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Light-'matter' interaction

  1. Apr 6, 2015 #1
    I have just started reading about a classical electromagnetic treatment of light-matter interaction (beginning with dispersion relations, and then moving on to the standard phenomena - reflection, refraction, etc.). The discussion begins with a forewarning that light is not 'continuous' as the classical electromagnetic treatment suggests. It touches upon the photonic nature of light, and this has got me confused:

    We say that light exhibits its particle nature only when it interacts with matter. But going by what quantum mechanics teaches us, isn't the matter light would interact with a wave (-particle) too?

    I mean, consider a grain of sand. Apparently, we cannot observe its wave-particle duality because even while sitting still, it always has a certain instantaneous momentum due to thermal vibrations, which give it a corresponding wavelength too less to be observable.

    Now, I want to apply 'light-logic' to this grain of sand: when light is not being observed (for example, as it propagates through vacuum) it behaves like a wave (a probability wave) and then collapses into a specific state when observed (i.e., when it interacts with 'matter'). By a similar token, one would suppose a sand particle to be a wave when not interacting with other matter or light, but it is clearly always a particle no matter what...how? I thought maybe it is because it is always interacting with 'ambient' photons, so it is always in a state of 'observation'. But then, the same should have been true for a photon as it moves between the slitted and solid planes in Young's double slit experiment - after all, it is always interacting with the particles in the air isn't it? Does being so small somehow give the photon a special status?

    Basically, I'm trying to analogically apply the concepts we use with microscopic particles to macroscopic objects, but things don't seem to fit. How am I supposed to imagine the wave (probability wave) of, say, a sand particle (which has vanishing wavelength)?
    Last edited: Apr 6, 2015
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  3. Apr 6, 2015 #2


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    I really have no idea what you mean by "it is clearly always a particle no matter what". First, what do you mean by "a particle" when you are working with quantum theory. Second, where did you get the idea a grain of sand "is clearly always a particle"? A much better way of thinking about quantum theory is that "particle" and "wave" are both themselves human "constructs"- that "reality" lies in a combination if those two concepts.

  4. Apr 6, 2015 #3
    Wow, I'm sorry - that sounds very childish and ambiguous now that I read it again. Let me explain. By particle, I mean to say that it is 'deterministic' - that is, we are observing it.

    As far as quantum theory is concerned, the wave-function is only an abstract quantity. My understanding is that as long as the entity exists as a wave, it is not being observed. That is, no measurement operator is being 'applied' on it. To be more mathematically precise - no action corresponding to a Hermitian operator is being applied on the entity, which would cause it to collapse into one of the eigenstates (and thus be observed in that eigenstate).

    I'm just trying to understand how to imagine the wave associated with the sand particle. For example, the way we make 'clouds' to represent electrons in atoms - they are the probability wave representations of the electron. Similarly, with light, we've got the Huygen's wavefronts to represent photons. But those are both microscopic objects. We can imagine them taking this form of waves when not interacting with other matter, which is a plausible situation. A sand particle may not find such a space free of any matter so easily, but if it did, how would we think of its (probability) 'wave'?
    Last edited: Apr 6, 2015
  5. Apr 6, 2015 #4
    I have been told to think of it this way, but I am having difficulty merging this picture with the one we are taught about in light-matter interactions. The picture in which light propagates through space as a 'probability wave', and shows up somewhere on the interference pattern when it interacts with the viewing plane.

    I want to know if my understanding here is correct - that observation in the quantum sense and interaction with other matter can be thought of as symmetrically.

    If so, how do we define 'interaction with matter'? Other matter that can be interacted with is itself either wave or particle...so do we need to think of interaction with two in separate ways?

    Finally, how is it that all of the macroscopic objects are always in a particle state (in the sense that I have tried to explain the concept in the previous post)? Is it possible for them to take on an abstract wave nature too?[/QUOTE]
    Last edited: Apr 6, 2015
  6. Apr 6, 2015 #5
    Modern physicists do not think in terms of a wave-particle duality. That concept has its limits, and your questions go past those limits. Physicists simply call everything a particle, and this includes the wave aspects.

    The difference between a macroscopic object and a microscopic particle is that a macroscopic object is constantly interacting with the environment. It is bombarded by countless photons and other particles, and emits them back to the environment. Every interaction with the environment has to be consistent with every other interaction (an interaction is like a measurement). Each interaction puts more complex constraints on the object's state, and this seems to collapse it into a macrostate. For a microscopic particle, there are relatively few interactions with the environment, so there are few constraints on the particle's state, so it is allowed to have a superposition state. It's a fuzzy boundary.
  7. Apr 8, 2015 #6
    So, what I understand from this is that wave-particle duality is not really a thing. That is kind of hard to accept for me, and the biggest hurdle to my acceptance is that Young's double slit was so elegantly explained when we took probability waves as the means of propagation of photons in between the slits and the viewing plane. If I could get some guidance on how to merge the two concepts into one homogeneous idea (with regard the double slit experiment), I think I'll finally understand it and accept it.

    But the photons that participate in the interference pattern in Young's double slit experiment must interact with the air molecules extensively, so shouldn't they be collapsing into 'macrostates' too? Shouldn't this destroy the interference pattern?
  8. Apr 8, 2015 #7
    You don't need quantum mechanics to calculate a two slit interference pattern. It's pure classical physics, described by electromagnetic waves. It's only when you try to combine the double slit experiment (which suggests light is a wave) with the photoelectric effect experiment (which suggests light is a particle) that things get weird. Light doesn't switch between being a particle and a wave. It's always a particle which is described by a wavefunction. As long as you know how to calculate the results of experiments, don't worry too much about the interpretation. That is secondary, and people don't agree on it, although we all agree on the calculations.

    The photons only interact with the air molecules weakly, since air is mostly transparent. This is a "weak" measurement of the photons, which doesn't destroy the interference pattern. The photon wavefunction is almost unconstrained by the air particles so the probabilities of interaction with the screen are almost unchanged.
  9. Apr 8, 2015 #8
    I think if the air becomes optically thick enough to significantly scatter the light, the photons going through the top slit will lose coherence with the photons going through the bottom slit, so they don't interfere, and the pattern disappears. It just looks smoothed out. This is a form of decoherence.
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