3) Also, if the wave travels out spherically from the source, then is the energy evenly distributed throughout the sphere of travel?

4) If so, then when it encounters something that can absorb it, wouldn't the energy, that is evenly distributed throughout the sphere of travel, have to basically instantaneously move to the point at where the photon is being absorbed?

Thanks for the help and insight,
Peter

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2. Mar 25, 2016

A. Neumaier

The photon starts like as a wave, travels as a wave (an excitation of the electromagnetic field - which permeates what you call ''free space''), and ends like as a wave. Its particle nature is only visible when and as long this wave is strongly localized as a wavelet. The form of the wave is spherical if the source is open in all directions, but more usually (e.g. for a laser source), the form of the wave is paraxial, focussed in a beam.

If you can visualize how gravity waves propagate - electromagnetic waves are very similar in most respect, except that gravity has twice as much spin.

When electromagnetic radiation (aka photons) hits an absorber, it is absorbed in whole pieces or not at all, not in arbitrarily dilute amounts. This discreteness is due to the quantum nature of the absorbing matter, which can attain an excited local state only when it gets a minimum amount of energy.

3. Mar 25, 2016

A. Neumaier

The photon wavelet wiggles forward with the speed of light, pulled by its momentum, always growing at its head and shrinking at its tail. And it becomes a little fatter with time, since its energy tends to spread.

4. Mar 25, 2016

Staff: Mentor

I don't think "wiggle" is a good word here, it leads to common misconceptions of things flying in curves.

In most interpretations of quantum mechanics, yes.

In some interpretations of quantum mechanics, yes. In others, no.

5. Mar 25, 2016

A. Neumaier

A wiggling tail dosn't fly in curves. It wiggles even in its rest frame. (Allowing the photon to be massive, as you suggested).

Peter99 said he can imagine how water waves and gravity waves propagate. So I tried to help him within his imagery.

In my magination, water waves wiggle up and down and at the same time forward. But it is difficult to get them in transversal wavelet format. So I think of photons as little fish made of wiggles that travel like a wave travels but at the same time keep their fish form. This is as good as one can make an intuitive picture of a single photon, and far better as the traditional one of a little cannon ball.

Of course the picture only works for single photons, as was asked. It starts to fail miserably if one want to use it to model entangled photon pairs traveling in opposite directions. You would have to imagine invisible, arbitrarily stretchable strings of love joining the pair of wavelets that keeps them one body and soul while they separate farther and farther. These strings would weaken and disappear only with enough decoherence, though this is the usual fate of love at a distance that gets too long.

Last edited: Mar 25, 2016
6. Mar 25, 2016

Staff: Mentor

Sure, but photons do not wiggle up and down. And I lost track how often I encountered and tried to correct that misconception, as wiggly photon lines appear everywhere in images.

7. Mar 25, 2016

A. Neumaier

Well, it is the standard picture for photon lines in Feynman diagrams. This cannot be altered.

But it is only imagery for the intuition, not reality. There is no good imagery for the more appropriate transversal wiggling, so one has to allow for a few more grains of salt. How would you paint the motion? Or do you simply suggest that one should not use any imagery at all?

8. Mar 25, 2016

Staff: Mentor

Yes, we know that. Others do not.
And while I was not thinking about Feynman diagrams: explaining that virtual particles are not real is another windmill-fight.
What would be wrong with a straight line? Not in Feynman diagrams, but at least in other diagrams (e.g. light bulb + light).

9. Mar 26, 2016

A. Neumaier

So this must be emphasized over and over again.
I know. Feynman opened a Pandora box with which we now have to live....
The problem here is that it immediately creates another puzzle: One then has to explain how waves can move along strainght lines. Geometric optics and all that. It is not intuitive enough to catch on and replace the common simpler but less appropriate imagery.

10. Mar 26, 2016

vanhees71

Well, it's said that Schwinger never used Feynman diagrams. So it's possible to do QFT without Feynman diagrams, but it's even more complicated than with them and, I'm pretty sure, much less fun ;-).

Then you asked, what's wrong with a straight line. Very simple, it usually depicts fermions (dashed lines depict (pseudo-)scalar bosons, dotted lines Faddeev-Popov ghosts, double straight lines $\Delta$ resonances, and pigtail lines non-abelian gauge bosons as gluons in QCD). So there's nearly no line style left to depict photons, and they where wiggly lines already in Feynman's original papers ;-)).

11. Mar 26, 2016

Staff: Mentor

Sure, but all those things are purely convention. We could use dotted lines for photons, and wavy lines for Faddeev-Popov ghosts.
Anyway, I was not talking about Feynman diagrams with the complaint above.

12. Mar 26, 2016

vanhees71

Sure, the trouble is that sometimes Feynman diagrams are sold as something depicting the real "mechanism" behind interactions between particles, but what they are in fact depicting when applied to relativistic quantum field theory "in vacuo" (i.e., to scattering processes of a few particles) are just formulae used to calculate transition matrix elements to describe scattering processes in perturbation theory. It's just an ingenious short-hand notation for these formulae invented by Feynman in a very intuitive way. At the famous Shelter Island conference in 1948 he and Schwinger, who used quantum-field theoretical formalism rather than any kind of pictures, to calculate such matrix elements, both coming to the same result. Also Tomonaga already during the war used techniques quite similar to Schwinger's. It was finally Dyson who used QFT to derive Feynman's diagrammatic fules. Unfortunately in the popular-science culture only the intuitive picture of Feynman diagrams are presented as if this intuition is the real thing, and then all kinds of weird ideas come up like "vacuum fluctuations" or "virtual particles", which have a definite meaning of course in the sense of QFT, but that's often completely deformed by the pop-sci writers :-(.

13. Mar 26, 2016

Peter99

Hi,

OK, well I had four questions, and now I have 20. But I guess that's the way of things. I appreciate the input!

That's OK about the wiggly fish imagery A. Neumaier put forth. I understood. It's interesting that you all are (casually) talking about a photon traveling and not a wave.

By "free space" I meant just space without matter present. Again, just trying to keep things simple as it all can get very complicated very quick. Would there be a better term or is there a conventional term to describe free space as I have defined it here?

Yes, but thanks for the clarification.

"Pulled by it's momentum"? Really? I always envisioned momentum to have more of an inertial quality to it... more pushed then pulled... or maybe not even pushed but not pulled either. But then we are talking about a massless particle. Is pulled the better way to envision this? If so, why?

This surprised me. I mean, a photon I think of as elementary and doesn't go through an actual energy-mass transfer from wave form to particle form. Could you elaborate on this?

So in question 3 and 4 you answered respectively:

So do the same people who say No to #3 also say No to #4?

How do they interpret collecting a photon, that has traveled billions of light years, as happening effectively in zero time? Are they saying that it does not take zero time but just an extremely small amount of time? Because then, conceivably, the energy is spread out over a HUGE area and the back end is (twice) billions of light years away. Say a ten billion light year diameter, so the energy at the back end would have to travel approximately 9.46E25 meters if it took a straight line to the detector. How quickly do modern detectors detect a photon in that what is the delta t for the transducer to make the detection? The actual gathering of the energy, not the instrumental overhead. I guess they can't gather the energy in less then 5E-44s (Planck Time) can they? However one looks at this, the energy has to travel hugely past the speed of light.

OK, a new question but related directly to the above, is there any evidence (empirical not theory) that electron tunneling happens with a time lag?

I remember my ex-wife, who was in graduate school studying particle physics (She did a lot of work a SLAC and a bit at CERN while I was doing the undergrad thing), bringing home a group of friends (all grad students) and we would all sit there and talk about this kind of stuff. I loved it then and I still love it!

Thanks for the input and insight,
Peter

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14. Mar 26, 2016

Peter99

Oh ya, and just to comment on this whole Feynman diagram thing. Obviously he just used the wiggly line symbolically. It's the same as on electrical schematics, they use the same wiggly line symbol to indicate light transmission (for photo-detectors and such) . I tutored for years and don't remember having any trouble with this in that students didn't seem confused. At least I don't remember anyone being confused about this. But I can see how perhaps one could be confused and obviously some of you have have run into trouble with students (?) getting the wrong idea about it. As A. Neumaier said, "A photon wavelet...(travels) always growing at its head and shrinking at its tail" and if one looks at the transverse wave of EMR then it invokes a wiggly feeling. I see no trouble with it, in fact, I think it's appropriate.

Peter

15. Mar 26, 2016

Staff: Mentor

Massive particles like electrons are way easier to work with because with them we can ignore the effects of relativity. Non-relativistic quantum mechanics works when the speeds are small compared with the speed of light and the energies are small compared with the amount of energy needed to create additional particles - and neither of these conditions applies to photons. Thus, for photons we need all the relativistic machinery of quantum electrodynamics where the concept of wave-particle duality never even appears.

However, even when working with massive particles and non-relativistic quantum mechanics, the notion of wave-particle duality often confuses more than it clarifies - and I submit this thread as empirical evidence in support of that proposition. Come to think of it.... That's another ongoing thread right now: https://www.physicsforums.com/threads/legitimacy-of-particle-wave-duality.863688/ and my opinion is down at #6.

I find it best to think of quantum objects as things that have some wave-like properties and some particle-like properties, but are neither. It is an unfortunate historical accident that physicists use the word "particle" to describe these quantum objects even though they do not behave much like the little tiny billiard balls that are implied by the colloquial English sense of the word.

16. Mar 26, 2016

A. Neumaier

There are three possibilities - pulled, pushed, or self-propelled. A photon cannot help moving with the speed of light; so I picture it as being pulled. Or pushed; it is perhaps a little better, but it doesn't really matter. Pictures are just that - little mental illustrations drastically simplifying the real thing. One can do it in any way one feels comfortable with, as long as one remembers that each mental picture has its (sometimes severe) limitations.
The traveling time doesn't matter. The detector responds with a firing rate proportional to the impinging intensity, no matter how low or irregular it is - and whenever it fires one says ''a photon arrived''. This is figurative talk, too, since in models where the electromagnetic field is not quantized (so that there are no photons in the model), the detector still behaves in the same way. All imagery and language used must be viewed in this light: It simplifies in order to allow being put into words and intuition, and the simplification is valid only to the extent it sensibly matches what can actually be observed.
Yes, though it is somewhat controversial. See, e.g., http://arxiv.org/abs/1301.2766. By the way, I think that tunneling is another one of these misguided imageries: Since it is impossible to tunnel through an infinitely high potential barrier, height matters, unlike in true tunnels, where only the distance to the next valley matters. This suggests that the tunneling electron was in fact climbing the barrier.

17. Mar 26, 2016

Staff: Mentor

There is no transfer. Particles are quantum objects. They are neither classical waves nor classical particles. Some properties of quantum objects are not so different from classical waves or classical particles, but those are always just similarities. The classical concepts never capture the properties of quantum objects properly.
The options:
- the energy spreads out evenly, and it does not get redistributed (e.g. many worlds)
- the energy spreads out evenly, and it gets instantly redistributed (some collapse interpretations)
- the energy goes in a specific direction but we cannot know which, it does not get instantly redistributed (e. g. de-Broglie-Bohm)
- we cannot make proper statements about the energy distribution before we measure it, so the questions are meaningless (ensemble interpretation and some others)
- probably some I forgot
Yes, some interpretations are nonlocal (they have effects that act faster than the speed of light).
Time lag between what and what? That is a highly non-trivial question.

18. Mar 27, 2016

vanhees71

Quantum theory predicts indeed long-ranged correlations that are stronger than possible in local hidden-variable theories (Bell's inequality being violated) for subsystems of entangled systems, and this has been indeed confirmed to high accuracy. This, however, has nothing to do with "effects that act faster than the speed of light". By construction, local relativistic quantum field theory does not admit such "spooky actions at a distance" (micro-causality).

19. Mar 27, 2016

Staff: Mentor

I said "some interpretations", not "quantum theory". Quantum theory is local - and you'll find countless threads where I highlight this. Some interpretations are nonlocal.

20. Mar 27, 2016

vanhees71

Well, then these interpretations go beyond quantum theory and thus are new theories rather than interpretations!