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Hearing about the wave-particle duality of light, I can’t stop wondering whether there is a field of study that attempts to understand the properties of light from a single perspective.

Is there such a field of study?

Dale
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user289714
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  • The answers you've gotten seem to disagree on what you mean by 'from a single perspective'. Could you clarify what you mean by this phrase? Do you mean a single perspective that explains both wave-y and particle-y behaviour, or do you mean a field that only uses one and ignores the other? – jacob1729 May 30 '21 at 14:36
  • One thing I think that may be confusing you is that I think you think that wave-particle duality of light is a theory or a law. What you need to know is that it is NOT A THEORY. It is what we observe how light behave. We look at light and we see it behaves as both a wave and a particle. It is FACT. Now that we accept the FACT that light has a wave-particle duality any theory we come up with to describe light must explain why light has a wave-particle duality – slebetman May 30 '21 at 19:24

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Yes, that theory is called quantum electrodynamics, or QED. It predicts all of the “particle” behaviors and all of the “wave” behaviors of light from a single unified mathematical model.

QED was the first successful quantum field theory. Since its development other similar field theories have been developed for the strong and weak nuclear forces. So currently duality is not a problem in physics.

Dale
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    I would contest that it predicts all of the particle properties. But that tangent would be too long for a comment (or even a brief answer). – doublefelix May 29 '21 at 18:29
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    @doublefelix Hmm I see your point. I meant “behaviors” instead of “properties”. Indeed it is the SM not QED that predicts the properties, and even then the free parameters of the SM are measured rather than predicted. Anyway, I will change the wording accordingly. – Dale May 29 '21 at 18:33
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Yes - quantum field theory; gauge theory in particular.

Oбжорoв
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The "duality" only comes in when you make a measurement. So the confusion about this duality is really just confusion about the measurement problem in Quantum Mechanics.

The fact that light is a wave is reflected in the description of the electromagnetic field as an element of a complex linear vector space.

The particle aspect is simply reflected in the fact that this vector space has an inner product defined on it that allows you to project a state onto a given state that you are interested in. In other words, it is a Hilbert space.

The quantum description of the electromagnetic field takes both these aspects into account by construction, so there is no need for a new perspective. Quantum mechanics was designed to solve this problem about 100 years ago.

(QED is such a theory, but it contains other things, namely the interactions with charged matter.)

Eric David Kramer
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  • Wow, I got two downvotes for this answer! Would anyone mind telling me what was so offensive about what I wrote? I know it sounds unconventional, but I believe my answer to be correct. Why do quantum states live in a Hilbert space? Why can't they live in any linear vector space? – Eric David Kramer Jun 06 '21 at 07:06
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I would argue that there is no such theory. It's tempting to consider quantum field theory to be that theory. That is, to see quantum fields as the unifying structures of waves and particles. But quantum fields can wave too. They can behave like particles too. It is even said that particles are excitations of a quantum field but I think this is too far-fetched. Particles are particles and quantum fields are just mathematical structures that show their (weird) behavior. The ultimate constituents of matter are (almost) pointlike particles, as assumed in both quantum field theory and quantum mechanics. In the case of light, this means that only photons are real. Sometimes, a collective of these particles shows wavelike behavior (a single particle never can), like in the double-slit experiment, sometimes they show particle behavior, like in high energy experiments.

So it's not QED that unites the two directions but the simple fact that waves are properties of collections of particles, be they photons or electrons or whatever elementary particle to which we can assign a quantum field. In this case point- like photons. Wavy Nature becomes apparent only if we consider a huge amount of these photons. Photons travel in all directions, at any speed, forward and backward in time in going from one point in spacetime to another, while these points are determined by the interaction of the photons with charged particles anywhere, while these charged particles on their turn are determined by interaction with other charged particles or macroscopic objects. All different paths have a certain probability amplitude and it's this amplitude that gives rise wavy behavior of collections of photons. This is the case when you look at a classical electromagnetic wave. All this photon craziness adds up, resulting in the classical em wave.

Deschele Schilder
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    "(a single particle never can)" - Isn't the double slit experiment with single photons contradicting this? You'll never see it hit certain places, so it must interfere with itself. – Jens May 30 '21 at 09:52
  • @Jens Can't you see it hit the screen? In the single-photon experiment, many photons are sent one by one to the screen. Each one hits the screen at a certain place (particle-like behavior), but the collective shows a wave-like pattern. There are indeed two parts of a wave (coming from the slits) interfering, but the single particle doesn't show this wavelight Nature. Only the collective will show this. – Deschele Schilder May 30 '21 at 10:05
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    "quantum fields can wave too" — this is just a side effect, and it's necessary, since otherwise correspondence principle will be broken. The point is that QFT is the single perspective from which all the seemingly dual properties appear. – Ruslan May 30 '21 at 10:32
  • @Ruslan But that perspective is particle-like. It's the way these particles behave that makes the wave Nature come forward. In classical electrodynamics, there were no such particles. Both in QFT and "ordinary" QM there are. The particle view unites both waves and, well, particles, just as it does for, massive particles. – Deschele Schilder May 30 '21 at 11:31
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    It's the classical-brain's perspective that ascribes the properties of particles to the localized wave packets formed by superpositions of $N$-photon states. The photons themselves, as described by the QFT, don't even resemble particles: $a^\dagger_{\vec k}$ creates an eigenmode of the field, which is maximally delocalized in space for any $\vec k$. Only after you do a Fourier integral over all $\vec k$-space do you get $a^\dagger_{\vec x}$, which is not the basic unit QFT is constructed from. – Ruslan May 30 '21 at 11:37
  • @Ruslan Aren't photons considered point-like in QFT? – Deschele Schilder May 30 '21 at 11:39
  • @DescheleSchilder yes, they are "point-like" in the sense "structureless". As for considering the size of the photons, there're lots of pitfalls; even position is ill-defined, see e.g. Born rule for photons: it works, but it shouldn't? – Ruslan May 30 '21 at 12:00
  • @Ruslan I think the basic difference between photons and massive particles in QFT is that photons can be absorbed or emitted by electrons (both point-like as far as we can tell). Photons don't literally bounce with electrons (like particles do in classical particle theory). Neither do electrons with electrons. This does not happen in classical theory. I'm not sure what Peierls means. It is said that photons can't have a well-defined position but that concerns only the measurement of a photon's position. If the electron has a position, so does the photon emitted. – Deschele Schilder May 30 '21 at 13:19
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    The statement "particles are particles and quantum fields are just mathematical structures" makes me think there's a conflation here between particle theories as mathematical models and analogies of particles to macroscopic objects like thrown rocks that we have an evolved intuition for. Mathematical models with analogies that feel more concrete/familiar might feel more real, but ultimately the important thing is how well the mathematical models make successful predictions. If any theory is 'more real', it's just that it is more complete in its predictive power. – Dan Bryant May 30 '21 at 15:13
  • @DanBryant I agree. But particles are real, mathematical models are not. They just describe the behavior of the particles. It's not the mathematics that exists out there. There has to be some kind of conflation to make a connection. The math describes physical things. It remains to be seen if the point particles (abstractions from things like rocks) are something real. How they behave can't be understood by looking at rocks. Photons can be absorbed, rocks can't. Point particles can take any path between two spacetime points (in both directions of time), rocks can't. Difficult to imagine... – Deschele Schilder May 30 '21 at 16:34
  • That which we call "particle" in physics class is unlike what we call "particle" outside. Does the atom emit a particle or does it excite a mode? QFT says it excites a mode. One thing is certain: whatever it is, our brains are not equipped to understand it. This debate has been ongoing for decades. I doubt we'll come to a conclusion here. – garyp May 30 '21 at 22:19
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    @garyp Yeah, whatever we call it (exciting a mode or sending a particle), the real Nature will always remain a mystery, as far as the imagination is concerned. Maybe we can know the "inside" by the very fact that we ourselves are part of the particles or modes of fields. It's the inside that gives us soul. – Deschele Schilder May 30 '21 at 22:42
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I interpret the question as follows: is there a field that studies electromagnetism either exclusively from the wave concept or exclusively from the particle perspective.

The answer to this question is no. There are fields of applied physics that use only the wave perspective, but this is a practical choice. Ray optics and wave optics are tools for optical and RF design. On the other hand, I don't see how there can be a field that studies electromagnetism exclusively in terms of photons, without considering the waves that describe these.

QED and QFT use both particle and wave aspects, so these theories are not based on a single perspective as meant in the OP.

my2cts
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    Well, there are -- they are ray optics and wave optics respectively. Sure, their predictions don't match with experiments but each of them does study light from a pure particle and a pure wave perspective respectively. –  May 30 '21 at 10:44
  • @DvijD.C. As I said, "There are fields of applied physics that use only the wave perspective, but this is a practical choice. " – my2cts May 30 '21 at 18:12
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    @DvijD.C. It is not true that ray optics approaches electromagnetism from a particle perspective. It is an approximation to wave optics. – my2cts May 30 '21 at 18:15